Plants as Food

The Nature of Plants

Definitions

Plant Foods Through History

Plant Foods and Health

Essential Nutrients in Fruits and Vegetables: Vitamins

Phytochemicals

Fiber

Toxins in Some Fruits and Vegetables

Fresh Produce and Food Poisoning

The Composition and Qualities of Fruits and Vegetables

Plant Structure: Cells, Tissues, and Organs

Texture

Color

Flavor

Handling and Storing Fruits and Vegetables

Post-Harvest Deterioration

Handling Fresh Produce

The Storage Atmosphere

Temperature Control: Refrigeration

Temperature Control: Freezing

Cooking Fresh Fruits and Vegetables

How Heat Affects the Qualities of Fruits and Vegetables

Hot Water: Boiling, Steaming, Pressure-Cooking

Hot Air, Oil, and Radiation: Baking, Frying, and Grilling

Microwave Cooking

Pulverizing and Extracting

Preserving Fruits and Vegetables

Drying and Freeze-Drying

Fermentation and Pickling: Sauerkraut and Kimchi, Cucumber Pickles, Olives

Sugar Preserves

Canning

We turn now from milk, eggs, meats, and fish, all expressions of animating protein and energizing fat, and enter the very different world that sustains them and us alike. The plant world encompasses earthy roots, bitter and pungent and refreshing leaves, perfumed flowers, mouth-filling fruits, nutty seeds, sweetness and tartness and astringency and pleasing pain, and aromas by the thousands! It turns out that this exuberantly diverse world was born of simple, harsh necessity. Plants can’t move as animals do. In order to survive their immobile, exposed condition, they became virtuosic chemists. They construct themselves from the simplest materials of the earth itself, water and rock and air and light, and thus transform the earth into food on which all animal life depends. Plants deter enemies and attract friends with colors, tastes, and scents, all chemical inventions that have shaped our ideas of beauty and deliciousness. And they protect themselves from the common chemical stresses of living with substances that protect us as well. So when we eat vegetables and fruits and grains and spices, we eat the foods that made us possible, and that opened our life to a kaleidoscopic world of sensation and delight.

Human beings have always been plant eaters. For a million years and more, our omnivorous ancestors foraged and lived on a wide range of wild fruits, leaves, and seeds. Beginning around 10,000 years ago they domesticated a few grains, seed legumes, and tubers, which are among the richest sources of energy and protein in the plant world, and can be grown and stored in large quantities. This control over the food supply made it possible for many people to be fed reliably from a small patch of land: so cultivation of the fields led to settlement, the first cities, and cultivation of the human mind. On the other hand, agriculture drastically reduced the variety of plant foods in the human diet. Millennia later, industrialization reduced it even further. Fruits and vegetables became accessory, even marginal foods in the modern Western diet. Only recently have we begun to understand how the human body still depends for its long-term health on a various diet rich in fruits and vegetables, herbs and spices. Happily, modern technologies now give us unprecedented access to the world’s cornucopia of edible plants. The time is ripe to explore this fascinating — and still evolving — legacy of natural and human inventiveness.

This chapter is a general introduction to the foods that we obtain from plants. Because there are so many of them, particular fruits and vegetables, herbs and spices are described in subsequent chapters. Foods derived from seeds — grains, legumes, nuts — have special properties, and are described separately in chapter 9.

Plants as Food

The Nature of Plants

Plants and animals are very different kinds of living things, and this is because they have evolved different solutions to a single basic challenge: how to obtain the energy and substance necessary to grow and reproduce. Plants essentially nourish themselves. They build their tissues out of water, minerals, and air, and run them on the energy in sunlight. Animals, on the other hand, can’t extract energy and construct complex molecules from such primitive materials. They must obtain them premade, and they do so by consuming other living things. Plants are independent autotrophs, while animals are parasitic heterotrophs. (Parasitism may not sound especially admirable, but without it there would be no need to eat and so none of the pleasures of eating and cooking!)

There are various ways of being an autotroph. Some archaic bacteria, which are microbes consisting of a single cell, manipulate sulfur, nitrogen, and iron compounds to produce energy. The most important development for the future of eating came more than 3 billion years ago with the evolution of a bacterium that could tap the energy in sunlight and store it in carbohydrate molecules (molecules built from carbon, hydrogen, and oxygen). Chlorophyll, the green pigment we see in vegetation all around us, is a molecule that captures sunlight and initiates this process of photosynthesis, which culminates in the creation of the simple sugar glucose.

The bacteria that managed to “invent” chlorophyll gave rise to algae and all green land plants — and indirectly to land animals as well. Before photosynthesis, the earth’s atmosphere contained little oxygen, and the sun’s killing ultraviolet rays penetrated all the way to the ground and several feet into the oceans. Living organisms could therefore survive only in deeper waters. When photosynthetic bacteria and early algae burgeoned, they liberated vast quantities of oxygen (O₂), which radiation in the upper atmosphere converted to ozone (O₃), which in turn absorbed ultraviolet light and prevented much of it from reaching the earth’s surface. Land life was now possible.

The challenging life of the plant. Plants are rooted to one spot in the earth, where they absorb water and minerals from the soil, carbon dioxide and oxygen from the air, and light energy from the sun, and transform these inorganic materials into plant tissues — and into nourishment for insects and other animals. Plants defend themselves against predators with a variety of chemical weapons, some of which also make them flavorful, healthful, or both. In order to spread their offspring far and wide, some plants surround their seeds with tasty and nourishing fruits that animals carry away and eat, often spilling some seeds in the process.

So we owe our very existence as oxygen-breathing, land-dwelling animals to the greenery we walk through and cultivate and consume every day of our lives.

Why Plants Aren’t Meaty Land-dwelling plants that can nourish themselves still need access to the soil for minerals and trapped water, to the atmosphere for carbon dioxide and oxygen, and to the sun for energy. All of these sources are pretty reliable, and plants have developed an economical structure that takes advantage of this reliability. Roots penetrate the soil to reach stable supplies of water and minerals; leaves maximize their surface area to capture sunlight and exchange gases with the air; and stalks support leaves and connect them with roots. Plants are essentially stationary chemical factories, made up of chambers for carbohydrate synthesis and carbohydrate storage, and tubes to transfer chemicals from one part of the factory to another, with structural reinforcement — also mainly carbohydrates — to provide mechanical rigidity and strength. Parasitic animals, by contrast, must find and feed on other organisms, so they are constructed mainly of muscle proteins that transform chemical energy into physical motion (p. 121).

Why Plants Have Strong Flavors and Effects Animals can also use their mobility to avoid becoming another creature’s meal, by fleeing or fighting. But stationary plants? They compensate for their immobility with a remarkable ability for chemical synthesis. These master alchemists produce thousands of strong-tasting, sometimes poisonous warning signals that discourage bacteria, fungi, insects, and us from attacking them. A partial list of their chemical warfare agents would include irritating compounds like mustard oil, hot-chilli capsaicin, and the tear-inducing factor in onions; bitter and toxic alkaloids like caffeine in coffee and solanine in potatoes; the cyanide compounds found in lima beans and many fruit seeds; and substances that interfere with the digestive process, including astringent tannins and inhibitors of digestive enzymes.

If plants are so well endowed with their own natural pesticides, then why isn’t the world littered with the corpses of their victims? Because animals have learned to recognize and avoid potentially harmful plants with the help of their senses of smell and taste, which can detect chemical compounds in very small concentrations. Animals have developed appropriate innate responses to significant tastes — aversion to the bitterness typical of alkaloids and cyanide, attraction to the sweetness of nutritionally important sugars. And some animals have developed specific detoxifying enzymes that enable them to exploit an otherwise toxic plant. The koala bear can eat eucalyptus leaves, and monarch butterfly caterpillars milkweed. Humans invented their own ingenious detoxifying methods, including plant selection and breeding and cooking. Cultivated varieties of such vegetables as cabbage, lima beans, potatoes, and lettuce are less toxic than their wild ancestors. And many toxins can be destroyed by heat or leached away in boiling water.

A fascinating wrinkle in this story is that humans actually prize and seek out certain plant toxins! We’ve managed to learn which irritating warning signals are relatively harmless, and have come to enjoy sensations whose actual purpose is to repel us. Hence our seemingly perverse love of mustard and pepper and onions. This is the essential appeal of herbs and spices, as we’ll see in chapter 8.

Why Ripe Fruits Are Especially Delicious The higher plants and animals reproduce by fusing genetic material from male and female sex organs, usually from different individuals. Animals have the advantage of being mobile: male and female can sense each other’s presence and move toward each other. Plants can’t move, and instead have to depend on mobile go-betweens. The male pollen of most land plants is carried to the female ovule by the wind or by animals. To encourage animals to help out, advanced plants evolved the flower, an organ whose shape, color, and scent are designed to attract a particular assistant, usually an insect. As it flies around and collects nutritious nectar or pollen for food, the insect spreads the pollen from one plant to another.

Once male and female cells have come together and developed into offspring, they must be given a good start. The animal mother can search out a promising location and deposit her young there. But plants need help. If the seeds simply dropped from the plant to the ground, they would have to compete with each other and with their overshadowing parent for sunlight and soil minerals. So successful plant families have developed mechanisms for dispersing their seeds far and wide. These mechanisms include seed containers that pop open and propel their contents in all directions, seed appendages that catch the wind or the fur of a passing animal — and structures that hitch a ride inside passersby. Fruits are plant organs that actually invite animals to eat them, so that the animals will carry their seeds away, and often pass them through their digestive system and deposit them in a nourishing pile of manure. (The seeds escape destruction in various ways, among them by being large and armored, or tiny and easily spilled, or poisonous.)

So, unlike the rest of the plant, fruit is meant to be eaten. This is why its taste, odor, and texture are so appealing to our animal senses. But the invitation to eat must be delayed until the seeds are mature and viable. This is the purpose of the changes in color, texture, and flavor that we call ripening. Leaves, roots, stalks can be eaten at any time, generally the earlier the tenderer. But we must wait for fruit to signal that it is ready to be eaten. The details of ripening are described in chapter 7 (p. 350).

Our Evolutionary Partners Like us, most of our food plants are relative newcomers to the earth. Life arose about 4 billion years ago, but flowering plants have been around for only about 200 million years, and dominant for the last 50 million. An even more recent development is the “herbaceous” habit of life. Most food plants are not long-lived trees, but relatively small, delicate plants that produce their seeds and die in one growing season. This herbaceous habit gives plants greater flexibility in adapting to changing conditions, and it has worked to our advantage as well. It allows us to grow crops to maturity in a few months, change plantings from year to year, rapidly breed new varieties, and eat plant parts that would be inedible were they toughened to endure for years. Herbaceous plants became widespread only in the last few million years, just as the human species was emerging. They made possible our rapid cultural development, and we in turn have used selection and breeding to direct their biological development. We and our food plants have been partners in each other’s evolution.

Definitions

We group the foods we obtain from plants into several loose categories.

Fruit and Vegetable Apart from such plant seeds as wheat and rice, which are described in chapter 9, the most prominent plant foods in our diet are fruits and vegetables. Vegetable took on its current sense just a few centuries ago, and essentially means a plant material that is neither fruit nor seed. So what is a fruit? The word has both a technical and a common meaning. Beginning in the 17th century, botanists defined it as the organ that develops from the flower’s ovary and surrounds the plant’s seeds. But in common usage, seed-surrounding green beans, eggplants, cucumbers, and corn kernels are called vegetables, not fruits. Even the United States Supreme Court has preferred the cook’s definition over the botanist’s. In the 1890s, a New York food importer claimed duty-free status for a shipment of tomatoes, arguing that tomatoes were fruits, and so under the regulations of the time, not subject to import fees. The customs agent ruled that tomatoes were vegetables and imposed a duty. A majority of the Supreme Court decided that tomatoes were “usually served at a dinner in, with, or after the soup, fish, or meat, which constitute the principal part of the repast, and not, like fruits, generally as dessert.” Ergo tomatoes were vegetables, and the importer had to pay.

The Key Distinction: Flavor Why do we customarily prepare vegetables as side dishes to the main course, and make fruits the centerpiece of the meal’s climax? Culinary fruits are distinguished from vegetables by one important characteristic: they’re among the few things we eat that we’re meant to eat. Many plants have engineered their fruits to appeal to the animal senses, so that animals will eat them and disperse the seeds within. These fruits are the natural world’s soft drinks and candies, flashily packaged in bright colors, and test-marketed through millions of years of natural selection. They tend to have a high sugar content, to satisfy the innate liking for sweetness shared by all animals. They have a pronounced and complex aroma, which may involve several hundred different chemicals, far more than any other natural ingredient. And they soften themselves to an appealingly tender, moist consistency. By contrast, the plant foods that we treat as vegetables remain firm, have either a very mild flavor — green beans and potatoes — or else an excessively strong one — onions and cabbage — and therefore require the craft of the cook to make them palatable.

The very words fruit and vegetable reflect these differences. Vegetable comes from the Latin verb vegere, meaning to invigorate or enliven. Fruit, on the other hand, comes from Latin fructus, whose cluster of related meanings includes gratification, pleasure, satisfaction, enjoyment. It’s the nature of fruit to taste good, to appeal to our basic biological interests, while vegetables stimulate us to find and create more subtle and diverse pleasures than fruits have to offer.

Herb and Spice The terms herb and spice are more straightforward. Both are categories of plant materials used primarily as flavorings, and in relatively small amounts. Herbs come from green parts of plants, usually leaves — parsley, thyme, basil — while spices are generally seeds, bark, underground stems — black pepper, cinnamon, ginger — and other robust materials that were well suited to international trade in early times. The word spice came from the medieval Latin species, which meant “kind of merchandise.”

Despite the fact that we consider them vegetables, capsicum “peppers,” pea pods, cucumbers, and even corn kernels are actually fruits: plant parts that originate in the flower’s ovary and surround one or more seeds.

Plant Foods Through History

How long has the Western world been eating the plant foods we eat today, and in the way that we eat them? Only a very few common vegetables have not been eaten since before recorded history (the relative newcomers include broccoli, cauliflower, brussels sprouts, celery). But it was only with the age of exploration in the 16th century that the variety of foods we now know became available to any single culture. In the Western world, fruit has been eaten as dessert at least since the Greeks; recognizable salads go back to the Middle Ages, and boiled vegetables in delicate sauces to 17th-century France.

Prehistory and Early Civilizations Many plants came under human cultivation by the unsophisticated but slowly effective means of gathering useful plants and leaving a few seeds in fertile refuse heaps. Judging from archaeological evidence, early Europeans seem to have relied on wheat, fava beans, peas, turnips, onions, radishes, and cabbage. In Central America, corn, beans, hard squashes, tomatoes, and avocados were staples around 3500 BCE, while Peruvian settlements relied heavily on the potato. Northern Asia started with millets, cabbage relatives, soybeans, and tree fruits in the apple and peach families; southern Asia had rice, bananas, coconuts, yams, cabbage relatives, and citrus fruits. Indigenous African crops included related but distinct millets, sorghum, rice, and bananas, as well as yams and cowpeas. Mustard seed flavored foods in Europe and in Asia, where ginger may also have been used. Chilli “pepper” was probably the chief spice in the Americas.

By the time of the earliest civilizations in Sumer and Egypt about 5,000 years ago, most of the plants native to that area and eaten today were already in use (see box, p. 250). Trade between the Middle East and Asia is also ancient. Egyptian records of around 1200 BCE document huge offerings of cinnamon, a product of Sri Lanka.

Greece, Rome, and the Middle Ages With the Greeks and Romans we begin to see the outlines of modern Western cuisine. The Greeks were fond of lettuce, and habitually ate fruit at the end of meals. Pepper from the Far East was in use around 500 BCE and quickly became the most popular spice of the ancient world. In Rome, lettuce was served at both the beginning and end of meals, and fruit as dessert. Thanks to the art of grafting growing shoots from desirable trees onto other trees, there were about 25 named apple varieties and 35 pears. Fruits were preserved whole by immersing them, stems and all, in honey, and the gastronome Apicius gave a recipe for pickled peaches. From the Roman recipes that survive, it would seem that few foods were served without the application of several strong flavors.

When the Romans conquered Europe they brought along tree fruits, the vine, and cultivated cabbage, as well as their heavy spice habit. Sauce recipes from the 14th century resemble those of Apicius, and the English lettuce-free salad would also have been quite pungent (see box, p. 251). Medieval recipe collections include relatively few vegetable dishes.

