THE FOUR BASIC FOOD MOLECULES

This chapter describes the characters of the four chemical protagonists in foods and the cooking process, the molecules referred to constantly in the first fourteen chapters.

• Water is the major component of nearly all foods—and of ourselves! It’s also a medium in which we heat foods in order to change their flavor, texture, and stability. One particular property of water solutions, their acidity or alkalinity, is a source of flavor, and has an important influence on the behavior of the other food molecules.

• Fats, oils, and their chemical relatives are water’s antagonists. Like water, they’re a component of living things and of foods, and they’re also a cooking medium. But their chemical nature is very different—so different that they can’t mix with water. Living things put this incompatibility to work by using fatty materials to contain the watery contents of cells. Cooks put this quality to work when they fry foods to crisp and brown them, and when they thicken sauces with microscopic but intact fat droplets. Fats also carry aromas, and produce them.

• Carbohydrates, the specialty of plants, include sugars, starch, cellulose, and pectic substances. They generally mix freely with water. Sugars give many of our foods flavor, while starch and the cell-wall carbohydrates provide bulk and texture.

• Proteins are the sensitive food molecules, and are especially characteristic of foods from animals: milk and eggs, meat and fish. Their shapes and behavior are drastically changed by heat, acid, salt, and even air. Cheeses, custards, cured and cooked meats, and raised breads all owe their textures to altered proteins.

WATER

Water is our most familiar chemical companion. It’s the smallest and simplest of the basic food molecules, just three atoms: H²O, two hydrogens and an oxygen. And its significance is hard to overstate. Leaving aside the fact that it shapes the earth’s continents and climate, all life, including our own, exists in a water solution: a legacy of life’s origin billions of years ago in the oceans. Our bodies are 60% water by weight; raw meat is about 75% water, and fruits and vegetables up to 95%.

WATER CLINGS STRONGLY TO ITSELF

The important properties of ordinary water can be understood as different manifestations of one fact. Each water molecule is electrically unsymmetrical, or polar: it has a positive end and a negative end. This is because the oxygen atom exerts a stronger pull than the hydrogen atoms on the electrons they share, and because the hydrogen atoms project from one side of the oxygen to form a kind of V shape: so there’s an oxygen end and a hydrogen end to the water molecule, and the oxygen end is more negative than the hydrogen end. This polarity means that the negative oxygen on one water molecule feels an electrical attraction to the positive hydrogens on other water molecules. When this attraction brings the two molecules closer to each other and holds them there, it’s called a hydrogen bond. The molecules in ice and liquid water are participating in from one to four hydrogen bonds at any given moment. However, the motion of the molecules in the liquid is forceful enough to overcome the strength of hydrogen bonds and break them: so the hydrogen bonds in liquid water are fleeting, and are constantly being formed and broken. This natural tendency of water molecules to form bonds with each other has a number of effects in life and in the kitchen.

WATER IS GOOD AT DISSOLVING OTHER SUBSTANCES

Water forms hydrogen bonds not only with itself, but with other substances that have at least some electrical polarity, some unevenness in the distribution of positive and negative electrical charges. Of the other major food molecules, which are much larger and more complex than water, both carbohydrates and proteins have polar regions. Water molecules are attracted to these regions and cluster around them. When they do this, they effectively surround the larger molecules and separate them from each other. If they do this more or less completely, so that each molecule is mostly surrounded by a cloud of water molecules, then that substance has dissolved in the water.

WATER AND HEAT: FROM ICE TO STEAM

The hydrogen bonds among its molecules have a strong effect on how water absorbs and transmits heat. At low temperatures, water exists as solid ice, its molecules immobilized in organized crystals. As it warms up, it first melts to become liquid water; and then the liquid water is vaporized to form steam. Each phase is affected by hydrogen bonding.

Ice Damages Cells.

Normally, the solid phase of a given substance is denser than the liquid phase. As the molecules’ attraction for each other becomes stronger than their movements, the molecules settle into a compact arrangement determined by their geometry. In solid water, however, the molecular packing is dictated by the requirement for even distribution of hydrogen bonds. The result is a solid with more space between molecules than the liquid phase has, by a factor of about one-eleventh. It’s because water expands when it freezes that water pipes burst when the heat fails in winter; that bottles of beer put in the freezer for a quick chill and then forgotten will pop open; that containers of leftover soup or sauce will shatter in the freezer if they’re too full for the liquid to expand freely. And it’s why raw plant and animal tissues are damaged when they’re frozen and leak liquid when thawed. During freezing, the expanding ice crystals rupture cell membranes and walls, which then lose internal fluids when the crystals melt.

