On Food and Cooking Harold Mcgee

Cooking is applied chemistry, and the basic concepts of chemistry — molecules, energy, heat, reactions — are keys to a clearer understanding of what our foods are and how we transform them. A casual acquaintance with these concepts is enough to follow most of the explanations in this book. For readers who’d like to get to know them better, here’s a brief review.

Atoms, Molecules, and Chemical Bonds

It was the ancient Greeks who gave us the idea of atoms, fundamental and invisibly small particles of matter, and also the word atom, which means “uncuttable,” “indivisible.” Greek philosophers proposed that there are just four basic kinds of particles in the world — atoms of earth, air, water, and fire — and that all material things, our bodies and our foods and everything else, are built from these primary particles. The modern scientific view of matter’s invisible innards is more complicated, but also more precise and illuminating.

Atoms and Molecules

All matter on earth is a mixture of around 100 pure substances, which we call the elements: hydrogen, oxygen, nitrogen, carbon, and so on. An atom is the smallest particle into which an element can be subdivided without losing its characteristic properties. Atoms are very small indeed: several million would fit into the period at the end of this sentence. All atoms are made up of smaller “subatomic” particles, electrons, protons, and neutrons. The different properties of the elements arise from the varying combinations of subatomic particles that make up their atoms, and in particular their quotas of protons and electrons. Hydrogen atoms contain one proton and one electron; oxygen carries 8 of each, and iron 26.

When two or more atoms bond together, which they do by sharing electrons with each other, they form a molecule (from the Latin for “mass,” “bulk”). The molecule is to a chemical compound what the atom is to an element: the smallest unit that retains the properties of the original material. Most matter on earth, including food, is a mixture of different chemical compounds.

Protons and Electrons Carry Positive and Negative Electrical Charges There’s one primary driving force behind all the chemical activity that makes life and cooking possible, and that’s the electrical attraction between protons and electrons. Protons carry a positive electrical charge, and electrons an exactly balancing negative charge. (The neutral neutron carries no charge.) Opposite electrical charges attract each other; similar electrical charges repel each other. In each atom, protons in the central nucleus attract a cloud of electrons that orbit constantly at various distances from the nucleus. Stable forms of the elements are electrically neutral, which means that their atoms contain equal numbers of protons and electrons.

(If like charges repel each other and opposite charges attract, then why is it that the protons in the nucleus don’t push each other away and the orbiting electrons fall straight into the nucleus? It turns out that there are forces besides electricity at work in the atom. The protons and neutrons are bound together by very strong nuclear forces, while it’s the nature of electrons to be in continual motion. So protons and electrons are always attracted to each other and move in response to the other’s presence, but they never consummate their attraction.)

Electrical Imbalance, Reactions, and Oxidation

The electrons in atoms are arranged around the nucleus in orbits that determine how strongly any particular electron is held there. Some electrons are held close and tightly to the nucleus, while others range far away and are held more weakly. The behavior of the outermost electrons largely determines the chemical behavior of the elements. For example, the elements classi-fied as metals — copper, aluminum, iron — hold their outermost electrons very weakly, and easily give them away to the atoms of other elements — oxygen, chlorine — that are hungrier for electrons, and that tend to grab up any that are loosely held. This imbalance in electrical pulls among different elements is the basis of most chemical reactions. Reactions are encounters among atoms and molecules that result in the loss, gain, or sharing of electrons, and thus changes in the properties of the atoms and molecules involved.

An atom of carbon. Carbon has six protons and six neutrons in its nucleus, and six electrons orbiting around the nucleus.

Of all the electron-grabbing elements, the most important is oxygen, so much so that chemists use the term oxidation to name the general chemical activity of grabbing electrons from other atoms, even if a chlorine atom is doing the grabbing. Oxidation is very important in the kitchen, because oxygen is always present in the air, and readily robs electrons from the carbon-hydrogen chains of fats, oils, and aroma molecules. This initial oxidation triggers a cascade of further oxidations and other reactions that end up breaking the original large fat molecules into small, strong-smelling fragments. Antioxidant substances — for example, phenolic compounds found in many foods made from plants — prevent this breakdown by giving oxygen the electrons it wants without starting a reaction cascade, thus sparing the fat molecules from oxidation.