New World, New Foods Plants — and especially the spice plants — helped shape world history in the last five centuries. The ancient European hunger for Asian spices was an important driving force in the development of Italy, Portugal, Spain, Holland, and England into major sea powers during the Renaissance. Columbus, Vasco da Gama, John Cabot, and Magellan were looking for a new route to the Indies in order to break the monopoly of Venice and southern Arabia on the ancient trade in cinnamon, cloves, nutmeg, and black pepper. They failed in that quest, but succeeded in opening the “West Indies” to European exploitation. The New World was initially disappointing in its yield of sought-for spices. But vanilla and chillis quickly became popular; and its wealth of new vegetables was largely adaptable to Europe’s climate: so the common bean, corn, squashes, tomatoes, potatoes, and sweet chillis eventually became staple ingredients in the new cuisines of the Old World.

The 17th and 18th centuries were a time of assimilating the new foods and advancing the art of cooking them. Cultivation and breeding received new attention; Louis XIV’s orchards and plantings at Versailles were legendary. And cooks took a greater interest in vegetables, and handled them with greater refinement, in part to make the meatless diet of Lent and other Catholic fasts more interesting. France’s first great culinary writer, Pierre François de La Varenne, chef to Henri IV, included meatless recipes for peas, turnips, lettuce, spinach, cucumbers, cabbage (five ways), chicory, celery, carrots, cardoons, and beets, as well as ordinary dishes of artichokes, asparagus, mushrooms, and cauliflower. And the recipes leave a major role for the vegetables’ own flavors. Similarly, the Englishman John Evelyn wrote a book-length disquisition on salads, once again firmly based on the lettuces, and emphasized the importance of balance.

With the 19th century, English vegetable cooking became ever simpler until it almost always meant boiled and buttered, a quick and simple method for homes and restaurants alike, while in France the elaborate professional style reached its apogee. The influential chef Antonin Carême declared in his Art of French Cooking in the 19th Century (1835) that “it is in the confection of the Lenten cuisine that the chef’s science must shine with new luster.” Carême’s enlarged repertoire included broccoli, truffles, eggplant, sweet potatoes, and potatoes, these last fixed à l’anglaise, dites, Mache-Potetesse (“in the English style, that is, mashed”). Of course, such luster tends to undermine the whole point of Lent. In his 366 Menus (1872), Baron Brisse asked: “Are the meatless meals of our Lenten enthusiasts really meals of abstinence?”

The Influence of Modern Technology The age of exploration and the advancement of fine cooking brought a new prominence to fruits and vegetables in Europe. Then the social and technical innovations of the industrial age conspired to make them both less available and less desirable. Beginning early in the 19th century, as industrialization drew people from the agricultural countryside to the cities, fruits and vegetables became progressively rarer in the diets of Europe and North America. Urban supplies did improve with the development of rail transportation in the 1820s, then canning at mid-century, and refrigeration a few decades later. Around the turn of the 20th century, vitamins and their nutritional significance were discovered, and fruits and vegetables were soon officially canonized as one of the four food groups that should be eaten at every meal. Still, the consumption of fresh produce continued to decline through much of the 20th century, at least in part because its quality and variety were also declining. In the modern system of food production, with crops being handled in massive quantities and shipped thousands of miles, the most important crop characteristics became productivity, uniformity, and durability. Rather than being bred for flavor and harvested at flavor’s peak, fruits and vegetables were bred to withstand the rigors of mechanical harvesting, transport, and storage, and were harvested while still hard, often weeks or months before they would be sold and eaten. A few mediocre varieties came to dominate the market, while thousands of others, the legacy of centuries of breeding, disappeared or survived only in backyard gardens.

At the end of the 20th century, several developments in the industrialized world brought renewed attention to plant foods, to their diversity and quality. One was a new appreciation of their importance for human health, thanks to the discovery of trace “phytochemicals” that appear to help fight cancer and heart disease (p. 255). Another was the growing interest in exotic and unfamiliar cuisines and ingredients, and their increasing availability in ethnic markets. Yet another, at the opposite extreme, was the rediscovery of the traditional system of food production and its pleasures: eating locally grown foods, often forgotten “heirloom” or other unusual varieties, that were harvested a matter of hours beforehand, then sold at farmers’ markets by the people who grew them. Allied to this trend was the growing interest in “organic” foods, produced without relying on the modern array of chemicals for controlling pests and disease. Organic practices mean different things to different people, and don’t guarantee either safer or more nutritious foods — agriculture is more complicated than that. But they represent an essential, prominent alternative to industrial farming, one that encourages attention to the quality of agricultural produce and the sustainability of agricultural practices.

These are good times for curious and adventurous eaters. There are many forgotten varieties of familiar fruits and vegetables to revive, and many new foods to taste. It’s estimated that there are 300,000 edible plant species on earth, and perhaps 2,000 that are cultivated to some extent. We have plenty of exploring to do!

Plant Foods
and Health

Plant foods can provide us all the nourishment we need in order to live and thrive. Our primate ancestors started out eating little else, and many cultures still do. But meat and other animal foods became important to our species at its birth, when their concentrated energy and protein probably helped accelerate our evolution (p. 119). Meat continued to have a deep biological appeal for us, and in societies that could afford to feed livestock on staple grains and roots, it became the most prized of foods. In the industrialized world, meat’s prestige and availability pushed grains, vegetables, and fruits to the side of the plate and the end of the meal. And for decades, nutritional science affirmed their accessory status. Fruits and vegetables in particular were considered to be the source of a few nutrients that we need only in small amounts, and of mechanically useful roughage. In recent years, though, we’ve begun to realize just how many valuable substances plant foods have always held for us. And we’re still learning.

Essential Nutrients in Fruits
and Vegetables: Vitamins

Most fruits and vegetables contribute only modestly to our intake of proteins and calories, but they’re our major source for several vitamins. They provide nearly all of our vitamin C, much of our folic acid, and half of our vitamin A. Each of these plays a number of roles in the metabolism of our cells. For example, vitamin C refreshes the chemical state of metal components in many enzymes, and helps with the synthesis of connective-tissue collagen. Vitamin A, which our bodies make from a precursor molecule in plants called beta-carotene (p. 267), helps regulate the growth of several different kinds of cells, and helps our eyes detect light. Folic acid, named from the Latin word for “leaf,” converts a by-product of our cells’ metabolism, homocysteine, into the amino acid methionine. This prevents homocysteine levels from rising, causing damage to blood vessels, and possibly contributing to heart disease and stroke.

Vitamins A, C, and E are also antioxidants (see below).

Phytochemicals

The first edition of this book reflected the prevailing nutritional wisdom circa 1980: we should eat enough fruits and vegetables to avoid vitamin and mineral deficiencies, and to keep our digestive system moving. Period.

What a difference 20 years makes!

Nutritional science has undergone a profound revolution in that time. For most of the 20th century it aimed to define an adequate diet. It determined our body’s minimal requirements for chemical building blocks (protein, minerals, fatty acids), for essential cogs in its machinery (vitamins), and for the energy it needs to run and maintain itself from day to day. Toward the end of the century, it became clear from laboratory studies and comparisons of health statistics in different countries that the major diseases of the adequately nourished developed world — cancer and heart disease — are influenced by what we eat. Nutritional science then began to focus on defining the elements of an optimal diet. So we discovered that minor, nonessential food components can have a cumulative effect on our long-term health. And plants, the planet’s biochemical virtuosos, turn out to be teeming with trace phytochemicals — from the Greek phyton, meaning “leaf” — that modulate our metabolism.

Antioxidants

Oxidative Damage: The Price of Living One major theme in modern nutrition is the body’s need to cope with the chemical wear and tear of life itself. Breathing is essential to human life because our cells use oxygen to react with sugars and fats and generate the chemical energy that keeps the cellular machinery functioning. Unfortunately, it turns out that energy generation and other essential processes involving oxygen generate chemical by-products called “free radicals,” very unstable chemicals that react with and damage our own complex and delicate chemical machinery. This damage is called oxidative because it usually originates in reactions involving oxygen. It can affect different parts of the cell, and different organs in the body. For example, oxidative damage to a cell’s DNA can cause that cell to multiply uncontrollably and grow into a tumor. Oxidative damage to the cholesterol-carrying particles in our blood can irritate the lining of our arteries, and initiate damage that leads to a heart attack or stroke. The high-energy ultraviolet rays in sunlight create free radicals in the eye that damage proteins in the lens and retina, and cause cataracts, macular degeneration, and blindness.

Our bodies stave off such drastic consequences by means of antioxidant molecules, which react harmlessly with free radicals before they have a chance to do any damage to the cells’ chemical machinery. We need a continuous and abundant supply of antioxidants to maintain our good health. The body does make a few important antioxidant molecules of its own, including some powerful enzymes. But the more help it gets, the better it’s able to defend itself from the constant onslaught of free radicals. And plants turn out to be a goldmine of antioxidants.

Antioxidants in Plants Nowhere in living things is oxidative stress greater than in the photosynthesizing leaf of a green plant, which harvests energetic particles of sunlight, and uses them to split water molecules apart into hydrogen and oxygen atoms in order to make sugars. Leaves and other exposed plant parts are accordingly chock-full of antioxidant molecules that keep these high-energy reactions from damaging essential DNA and proteins. Among these plant antioxidants are the carotenoid pigments, including orange beta-carotene, yellow lutein and zeaxanthin, and the red lycopene that colors tomato fruits. Green chlorophyll itself is an antioxidant, as are vitamins C and E. Then there are thousands of different “phenolic” compounds built from rings of 6 carbon atoms, which play several roles in plant life, from pigmentation to antimicrobial duty to attracting and repelling animals. All fruits, vegetables, and grains probably contain at least a few kinds of phenolic compounds; and the more pigmented and astringent they are, the more they’re likely to be rich in phenolic antioxidants.

Each plant part, each fruit and vegetable, has its own characteristic cluster of antioxidants. And each kind of antioxidant generally protects against a certain kind of molecular damage, or helps regenerate certain other protective molecules. No single molecule can protect against all kinds of damage. Unusually high concentrations of single types can actually tip the balance the wrong way and cause damage. So the best way to reap the full benefits of the antioxidant powers of plants is not to take manufactured supplements of a few prominent chemicals: it is to eat lots of different vegetables and fruits.

Other Beneficial Phytochemicals Antioxidants may be the most important group of ingredients for maintaining long-term health, but they’re not the only one. Trace chemicals in plants, including herbs and spices, are turning out to have helpful effects on many other processes that affect the balance between health and disease. For example, some act like aspirin (originally found in plants) to prevent the body from overreacting to minor damage with an inflammation that can lead to heart disease or cancer; some prevent the body from turning mildly toxic chemicals into more powerful toxins that damage DNA and cause cancer; some inhibit the growth of cells that are already cancerous. Others slow the loss of calcium from our bones, encourage the growth of beneficial bacteria in our system, and discourage the growth of disease bacteria.

The box on p. 256 lists some of these effects, and the chemicals and plants that cause them. Our knowledge of this aspect of nutrition is still in its infancy, but we know enough right now for at least one conclusion to be evident: no single fruit or vegetable offers the many kinds of protections that a varied diet can provide.

So today’s provisional nutritional wisdom goes like this: fruits and vegetables, herbs and spices supply us with many different beneficial substances. Within an otherwise adequate diet, we should eat as much of them as we can, and as great a variety as we can.

Estimating Healthfulness by Eye There’s a useful guideline for estimating the relative healthfulness of vegetables and fruits: the deeper its color, the more healthful the food is likely to be. The more light a leaf gets, the more pigments and antioxidants it needs to handle the energy input, and so the darker the coloration of the leaf. For example, the light-colored inner leaves of lettuce and cabbage varieties that form tight heads contain a fraction of the carotene found in the darker outer leaves and in the leaves of more open varieties. Similarly, the dark leaves of open romaine lettuce contain nearly 10 times the eye-protecting lutein and zeaxanthin of the pale, tight heads of iceberg lettuce. Other deeply colored fruits and vegetables also contain more beneficial carotenoids and phenolic compounds than their pale counterparts. Their skins are especially rich sources. Among the fruits highest in antioxidant content are cherries, red grapes, blueberries, and strawberries; among vegetables, garlic, red and yellow onions, asparagus, green beans, and beets.

Fiber

Fiber is defined as the material in our plant foods that our digestive enzymes can’t break down into absorbable nutrients. These substances therefore aren’t absorbed in the small intestine, and pass intact into the large intestine, where some are broken down by intestinal bacteria, and the rest are excreted. The four main components of fiber come from plant cell walls (p. 265). Cellulose and lignin form solid fibers that don’t dissolve in our watery digestive fluids, while pectins and hemicelluloses do dissolve into their individual molecules. Minor components of fiber include uncooked starch and various gums, mucilages, and other unusual carbohydrates (e.g., mushroom chitin, seaweed agar and carrageenan, inulin in onions, artichokes, and sunchokes). Particular foods offer particular kinds of fiber. Wheat bran — the dry outer coat of the grain — is a rich source of insoluble cellulose, while oat bran is a rich source of soluble glucan (a carbohydrate), and juicy ripe fruits are a relatively dilute source of soluble pectins.

The different fiber components contribute to health in different ways. Insoluble cellulose and lignin mainly provide bulk to the intestinal contents, and thus increase the rate and ease with which they pass through the large intestine. It’s thought that rapid excretion may help minimize our exposure to DNA-damaging chemicals and other toxins in our foods, and the fiber materials may bind some of these toxins and prevent them from being absorbed by our cells. Soluble fiber components make the intestinal contents thicker, so that there is slower mixing and movement of both nutrients and toxins. They, too, probably bind certain chemicals and prevent their absorption. Soluble fiber has been shown to lower blood cholesterol and slow the rise of blood sugar after a meal. Inulin in particular encourages the growth of beneficial intestinal bacteria, while reducing the numbers of potential troublemakers. The details are complex, but overall it appears that soluble fiber helps protect against heart disease and diabetes.

In sum, the indigestible portion of fruits and vegetables does us good. It’s a mistake to think that a juiced orange or carrot is as valuable as the whole fruit or vegetable.

Toxins in Some Fruits
and Vegetables

Many plants, perhaps all plants, contain chemicals meant to discourage animals from eating them. The fruits and vegetables that we eat are no exception. While domestication and breeding have reduced their toxin contents to the point that they’re not generally hazardous, unusual preparations or serving sizes can cause problems. The following plant toxins are worth being aware of.

Alkaloids Alkaloids are bitter-tasting toxins that appeared in plants about the time that mammals evolved, and seem especially effective at deterring our branch of the animal family by both taste and aftereffects. Almost all known alkaloids are poisonous at high doses, and most alter animal metabolism at lower doses: hence the attractions of caffeine and nicotine. Among familiar foods, only the potato accumulates potentially troublesome alkaloid levels, which make greened potatoes and potato sprouts bitter and toxic (p. 302).

Cyanogens Cyanogens are molecules that warn and poison animals with bitter hydrogen cyanide, a deadly poison of the enzymes that animals use to generate energy. When the plant’s tissue is damaged by chewing, the cyanogens are mixed with the plant enzyme that breaks them apart and releases hydrogen cyanide (HCN). Cyanogen-rich foods, including manioc, bamboo shoots, and tropical varieties of lima beans, are made safe for consumption by open boiling, leaching in water, and fermentation. The seeds of citrus, stone, and pome fruits generate cyanide, and stone-fruit seeds are prized because their cyanogens also produce benzaldehyde, the characteristic odor of almond extract (p. 506).

Hydrazines Hydrazines are nitrogen-containing substances that are found in relatively large amounts (500 parts per million) in the common white mushroom and other mushroom varieties, and that persist after cooking. Mushroom hydrazines cause liver damage and cancer when fed to laboratory mice, but have no effect in rats. It’s not yet clear whether they pose a significant hazard to humans. Until we know, it’s best to eat mushrooms in moderation.

Protease Inhibitors and Lectins These are proteins that interfere with digestion: inhibitors block the action of protein-digesting enzymes, and lectins bind to intestinal cells and prevent them from absorbing nutrients. Lectins can also enter the blood-stream and bind red blood cells to each other. They’re found mainly in soy, kidney, and lima beans. Both inhibitors and lectins are inactivated by prolonged boiling. But they can survive in beans that are eaten raw or undercooked, and cause symptoms similar to food poisoning.

Flavor Chemicals Flavor chemicals are generally consumed in only tiny amounts, but a few may cause problems when overindulged in. Safrole, the main aromatic in oil of sassafras and therefore of traditional root beer, causes DNA damage and was banned as an additive in 1960 (root beer is now made with safe sarsaparilla or artificial flavorings). Myristicin, the major flavor contributor in nutmeg, seems largely responsible for intoxication and hallucinations that result from ingesting large amounts. Glycyrrhizin, an intensely sweet-tasting substance in true licorice root, induces high blood pressure. Coumarin, which gives sweet clover its sweet aroma and is also found in lavender and vanilla-like tonka beans (Dipteryx odorata), interferes with blood clotting.