Liquid Water Is Slow to Heat Up

Again thanks to the hydrogen bonding between water molecules, liquid water has a high specific heat, the amount of energy required to raise its temperature by a given amount. That is, water absorbs a lot of energy before its temperature rises. For example, it takes 10 times the energy to heat an ounce of water 1º as it does to heat an ounce of iron 1º. In the time that it takes to get an iron pan too hot to handle on the stove, water will have gotten only tepid. Before the heat energy added to the water can cause its molecules to move faster and its temperature to rise, some of the energy must first break the hydrogen bonds so that the molecules are free to move faster.

The basic consequence of this characteristic is that a body of water—our body, or a pot of water, or an ocean—can absorb a lot of heat without itself quickly becoming hot. In the kitchen, it means that a covered pan of water will take more than twice as long as a pan of oil to heat up to a given temperature; and conversely, it will hold that temperature longer after the heat is removed.

Liquid Water Absorbs a Lot of Heat as It Vaporizes into Steam.

Hydrogen bonding also gives water an unusually high “latent heat of vaporization,” or the amount of energy that water absorbs without a rise in temperature as it changes from a liquid to a gas. This is how sweating cools us: as the water on the skin of our overheated body evaporates, it absorbs large amounts of energy and carries it away into the air. Ancient cultures used the same principle to cool their drinking water and wine, storing them in porous clay vessels that evaporate moisture continuously. Cooks take advantage of it when they bake delicate preparations like custards gently by partly immersing the containers in an open water bath, or oven-roast meats slowly at low temperatures, or simmer stock in an open pot. In each case, evaporation removes energy from the food or its surroundings and causes it to cook more gently.

Steam Releases a Lot of Heat When It Condenses into Water

Conversely, when water vapor hits a cool surface and condenses into liquid water, it gives up that same high heat of vaporization. This is why steam is such an effective and quick way of cooking foods compared with plain air— also a gas—at the same temperature. We can put a hand into an oven at 212ºF/ 100ºC and hold it there for some time before it gets uncomfortably warm; but a steaming pot will scald us in a second or two. In bread baking, an initial blast of steam increases the dough’s expansion, or oven spring, and produces a lighter loaf.

WATER AND ACIDITY: THE PH SCALE

Acids and Bases Despite the fact that the molecular formula for water is H2O, even absolutely pure water contains other combinations of oxygen and hydrogen. Chemical bonds are continually being formed and broken in matter, and water is no exception. It tends to “dissociate” to a slight extent, with a hydrogen occasionally breaking off from one molecule and rebonding to a nearby intact water molecule. This leaves one negatively charged OH combination, and a positively charged H3O. Under normal conditions, a very small number of molecules exist in the dissociated state, something on the order of two ten-millionths of a percent. This is a small number but a significant one, because the presence of relatively mobile hydrogen ions, which are the basic units of positive charge (protons), can have drastic effects on other molecules in solution. A structure that is stable with a few protons around may be unstable when many protons are in the vicinity. So significant is the proton concentration that humans have a specialized taste sensation to estimate it: sourness. Our term for the class of chemical compounds that release protons into solutions, acids, derives from the Latin acere, meaning to taste sour. We call the complementary chemical group that accepts protons and neutralizes them, bases or alkalis.

The properties of acids and bases affect us continually in our daily life. Practically every food we eat, from steak to coffee to oranges, is at least slightly acidic. And the degree of acidity of the cooking medium can have great influence on such characteristics as the color of fruits and vegetables and the texture of meat and egg proteins. Some measure of acidity would clearly be quite useful. A simple scale has been devised to provide just that.

The pH Scale 

The standard measure of proton activity in solutions is pH, a term suggested by the Danish chemist S. P. L. Sørenson in 1909. It’s essentially a more convenient version of the minuscule percentages of molecules involved (for some details, see box below). The pH scale runs from 0 to 14. The pH of neutral, pure water, with equal numbers of protons and OH ions, is set at 7. A pH lower than 7 indicates a greater concentration of protons and so an acidic solution, while a pH above 7 indicates a greater prevalence of proton-accept-ing groups, and so a basic solution. Here’s a list of common solutions and their usual pH.