Electrical Imbalance and Chemical Bonds

Electron hunger is also the basis for the chemical bond, an interaction between atoms or molecules that holds them together, either loosely or tightly, momentarily or indefinitely. There are several different kinds of chemical bonds that are important in the kitchen, as they are throughout nature.

Ionic Bonds; Salt One kind of chemical bond is the ionic bond, in which one atom completely captures the electron(s) of another, so great is the difference between their electron hungers. Chemical compounds held together by ionic bonds don’t simply dissolve in water; they come apart into separate ions, or atoms that are electrically charged because they either carry extra electrons or gave up some of their electrons. (The term was coined by the pioneer of electricity, Michael Faraday, from the Greek word for “going,” to name those electrically charged particles that move when an electrical field is set up in a water solution.) Salt, our most common seasoning, is a compound of sodium and chlorine held together with ionic bonds. In a solid crystal of pure salt, positively charged sodium ions alternate with negatively charged chloride ions, the sodiums having lost their electrons to the chlorines. Because several positive sodium ions are always in a state of attraction to several negative chloride ions, we can’t really speak of individual molecules of salt, with one particular sodium atom bonded to a particular chlorine atom. In water, salt dissolves into separate positive sodium ions and negative chloride ions.

Ionic and covalent bonds. Left: An ionic bond results when an atom completely captures one or more electrons of another atom, and the two atoms experience an attractive force (dotted line) due to their opposite electrical charges. Right: In the covalent bond, atoms share electrons, and thereby form stable combinations called molecules.

Strong Bonds That Make Molecules A second kind of chemical bond, called covalent (from the Latin, “of equal power”), produces stable molecules. When two atoms have roughly similar affinities for electrons, they will share them rather than gain or lose them entirely. In order for sharing to occur, the electron clouds of two atoms must overlap, and this condition results in a fixed arrangement in space between two particular atoms, which thus form a stable combined structure. The bonding geometry determines the overall shape of the molecule, and molecular shape in turn defines the ways in which one molecule can react with others.

The elements most important to life on earth — hydrogen, oxygen, carbon, nitrogen, phosphorus, sulfur — all tend to form covalent bonds, and these make possible the complex, stable assemblages that constitute our bodies and our foods. The most familiar pure chemical compounds in the kitchen are water, a covalent combination of two hydrogen atoms and an oxygen; and sucrose, or table sugar, a combination of carbon, oxygen, and hydrogen atoms. Covalent bonds are generally strong and stable at room temperature: that is, they’re not broken in significant numbers unless subjected to heat or to reactive chemicals, including enzymes. Unlike salt, which dissolves into electrically charged ions, covalently bonded molecules that can dissolve in water generally do so as intact, electrically neutral molecules.

Weak Bonds Between Polar Molecules: Water A third kind of chemical bond, about a tenth as strong and stable as covalent bonds, is the hydrogen bond. The hydrogen bond is one of several “weak” bonds that do not form molecules, but do make temporary links between different molecules, or between different parts of one large molecule. Weak bonds come about because most covalent bonds leave at least a slight electrical imbalance among the participating atoms. Consider water, whose chemical formula is H₂O. The oxygen atom has a greater hunger for electrons than the two hydrogen atoms, and so the shared electrons are held closer to the oxygen than to the hydrogens. As a result, there’s an overall negative charge in the vicinity of the oxygen, and an overall positive charge around the hydrogen atoms. This unequal distribution of charge, together with the geometry of the covalent bonds, results in a molecule with a positive end and a negative end. Such a molecule is called polar because it has two separate centers, or poles, of charge.

A hydrogen bond results from the attraction between oppositely charged ends of polar molecules (or portions of molecules). This kind of bond is important because it’s very common in materials that contain water, because it brings different kinds of molecules into close association, and because it’s weak enough that these molecular associations can change rapidly at room temperature. Many of the chemical interactions in plant and animal cells occur via hydrogen bonds.