Toxic Amino Acids Toxic amino acids are unusual versions of the building blocks for our proteins that interfere with proper protein functioning. Canavanine interferes with several cell functions and has been associated with the development of lupus; it’s found in large quantities in alfalfa sprouts and the jack bean. Vicine and convicine in the fava bean cause a blood-cell-destroying anemia, favism, in susceptible people (p. 490).

Oxalates Oxalates are various salts of oxalic acid, a waste product of plant metabolism found in a number of foods, notably spinach, chard, beets, amaranth, and rhubarb. The sodium and potassium salts are soluble, while the calcium salts are insoluble and form crystals that irritate the mouth and digestive system. Soluble oxalates can combine with calcium in the human kidney to form painful kidney stones. In very large doses — a few grams — oxalic acid is corrosive and can be fatal.

Bracken-Fern Toxins Bracken-fern toxins cause several blood disorders and cancer in animals that graze on this common fern (Pteridium), which is sometimes collected in the young “fiddlehead” stage for human consumption. Ostrich ferns, Matteuccia species, are thought to be a safer source of fiddleheads, but there’s little solid information about the safety of eating ferns. It’s prudent to eat fiddleheads in moderation, and to avoid bracken ferns by checking labels and asking produce sellers.

Psoralens Psoralens are chemicals that damage DNA and cause blistering skin inflammations. They’re found occasionally in badly handled celery and celery root, parsley, and parsnips, when these vegetables have been stressed by near-freezing temperatures, intense light, or infection by mold. Psoralens are absorbed through the skin during handling, or by being ingested with the vegetable, either raw or cooked. They lie dormant in skin cells until they’re struck by ultraviolet rays in sunlight, which causes them to bind to and damage DNA and important cell proteins. The psoralen-generating vegetables should be bought as fresh as possible and used quickly.

In addition to their own chemical defenses, fruits and vegetables can carry other toxins that come from contaminating molds (patulin in apple juice, from a Penicillium mold growing on damaged fruit), agricultural chemicals (pesticides, herbicides, fungicides), and soil and air pollutants (dioxins, polycyclic aromatic hydrocarbons). In general, it’s thought that the usual levels of these contaminants do not constitute an immediate health hazard. On the other hand, they are toxins, and therefore undesirable additions to our diet. We can reduce our intake of them by washing produce, by peeling off surface layers, and by buying certified organic produce, which is grown in relatively clean soil without the use of most agricultural chemicals.

Fresh Produce and Food Poisoning

Though we generally associate outbreaks of food poisoning with foods derived from animals, fruits and vegetables are also a significant source. They have caused outbreaks of nearly every major food pathogen known (see box below). There are several reasons for this. Fruits and vegetables are grown in the soil, a vast reservoir of microbes. Field facilities for the harvesting crew (toilets, wash water) and for processing and packing may not be hygienic, so the produce is easily contaminated by people, containers, and machinery. And produce is often eaten raw. Salad bars in restaurants and cafeterias can collect and grow bacteria for hours, and have been associated with many outbreaks of food poisoning. Fruit juices, often made by crushing whole fruits, are readily contaminated by a small number of infected pieces; so fresh cider has become hard to find. Nearly all juice production in the United States is now pasteurized.

The prudent consumer will thoroughly wash all produce, including fruits whose skins will be discarded (knives and fingers can introduce surface bacteria to the flesh). Soapy water and commercial produce washes are more effective than water alone. Washing can reduce microbial populations a hundredfold, but it’s impossible to eliminate all microbes from uncooked lettuce and other produce — they can evade even heavily chlorinated water by hiding in microscopic pores and cracks in the plant tissue. Raw salads are therefore not advised for people who are especially vulnerable to infections. Once fruits and vegetables have been cut up, they should be kept refrigerated and used as soon as possible.

The Composition
and Qualities of Fruits
and Vegetables

What makes a vegetable tender or tough? Why do leafy greens shrink so much when cooked? Why do apples and avocados turn brown when cut open? Why are green potatoes dangerous? Why do some fruits get sweet in the bowl, and others just older? The key to understanding these and other characteristics is a familiarity with the structural and chemical makeup of plant tissues.

Plant Structure:
Cells, Tissues, and Organs

The Plant Cell Like animals, plants are built up out of innumerable microscopic chambers called cells. Each cell is surrounded and contained by a thin, balloon-like cell membrane constructed from certain fat-like molecules and proteins, and permeable to water and other small molecules. Immediately inside the membrane is a fluid called the cytoplasm, which is filled with much of the complex chemical machinery necessary to the cell’s growth and function. Then within the cytoplasm float a variety of other membrane-contained bags, each with its own chemical nature. Nearly all plant cells contain a large watery vacuole, which may be filled with enzymes, sugars, acids, proteins, water-soluble pigments, and waste or defensive compounds. Often one large vacuole will fill 90% of the cell volume and squeeze the cytoplasm and nucleus (the body that contains most of the cell’s DNA) up against the cell membrane. Leaf cells contain dozens to hundreds of chloroplasts, bags filled with green chlorophyll and other molecules that do the work of photosynthesis. The cells of fruits often contain chromoplasts, which concentrate yellow, orange, and red pigments that are soluble in fat. And storage cells are often filled with amyloplasts, which hold many granules of the long sugar chains called starch.

Cross-section through a typical plant cell.

The Cell Wall One last and very important component of the plant cell is its cell wall, something that animal cells lack entirely. The plant cell wall surrounds the membrane and is strong and rigid. Its purpose is to lend structural support to the cell and the tissue of which it is a part. Neighboring cells are held together by the outer, glue-like layers of their cell walls. Some specialized strengthening cells become mostly cell wall and do their job even after their death. The gritty grains in pear flesh, the fibers in celery stalks, the stone that surrounds a peach seed, and the seed coats of beans and peas are all mainly the cell-wall material of strengthening cells.

Broadly speaking, the texture of plant foods is determined by the fullness of the storage vacuole, the strength of the cell walls, and the absence or presence of starch granules. Color is determined by the chloroplasts and chromoplasts, and sometimes by water-soluble pigments in vacuoles. Flavor comes from the contents of the storage vacuoles.

Plant Tissues Tissues are groups of cells organized to perform a common function. Plants have four basic tissues.

Ground tissue is the primary mass of cells. Its purpose depends on its location in the plant. In leaves the ground tissue performs photosynthesis; elsewhere it stores nutrients and water. Cells in the ground tissue usually have thin cell walls, so the tissue is generally tender. Most of our fruits and vegetables are mainly ground tissue.

Vascular tissue runs through the ground tissue, and resembles our veins and arteries. It is the system of microscopic tubes that transport nutrients throughout the plant. The work is divided between two subsystems: xylem, which takes water and minerals from the roots to the rest of the plant, and phloem, which conducts sugars down from the leaves. Vascular tissue usually provides mechanical support as well, and is often tough and fibrous compared to the surrounding tissue.

Dermal tissue forms the outer surface of the plant, the layer that protects it and helps it retain its moisture. It may take the form of either epidermis or periderm. The epidermis is usually a single layer of cells that secretes several surface coatings, including a fatty material called cutin, and wax (long molecules made by joining fatty acids with alcohols), which is what makes many fruits naturally take a shine. Periderm is found instead of epidermis on underground organs and older tissues, and has a dull, corky appearance. Our culinary experience of periderm is usually limited to the skins of potatoes, beets, and so on.

Secretory tissue usually occurs as isolated cells on the surface or within the plant. These cells correspond to the oil and sweat glands in our skin, and produce and store various aroma compounds, often to attract or repel animals. The large mint family, which includes other common herbs like thyme and basil, is characterized by glandular hairs on stems and leaves that contain aromatic oils. Vegetables in the carrot family concentrate their aromatic substances in inner secretory cells.

The three kinds of plant tissue in a stem. Fibrous vascular tissue and thick dermal layers are common causes of toughness in vegetables.

Plant Organs There are six major plant organs: the root, the stem, the leaf, the flower, the fruit, and the seed. We’ll take a closer look at seeds in chapter 9.

Roots Roots anchor the plant in the ground, and absorb and conduct moisture and nutrients to the rest of the plant. Most roots are tough, fibrous, and barely edible. The exceptions are roots that swell up with nonfibrous storage cells; they allow plants to survive temperate-zone winter to flower in their second year (carrots, parsnips, radishes) or seasonal dryness in the tropics (sweet potatoes, manioc). Root vegetables develop this storage area in different ways, and so have different anatomies. In the carrot, storage tissue forms around the central vascular core, which is less flavorful. The beet produces concentric layers of storage and vascular tissue, and in some varieties these accumulate different pigments, so their slices appear striped.

Stems, Stalks, Tubers, and Rhizomes Stems and stalks have the main function of conducting nutrients between the root and leaves, and providing support for the aboveground organs. They therefore tend to become fibrous, which is why asparagus and broccoli stems often need to be peeled before cooking, celery and cardoon stalks deveined. The junction between stem and root, which is called the hypocotl, can swell into a storage organ; turnips, celery “root,” and beets are actually part stem, part root. And some plants, including the potato, yam, sunchoke, and ginger, have developed special underground stem structures for nonsexual reproduction: they “clone” themselves by forming a storage organ that can produce its own roots and stem and become an independent — but genetically identical — plant. The common potato and true yam are such swollen underground stem tips called tubers, while the sunchoke and ginger “root” are horizontal underground stems called rhizomes.

Leaves Leaves specialize in the production of high-energy sugar molecules via photosynthesis, a process that requires exposure to sunlight and a good supply of carbon dioxide. They therefore contain very little storage or strengthening tissue that would interfere with access to light or air, and are the most fragile and short-lived parts of the plant. To maximize light capture, the leaf is flattened out into a thin sheet with a large surface area, and the photosynthetic cells are heavily populated with chloroplasts. To promote gas exchange, the leaf interior is filled with thousands of tiny air pockets, which further increase the area of cells exposed to the air. Some leaves are as much as 70% air by volume. This structure helps explain why leafy vegetables shrink so much when cooked: heat collapses the spongy interior. (It also wilts the leaves so that they pack together more compactly.)

Cross section of a leaf. Because photosynthesis requires a continuous supply of carbon dioxide, leaf tissue often has a spongy structure that directly exposes many inner cells to the air.

An exception to the rule against storage tissue in leaves is the onion family (tulips and other bulb ornamentals are exceptions as well). The many layers of the onion (and the single layer of a garlic clove) surrounding the small inner stem are the swollen bases of leaves whose tops die and fall off. The leaf bases store water and carbohydrates during the plant’s first year of growth so that they can be used during the second, when it will flower and produce seed.

Flowers Flowers are the plant’s reproductive organs. Here the male pollen and female ovules are formed; here too they unite in the chamber that contains the ovules, the ovary, and develop into embryos and seeds. Flowers are often brilliantly colored and aromatic to attract pollinating insects, and can be a striking ingredient. However, some familiar plants protect their flowers from animal predators with toxins, so their edibility should be checked before use (p. 326). We also eat a few flowers or their supporting tissues before they mature; broccoli, cauliflower, and artichokes are examples.

Fruits The fruit is the organ derived from the flower’s ovary (or adjacent stem tissue). It contains the seeds, and promotes their dispersal away from the mother plant. Some fruits are inedible — they’re designed to catch the wind, or the fur of a passing animal — but the fruits that we eat were made by the plant to be eaten, so that an animal would intentionally take it and the seeds away. The fruit has no support, nutrition, or transport responsibilities to the other organs. It therefore consists almost entirely of storage tissue filled with appealing and useful substances for animals. When ready and ripe, it’s usually the most flavorful and tenderest part of the plant.

Texture

The texture of raw fruits and vegetables can be crisp and juicy, soft and melting, mealy and dry, or flabby and chewy. These qualities are a reflection of the way the plant tissues break apart as we chew. And their breaking behavior depends on two main factors: the construction of their cell walls, and the amount of water held in by those walls.

The cell walls of our fruits and vegetables have two structural materials: tough fibers of cellulose that act as a kind of framework, and a semisolid, flexible mixture of water, carbohydrates, minerals, and proteins that cross-link the fibers and fill the space between them. We can think of the semisolid mixture as a kind of cement whose stiffness varies according to the proportions of its ingredients. The cellulose fibers act as reinforcing bars in that cement. Neighboring cells are held together by the cement where their walls meet.

Crisp Tenderness: The Roles of Water Pressure and Temperature Cell walls are thus firm but flexible containers. The cells that they contain are mostly water. When water is abundant and a cell approaches its maximum storage capacity, the vacuole swells and presses the surrounding cytoplasm (p. 261) against the cell membrane, which in turn presses against the cell wall. The flexible wall bulges to accommodate the swollen cell. The pressure exerted against each other by many bulging cells — which can reach 50 times the pressure of the surrounding air — results in a full, firm, turgid fruit or vegetable. But if the cells are low on water, the mutually supporting pressure disappears, the flexible cell walls sag, and the tissue becomes limp and flaccid.

Water and walls determine texture. A vegetable that is fully moist and firm will seem both crisp and more tender than the same vegetable limp from water loss. When we bite down on a vegetable turgid with water, the already-stressed cell walls readily break and the cells burst open; in a limp vegetable, chewing compresses the walls together, and we have to exert much more pressure to break through them. The moist vegetable is crisp and juicy, the limp one chewy and less juicy. Fortunately, water loss is largely reversible: soak a limp vegetable in water for a few hours and its cells will absorb water and reinflate. Crispness can also be enhanced by making sure that the vegetable is icy cold. This makes the cell-wall cement stiff, so that when it breaks under pressure, it seems brittle.

Mealiness and Meltingness: The Role of Cell Walls Fruits and vegetables can sometimes have a mealy, grainy, dry texture. This results when the cement between neighboring cells is weak, so that chewing breaks the cells apart from each other rather than breaking them open, and we end up with lots of tiny separate cells in our mouth. Then there’s the soft, melting texture of a ripe peach or melon. This too is a manifestation of weakened cell walls, but here the weakening is so extreme that the walls have practically disintegrated, and the watery cell interior oozes out under the least pressure. The contents of the cells also have an effect: a ripe fruit’s vacuole full of sugar solution will give a melting, succulent impression, while a potato’s solid starch grains will contribute a firm chalkiness. Because starch absorbs water when heated, cooked starchy tissue becomes moist but mealy or pasty, never juicy.

The changes in texture that occur during ripening and cooking result from changes in the cell-wall materials, in particular the cement carbohydrates. One group is the hemicelluloses, which form strengthening cross-links between celluloses. They are built up from glucose and xylose sugars, and can be partly dissolved and removed from cell walls during cooking (p. 282). The other important component is the pectic substances, large branched chains of a sugar-like molecule called galacturonic acid, which bond together into a gel that fills the spaces between cellulose fibers. Pectins can be either dissolved or consolidated by cooking, and their gel-like consistency is exploited in the making of fruit jellies and jams (p. 296). When fruits soften during ripening, their enzymes weaken the cell walls by modifying the pectins.

Wilting in vegetables. Plant tissue that is well supplied with water is filled with fluids and mechanically rigid (left) . Loss of water causes cell vacuoles to shrink. The cells become partly empty, the cell walls sag, and the tissue weakens (right).

Tough Cellulose and Lignin Cellulose, the other major cell-wall component, is very resistant to change, and this is one reason that it’s the most abundant plant product on earth. Like starch, cellulose consists of a chain of glucose sugar molecules. But a difference in the way they’re linked to each other allows neighboring chains to bond tightly together into fibers that are invulnerable to human digestive enzymes and all but extreme heat or chemical treatment. Cellulose becomes most visible to us in the winter as hay, a stubble field, or the fine skeletons of weeds. This remarkable stability makes cellulose valuable to long-lived trees and to the human species as well. Wood is one-third cellulose, and cotton and linen fibers are almost pure cellulose. However, cellulose is a problem for the cook: it simply can’t be softened by normal kitchen techniques. Sometimes, as in the gritty “stone cells” of pears, quince, and guava, this is a relatively minor distraction. But when it’s concentrated to provide structural support in stems and stalks — in celery and cardoons, for example — cellulose makes vegetables permanently stringy, and the only remedy is to pull the fibers from the tissue.

One last cell wall component is seldom significant in food. Lignin is also a strengthening agent and very resistant to breakdown; it’s the defining ingredient of wood. Most vegetables are harvested well in advance of appreciable lignin formation, but occasionally we do deal with woody asparagus and broccoli stems. The only remedy for this kind of toughness is to peel away the lignified areas.