FATS, OILS, AND RELATIVES: LIPIDS

LIPIDS DON’T MIX WITH WATER

Fats and oils are members of a large chemical family called the lipids, a term that comes from the Greek for “fat.” Fats and oils are invaluable in the kitchen: they provide flavor and a pleasurable and persistent smoothness; they tenderize many foods by permeating and weakening their structure; they’re a cooking medium that allows us to heat foods well above the boiling point of water, thus drying out the food surface to produce a crisp texture and rich flavor. Many of these qualities reflect a basic property of the lipids: they are chemically unlike water, and largely incompatible with it. And thanks to this quality, they have played an essential role in the function of all living cells from the very beginnings of life. Because they don’t mix with water, lipids are well suited to the job of forming boundaries—membranes— between watery cells. This function is performed mainly by phospholipids similar to lecithin (p. 802), molecules that cooks also use to form membranes around tiny oil droplets. Fats and oils themselves are created and stored by animals and plants as a concentrated, compact form of chemical energy, packing twice the calories as the same weight of either sugar or starch.

In addition to fats, oils, and phospholipids, the lipid family includes beta-carotene and similar plant pigments, vitamin E, cholesterol, and waxes. These are all molecules made by living things that consist mainly of chains of carbon atoms, with hydrogen atoms projecting from the chain. Each carbon atom can form four bonds with other atoms, so a given carbon atom in the chain is usually bonded to two carbon atoms, one on each side, and two hydrogens.

This carbon-chain structure has one overriding consequence: lipids can’t dissolve in water. They are “hydrophobic” or “water-fearing” substances. The reason for this is that carbon and hydrogen atoms pull with a similar force on their shared electrons. So unlike the oxygen-hydrogen bond, the carbon-hydrogen bond is not polar, and the hydrocarbon chain as a whole is nonpolar. When polar water and nonpolar lipids are mixed together, the polar water molecules form hydrogen bonds with each other, the long lipid chains form a weaker kind of bond with each other (van der Waals bonds, p. 814), and the two substances segregate themselves. Oils minimize the surface at which they contact water by coalescing into large blobs, and resist being divided into smaller droplets.

Thanks to their chemical relatedness, different lipids can dissolve in each other. This is why the carotenoid pigments—the beta-carotene in carrots, the lycopene in tomatoes—and intact chlorophyll, whose molecule has a lipid tail, color cooking fats much more intensely than they do cooking water.

Lipids share two other characteristics. One is their clingy, viscous, oily consistency, which results from the many weak bonds formed between their long carbon-hydrogen molecules. And those same molecules are so bulky that all natural fats, solid or liquid, float on water. Water is a denser substance due to its extensive hydrogen bonding, which packs its small molecules more tightly together.

THE STRUCTURE OF FATS

Fats and oils are members of the same class of chemical compounds, the triglycerides. They differ from each other only in their melting points: oils are liquid at room temperature, fats solid. Rather than use the technical triglyceride to denote these compounds, I’ll use fats as the generic term. Oils are liquid fats. These are invaluable ingredients in cooking. Their clingy viscosity provides a moist, rich quality to many foods, and their high boiling point makes them an ideal cooking medium for the production of intense browning-reaction flavors.

Glycerol and Fatty Acids 

Though they contain traces of other lipids, natural fats and oils are triglycerides, a combination of three fatty acid molecules with one molecule of glycerol. Glycerol is a short 3carbon chain that acts as a common frame to which three fatty acids can attach themselves. The fatty acids are so named because they consist of a long hydrocarbon chain with one end that has an oxygen-hydrogen group and that can release the hydrogen as a proton. It’s the acidic group of the fatty acid that binds to the glycerol frame to construct a glyceride: glycerol plus one fatty acid makes a monoglyceride, glycerol plus two fatty acids makes a diglyceride, and glycerol plus three fatty acids makes a triglyceride. Before it bonds to the glycerol frame, the acidic end of the fatty acid is polar, like water, and so it gives the free fatty acid a partial ability to form hydrogen bonds with water.

Fatty acid chains can be from 4 to about 35 carbons long, though the most common in foods are from 14 to 20. The properties of a given triglyceride molecule depend on the structure of its three fatty acids and their relative positions on the glycerol frame. And the properties of a fat depend on the particular mixture of triglycerides it contains.