Very Weak Bonds Between Nonpolar Molecules: Fats and Oils A fourth kind of chemical bond is very weak indeed, between a hundredth and a ten-thousandth as strong as a molecule-making covalent bond. These van der Waals bonds, named after the Dutch chemist who first described them, are the kind of flickering electrical attraction that even nonpolar molecules can feel for each other, thanks to brief fluctuations in their structures. Where electrically polar water is held together as a liquid by hydrogen bonds, nonpolar fat molecules are held together as a liquid and given their appealingly thick consistency by van der Waals bonds. Though these bonds are indeed weak, their effect can add up to a significant force: fat molecules are long chains and include dozens of carbon atoms, so each fat molecule can interact with many more other molecules than a small water molecule can.

Energy

Energy Causes Change

The paragraphs directly above describe various bonds as “weak” and “strong,” easily or not so easily formed and broken. The idea of bond strength is useful because most cooking is a matter of the systematic breaking of certain chemical bonds and the formation of others. The key to the behavior of chemical bonds is energy. The word is a Greek compound of “in” and “force” or “activity,” and now has as its standard definition “the capacity for doing work,” or “the exertion of a force across a distance.” Most simply, energy is that property of physical systems that makes possible change. A system with little energy is largely unchanging. Conversely, the more energy available to an object, the more likely that object is to be changed, or to change its surroundings. Our kitchens are organized around this principle. Stoves and ovens change the qualities of food by pouring heat energy into it, while the refrigerator preserves food by removing heat and thus slowing down the chemical changes that constitute spoilage.

Atoms and molecules can absorb or release energy in several different forms, two of which are important in the kitchen.

The Nature of Heat: Molecular Movement

One kind of energy is the energy of motion, or kinetic energy. Atoms and molecules can move from one place to another; or spin in place, or vibrate, and all of these changes in position or orientation require energy. Heat is a manifestation of a material’s kinetic energy, and temperature is a measure of that energy: the higher the temperature of a food or pan, the hotter it is, the faster its molecules are moving and colliding with each other. And simple movement is the key to transforming molecules and foods. As molecules move faster and more forcefully, their motions and collisions begin to overcome the electrical forces holding them together. This frees some atoms to find new partners and rearrange themselves in new molecules. Heat thus encourages chemical reactions and chemical change.

Bond Energy

The second kind of energy that’s important in the kitchen is the energy of the chemical bonds that hold molecules together. When two or more atoms become a molecule by sharing electrons and bonding with each other, they’re pulled together by an electrical force. So in the process of forming the bond, some of their electrical energy is transformed into energy of motion. And the stronger the electrical force, the more rapidly they accelerate toward each other. The stronger the bond, the more energy is released — lost — from the molecule in the form of motion. Strong bonds, then, “contain” less energy than weak bonds. This is another way of saying that they are more stable, less susceptible to change, than weak bonds.

Van der Waals bonds. Thanks to fluctuations in the positions of their shared electrons, even the nonpolar chains of carbon and hydrogen atoms in fats experience weakly attractive electrical forces (dotted lines).

Bond strength is defined as the amount of energy released from the participating atoms when they form the bond. This is the same as the amount of energy required to break that bond once it’s formed. When the atoms in a molecule are heated up so they move with the same kinetic energy that they had released when they bonded to each other, then those bonds begin to break apart, and the molecule begins to react and change.

The strong covalent bonds typical of our major food molecules — proteins, carbohydrates, fats — are broken by about 100 times the average kinetic energy of molecules at room temperature. This means that they break very rarely at room temperature, and don’t change at a significant rate until we heat them. The weaker, temporary hydrogen and van der Waals bonds between molecules are constantly being broken and re-formed at room temperature, and this welter of activity increases as the temperature rises. This is why fats melt and become thinner in consistency as we heat them: the energy of their motion increasingly overpowers the forces attracting them to each other.

The Phases of Matter

In our everyday life, we encounter matter in three different states, or phases (the word comes from the Greek for “appearance” or “manifestation”). These states are the solid phase, the liquid phase, and the gas phase. The temperatures at which a material melts — changes from solid to liquid — and boils — changes from liquid to gas — are determined by the bonding forces among the atoms or molecules. The stronger the bonds, the more energy needed to overcome them, and so the higher the temperature at which the material shifts from one phase to another. During that shift, all the heat added to the material goes into completing the phase change. The temperature of a solid-liquid mix will remain fairly constant until all the solid has melted. Similarly, the temperature of a pot of boiling water on a high flame remains constant — at the boiling point — until all the liquid water has been turned into steam.