Color

Plant pigments are one of life’s glories! The various greens of forest and field, the purples and yellows and reds of fruits and flowers — these colors speak to us of vitality, renewal, and the sheer pleasure of sensation. Some pigments are designed to catch our eye, some actually become part of our eye, and some made possible the very existence of us and our eyes (see box, p. 271). Many turn out to have beneficial effects on our health. The cook’s challenge is to preserve the vividness and appeal of these remarkable molecules.

There are four families of plant pigments, each with different functions in the plant’s life and different behaviors in the kitchen. All of them are large molecules that appear to be a certain color because they absorb certain wavelengths of light, and thus leave only parts of the spectrum for our eyes to detect. Chlorophylls are green, for example, because they absorb red and blue wavelengths.

The softening of plant cell walls. The walls are made up of a framework of cellulose fibers embedded in a mass of amorphous materials, including the pectins (left). When cooked in boiling water, the cellulose fibers remain intact, but the amorphous materials are partly extracted into fluids from within the cells, thus weakening the walls (right) and tenderizing the vegetable or fruit.

Green Chlorophylls The earth is painted green with chlorophylls, the molecules that harvest solar energy and funnel it into the photosynthetic system that converts it into sugar molecules. Chlorophyll a is bright blue-green, chlorophyll b a more muted olive color. The a form dominates the b by 3 to 1 in most leaves, but the balance is evener in plants that grow in the shade, and in aging tissues, where the a form is degraded faster. The chlorophylls are concentrated in cell bodies called chloroplasts, where they’re embedded in the many folds of a membrane along with the other molecules of the photosynthetic system. Each chlorophyll molecule is made up of two parts. One is a ring of carbon and nitrogen atoms with a magnesium atom at the center, quite similar to the heme ring in the meat myoglobin pigment (p. 133). This ring portion is soluble in water, and does the work of absorbing light. The second part is a fat-soluble tail of 16 carbon atoms, which anchors the whole molecule in the chloroplast membrane. This part is colorless.

These complex molecules are readily altered when their membrane home is disrupted during cooking. This is why the bright green of fresh vegetables is fragile. Ironically, prolonged exposure to intense light also damages chlorophylls. Attention to cooking times, temperatures, and acidities are thus essential to serving bright green vegetables (p. 280).

Yellow, Orange, Red Carotenoids Carotenoids are so named because the first member of this large family to be chemically isolated came from carrots. These pigments absorb blue and green wavelengths and are responsible for most of the yellow and orange colors in fruits and vegetables (beta-carotene, xanthophylls, zeaxanthin), as well as the red of tomatoes, watermelons, and chillis (lycopene, capsanthin, and capsorubin; most red colors in plants are caused by anthocyanins). Carotenoids are zigzag chains of around 40 carbon atoms and thus resemble fat molecules (p. 797). They’re generally soluble in fats and oils and are relatively stable, so they tend to stay bright and stay put when a food is cooked in water. Carotenoids are found in two different places in plant cells. One is in special pigment bodies, or chromoplasts, which signal animals that a flower is open for business or a fruit is ripe. Their other home is the photosynthetic membranes of chloroplasts, where there is one carotenoid molecule for every five or so chlorophylls. Their main role there is to protect chlorophyll and other parts of the photosynthetic system. They absorb potentially damaging wavelengths in the light spectrum, and act as antioxidants by soaking up the many high-energy chemical by-products generated in photosynthesis. They can do the same in the human body, particularly in the eye (p. 256). Chloroplast carotenoids are usually invisible, their presence masked by green chlorophyll, but it’s a good rule of thumb that the darker green the vegetable, the more chloroplasts and chlorophyll it contains, and the more carotenoids as well.

About ten carotenoids have a nutritional as well as aesthetic significance: they are converted to vitamin A in the human intestinal wall. Of these the most common and active is beta-carotene. Strictly speaking, only animals and animal-derived foods contain vitamin A itself; fruits and vegetables contain only its precursors. But without these pigment precursors there would be no vitamin A in animals either. In the eye, vitamin A becomes part of the receptor molecule that detects light and allows us to see. Elsewhere in the body it has a number of other important roles.

Red and Purple Anthocyanins, Pale Yellow Anthoxanthins Anthocyanins (from the Greek for “blue flower”) are responsible for most of the red, purple, and blue colors in plants, including many berries, apples, cabbage, radishes, and potatoes. A related group, the anthoxanthins (“yellow flower”) are pale yellow compounds found in potatoes, onions, and cauliflower. This third major class of plant pigments is a subgroup of the huge phenolic family, which is based on rings of 6 carbon atoms with two-thirds of a water molecule (OH) attached to some of them, which makes phenolics soluble in water. The anthocyanins have 3 rings. There are about 300 known anthocyanins, and a given fruit or vegetable will usually contain a mixture of a dozen or more. Like many other phenolic compounds, they are valuable antioxidants (p. 255).

Anthocyanins and anthoxanthins reside in the storage vacuole of plant cells, and readily bleed into surrounding tissues and ingredients when cell structures are damaged by cooking. This is why the lovely color of purple-tinted asparagus, beans, and other vegetables often disappears with cooking: the pigment is stored in just the outer layers of cells, and gets diluted to invisibility when the cooked cells break open. The main function of the anthocyanins is to provide signaling colors in flowers and fruits, though they may have begun their career as light-absorbing protection for the photosynthetic systems in young leaves (see box, p. 271). Anthocyanins are very sensitive to the acid-alkaline balance of foods — alkalinity shifts their color to the blue — and they’re altered by traces of metals, so they are often the source of strange off-colors in cooked foods (p. 281).

Red and Yellow Betains A fourth group of plant pigments is the betains, which are only found in a handful of distantly related species. However, these include three popular and vividly colored vegetables: beets and chard (both varieties of the same species), amaranth, and the prickly pear, the fruit of a cactus. The betains (sometimes called betalains) are complex nitrogen-containing molecules that are otherwise similar to anthocyanins: they are water-soluble, sensitive to heat and light, and tend toward the blue in alkaline conditions. There are about 50 red betains and 20 yellow betaxanthins, combinations of which produce the almost fluorescent-looking stem and vein colors of novelty chards. The human body has a limited ability to metabolize these molecules, so a large dose of red beets or prickly pears can give a startling but harmless tinge to the urine. The red betains contain a phenolic group and are good antioxidants; yellow betaxanthins don’t and aren’t.

The three major kinds of plant pigments. For the sake of clarity, most hydrogen atoms are left unlabeled; dots indicate carbon atoms. Top: Beta-carotene, the most common carotenoid pigment, and the source of the orange color of carrots. The long fat-like carbon chain makes these pigments much more soluble in fats and oils than in water. Bottom left: Chlorophyll a, the main source of the green in vegetables and fruits, with a heme-like region (p. 133) and a long carbon tail that makes chlorophyll more soluble in fats and oils than in water. Bottom right: Cyanidin, a blue pigment in the anthocyanin family. Thanks to their several hydroxyl (OH) groups, anthocyanins are water-soluble, and readily leak out of boiled vegetables.

Discoloration: Enzymatic Browning Many fruits and vegetables — for example apples, bananas, mushrooms, potatoes — quickly develop a brown, red, or gray discoloration when cut or bruised. This discoloration is caused by three chemical ingredients: 1- and 2-ring phenolic compounds, certain plant enzymes, and oxygen. In the intact fruit or vegetable, the phenolic compounds are kept in the storage vacuole, the enzymes in the surrounding cytoplasm. When the cell structure is damaged and phenolics are mixed with enzymes and oxygen, the enzymes oxidize the phenolics, forming molecules that eventually react with each other and bond together into light-absorbing clusters. This system is one of the plant’s chemical defenses: when insects or microbes damage its cells, the plant releases reactive phenolics that attack the invaders’ own enzymes and membranes. The brown pigments that we see are essentially masses of spent weapons. (A similar kind of enzyme acting on a similar compound is responsible for the “browning” of humans in the sun; here the pigment itself is the protective agent.)

Minimizing Brown Discoloration Enzymatic browning can be discouraged by several means. The single handiest method for the cook is to coat cut surfaces with lemon juice: the browning enzymes work very slowly in acidic conditions. Chilling the food below about 40ºF/4ºC will also slow the enzymes down somewhat, as will immersing the cut pieces in cold water, which limits the availability of oxygen. In the case of precut lettuce for salads, enzyme activity and browning can be reduced by immersing the freshly cut leaves in a pot of water at 115ºF/47ºC for three minutes before chilling and bagging them. Boiling temperatures will destroy the enzyme, so cooking will eliminate the problem. However, high temperatures can encourage phenolic oxidation in the absence of enzymes: this is why the water in which vegetables have been cooked sometimes turns brown on standing. Various sulfur compounds will combine with the phenolic substances and block their reaction with the enzyme, and these are often applied commercially to dried fruits. Sulfured apples and apricots retain their natural color and flavor, while unsulfured dried fruits turn brown and develop a more cooked flavor.

Brown discoloration caused by plant enzymes. When the cells in certain fruits and vegetables are damaged by cutting, bruising, or biting, browning enzymes in the cell cytoplasm come into contact with small, colorless phenolic molecules from the storage vacuole. With the help of oxygen from the air, the enzymes bind the phenolic molecules together into large, colored assemblies that turn the damaged area brown.

Another acid that inhibits browning by virtue of its antioxidant properties is ascorbic acid, or vitamin C. It was first identified around 1925 when the Hungarian biochemist Albert Szent-Györgyi found that the juice of some nonbrowning plants, including the chillis grown for paprika, could delay the discoloration of plants that do brown, and he isolated the responsible substance.

Flavor

The overall flavor of a fruit or vegetable is a composite of several distinct sensations. From the taste buds on our tongues, we register salts, sweet sugars, sour acids, savory amino acids, and bitter alkaloids. From the cells in our mouth sensitive to touch, we notice the presence of astringent, puckery tannins. A variety of cells in and near the mouth are irritated by the pungent compounds in peppers, mustard, and members of the onion family. Finally, the olfactory receptors in our nasal passages can detect many hundreds of volatile molecules that are small and chemically repelled by water, and therefore fly out of the food and into the air in our mouth. The sensations from our mouth give us an idea of a food’s basic composition and qualities, while our sense of smell allows us to make much finer discriminations.

Taste: Salty, Sweet, Sour, Savory, Bitter Of the five generally recognized tastes, three are especially prominent in fruits and vegetables. Sugar is the main product of photosynthesis, and its sweetness is the main attraction provided by fruits for their animal seed dispersers. The average sugar content of ripe fruit is 10 to 15% by weight. Often the unripe fruit stores its sugar as tasteless starch, which is then converted back into sugar during ripening to make the fruit more appealing. At the same time, the fruit’s acid content usually drops, a development that makes the fruit seem even sweeter. There are several organic acids — citric, malic, tartaric, oxalic — that plants can accumulate in their vacuoles and variously use as alternative energy stores, chemical defenses, or metabolic wastes, and that account for the acidity of most fruits and vegetables (all are acid to some degree). The sweet-sour balance is especially important in fruits.

Most vegetables contain only moderate amounts of sugar and acid, and these are quickly used up by the plant cells after harvest. This is why vegetables picked just before cooking are more full-flavored than store-bought produce, which is usually days to weeks from the field.

Bitter tastes are generally encountered only among vegetables and seeds (for example, coffee and cocoa beans), which contain alkaloids and other chemical defenses meant to discourage animals from eating them. Farmers and plant breeders have worked for thousands of years to reduce the bitterness of such crops as lettuce, cucumbers, eggplants, and cabbage, but chicory and radicchio, various cabbage relatives, and the Asian bitter gourd are actually prized for their bitterness. In many cultures, bitterness is thought to be a manifestation of medicinal value and therefore of healthfulness, and there may be some truth to this association (p. 334).

Though savory, mouth-filling amino acids are more characteristic of protein-rich animal foods, some fruits and vegetables do contain significant quantities of glutamic acid, the active portion of MSG. Notable among them are tomatoes, oranges, and many seaweeds. The glutamic acid in tomatoes, together with its balanced sweetness and acidity, may help explain why this fruit is so successfully used as a vegetable, both with meats and without.

Touch: Astringency Astringency is neither a taste nor an aroma, but a tactile sensation: that dry, puckery, rough feeling that follows a sip of strong tea or red wine, or a bite into an unripe banana or peach. It is caused by a group of phenolic compounds consisting of 3 to 5 carbon rings, which are just the right size to span two or more normally separate protein molecules, bond to them, and hold them together. These phenolics are called tannins because they have been used since prehistory to tan animal hides into tough leather by bonding with the skin proteins. The sensation of astringency is caused when tannins bond to proteins in our saliva, which normally provide lubrication and help food particles slide smoothly along the mouth surfaces. Tannins cause the proteins to clump together and stick to particles and surfaces, increasing the friction between them. Tannins are another of the plant kingdom’s chemical defenses. They counteract bacteria and fungi by interfering with their surface proteins, and deter plant-eating animals by their astringency and by interfering with digestive enzymes. Tannins are most often found in immature fruit (to prevent their consumption before the seeds are viable), in the skins of nuts, and in plant parts strongly pigmented with anthocyanins, phenolic molecules that turn out to be the right size to cross-link proteins. Red-leaf lettuces, for example, are noticeably more astringent than green.

Though a degree of astringency can be desirable in a dish or drink — it contributes a feeling of substantialness — it often becomes tiresome. The problem is that the sensation becomes stronger with each dose of tannins (whereas most flavors become less prominent), and it lingers, with the duration also increasing with each exposure. So it’s worth knowing how to control astringency (p. 284).

Irritation: Pungency The sensations caused by “hot” spices and vegetables — chillis, black pepper, ginger, mustard, horse-radish, onions, and garlic — are most accurately described as irritation and pain (for why we can enjoy such sensations, seep. 394). The active ingredients in all of these are chemical defenses that are meant to annoy and repel animal attackers. Very reactive sulfur compounds in the mustard and onion families apparently do mild damage to the unprotected cell membranes in our mouth and nasal passages, and thus cause pain. The pungent principles of the peppers and ginger, and some of the mustard compounds, work differently; they bind to a specific receptor on the cell membranes, and the receptor then triggers reactions in the cell that cause it to send a pain signal to the brain. The mustard and onion defenses are created only when tissue damage mixes together normally separate enzymes and their targets. Because enzymes are inactivated by cooking temperatures, cooking will moderate the pungency of these foods. By contrast, the peppers and ginger stockpile their defenses ahead of time, and cooking doesn’t reduce their pungency.

The nature and use of pungent ingredients are described in greater detail in the next several chapters, in entries on particular vegetables and spices.

Aroma: Variety and Complexity The subject of aroma is both daunting and endlessly fascinating! Daunting because it involves many hundreds of different chemicals and sensations for which we don’t have a good everyday vocabulary; fascinating because it helps us perceive more, and find more to enjoy, in the most familiar foods. There are two basic facts to keep in mind when thinking about the aroma of any food. First, the distinctive aromas of particular foods are created by specific volatile chemicals that are characteristic of those foods. And second, nearly all food aromas are composites of many different volatile molecules. In the case of vegetables, herbs, and spices, the number may be a dozen or two, while fruits typically emit several hundred volatile molecules. Usually just a handful create the dominant element of an aroma, while the others supply background, supporting, enriching notes. This combination of specificity and complexity helps explain why we find echoes of one food in another, or find that two foods go well together. Some affinities result when the foods happen to share some of the same aroma molecules.

One way to approach the richness of plant flavors is to taste actively and with other people. Rather than simply recognizing a familiar flavor as what you expect, try to dissect that flavor into some of its component sensations, just as a musical chord can be broken down into its component notes. Run through a checklist of the possibilities, and ask: Is there a green-grass note in this aroma? A fruity note? A spicy or nutty or earthy note? If so, which kind of fruit or spice or nut? Chapters 6–8 give interesting facts about the aromas of particular fruits, vegetables, herbs, and spices.