SATURATED AND UNSATURATED FATS, HYDROGENATION, AND TRANS FATTY ACIDS

The Meaning of Saturation 

The terms “saturated” and “unsaturated” fats are familiar from nutrition labels and ongoing discussions of diet and health, but their meaning is seldom explained. A saturated lipid is one whose carbon chain is satu-rated—filled to capacity—with hydrogen atoms: there are no double bonds between carbon atoms, so each carbon within the chain is bonded to two hydrogen atoms. An unsaturated lipid has one or more double bonds between carbon atoms along its backbone. The double-bonded carbons therefore have only one bond left for a hydrogen atom. A fat molecule with more than one double bond is called polyunsaturated.

Fat Saturation and Consistency

Saturation matters in the behavior of fats because double bonds significantly alter the geometry and the regularity of the fatty-acid chain, and so its chemical and physical properties. A saturated fatty acid is very regular and can stretch out completely straight. But because a double bond between carbon atoms distorts the usual bonding angles, it has the effect of adding a kink to the chain. Two or more kinks can make it curl.

A group of identical and regular molecules fits more neatly and closely together than different and irregular molecules. Fats composed of straight-chain saturated fatty acids fall into an ordered solid structure— the process has been described as “zipper-ing”—more readily than do kinked unsaturated fats. Animal fats are about half saturated and half unsaturated, and solid at room temperature, while vegetable fats are about 85% unsaturated, and are liquid oils in the kitchen. Even among the animal fats, beef and lamb fats are noticeably harder than pork or poultry fats, because more of their triglycerides are saturated.

Double bonds are not the only factor in determining the melting point of fats. Short-chain fatty acids are not as readily “zippered” together as the longer chains, and so tend to lower the melting point of fats. And the more variety in the structures of their fatty acids, the more likely the mixture of triglycerides will be an oil.

Fat Saturation and Rancidity

Saturated fats are also more stable, slower to become rancid than unsaturated fats. The double bond of an unsaturated fat opens a space unprotected by hydrogen atoms on one side of the chain. This exposes the carbon atoms to reactive molecules that can break the chain and produce small volatile fragments. Atmospheric oxygen is just such a reactive molecule, and is one of the major causes of flavor deterioration in foods containing fats. Water and metal atoms from other food ingredients also help fragment fats and cause rancidity. The more unsaturated the fat, the more prone it is to deterioration. Beef has a longer shelf life than chicken, pork, or lamb because its fat is more saturated and so more stable.

Some small volatile fragments of unsaturated lipids actually have desirable and distinctive aromas. The typical aroma of crushed green leaves and of cucumber both come from fragments of membrane phospholipids generated not just by oxygen, but by special plant enzymes. And the characteristic aroma of deep-fried foods comes in part from particular fatty-acid fragments created at high temperatures.

Hydrogenation: Altering Fat Saturation

For more than a century now, manufacturers have been making solid, fat-like shortenings and margarines from liquid seed oils to obtain both the desired texture and improved keeping qualities. There are several ways to do this, the simplest and most common being to saturate the unsaturated fatty acids artificially. This process is called hydrogenation, because it adds hydrogen atoms to the unsaturated chains. A small amount of nickel is added to the oil as a catalyst, and the mixture is then exposed to hydrogen gas at high temperature and pressure. After the fat has absorbed the desired amount of hydrogen, the nickel is filtered out.

Trans Fatty Acids 

It turns out that the hydrogenation process straightens a certain proportion of the kinks in unsaturated fatty acids not by adding hydrogen atoms to them, but by rearranging the double bond, twisting it so that its bend is less extreme. These molecules remain chemically unsaturated—the double bond between two carbons remains—but they have been transformed from an acutely irregular cis geometry to a more regular trans structure . Cis is Latin for “on this side of,” and trans for “across from”; the terms describe the positions of neighboring hydrogen atoms on the double bond between carbon atoms. Because the trans fatty acids are less kinked, more like a saturated fat chain in structure, they make it easier for the fat to crystallize and so make it firmer. They also make the fatty acid less prone to attack by oxygen, so it’s more stable. Unfortunately, trans fatty acids also resemble saturated fats in raising blood cholesterol levels, which can contribute to the development of heart disease (p. 38). Manufacturers will soon be required to list the trans fatty acid content of their foods, and they’re beginning to implement other processing techniques that harden fat consistency without creating trans fatty acids.