States of matter. Crystalline solids such as salt and sugar are made up of atoms or molecules bonded together in highly ordered, regular arrays. Amorphous solids, such as hard candies and glass, are masses of atoms or molecules that have bonded to each other in a random arrangement. Liquids are a loosely bonded, fluid mass of atoms or molecules, while a gas is a fluid and dispersed group of atoms or molecules.

Solids

At low temperatures, atomic motion is limited to rotation and vibration, and the immobilized atoms or molecules bond tightly to each other in solid, closely packed, well-defined structures. Such structures define the solid phase. In a crystalline solid — salt, sugar, tempered chocolate — the particles are arranged in a regular, repeating array, while in amorphous solids — boiled candies, glass — they are randomly oriented. Large, irregular molecules like proteins and starch often form both highly ordered, crystalline regions and disordered amorphous regions in the same chunk of material. Ionic bonds, hydrogen bonds, and van der Waals bonds may be involved in holding the particles of a solid together.

Liquids

At a temperature that is characteristic of each solid substance, the rotation and vibration of individual molecules in that substance becomes forceful enough that the electrical forces holding them in place are overpowered. The fixed structure then breaks up, leaving the molecules free to move from one place to another. However, most of the molecules are still moving slowly enough that they remain influenced by the forces that once immobilized them, and so they remain loosely associated with each other. They’re free to move, but move together. This fluid but cohesive phase is a liquid.

Gases

If the temperature continues to rise and the molecules move with a kinetic energy high enough that they can break away from each other’s influence completely and move freely into the air, the substance become a different kind of fluid, a gas. The most familiar transition from the liquid phase to the gas phase is boiling, in which we transform liquid water into bubbles of water vapor, or steam. Less obvious to the eye, because it’s so gradual, is the evaporation of water at temperatures below the boiling point. The molecules in a liquid move with a wide range of kinetic energies, and a small portion of the molecules in room-temperature water are moving fast enough to escape from the surface and move into the air.

In fact, water molecules can even escape as a gas from solid ice! This direct transformation of a solid into a gas is called sublimation, and is the cause of that deterioration in foods known as “freezer burn,” in which crystalline water evaporates into the freezer’s cold, dry air. Freeze-drying is a controlled version of the same process.

Many Food Molecules Can’t Change Phase

Most of the molecules that the cook works with can’t simply change from one phase to another when heated. Instead, they react to form entirely different kinds of molecules. This is because food molecules are large, and form so many weak bonds between molecules that they’re in fact held very strongly together. It takes as much energy to break them apart from each other as it does to break the molecules themselves apart: and so rather than melting or evaporating, the molecules become transformed. For example, sugar will melt from a solid into a liquid, but rather than then vaporize into a gas as water does, it breaks apart and forms hundreds of new compounds: a process we call caramelization. Fats and oils melt, but break down and smoke before they begin to boil. Starch, which is a long chain of sugar molecules joined together, won’t even melt: it and proteins, also very large molecules, begin to break down as solids.

Mixtures of Phases: Solutions, Suspensions, Emulsions, Gels, Foams

Cooks seldom deal with pure chemical compounds or even single phases. Foods are mixtures of different molecules, different phases, and even different kinds of mixtures! Here are brief definitions of mixtures that are important in the kitchen.

A solution is a material in which individual ions or molecules are dispersed in a liquid. Salt brines and sugar syrups are simple culinary examples.
A suspension is a material in which a substance is dispersed in a liquid in clusters or particles consisting of many molecules. Nonfat milk is a suspension of milk-protein particles in water. Suspensions are usually cloudy because the clusters are large enough to deflect light rays (individual dissolved molecules are too small to do so, so solutions are clear). •An emulsion is a special kind of suspension, one in which the dispersed substance is a liquid that can’t mix evenly with the containing liquid. Cream is an emulsion of butterfat in water, and an oil-and-vinegar dressing is usually an emulsion of vinegar in oil.
A gel is a dispersion of water in a solid: the molecules of the solid form a sponge-like network, and the pockets of water are trapped in the network. Examples are savory or sweet jellies made with gelatin, and fruit jellies made with pectin.
A foam is a dispersion of gas bubbles in a liquid or solid. Soufflés, bread, and the head on a glass of beer are all foams.

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