Aroma Families The box on pp. 274–275 identifies some of the more prominent aromas to be found in plant foods. Though I’ve divided them by type of food, this division is arbitrary. Fruits may have green-leaf aromas; vegetables may contain chemicals more characteristic of fruits or spices; spices and herbs share many aromatics with fruits. Some examples: cherries and bananas contain the dominant element of cloves; coriander contains aromatics that are prominent in citrus flowers and fruits; carrots share piney aromatics with Mediterranean herbs. While a given plant does usually specialize in the production of a certain kind of aromatic, plants in general are biochemical virtuosos, and may operate a number of different aromatic production lines at once. Some of the most important production lines are these:

• “Green,” cucumber/melon, and mushroom aromas, produced from unsaturated fatty acids in cell membranes when tissue damage mixes an oxidizing enzyme (lipoxygenase) with unsaturated fatty acids in cell membranes. This enzyme breaks the long fatty acid chains into small, volatile pieces, and other enzymes then modify the pieces.
• “Fruity” aromas, produced when enzymes in the intact fruit combine an acid molecule with an alcohol molecule to produce an ester.
• “Terpene” aromas, produced by a long series of enzymes from small building blocks that also get turned into carotenoid pigments and other important molecules. They range from flowery to citrusy, minty, herbaceous, and piney (p. 391).
• “Phenolic” aromas, produced by a series of enzymes from an amino acid with a 6-carbon ring. These are offshoots of the biochemical pathway that makes woody lignin (p. 266), and include many spicy, warming, and pungent molecules (p. 391).
• “Sulfur” aromatics, usually produced when tissue damage mixes enzymes with nonaromatic aroma precursors. Most sulfur aromatics are pungent chemical defenses, though some give a more subtle depth to a number of fruits and vegetables.

Fascinating and useful as it is to analyze the flavors of the plant world, the greatest pleasure still comes from savoring them whole. This is one of the great gifts of life in the natural world, as Henry David Thoreau reminded us:

Handling and Storing
Fruits and Vegetables

Post-Harvest Deterioration

There’s no match for the flavor of a vegetable picked one minute and cooked the next. Once a vegetable is harvested it begins to change, and that change is almost always for the worse. (Exceptions include plant parts designed to hibernate, for example onions and potatoes.) Plant cells are hardier than animal cells, and may survive for weeks or even months. But cut off from their source of renovating nutrients, they consume themselves and accumulate waste products, and their flavor and texture suffer. Many varieties of corn and peas lose half their sugar in a few hours at room temperature, either by converting it to starch or using it for energy to stay alive. Bean pods, asparagus, and broccoli begin to use their sugar to make tough lignified fibers. As crisp, crunchy lettuce and celery use up their water, their cells lose turgor pressure and they become limp and chewy (p. 265).

Fruits are a different story. Some fruits may actually get better after harvest because they continue to ripen. But ripening soon runs its course, and then fruits also deteriorate. Eventually fruit and vegetable cells alike run out of energy and die, their complex biochemical organization and machinery break down, their enzymes act at random, and the tissue eats itself away.

The spoilage of fruits and vegetables is hastened by microbes, which are always present on their surfaces and in the air. Bacteria, molds, and yeasts all attack weakened or damaged plant tissue, break down its cell walls, consume the cell contents, and leave behind their distinctive and often unpleasant waste products. Vegetables are mainly attacked by bacteria, which grow faster than other microbes. Species of Erwinia and Pseudomonas cause familiar “soft rot.” Fruits are more acidic than vegetables, so they’re resistant to many bacteria but more readily attacked by yeasts and molds (Penicillium, Botrytis).

Precut fruits and vegetables are convenient but especially susceptible to deterioration and spoilage. Cutting has two important effects. The tissue damage induces nearby cells to boost their defensive activity, which depletes their remaining nutrients and may cause such changes as toughening, browning, and the development of bitter and astringent flavors. And it exposes the normally protected nutrient-rich interior to infection by microbes. So precut produce requires special care.

Handling Fresh Produce

The aim in storing fruits and vegetables is to slow their inevitable deterioration. This begins with choosing and handling the produce. Mushrooms as well as some ripe fruits — berries, apricots, figs, avocados, papayas — have a naturally high metabolism and deteriorate faster than lethargic apples, pears, kiwi fruits, cabbages, carrots, and other good keepers. “One rotten apple spoils the barrel”: moldy fruit or vegetables should be discarded and refrigerator drawers and fruit bowls should be cleaned regularly to reduce the microbial population. Produce shouldn’t be subjected to physical stress, whether dropping apples on the floor or packing tomatoes tightly into a confined space. Even rinsing in water can make delicate berries more susceptible to infection by abrading their protective epidermal layer with clinging dirt particles. On the other hand, soil harbors large numbers of microbes, and should be removed from the surfaces of sturdier fruits and vegetables before storing them.

The Storage Atmosphere

The storage life of fresh produce is strongly affected by the atmosphere that surrounds it. All plant tissues are mostly water, and require a humid atmosphere to avoid drying out, losing turgidity, and damaging their internal systems. Practically, this means it’s best to keep plant foods in restricted spaces — plastic bags, or drawers within a refrigerator — to slow down moisture loss to the compartment as a whole and to the outside. At the same time, living produce exhales carbon dioxide and water, so moisture can accumulate and condense on the food surfaces, which encourages microbial attack. Lining the container with an absorbent material — a paper towel or bag — will delay condensation.

The metabolic activity of the cells can also be slowed by limiting their access to oxygen. Commercial packers fill their bags of produce with a well-defined mixture of nitrogen, carbon dioxide, and just enough oxygen (8% or less) to keep the plant cells functioning normally; and they use bags whose gas permeability matches the respiration rate of the produce. (Too little oxygen and fruits and vegetables switch to anaerobic metabolism, which generates alcohol and other odorous molecules characteristic of fermentation, and causes internal tissue damage and browning.)

Home and restaurant cooks can approximate such a controlled atmosphere by packing their produce in closed plastic bags with most of the air squeezed out of them. The plant cells consume oxygen and create carbon dioxide, so the oxygen levels in the bags slowly decline. However, a major disadvantage of a closed plastic bag is that it traps the gas ethylene, a plant hormone that advances ripening in fruits and induces defensive activity and accelerated aging in other tissues. This means that bagged fruits may pass from ripe to overripe too quickly, and one damaged lettuce leaf can speed the decline of a whole head. Recently, manufacturers have introduced produce containers with inserts that destroy ethylene and extend storage life (the inserts contain permanganate).

A very common commercial treatment that slows both water loss and oxygen uptake in whole fruits and fruit-vegetables — apples, oranges, cucumbers, tomatoes — is to coat them at the packing facility with a layer of edible wax or oil. A number of different materials are used, including natural beeswax and carnauba, candellila, and rice-bran waxes and vegetable oils, and such petrochemical by-products as paraffin, polyethylene waxes, and mineral oil. These treatments are harmless, but they can make produce surfaces unpleasantly waxy or hard.

Temperature Control:
Refrigeration

The most effective way to prolong the storage life of fresh produce is to control its temperature. Cooling slows chemical reactions in general, so it slows the metabolic activity of the plant cells themselves, and the growth of the microbes that attack them. A reduction of just 10ºF/5ºC can nearly double storage life. However, the ideal storage temperature is different for different fruits and vegetables. Those native to temperate climates are best kept at or near the freezing point, and apples may keep for nearly a year if the storage atmosphere is also controlled. But fruits and vegetables native to warmer regions are actually injured by temperatures that low. Their cells begin to malfunction, and uncontrolled enzyme action causes damage to cell walls, the development of off-flavors, and discoloration. Chilling injury may become apparent during storage, or only after the produce is brought back to room temperature. Banana skins turn black in the refrigerator; avocados darken and fail to soften further; citrus fruits develop spotted skins. Foods of tropical and subtropical origin keep best at the relatively high temperature of 50ºF/10ºC, and are often better off at room temperature than in the refrigerator. Among them are melons, eggplants, squash, tomatoes, cucumbers, peppers, and beans.

Temperature Control:
Freezing

The most drastic form of temperature control is freezing, which stops cold the overall metabolism of fruits, vegetables, and spoilage microbes. It causes most of the water in the cells to crystallize, thus immobilizing other molecules and suspending most chemical activity. The microbes are hardy, and most of them revive on warming. But freezing kills plant tissues, which suffer two kinds of damage. One is chemical: as the water crystallizes, enzymes and other reactive molecules become unusually concentrated and react abnormally. The other damage is physical disruption caused by the water crystals, whose edges puncture cell walls and membranes. When the food is thawed, the cell fluids leak out of the cells, and the food loses crispness and becomes limp and wet. Producers of frozen foods minimize the size of the ice crystals, and so the amount of damage done, by freezing the food as quickly as possible to as low a temperature as possible, often –40ºF/–40ºC. Under these conditions, many small ice crystals form; at higher temperatures fewer and larger crystals form, and do more damage. Home and restaurant freezers are warmer than commercial freezers and their temperatures fluctuate, so during storage some water melts and refreezes into larger crystals, and the food’s texture suffers.

Although freezing temperatures generally reduce enzymatic and other chemical activity, some reactions are actually enhanced by the concentrating effects of ice formation, including enzymatic breakdown of vitamins and pigments. The solution to this problem is blanching. In this process the food is immersed in rapidly boiling water for a minute or two, just enough time to inactivate the enzymes, and then just as rapidly immersed in cold water to stop further cooking and softening of the cell walls. If vegetables are to be frozen for more than a few days, they should be blanched first. Fruits are less commonly blanched because their cooked flavor and texture are less appealing. Enzymatic browning in frozen fruit can be prevented by packing it in a sugar syrup supplemented with ascorbic acid (between ¼ and ¾ teaspoon per quart, 750–2,250 mg per liter, depending on the fruit’s susceptibility to browning). Sugar syrup (usually around 40%, or 1.5 lb sugar per quart water, 680 gm per liter) can also improve the texture of frozen fruit by being absorbed into the cell-wall cement, which becomes stiffer. Frozen produce should be wrapped as air- and watertight as possible. Surfaces left exposed to the relatively dry atmosphere of the freezer will develop freezer burn, the slow, patchy drying out caused by the evaporation of frozen water molecules directly into vapor (this is called “sublimation”). Freezer-burned patches develop a tough texture and stale flavor.

Cooking Fresh Fruits
and Vegetables

Compared to meats, eggs, and dairy products, vegetables and fruits are easy to cook. Animal tissues and secretions are mainly protein, and proteins are sensitive molecules; moderate heat (140ºF/60ºC) causes them to cling tightly to each other and expel water, and they quickly become hard and dry. Vegetables and fruits are mainly carbohydrates, and carbohydrates are robust molecules; even boiling temperatures simply disperse them more evenly in the tissue moisture, so the texture becomes soft and succulent. However, the cooking of vegetables and fruits does have its fine points. Plant pigments, flavor compounds, and nutrients are sensitive to heat and to the chemical environment. And even carbohydrates sometimes behave curiously! The challenge of cooking vegetables and fruits is to create an appealing texture without compromising color, flavor, and nutrition.

How Heat Affects the Qualities
of Fruits and Vegetables

Color Many plant pigments are altered by cooking, which is why we can often judge by their color how carefully vegetables have been prepared. The one partial exception to this rule is the yellow-orange-red carotenoid group, which is more soluble in fat than in water, so the colors don’t readily leak out of the tissue, and are fairly stable. However, even carotenoids are changed by cooking. When we heat carrots, their beta-carotene shifts structure and hue, from red-orange toward the yellow. Apricots and tomato paste dried in the sun lose much of their intact carotenoids unless they’re treated with antioxidant sulfur dioxide (p. 291). But compared to the green chlorophylls and multihued anthocyanins, the carotenoids are the model of steadfastness.

Green Chlorophyll One change in the color of green vegetables as they are cooked has nothing to do with the pigment itself. That wonderfully intense, bright green that develops within a few seconds of throwing vegetables into boiling water is a result of the sudden expansion and escape of gases trapped in the spaces between cells. Ordinarily, these microscopic air pockets cloud the color of the chloroplasts. When they collapse, we can see the pigments much more directly.

The Enemy of Green: Acids Green chlorophyll is susceptible to two chemical changes during cooking. One is the loss of its long carbon-hydrogen tail, which leaves the pigment water-soluble — so that it leaks out into the cooking liquid — and more susceptible to further change. This loss is encouraged by both acid and alkaline conditions and by an enzyme called chlorophyllase, which is most active between 150–170ºF/66–77ºC and only destroyed near the boiling point. The second and more noticeable change in chlorophyll is the dulling of its color, which is caused when either heat or an enzyme nudge the magnesium atom from the center of the molecule. The replacement of magnesium by hydrogen is by far the most common cause of color change in cooked vegetables. In even slightly acidic water, the plentiful hydrogen ions displace the magnesium, a change that turns chlorophyll a into grayish-green pheophytin a, chlorophyll b into yellowish pheophytin b. Cooking vegetables without water — stir-frying, for example — will also cause a color change, because when the temperature of the plant tissue rises above 140ºF/60ºC, the organizing membranes in and around the chloroplast are damaged, and chlorophyll is exposed to the plant’s own natural acids. Freezing, pickling, dehydration, and simple aging also damage chloroplasts and chlorophyll. This is why dull, olive-green vegetables are so common.

Changes in chlorophyll during cooking. Left: The normal chlorophyll molecule is bright green and has a fat-like tail that makes it soluble in fats and oils. Center: Enzymes in the plant cells can remove the fat-like tail, producing a tailless form that is water-soluble and readily leaks into cooking liquids. Right: In acid conditions, the central magnesium atom is replaced by hydrogens, and the resulting chlorophyll molecule is a dull olive green.

Traditional Aids: Soda and Metals There are two chemical tricks that can help keep green vegetables bright, and cooks have known about them for hundreds and even thousands of years. One is to cook them in alkaline water, which has very few hydrogen ions that are free to displace the magnesium in chlorophyll. The great 19th-century French chef Antonin Carême de-acidified his cooking water with wood ash; today baking soda (sodium bicarbonate) is the easiest. The other chemical trick is to add to the cooking water other metals — copper and zinc — that can replace magnesium in the chlorophyll molecule, and resist displacement by hydrogen. However, both tricks have disadvantages. Copper and zinc are essential trace nutrients, but in doses of more than a few milligrams they can be toxic. And while there’s nothing toxic about sodium bicarbonate, excessively alkaline conditions can turn vegetable texture to mush (p. 282), speed the destruction of vitamins, and leave a soapy off-taste.

Watch the Water, Time, and Sauce Dulling of the greens can be minimized by keeping cooking times short, between five and seven minutes, and protecting chlorophyll from acid conditions. Stir-frying and microwaving can be very quick, but they expose chlorophyll fully to the cells’ own acids. Ordinary boiling in copious water has the advantage of diluting the cells’ acids. Most city tap water is kept slightly alkaline to minimize pipe corrosion, and slightly alkaline water is ideal for preserving chlorophyll’s color. Check the pH of your water: if it’s acid, its pH below 7, then experiment with adding small amounts of baking soda (start with a small pinch per gallon/4 liters) to adjust it to neutral or slightly alkaline. Once the vegetables are cooked, either serve them immediately or plunge them briefly in ice water so that they don’t continue to cook and get dull. Don’t dress the vegetables with acidic ingredients like lemon juice until the last minute, and consider protecting them first with a thin layer of oil (as in a vinaigrette) or butter.

Red-Purple Anthocyanins and Pale Anthoxanthins The usually reddish anthocyanins and their pale yellow cousins, the anthoxanthins, are chlorophyll’s opposites. They’re naturally water-soluble, so they always bleed into the cooking water. They too are sensitive to pH and to the presence of metal ions, but acidity is good for them, metals bad. And where chlorophyll just gets duller or brighter according to these conditions, the anthocyanins change color completely! This is why we occasionally see red cabbage turn blue when braised, blueberries turn green in pancakes and muffins, and garlic turn green or blue when pickled. (The betacyanins and betaxanthins in beets and chard are different compounds and somewhat more stable.)

The Enemies: Dilution, Alkalinity, and Metals Anthocyanins and anthoxanthins are concentrated in cell vacuoles, and sometimes (as in purple beans and asparagus) just in a superficial layer of cells. So when the food is cooked and the vacuoles damaged, the pigments escape and can become so diluted that their color fades or disappears — especially if they’re cooked in a pot of water. The pigments that remain are affected by the new chemical environment of the cooked plant tissue. The vacuoles in which anthocyanins are stored are generally acid, while the rest of the cell fluids are less so. Cooking water is often somewhat alkaline, and quick breads include distinctly alkaline baking soda. In acid conditions, anthocyanins tend toward the red; around neutral pH, they’re colorless or light violet; and in alkaline conditions, bluish. And pale anthoxanthins become more deeply yellow as alkalinity rises. So red fruits and vegetables can fade and even turn blue when cooked, while pale yellow ones darken. And traces of metals in the cooking liquid can generate very peculiar colors: some anthocyanins and anthoxanthins form grayish, green, blue, red, or brown complexes with iron, aluminum, and tin.

The Aid: Acids The key to maintaining natural anthocyanin coloration is to keep fruits and vegetables sufficiently acidic, and avoid supplying trace metals. Lemon juice in the cooking water or sprinkled on the food can help with both aims: its citric acid binds up metal ions. Cooking red cabbage with acidic apples or vinegar keeps it from turning purple; dispersing baking soda evenly in batters, and using as little as possible to keep the batter slightly acidic, will keep blueberries from turning green.