FATS AND HEAT

Most fats do not have sharply defined melting points. Instead, they soften gradually over a broad temperature range. As the temperature rises, the different kinds of fat molecules melt at different points and slowly weaken the whole structure. (An interesting exception to this rule is cocoa butter). This behavior is especially important in making pastries and cakes, and it’s what makes butter spreadable at room temperature.

Melted fats do eventually change from a liquid to a gas: but only at very high temperatures, from 500º to 750ºF/260– 400ºC. This high boiling point, far above water’s, is the indirect result of the fats’ large molecular size. While they can’t form hydrogen bonds, the carbon chains of fats do form weaker bonds with each other. Because fat molecules are capableof forming so many bonds along their lengthy hydrocarbon chains, the individually weak interactions have a large net effect: it takes a lot of heat energy to knock the molecules apart from each other.

The Smoke Point

Most fats begin to decompose at temperatures well below their boiling points, and may even spontaneously ignite on the stovetop if their fumes come into contact with the gas flame. These facts limit the maximum useful temperature of cooking fats. The characteristic temperature at which a fat breaks down into visible gaseous products is called the smoke point. Not only are the smoky fumes obnoxious, but the other materials that remain in the liquid, including chemically active free fatty acids, tend to ruin the flavor of the food being cooked.

The smoke point depends on the initial free fatty acid content of the fat: the lower the free fatty acid content, the more stable the fat, and the higher the smoke point. Free fatty acid levels are generally lower in vegetable oils than in animal fats, lower in refined oils than unrefined ones, and lower in fresh fats and oils than in old ones. Fresh refined vegetable oils begin to smoke around 450ºF/230ºC, animal fats around 375ºF/190ºC. Fats that contain other substances, such as emulsifiers, preservatives, and in the case of butter, proteins and carbohydrates, will smoke at lower temperatures than pure fats. Fat breakdown during deep frying can be slowed by using a tall, narrow pan and so reducing the area of contact between fat and atmosphere. The smoke point of a deep-frying fat is lowered every time it’s used, since some breakdown is inevitable even at moderate temperatures, and trouble-making particles of food are always left behind.

EMULSIFIERS: PHOSPHOLIPIDS, LECITHIN, MONOGLYCERIDES

Some very useful chemical relatives of the true fats, the triglycerides, are the diglycerides and monoglycerides. These molecules act as emulsifiers to make fine, cream-like mixtures of fat and water—such sauces as mayonnaise and hollandaise— even though fat and water don’t normally mix with each other. The most prominent natural emulsifiers are the diglyceride phospholipids in egg yolks, the most abundant of which is lecithin (it makes up about a third of the yolk lipids). Diglycerides have only two fatty-acid chains attached to the glycerol frame, and monoglycerides just one, with the remaining positions on the frame being occupied by small polar groups of atoms. These molecules are thus water-soluble at the head, and fat-soluble at the tail. In cell membranes, the phospholipids assemble themselves in two layers, with one set of polar heads facing the watery interior, the other set the watery exterior, and the tails of both sets mingling in between. When the cook whisks some fat into a water-based liquid that contains emulsifiers—oil into egg yolks, for exam-ple—the fat forms tiny droplets that would normally coalesce and separate again. But the emulsifier tails become dissolved in the droplets, and the electrically charged heads project from the droplets and shield the droplets from each other. The emulsion of fat droplets is now stable.

These “surface-active” molecules have many other applications as well. For example, monoglycerides have been used for decades in the baking business because they help retard staling, apparently by complexing with amylose and blocking starch retrogradation.

CARBOHYDRATES

The name for this large group of molecules comes from the early idea that they were made up of carbon and water. They are indeed made up of carbon, hydrogen, and oxygen atoms, though the oxygen and hydrogen are not found as intact water complexes within the molecules. Carbohydrates are produced by all plants and animals for the purpose of storing chemical energy, and by plants to make a supporting skeleton for its cells. Simple sugars and starch are energy stores, while pectins, cellulose, and other cell-wall carbohydrates are the plant’s structural materials.

SUGARS

Sugars are the simplest carbohydrates. There are many different kinds of sugar molecules, each distinguished by the number of carbon atoms it contains, and then by the particular arrangement it assumes. Five-carbon sugars are especially important to all life because two of them, ribose and deoxyribose, form the backbones of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), the carriers of the genetic code. And the 6-carbon sugar glucose is the molecule from which most living things obtain the energy to run the biochemical machinery of their cells. Sugars are such an important nutrient that we have a special sense designed specifically to detect them. Sugars taste sweet, and sweetness is a nearly universal source of pleasure. It’s the essence of the dishes we serve at the end of the meal, as well as of candies and confections.

OLIGOSACCHARIDES

The oligosaccharides (“several-unit sugars”) raffinose, stachyose, and verbascose are 3-, 4-, and 5-ring sugars, respectively, all too large to trigger our sweet detectors, so they’re tasteless. They’re commonly found in the seeds and other organs of plants, where they make up part of the energy supply. These sugars all affect our digestive system, thanks to the fact that we don’t have digestive enzymes capable of breaking them down into single sugars that can be absorbed by the intestine. As a result, the oligosaccharides are not digested and pass intact into the colon, where various bacteria do digest them, producing large quantities of carbon dioxide and other gases in the process.

POLYSACCHARIDES: STARCH, PECTINS, GUMS

Polysaccharides, which include starch and cellulose, are sugar polymers, or molecules composed of numerous individual sugar units, as many as several thousand. Usually only one or a very few kinds of sugars are found in a given polysaccharide. Polysaccharides are classified according to the overall characteristics of the large molecules: a general size range, an average composition, and a common set of properties. Like the sugars of which they’re composed, polysaccharides contain many exposed oxygen and hydrogen atoms, so they can form hydrogen bonds and absorb water. However, they may or may not dissolve in water,depending on the attractive forces among the polymers themselves.

Starch 

By far the most important polysaccharide for the cook is starch, the compact, unreactive polymer in which plants store their supply of sugar. Starch is simply a chain of glucose sugars. Plants produce starch in two different configurations: a completely linear chain called amylose, and a highly branched form called amylopectin, each of which may contain thousands of glucose units. Starch molecules are deposited together in a series of concentric layers to form solid microscopic granules. When starchy plant tissue is cooked in water, the granules absorb water, swell, and release starch molecules; when cooled again, the starch molecules rebond to each other and can form a moist but solid gel. Various aspects of starch—the way it determines the texture of cooked rice, its formation into pure starch noodles.

Glycogen 

Glycogen, or “animal starch,” is an animal carbohydrate similar to amylopectin, though more highly branched. It’s a fairly minor component of animal tissue and so of meats, although its concentration at the time of slaughter will affect the ultimate pH of the meat, and thereby its texture.

Cellulose. 

Cellulose is, like amylose, a linear plant polysaccharide made up solely of glucose sugars. Yet thanks to a minor difference in the way the sugars are linked to each other, the two compounds have very different properties: cooking dissolves starch granules but leaves cellulose fibers intact; most animals can digest starch, but not cellulose. Cellulose is a structural support that’s laid down in cell walls in the form of tiny fibers analogous to steel reinforcing bars, and it’s made to be durable. Few animals can digest cellulose, and hay-eating cattle and wood-eating termites can do so only because their guts are populated by cellulose-digesting bacteria. To other animals, including ourselves, cellulose is indigestible fiber.

Hemicelluloses and Pectic Substances

These polysaccharides (made from a variety of sugars, including galactose, xylose, arabinose) are found together with cellulose in the plant cell walls. If the cellulose fibrils are the reinforcing bars in the cell walls, the amorphous hemicelluloses and pectic substances are a sort of jelly-like cement in which the bars are embedded. Their significance for the cook is that, unlike cellulose, they are partly soluble in water, and therefore contribute to the softening of cooked vegetables and fruits. Pectin is abundant enough to be extracted from citrus fruits and apples and used to thicken fruit syrups into jams and jellies.

Inulin 

Inulin is a polymer of fructose sugars, from a handful to hundreds per molecule. Inulin is a form of energy storage and a source of antifreeze (sugars lower the freezing point of a water solution) in members of the onion and lettuce families, notably garlic and the sunchoke. Like the oligosaccharides, inulin is not digestible, and so feeds bacteria in our large intestine and generates gas.