Creating Color from Tannins On rare and wonderful occasions, cooking can actually create anthocyanins: in fact, it transforms touch into color! Colorless quince slices cooked in a sugar syrup lose their astringency and develop a ruby-like color and translucency. Quinces and certain varieties of pear are especially rich in phenolic chemicals, including aggregates (proanthocyanidins) of from 2 to 20 anthocyanin-like subunits. The aggregates are the right size to cross-link and coagulate proteins, so they feel astringent in our mouth. When these fruits are cooked for a long time, the combination of heat and acidity causes the subunits to break off one by one; and then oxygen from the air reacts with the subunits to form true anthocyanins: so the tannic, pale fruits become more gentle-tasting and anything from pale pink to deep red. (Interestingly, the similar development of pinkness in canned pears is considered discoloration. It’s accentuated by tin in unenameled cans.)

Texture We’ve seen that the texture of vegetables and fruits is determined by two factors: the inner water pressure of the tissue’s cells, and the structure of the cell walls (p. 265). Cooking softens plant tissues by releasing the water pressure and dismantling the cell walls. When the tissue reaches 140ºF/60ºC, the cell membranes are damaged, the cells lose water and deflate, and the tissue as a whole goes from firm and crisp to limp and flabby. (Even vegetables surrounded by boiling water lose water during cooking, as weighings before and after will prove.) At this stage, vegetables often squeak against the teeth: they’ve lost the crunch of turgid tissue, but the cell walls are still strong and resist chewing. Then as the tissue temperature approaches the boiling point, the cell walls begin to weaken. The cellulose framework remains mostly unchanged, but the pectin and hemicellulose “cement” softens, gradually breaks down into shorter chains, and dissolves. Teeth now easily push adjacent cells apart from each other, and the texture becomes tender. Prolonged boiling will remove nearly all of the cell-wall cement and cause the tissue to disintegrate, thus transforming it into a puree.

Acid and Hard Water Maintain Firmness; Salt and Alkalinity Speed Softening The wall-dissolving, tenderizing phase of fruit and vegetable cooking is strongly influenced by the cooking environment. Hemicelluloses are not very soluble in acid conditions, and readily soluble in alkaline conditions. This means that fruits and vegetables cooked in an acid liquid — a tomato sauce for example, or other fruit juices and purees — may remain firm during hours of cooking, while in neutral boiling water, neither acid nor alkaline, the same vegetables soften in 10 or 15 minutes. In distinctly alkaline water, fruits and vegetables quickly become mushy. Table salt in neutral cooking water speeds vegetable softening, apparently because its sodium ions displace the calcium ions that cross-link and anchor the cement molecules in the fruit and vegetable cell walls, thus breaking the cross-links and helping to dissolve the hemicelluloses. On the other hand, the dissolved calcium in hard tap water slows softening by reinforcing the cement cross-links. When vegetables are cooked without immersion in water — when they’re steamed or fried or baked — the cell walls are exposed only to the more or less acid cell fluids (steam itself is also a somewhat acidic pH 6), and a given cooking time often produces a firmer result than boiling.

Cooking starchy vegetables. Left: Before cooking, the plant cells are intact, the starch granules compact and hard. Right: Cooking causes the starch granules to absorb water from the cell fluids, swell, and soften.

The cook can make use of these influences to diagnose the cause of excessively rapid or slow softening and adjust the preparation — for example, precooking vegetables in plain water before adding them to a tomato sauce, or compensating for hard water with a softening pinch of alkaline baking soda. In the case of green vegetables, shortening the softening time with the help of salt and a discreet dose of baking soda helps preserve the bright green of the chlorophyll (p. 280).

Starchy Vegetables Potatoes, sweet potatoes, winter squashes, and other starchy vegetables owe their distinctive cooked texture to their starch granules. In the raw vegetables, starch granules are hard, closely packed, microscopic agglomerations of starch molecules, and give a chalky feeling when chewed out of the cells. They begin to soften at about the same temperature at which the membrane proteins denature, the “gelation range,” which in the potato is from 137–150ºF/58–66ºC (it varies from plant to plant). In this range the starch granules begin to absorb water molecules, which disrupt their compact structure, and the granules swell up to many times their original size, forming a soft gel, or sponge-like network of long chains holding water in the pockets between chains. The overall result is a tender but somewhat dry texture, because the tissue moisture has been soaked up into the starch. (Think of the textural difference between cooked high-starch potatoes and low-starch carrots.) In starchy vegetables with relatively weak cell walls, the gel-filled cells may be cohesive enough to pull away from each other as separate little particles, giving a mealy impression. This water absorption and the large surface area of separate cells are the reasons that mashed potatoes and other cooked starchy purees benefit from and accommodate large amounts of lubricating fat.

Precooking Can Give a Persistent Firmness to Some Vegetables and Fruits It turns out that in certain vegetables and fruits — including potatoes, sweet potatoes, beets, carrots, beans, cauliflower, tomatoes, cherries, apples — the usual softening during cooking can be reduced by a low-temperature precooking step. If preheated to 130–140ºF/55–60ºC for 20–30 minutes, these foods develop a persistent firmness that survives prolonged final cooking. This can be valuable for vegetables meant to hold their shape in a long-cooked meat dish, or potatoes in a potato salad, or for foods to be preserved by canning. It’s also valuable for boiled whole potatoes and beets, whose outer regions are inevitably over-softened and may begin to disintegrate while the centers cook through. These and other long-cooked root vegetables are usually started in cold water, so that the outer regions will firm up during the slow temperature rise. Firm-able vegetables and fruits have an enzyme in their cell walls that becomes activated at around 120ºF/50ºC (and inactivated above 160ºF/70ºC), and alters the cell-wall pectins so that they’re more easily cross-linked by calcium ions. At the same time, calcium ions are being released as the cell contents leak through damaged membranes, and they cross-link the pectin so that it will be much more resistant to removal or breakdown at boiling temperatures.

Persistently Crisp Vegetables A few underground stem vegetables are notable for retaining some crunchiness after prolonged cooking and even canning. These include the Chinese water chestnut, lotus root, bamboo shoots, and beets. Their textural robustness comes from particular phenolic compounds in their cell walls (ferulic acids) that form bonds with the cell-wall carbohydrates and prevent them from being dissolved away during cooking.

Flavor The relatively mild flavor of most vegetables and fruits is intensified by cooking. Heating makes taste molecules — sweet sugars, sour acids — more prominent by breaking down cell walls and making it easier for the cell contents to escape and reach our taste buds. Carrots, for example, taste far sweeter when cooked. Heat also makes the food’s aromatic molecules more volatile and so more noticeable, and it creates new molecules by causing increased enzyme activity, mixing of cell contents, and general chemical reactivity. The more prolonged or intense the heating, the more the food’s original aroma molecules are modified and supplemented, and so the more complex and “cooked” the flavor. If the cooking temperature exceeds the boiling point — in frying and baking, for example — then these carbohydrate-rich materials will begin to undergo browning reactions, which produce characteristic roasted and caramelized flavors. Cooks can create several layers of flavor in a dish by combining well-cooked, lightly cooked, and even raw batches of the same vegetables or herbs.

One sensory quality unique to plants is astringency (p. 271), and it can make such foods as artichokes, unripe fruits, and nuts less than entirely pleasant to eat. There are ways to control the influence of tannins in these foods. Acids and salt increase the perception of astringency, while sugar reduces it. Adding milk, gelatin, or other proteins to a dish will reduce its astringency by inducing the tannins to bind to food proteins before they can affect salivary proteins. Ingredients rich in pectin or gums will also take some tannins out of circulation, and fats and oils will slow the initial binding of tannins and proteins.

Nutritional Value Cooking destroys some of the nutrients in food, but makes many nutrients more easily absorbed. It’s a good idea to include both raw and cooked fruits and vegetables in our daily diet.

Some Diminishment of Nutritional Value… Cooking generally reduces the nutritional content of fruits and vegetables. There are some important exceptions to this rule, but the levels of most vitamins, antioxidants, and other beneficial substances are diminished by the combination of high temperatures, uncontrolled enzyme activity, and exposure to oxygen and to light. They and minerals can also be drawn out of plant tissues by cooking water. These losses can be minimized by rapid and brief cooking. Baked potatoes, for example, heat up relatively slowly and lose much more vitamin C to enzyme action than do boiled potatoes. However, some techniques that speed cooking — cutting vegetables into small pieces, and boiling in a large volume of water, which maintains its temperature — can result in increased leaching of water-soluble nutrients, including minerals and the B and C vitamins. To maximize the retention of vitamins and minerals, cook small batches of vegetables and fruits in the microwave oven, in a minimal amount of added water.

…And Some Enhancement Cooking has several general nutritional benefits. It eliminates potentially harmful microbes. By softening and concentrating foods, it also makes them easier to eat in significant quantities. And it actually improves the availability of some nutrients. Two of the most important are starch and the carotenoid pigments. Starch consists of long chains of sugar molecules crammed into masses called granules. Our digestive enzymes can’t penetrate past the outer layer of raw starch granules, but cooking unpacks the starch chains and lets our enzymes break them down. Then there are beta-carotene, the precursor to vitamin A, its chemical relative lycopene, an important antioxidant, and other valuable carotenoid pigments. Because they’re not very soluble in water, we simply don’t extract these chemicals very efficiently by just chewing and swallowing. Cooking disrupts the plant tissues more thoroughly and allows us to extract much more of them. (Added fat also significantly improves our absorption of fat-soluble nutrients.)

There are many different ways of cooking vegetables and fruits. What follows is a brief outline of the most common methods and their general effects. They can be divided into three groups: moist methods that transfer heat by means of water; dry methods that transfer heat by means of air, oil, or infrared radiation; and a more miscellaneous group that includes ways of restructuring the food, either turning it into a fluid version of itself, or extracting the essence of its flavor or color.

Hot Water: Boiling, Steaming,
Pressure-Cooking

Boiling and steaming are the simplest methods for cooking vegetables, because they require no judgment of cooking temperature: whether water is boiling on a high flame or low, its temperature is 212ºF/100ºC (near sea level, with predictably lower temperatures at higher elevations). And because hot water and steam are excellent carriers of heat, these are efficient methods as well, ideal for the rapid cooking of green vegetables that minimizes their loss of color (p. 280). One important difference is that hot water dissolves and extracts some pectin and calcium from cell walls, while steaming leaves them in place: so boiling will soften vegetables faster and more thoroughly.

Boiling In the case of boiling green vegetables, it’s good to know the pH and dissolved mineral content of your cooking water. Ideally it should be neutral or just slightly alkaline (pH 7–8), and not too hard, because acidity dulls chlorophyll, and acidity and calcium both slow softening and so prolong the cooking. A large volume of rapidly boiling water will maintain a boil even after the cold vegetables are added, cut into pieces small enough to cook through in about five minutes. Salt in the cooking water at about the concentration of seawater (3%, or 2 tablespoons/30 gm per quart/liter) will speed softening (p. 282) and also minimize the loss of cell contents to the water (cooking water without its own dissolved salt will draw salts and sugars from the plant cells). When just tender enough, the vegetables should be removed and either served immediately or scooped briefly into ice water to stop the cooking and prevent further dulling of the color.

Starchy vegetables, especially potatoes cooked whole or in large pieces, benefit from a different treatment. Their vulnerability is a tendency for the outer portions to soften excessively and fall apart while the interiors cook through. Hard and slightly acid water can help them maintain their surface firmness, as will starting them in cold water and raising the temperature only gradually to reinforce their cell walls (p. 283). Salt is best omitted from the water, since it encourages early softening of the vulnerable exterior. Nor is it necessarily best to cook them at the boiling point: 180–190ºF/80–85ºC is sufficient to soften starch and cell walls and won’t overcook the exterior as badly, though the cooking through will take longer.

When vegetables are included in a meat braise or stew and are expected to have a tender integrity, their cooking needs as much attention as the meat’s. A very low cooking temperature that keeps the meat tender may leave the vegetables hard, while repeated bouts of simmering to dissolve a tough cut’s connective tissue may turn them to mush. The vegetables can be precooked separately, either to soften them for a low-temperature braise or firm them for long simmering; or they can be removed from a long-simmered dish when they reach the desired texture and added back when the meat is done.

Steaming Steaming is a good method for cooking vegetables at the boiling point, but without the necessity of heating a whole pot of water, exposing the food directly to turbulent water, and leaching out flavor or color or nutrients. It doesn’t allow the cook to control saltiness, calcium cross-linking, or acidity (steam itself is a slightly acid pH 6, and plant cells and vacuoles are also more acid than is ideal for chlorophyll); and evenness of cooking requires that the pieces be arranged in a single layer, or that the pile be very loose to allow the steam access to all food surfaces. Steaming leaves the food tasting exclusively of its cooked self, though the steam can also be aromatized by the inclusion of herbs and spices.

Pressure Cooking Pressure cooking is sometimes applied to vegetables, especially in the canning of low-acid foods. It is essentially cooking by a combination of boiling water and steam, except that both are at about 250ºF/120ºC rather than 212ºF/100ºC. (Enclosing the water in an airtight container traps the water vapor, which in turn raises the boiling point of the water.) Pressure cooking heats foods very rapidly, which means that it’s also very easy to overcook fresh vegetables. It’s best to follow specialized recipes closely.

Hot Air, Oil, and Radiation:
Baking, Frying, and Grilling

These “dry” cooking methods remove moisture from the food surface, thus concentrating and intensifying flavor, and can heat it above the boiling point, to temperatures that generate the typical flavors and colors of the browning reactions (p. 777).

Baking The hot air in an oven cooks vegetables and fruits relatively slowly, for several reasons. First, air is not as dense a medium as water or oil, so air molecules collide with the food less often, and take longer to impart energy to it. Second, a cool object in a hot oven develops a stagnant “boundary layer” of air molecules and water vapor that slows the collision rate even further. (A convection fan speeds cooking by circulating the air more rapidly and disrupting the boundary layer.) Third, in a dry atmosphere the food’s moisture evaporates from the surface, and this evaporation absorbs most of the incoming energy, only a fraction of which gets to the center. So baking is much less efficient than boiling or frying.

Of course, the oven’s thin medium is why the oven is a good means for drying foods, either partly — for example, to concentrate the flavor of watery tomatoes — or almost fully, to preserve and create a chewy or crisp texture. And once the surface has dried and its temperature rises close to the oven’s, then carbohydrates and proteins can undergo the browning reactions, which generate hundreds of new taste and aroma molecules and so a greater depth of flavor.

Often vegetables are coated with oil before baking, and this simple pretreatment has two important consequences. The thin surface layer of oil doesn’t evaporate the way the food moisture does, so all the heat the oil absorbs from the oven air goes to raising its and the food’s temperature. The surface therefore gets hotter than it would without the oil, and the food is significantly quicker both to brown and to cook through. Second, some of the oil molecules participate in the surface browning reactions and change the balance of reaction products that are formed; they create a distinctly richer flavor.

Frying and Sautéing Baking oiled vegetables is sometimes called “oven frying,” and indeed true frying in oil also desiccates the food surface, browns it, and enriches the flavor with the characteristic notes contributed by the oil itself. A food may be fried partly or fully immersed in oil, or just well lubricated with it (sautéing); and typical oil temperatures range from 325–375ºF/160–190ºC. True frying is faster than oven frying because oil is much denser than air, so energetic oil molecules collide with the food much more frequently. The key to successful frying is getting the piece size and frying temperature right, so that the pieces cook through in the time that the surfaces require to be properly browned. Starchy vegetables are the most commonly fried plant foods, and I describe the important example of potatoes in detail in chapter 6 (p. 303). Many more delicate vegetables and even fruits are fried with a protective surface coating of batter (p. 553) or breading, which browns and crisps while the food inside is insulated from direct contact with the high heat.

Stir-Frying and Sweating Two important variations on frying exploit opposite ends of the temperature scale. One is high-temperature stir-frying. The vegetables are cut into pieces sufficiently small that they heat through in about a minute, and they’re cooked on a smoking-hot metal surface with just enough oil to lubricate them, and with constant stirring to ensure even heating and prevent burning. In stir-frying it’s important to preheat the pan alone and add the oil just a few seconds before the vegetables; otherwise the high heat will damage the oil and make it unpalatable, viscous, and sticky. The rapidity of stir-frying makes it a good method for retaining pigments and nutrients. At the other extreme is a technique sometimes called “sweating” (Italian soffrito or Catalan soffregit, both meaning “underfrying”): the very slow cooking over low heat of finely chopped vegetables coated with oil, to develop a flavor base for a dish featuring other ingredients. Often the cook wants to avoid browning, or to minimize it; here the low heat and oil function to soften the vegetables, develop and concentrate their flavors, and blend those flavors together. Vegetables cooked in a version of the confit (p. 177) are immersed in oil and slowly cooked to soften them and infuse them with the oil’s flavor and richness.

Grilling Grilling and broiling cook by means of the intense infrared radiation emitted from burning coals, flames, and glowing electrical elements. This radiation can desiccate, brown, and burn in rapid succession, so it’s important to adjust the distance between heat source and food to make sure that the food can heat through before the surface chars. As in baking, a coating of oil speeds the cooking and improves flavor. Enclosing the food in a wrapper — fresh corn in its husk, plantains in their skin, potatoes in aluminum foil — can give some protection to the surface and essentially steam the food in its own moisture, while allowing in some of the smoky aroma from the heat source and smoldering wrapper. And some foods actually benefit from charring. Large sweet and hot chillis have a thick, tough cuticle or “skin” that is tedious to peel away. Because it’s relatively dry compared to the underlying flesh, and made up in part of flammable waxes, it can be burned to a crisp before the flesh gets soft. Once burned, the skin can be scraped or rinsed off with ease. Similarly, the flesh of eggplants is smokily perfumed and easily scraped from the skin when the whole vegetable is grilled until the flesh softens and the skin dries and toughens.

Microwave Cooking

Microwave radiation selectively energizes the water molecules in fruits and vegetables, and the water molecules then heat up the cell wall, starch, and other plant molecules (p. 786). Because radiation penetrates into food an inch/2 cm or so, it can be a fairly rapid method, and is an excellent one for retaining vitamins and minerals. However, it has several quirks that the cook must anticipate and compensate for. Because the microwaves penetrate a limited distance into the food, they will cook evenly only if the food is cut into similar-sized thin pieces, and the pieces arranged in a single layer or very loose pile. Energetic water molecules turn into water vapor and escape from the food: so microwaves tend to dry foods out. Vegetables should be enclosed in an almost steam-tight container, and often benefit from starting out with a small amount of added water so that their surfaces don’t lose too much moisture and shrivel. And because the foods must be enclosed, they retain some volatile chemicals that would otherwise escape — so their flavor can seem strong and odd. The inclusion of other aromatics can help mask this effect.

Cooks can exploit the drying quality of microwave radiation to crisp thin slices of fruits and vegetables. This is best done at a low power setting so that the heating is gentle and even, and doesn’t rapidly progress to browning or burning. When there’s little water left in a patch of tissue, it takes more energy to break it free, so the local boiling point rises to a temperature high enough to break apart carbohydrates and proteins, and this causes browning and then blackening.

Pulverizing and Extracting

In addition to preparing fruits and vegetables more or less as is, with their tissue structure intact, cooks often deconstruct them completely. In some preparations, we blend the contents of the plant cells with the walls that normally separate and contain them. In others, we separate the food’s flavor or color from its flavorless, colorless cell-wall fibers or abundant water, and produce a concentrated extract of that food’s essence.

Purees The simplest deconstructed version of fruits and vegetables is the puree, which includes such preparations as tomato and apple sauces, mashed potatoes, carrot soup, and guacamole. We make purees by applying enough physical force to crush the tissue, break apart and break open its cells, and mix cell innards with fragments of the cells’ walls. Thanks to the high water content of the cells, most purees are fluid versions of the original tissue. And thanks to the thickening powers of the cell-wall carbohydrates, which bind up water molecules and get entangled with each other, they also have a considerable, velvety body — or can develop such a body when we boil off excess water and concentrate the carbohydrates. (Potatoes and other starchy vegetables are a major exception: starch granules in the cells absorb all the free moisture in the tissue, and are best left intact in unbroken cells so the solid puree doesn’t become gluey. See the discussion of mashed potatoes on p. 303.) Purees are made into sauces and soups, frozen into ices, and dried into “leathers.” For purees as sauces, see p. 620.

Many ripe fruits have sufficiently weakened cell walls that they are easily pureed raw, while most vegetables are first cooked to soften the cell walls. Precooking has the additional advantage of inactivating cell enzymes which, when cellular organization is disrupted, would otherwise destroy vitamins and pigments, alter flavor, and cause unsightly browning (p. 269). The size of solid particles in the puree, and so its textural fineness, is determined by how thoroughly ripening or cooking have dismantled the cell walls, and by the method used to crush the tissue. Mashing by hand leaves large cell aggregates intact; the screens in food mills and strainers produce smaller pieces; machine-powered food processor blades chop very finely, and blender blades, working in a more confined space, chop and shear more finely still. Persistent cellulose-rich fibers can be removed only by passing the puree through a strainer.

Juices Juices are refined versions of the puree: they are mainly the fluid contents of fruit and vegetable cells, made by crushing the raw food and separating off most of the solid cell-wall materials. Some of these materials inevitably end up in the juice — for example, the pulp in orange juice — and can cause both desirable and undesirable haziness and body. Because juicing mixes together the contents of living cells, including active enzymes and various reactive and oxygen-sensitive substances, fresh juices are unstable and change rapidly. Apple and pear juices turn brown, for example, thanks to the action of browning enzymes and oxygen (p. 269). If not used immediately, they’re best kept chilled or frozen, perhaps after a heat treatment just short of the boil to inactivate enzymes and kill microbes. Modern juicing machines can apply very strong forces, and make it possible to extract juice from any fruit or vegetable, not just the traditional ones.

Foams and Emulsions The cell-wall carbohydrates in purees and juices can be used to stabilize two otherwise fleeting physical structures, a foam of air bubbles and an emulsion of oil droplets (pp. 638, 625), which are especially easy to prepare with modern electrical blenders and mixers. If a puree or juice is whipped to fill it with air bubbles, the cell-wall carbohydrates slow the flow of water out of the bubble walls, so the bubbles take longer to collapse. This allows the cook to make a foam or mousse that lasts long enough to be savored; foams from juice are especially ethereal. Similarly, when oil is whisked into a puree or juice, the plant carbohydrates insulate the oil droplets from each other, and the oil and water phases separate more slowly. The cook can therefore incorporate oil into a puree or juice to form a temporary emulsion, with richer dimensions of flavor and texture than the puree alone. The thicker the puree, the more stable and less delicate the foam or emulsion. The consistency of a thick preparation can be lightened by adding liquid (water, juice, stock).

Frozen Purees and Juices: Ices, Sorbets, Sherbets When purees and juices are frozen, they form a refreshing semisolid mass that’s known by a variety of names, including ice, sorbet, granita, and sherbet. This kind of preparation was first refined in 17th-century Italy, which gave us the term sorbet (via sorbetto from the Arabic sharab, or “syrup”). Its flavor is essentially that of the fruit (sometimes an herb, spice, flower, coffee, or tea), usually heightened with added sugar and acid (to 25–35% and 0.5% respectively), and with an overall sugar-acid ratio similar to that of the melons (30–60:1; see p. 382). The puree or juice is often diluted with some water as well, sometimes to reduce the acidity (lemon and lime juices), sometimes to stretch an ingredient in short supply, and sometimes to improve the flavor, which is interestingly affected by the very cold serving temperature: for example, undiluted melon can taste too much like its close relative the cucumber, and thinned pear puree tastes less like frozen fruit, more delicate and perfumed. In the United States, “sherbet” is the term applied to fruit ices with milk solids included (3–5%) to fill out the flavor and help soften the texture.

Though traditional ices are made with fruits, vegetable ices can be refreshing too, as a cool mouthful and as a surprise.

The Texture of Frozen Purees and Juices Ice texture can vary from rocky to coarse to creamy, depending on the proportions of ingredients, how the ice is made, and the temperature at which it’s served. During the freezing process, water in the mix solidifies into millions of tiny ice crystals, which are surrounded by all the other substances in the mix: mainly leftover liquid water that forms a syrup with dissolved sugars, both from the fruit and added by the cook, as well as contents of the plant cells and cell walls. The more syrup and plant debris there are, the more the solid crystals are lubricated, the more easily they slide past each other when we press with spoon or tongue, and the softer the ice’s texture. Most ices are made with about double the sugar of ice cream (whose substantial fat and protein content helps soften the texture, p. 40), between 25 and 35% by weight. Sweet fruits require less added sugar to reach this proportion, and purees rich in pectins and other plant debris (pineapple, raspberry) require less total sugar for softening. Many cooks replace a quarter to a third of the added table sugar (sucrose) with corn syrup or glucose, which helps soften without adding as much perceptible sweetness. The size of the ice crystals, and so the ice’s coarseness or creaminess, is determined by the content of sugar and plant solids, and by agitation during freezing. Sugar and solids encourage the formation of many small crystals rather than a few large ones, and so do stirring and churning (p. 44). Ices served right from the freezer are relatively hard and crystalline; allowing them to warm and thus partly melt produces a softer, smoother consistency.

Vegetable Stocks A vegetable stock is a water extract of several vegetables and herbs that can serve as a flavorful base for soups, sauces, and other preparations. By simmering the vegetables until soft, the cook breaks down their cell walls and releases the cell contents into the water. These contents include salts, sugars, acids, and savory amino acids, as well as aromatic molecules. Carrots, celery, and onions are almost always included for their aromatics, and mushrooms and tomatoes are the richest source of savory amino acids. The vegetables are finely chopped to maximize their surface area for extraction. Precooking some or all of the vegetables in a small amount of fat or oil has two advantages: it adds new flavors, and the fat it contributes is a better solvent than water for many aromatic molecules. It’s important not to dilute the extracted flavors in too much water; good proportions by weight (volume varies by piece size) are 1 part vegetables to 1.5 or 2 parts water. The vegetables and water are simmered uncovered (to allow evaporation and concentration) for no more than an hour, after which it’s generally agreed that the stock flavor ceases to improve and even deteriorates. Once the vegetables are strained out, the stock can be concentrated by boiling it down.

Flavored Oils, Vinegars, Syrups, Alcohols Cooks extract the characteristic aroma chemicals of fruits and vegetables, herbs and spices, into a variety of liquids that then serve as convenient ready-made flavorings for sauces, dressings, and other preparations. In general, the freshest-tasting extracts come from slowly steeping intact raw fruits or herbs at room or refrigerator temperature for days or weeks. The flavors of dried herbs and spices are less altered by heat, and can be extracted more rapidly in hot liquids.

The growth of microbes that cause spoilage or illness is inhibited by the acidity of vinegar, the concentrated sugar in syrups, and the alcohol in vodka (whose own neutral flavor makes it a good medium for flavor extraction), so flavored vinegars, syrups, and alcohols are relatively trouble-free preparations. However, flavored oils require special care. The air-free environment within the oil can encourage the growth of botulism bacteria, which live in the soil, are found on most field-grown foods, and have spores that survive ordinary cooking temperatures. Cold temperatures inhibit their growth. Uncooked oils flavored with garlic or herbs are safest when made in the refrigerator, and both uncooked and cooked flavored oils should be stored in the refrigerator.

“Chlorophyll” A somewhat arcane but fascinating vegetable extract is culinary chlorophyll, an intensely green coloring agent that is not identical to biochemical chlorophyll, but is certainly a concentrated source of it. Culinary chlorophyll is made by finely grinding dark green leaf vegetables to isolate and break open cells; soaking them in water to dilute pigment-damaging enzymes and acids, and separate off solid fibers and cell-wall debris; gently simmering the water to inactivate enzymes and cause the cells and free chloroplasts to rise to the surface; and straining off and draining the green mass. Though the chemical chlorophyll in culinary chlorophyll will still turn drab when heated in an acid food, it can be added at the last minute to acid and other sauces and maintain its vibrant green through the meal.

Preserving Fruits
and Vegetables

Fruits and vegetables can be preserved indefinitely by killing the living tissue and thus inactivating its enzymes, and then making it either inhospitable or unavailable to microbes. Some of these techniques are ancient, some a product of the industrial age.

Drying and
Freeze-Drying

Drying Drying preserves foods by reducing the tissue’s water content from around 90% to between 5 and 35%, a range in which very little can grow on it. This is one of the oldest preservative techniques; the sun, fire, and mounds of hot sand have been used to dry foods since prehistory. Fruits and vegetables usually benefit from treatments to inactivate the enzymes that cause vitamin and color damage. Commercially dried vegetables are usually blanched; and fruits are dipped or sprayed with a number of sulfur compounds that prevent oxidation and thereby enzymatic browning and the loss of antioxidant phenolic compounds, vitamins, and flavor. While sun-drying used to be the most common treatment for prunes, raisins, apricots, and figs, forced hot air-drying is now widely used because it is more predictable. Home and restaurant cooks can use the oven or small electric driers whose temperature is easier to control. Fruits and vegetables are dried at relatively low temperatures, 130–160ºF/55–70ºC, to minimize the loss of flavor and color and prevent the surface from drying too fast and impeding moisture loss from within. Pureed fruits are spread out into thin sheets to make “fruit leather.” Relatively moist dried fruits and vegetables are nicely soft, but they’re also vulnerable to some hardy yeasts and molds, and therefore are best stored in the refrigerator.

Freeze-Drying Freeze-drying is a controlled version of freezer burn: it removes moisture not by evaporation but by sublimation, the transformation of ice directly into water vapor. Although we think of freeze-drying as a recent industrial innovation, the natives of Peru have been freeze-drying potatoes in the Andes for millennia. To make chuño, which can be stored indefinitely, they trample potatoes to break down their structure and expose them constantly to the dry, cold mountain air, so that they freeze at night and lose some moisture by sublimation, then thaw during the day and lose more water by evaporation. Chuño develops a strong flavor from the disruption of the potato tissues and long exposure to the air and sun, and is reconstituted in water to make stews.

In modern industrial freeze-drying, foods are quickly chilled to as low as –70ºF/–57ºC, then slightly warmed and subjected to a vacuum, which pulls their water molecules out and dries them. Because the foods aren’t heated or exposed to oxygen, their flavor and color remain relatively fresh. Many fruits and vegetables are freeze-dried today and used as is for snack foods, or reconstituted with water in instant soup mixes, emergency rations, and camping foods.

Fermentation and Pickling:
Sauerkraut and Kimchi,
Cucumber Pickles, Olives

Fermentation is one of the oldest and simplest means of preserving foods. It requires no particular kind of climate, no cooking, and so no expenditure of fuel: just a container, which can be a mere hole in the ground, and perhaps some salt or seawater. Olives and sauerkraut — fermented cabbage — are familiar examples of fermented fruits and vegetables. An overlapping category is the pickle, a food preserved by immersion in brine or a strong acid such as vinegar. Brines often encourage fermentation, and fermentation generates preservative acids, so the term “pickle” is applied to both fermented and unfermented preparations of cucumbers and other foods. Less familiar but intriguing relatives of sauerkraut and olives include North African preserved lemons, the pickled plums, radishes, and other vegetables of Japan, and the highly spiced, multifarious pickled fruits and vegetables of India.

The Nature of Fermentation Preserving fruits and vegetables by fermentation is based on the fact that plants are the natural home of certain benign microbes which in the right conditions — primarily the absence of air — will flourish and suppress the growth of other microbes that cause spoilage and disease. They accomplish this suppression by being the first to consume the plant material’s readily metabolized sugars, and by producing a variety of antimicrobial substances, including lactic and other acids, carbon dioxide, and alcohol. At the same time, they leave most of the plant material intact, including its vitamin C (protected from oxidation by the carbon dioxide they generate); they often add significant amounts of B vitamins; and they generate new volatile substances that enrich the food’s aroma. These benign “lactic acid bacteria” apparently evolved eons ago in oxygen-poor piles of decaying vegetation, and now transform our carefully gathered harvests into dozens of different foods across the globe (see box, p. 308), as well as turning milk into yogurt and cheese and chopped meat into tangy sausages (pp. 44 and 176).

Fermentation Conditions and Results While some fruits and vegetables are fermented alone in tightly covered pits or jars, most are either dry-salted or brined to help draw water, sugars, and other nutrients out of the plant tissues, and to provide a liquid to cover the food and limit its exposure to oxygen. The characteristics of the pickle depend on the salt concentration and the fermentation temperature, which determine which microbes dominate and the substances they produce. Low salt concentrations and temperatures favor Leuconostoc mesenteroides, which generates a mild but complex mixture of acids, alcohol, and aroma compounds; higher temperatures favor Lactobacillus plantarum, which produces lactic acid almost exclusively. Many pickles undergo a microbial succession, with Leuconostoc dominating early and then giving way to Lactobacillus as the acidity rises. Some Asian pickles are made not by spontaneous lactic fermentations, but by the addition of another fermented “starter” material, the by-products of producing wine or miso or soy sauce. Japanese nukazuke are unique in employing rice bran, whose abundant B vitamins end up enriching the pickled daikon and other vegetables.

Problems Problems in vegetable fermentations are generally caused by inadequate or excessive salt concentrations or temperatures, or exposure to the air, all conditions that favor the growth of undesirable microbes. In particular, if the vegetables are not weighted down to keep them below the brine surface, or if the brine surface is itself not tightly covered, a film of yeasts, molds, and air-requiring bacteria will form, lower the brine acidity by consuming its lactic acid, and encourage the growth of spoilage microbes. The results may include discoloration, softening, and rotten smells from the breakdown of fats and proteins. Even the helpful Lactobacillus plantarum can generate an undesirably harsh acidity if the fermentation is too vigorous or prolonged.

Unfermented, Directly Acidified Pickles There are also a host of fruit and vegetable products that are pickled not by fermentation, but by the direct addition of acid in the form of wine or vinegar, which inhibits the growth of spoilage microbes. This ancient technique is much faster than fermentation and allows greater control over texture and salt content, but it produces a simpler flavor. Today, the usual method is to add enough hot vinegar to produce a final acetic acid concentration of around 2.5% (half that of standard vinegar) in such materials as beans, carrots, okra, pumpkin, mushrooms, watermelon rind, pears, and peaches. Nonfermented pickles are usually heat-treated (185ºF/85ºC for 30 minutes) to prevent spoilage. The simple flavor of directly acidified pickles is often augmented by the addition of spices and/or sugar.

Pickle Texture Most pickled fruits and vegetables are eaten raw as a condiment, and are preferred crisp. The use of unrefined sea salt improves crispness thanks to its calcium and magnesium impurities, which help cross-link and reinforce cell-wall pectins. Especially crisp cucumber and watermelon-rind pickles are made by adding alum (aluminum hydroxide), whose aluminum ions cross-link cell-wall pectins, or by presoaking the raw materials in a solution of “pickling lime,” or calcium hydroxide, whose calcium ions do the same. (Lime is strongly alkaline and its excess must be washed from the ingredients before pickling to avoid neutralizing the pickles’ acidity.) When subsequently cooked, pickles may not soften because their acidity stabilizes cell walls (p. 282). Tender pickles are produced by precooking the vegetable until soft.

Some Fermented Vegetables and Fruits

Adapted from G. Campbell Platt, Fermented Foods of the World — A Dictionary and Guide (London: Butterworth, 1987).

Fermented Cabbage: Sauerkraut and Kimchi Two popular styles of cabbage pickles illustrate the kind of distinctiveness that can be achieved with slight variations in the fermentation process. European sauerkraut is a refreshing side dish for rich meats, and Korean kimchi is a strong accompaniment to bland rice. Sauerkraut — the word is German for “sour cabbage” — is made by fermenting finely shredded head cabbage with a small amount of salt at a cool room temperature; it’s allowed to become quite tart and develops a remarkable, almost flowery aroma thanks to some yeast growth. Kimchi is made by fermenting intact stems and leaves of Chinese cabbage together with hot peppers and garlic, and sometimes other vegetables, fruits (apple, pear, melon), and fish sauce. More salt is used, and the fermentation temperature is significantly lower, a reflection of its original production in pots partly buried in the cold earth of late autumn and winter. The result is a crunchy, pungent pickle that is noticeably less acid but saltier than sauerkraut, and may even be fizzy due to the dominance of gas-producing bacteria below about 58ºF/14ºC.

Cucumber Pickles Today there are three different styles of cucumber pickle in the United States, and the two most common are really flavored cucumbers; they don’t keep unless refrigerated. True fermented cucumbers have become relatively hard to find.

All cucumber pickles start with thin-skinned varieties that are harvested while immature so that the seed region hasn’t yet begun to liquefy, and cleaned of flower remnants that harbor microbes with enzymes that cause softening. Fermented cucumbers are cured in a 5–8% brine at 64–68ºF/18–20ºC for two to three weeks, and accumulate 2–3% salt and 1–1.5% lactic acid: so they’re relatively strong. Such pickles are sometimes moderated before bottling by soaking out some salt and lactic acid, and adding acetic acid. The most common style of cucumber pickle, crisper and more gentle in flavor, is made by soaking the cucumbers briefly in vinegar and salt until they reach 0.5% acetic acid and 0–3% salt, and then pasteurizing them before bottling. Such pickles need to be refrigerated after opening. Finally, there are the freshest-tasting but very perishable pickles, which are soaked in vinegar and salt but not pasteurized. They are kept refrigerated from the moment they’re packaged.

Common problems in home-pickled cucumbers include cheesy and rancid off-flavors, which come from the growth of undesirable bacteria when there’s not enough salt or acidity to inhibit them, and hollow “bloaters,” which are pickles swollen with carbon dioxide produced by yeasts (or sometimes by Lactobacillus brevis or mesentericus) when the salt level is too high.

Olives Fresh olives are practically inedible thanks to their ample endowment of a bitter phenolic substance, oleuropein, and its relatives. The olive tree was first cultivated in the eastern Mediterranean around 5,000 years ago, probably as a source of oil. Olive fermentation may have been discovered when early peoples learned to remove the bitterness by soaking the fruit in changes of water. By Roman times, the soaking water was often supplemented with alkaline wood ashes, which cut the debittering period from weeks to hours. (The modern industrial treatment is a 1–3% solution of sodium hydroxide, or lye.) Alkaline conditions actually break bitter oleuropein down, and also breach the waxy outer cuticle and dissolve cell-wall materials. These effects make the fruit as a whole more permeable to the salt brine that follows (after a wash and acid treatment to neutralize the alkalinity), and help the fermentation proceed faster. Lactic acid bacteria are the main fermenters, though some yeasts also grow and contribute to the aroma. Olives may be debittered and fermented while still green (“Spanish” style, the major commercial type) or once their skin has turned dark with purplish anthocyanins, when they are less bitter.

Olives are also fermented without any preliminary leaching or alkaline treatment, but this results in a different kind of fermentation. Nutrients for the microbes in the brine diffuse very slowly from the flesh through the waxy cuticle, and the intact phenolic materials inhibit microbial growth. So the temperature is kept low (55–64ºF/13–18ºC), and yeasts rather than lactic acid bacteria dominate in a slow alcoholic fermentation that takes as long as a year. This method is usually applied to black ripe olives (Greek, Italian Gaeta, French Niçoise). They turn out more bitter and less tart than the pretreated kinds (an acidity of 0.3–0.5% rather than 1%), and have a distinctively winey, fruity aroma.

Unfermented “ripe black olives” are an invention of the California canning industry. They’re made from unripe green olives, which may undergo an incidental and partial fermentation while being stored in brine before processing. But their unique character is determined by repeated brief lye treatments to leach out and break down oleuropein, and the addition of an iron solution and dissolved oxygen to react with phenolic compounds and turn the skin black. Olives so treated are then packed in a light 3% brine, canned, and sterilized. They have a bland, cooked flavor and often some residual alkalinity, which gives them a slippery quality.

Unusual Fermentations: Poi, Citron, Preserved Lemons Poi is a Hawaiian preparation of taro root (p. 306). The starchy taro is cooked, mashed, thinned with water, and then allowed to stand for one to three days. Lactic acid bacteria sour it, and produce some volatile acids as well (vinegary acetic, cheesy propionic). In longer fermentations, yeasts and Geotrichum molds also grow and contribute fruity and mushroomy notes.

Citron peel, candied from a relative of the lemon, owes its traditionally complex flavor to fermentation. Originally the citron fruits were preserved for some weeks in seawater or a 5 to 10% brine while they were shipped from Asia and the Middle East to Europe; now they’re brined to develop flavor. Yeasts grow on the peel and produce alcohol, which then supports acetic acid bacteria. The result is the production of volatile esters that deepen the aroma of the peel. The preserved lemons of Morocco and other north African countries have a similar character; they’re made by packing cut lemons with salt and fermenting for days to weeks.

Sugar Preserves

Another venerable technique for preserving fruits is to boost their sugar content. Like salt, sugar makes the fruit inhospitable to microbes: it dissolves, binds up water molecules, and draws moisture out of living cells, thus crippling them. Sugar molecules are quite heavy compared to the sodium and chloride ions in salt, so it takes a larger mass of sugar to do the same job of preserving. The usual proportion by weight of added sugar to fruit is about 55 to 45, with sugar accounting for nearly two-thirds of the final cooked mixture. Of course sugar preserves are very sweet, and this is a large part of their appeal. But they also develop an intriguing consistency otherwise found only in meat jellies — a firm yet moist solidity that can range from stiff and chewy to quiveringly tender. And they can delight the eye with a crystalline clarity: in the 16th century, Nostradamus described a quince jelly whose color “is so diaphanous that it resembles an oriental ruby.” These remarkable qualities arise from the nature of pectin, one of the components of the plant cell wall, and its fortuitous interaction with the fruit’s acids and the cook’s added sugar.

The Evolution of Sugar Preserves The earliest sugar preserves were probably fruit pieces immersed in syrupy honey (the Greek term for quinces packed in honey, melimelon, gave us the word marmalade) or in the boiled-down juice of wine grapes. The first step toward jams and jellies was the discovery that when they were cooked together, sugar and fruit developed a texture that neither could achieve on its own. In the 4th century CE, Palladius gave directions for cooking down shredded quince in honey until its volume was reduced by half, which would have made a stiff, opaque paste similar to today’s “fruit cheese” (spreadable “fruit butter” is less reduced). By the 7th century there were recipes for what were probably clear and delicate jellies made by boiling the juice of quince with honey. A second important innovation was the introduction from Asia of cane sugar, which unlike honey is nearly pure sugar, with no moisture that needs boiling off, and no strong flavor that competes with the flavor of the fruit. The Arab world was using cane sugar by the Middle Ages, and brought it to Europe in the 13th century, where it soon became the preferred sweetener for fruit preserves. However, jams and jellies didn’t become common fare until the 19th century, when sugar had become cheap enough to use in large quantities.

Pectin Gels Fruit preserves are a kind of physical structure called a gel: a mixture of water and other molecules that is solid because the other molecules bond together into a continuous, sponge-like network that traps the water in many separate little pockets. The key to creating a fruit gel is pectin, long chains of several hundred sugar-like subunits, which seems to have been designed to help form a highly concentrated, organized gel in plant cell walls (p. 265). When fruit is cut up and heated near the boil, the pectin chains are shaken loose from the cell walls and dissolve into the released cell fluids and any added water. They can’t simply re-form their gel for a couple of reasons. Pectin molecules in water accumulate a negative electrical charge, so they repel each other rather than bond to each other; and they’re now so diluted by water molecules that even if they did bond, they couldn’t form a continuous network. They need help to find each other again.

The cook does three things to cooked fruit to bring pectin molecules back together into a continuous gel. First, he adds a large dose of sugar, whose molecules attract water molecules to themselves, thus pulling the water away from the pectin chains and leaving them more exposed to each other. Second, he boils the mixture of fruit and sugar to evaporate some of the water away and bring the pectin chains even closer together. Finally, he increases the acidity, which neutralizes the electrical charge and allows the aloof pectin chains to bond to each other into a gel. Food scientists have found that the optimal conditions for pectin gelation are a pH between 2.8 and 3.5 — about the acidity of orange juice, and 0.5% acid by weight — a pectin concentration of 0.5 to 1.0%, and a sugar concentration of 60 to 65%.

Preparing Preserves Preserve making begins with cooking the fruit to extract its pectin. Quince, apples, and citrus fruits are especially rich in pectin and often included to supplement other pectin-poor fruits, including most berries. The combination of heat and acid will eventually break pectin chains into pieces too small to form a network, so this preliminary cooking should be as brief and gentle as possible. (If a sparkling, clear jelly is desired, then the cooked fruit is gently strained to remove all solid particles of cell debris.) Then sugar is added, supplemental pectin if necessary, and the mixture rapidly brought to the boil to remove water and concentrate the other ingredients. The boiling is continued until the temperature of the mix reaches 217–221ºF/103–105ºC (at sea level; 2ºF/1ºC lower for every 500 ft/165 m elevation), which indicates that the sugar concentration has reached 65% (for the relationship between sugar content and boiling point, see p. 680). A fresher flavor results when this cooking is done at a gentle simmer in a wide pot with a large surface area for evaporation. (Industrial manufacturers cook the water out under a vacuum at much lower temperatures, 100–140ºF/38–60ºC, to maintain as much fresh flavor and color as possible.) Now supplemental acid is added (late in the process, to avoid breaking down the pectin chains), and the readiness of the mix is tested by placing a drop on a cold spoon or saucer to see whether it gels. Finally, the mix is poured into sterilized jars. The mix sets as it cools below about 180ºF/80ºC, but firms most rapidly at 86ºF/30ºC and continues to get firmer for some days or weeks.

Two kinds of pectin gels. Left: In ordinary fruit preserves, the cook causes pectin molecules to bond directly to each other and form a continuous meshwork by carefully adjusting acidity and sugar content. Right: A modified form of pectin (low methoxy) can be bonded into a continuous meshwork by means of added calcium ions (the black dots), no matter what the sugar content. This is how low-sugar preserves are made.

The usual problem with preserve making is failure of the mix to set even at the proper boiling temperature and sugar concentration. This can be caused by three different factors: inadequate amounts of either acid or good-quality pectin, or prolonged cooking that damages the pectin. Failures can sometimes be rescued by the addition of a commercial liquid pectin preparation and/or cream of tartar or lemon juice, and a brief reboiling. Too much acid can cause weeping of fluid from an overfirm gel.

Uncooked and Unsweetened “Preserves” Modern preserve making has been transformed by the availability of concentrated pectin, extracted and purified from citrus and apple wastes, which can be added to any crushed fruit, cooked or not, to guarantee a firm gel. “Freezer jams” are made by loading up crushed fresh fruit with supplemental pectin and sugar, letting them sit for a day while the pectin molecules slowly form their network and form the gel, and then “preserving” them in the refrigerator or freezer (the uncooked fruit would otherwise soon be spoiled by sugar-tolerant molds and yeasts). Pectin is also used to make clear jelly candies and other confections.

Food chemists have developed several different versions of pectin for special commercial applications. The most notable of these is a pectin that sets without the need for any added sugar to pull water molecules away from the long pectin chains; instead, the chains bond to each other strongly by means of cross-linking calcium, which is added after the fruit and pectin mixture has been cooked. This pectin is what makes it possible to produce low-calorie “preserves” with artificial sweeteners.

Candied Fruits Candied fruits are small whole fruits or pieces that are impregnated with a saturated sugar syrup, then drained, dried, and stored at room temperature as separate pieces. Fruit cooked in a sugar syrup remains relatively firm and maintains its shape thanks to the interaction of sugar molecules with the cell-wall hemicelluloses and pectins. Candying can be a tedious process because it takes time for sugar to diffuse from the syrup evenly into the fruit. Typically the fruit is gently cooked to soften it and make its tissues more permeable, then soaked for several days at room temperature in a syrup that starts out at 15–20% sugar, and is made more concentrated each day until it reaches 70–74%.

Canning

Canning was a cause for wonder when it was invented by Nicolas Appert around 1810: contemporaries said that it preserved fruits and vegetables almost as if fresh! True, it preserves them without the desiccated texture of drying, the salt and sourness of fermentation, or the sweetness of sugar preserves; but there’s no mistaking that canned foods have been cooked. Canning is essentially the heating of food that has been isolated in hermetically sealed containers. The heat deactivates plant enzymes and destroys harmful microbes, and the tight seal prevents recontamination by microbes in the environment. The food can then be stored at room temperature without spoiling.

The arch villain of the canning process is the bacterium Clostridium botulinum, which thrives in low-acid, airless conditions — oxygen is toxic to it — and produces a deadly nerve toxin. The botulism toxin is easily destroyed by boiling, but the dormant bacterial spores are very hardy and can survive prolonged boiling. Unless they are killed by the extreme condition of higher-than-boiling temperatures (which require a pressure cooker), the spores will proliferate into active bacteria when the can cools down, and the toxin will accumulate. One precautionary measure is to boil any canned produce after opening to destroy any toxin that may be there. But all suspect cans, especially those bulging from the pressure of gases produced by bacterial growth, should be discarded.

The low pH (high acidity) of tomatoes and many common fruits inhibits the growth of botulism bacteria, so these foods require the least severe canning treatment, usually about 30 minutes in a bath of boiling water to heat the contents to 185–195ºF/85–90ºC. Most vegetables, however, are only slightly acid, with a pH of 5 or 6, and are much more vulnerable to bacteria and molds. They’re typically heated in a pressure cooker at 240ºF/116ºC for 30 to 90 minutes.