Plant Gums 

There are a number of other plant carbohydrates that cooks and manufacturers have found useful for thickening and gelling liquid foods, helping to stabilize emulsions, and producing smoother consistencies in frozen goods and candies. Like the cell-wall cements, they’re generally complex polymers of several different sugars or related carbohydrates. They include:

• Agarose, alginates, and carrageenans, cell-wall polymers from various seaweeds

• Gum arabic, which exudes from cuts in various species of Acacia trees

• Gum tragacanth, an exudate from various species of Astralagus shrubs

• Guar gum, from seeds of a shrub in the bean family (Cyamopsis tetragonobola)

• Locust-bean gum, from seeds of the carob tree, Ceratonia siliqua

• Xanthan gum and gellan, polysaccharides produced by certain bacteria in industrial fermentation.

PROTEINS

Of all the major food molecules, proteins are the most challenging and mercurial. The others, water and fats and carbohydrates, are pretty stable and staid. But expose proteins to a little heat, or acid, or salt, or air, and their behavior changes drastically. This changeability reflects their biological mission. Carbohydrates and fats are mainly passive forms of stored energy, or structural materials. But proteins are the active machinery of life. They assemble all the molecules that make a cell, themselves included, and tear them down as well; they move molecules from one place in the cell to another; in the form of muscle fibers, they move whole animals. They’re at the heart of all organic activity, growth, and movement. So it’s the nature of proteins to be active and sensitive. When we cook foods that contain them, we take advantage of their dynamic nature to make new structures and consistencies.

AMINO ACIDS AND PEPTIDES

Like starch and cellulose, proteins are large polymers of smaller molecular units. The smaller units are called amino acids. They consist of between 10 and 40 atoms, mainly carbon, hydrogen, and oxygen, with at least one nitrogen atom in an amine group—NH2—that gives the amino acids their family name. A couple of amino acids include sulfur atoms. There are about 20 different kinds of amino acids that occur in significant quantities in food. Particular protein molecules are dozens to hundreds of amino acids long, and often contain many of the 20 different kinds. Short chains of amino acids are called peptides.

Amino Acids and Peptides Contribute Flavor

Three aspects of amino acids are especially important to the cook. First, amino acids participate in the browning reactions that generate flavor at high cooking temperatures. Second, many single amino acids and short peptides have tastes of their own, and in foods where proteins have been partly broken down— aged cheeses, cured hams, soy sauce—these tastes can contribute to the overall flavor. Most tasty amino acids are either sweet or bitter to some degree, and a number of peptides are also bitter. But glutamic acid, better known in its concentrated commercial form MSG (monosodium glutamate), and some peptides have a unique taste that is designated by such words as savory, brothy, and umami (Japanese for “delicious”). They lend an added dimension of flavor to foods that are rich in them, including tomatoes and certain seaweeds as well as salt-cured and fermented products. When heated, sulfur-containing amino acids break down and contribute eggy, meaty aroma notes.

Amino Acids Influence Protein Behavior 

The third important characteristic of amino acids is that they have a variety of chemical natures, and these influence the structure and behavior of the protein they’re a part of. Some amino acids have portions resembling water and can form hydrogen bonds with other molecules, including water. Some have short carbon chains or carbon rings that resemble fats, and can form van der Waals bonds with other similar molecules. And some, especially those that include a sulfur atom, are especially reactive, and can form strong covalent bonds with other molecules, including other sulfur-containing amino acids. This means that a single protein has many different chemical environments along its chain: parts that attract water molecules, parts that avoid water molecules, and parts that are ready to form strong bonds with similar parts on other proteins, or on other parts of the same protein.

PROTEIN STRUCTURE

Proteins are formed by linking the amine nitrogen of one amino acid with a carbon atom on another amino acid, and then repeating this “peptide bond” to make a chain dozens or hundreds of amino acids long. The carbon-nitrogen backbone of the protein molecule forms a sort of zigzag pattern, with the “side groups”—the other atoms on each amino acid—sticking out to the sides.

The Protein Helix 

One effect of the peptide bond is a certain kind of regularity that causes the molecule as a whole to twist and form a spiral, or helix. Very few proteins exist as a simple regular helix, but those that do tend to join together in strong fibers. These include connective-tis-sue collagen in meat, an important factor in its tenderness, and the source of gelatin.

Protein Folds 

The other influence on pro-tein structure is the side groups of its aminoacids. Because the protein chain is so long,it can bend back on itself and bringtogether amino acids that are some dis-tance along the chain from each other.Amino acids with similar side groups canthen bond to each other in various ways, including via hydrogebonds, van der Waals bonds, ionic bonds and strong covalent bonds (especially between sulfur atoms). This bonding is what gives a particular protein molecule the characteristic shape that allows it to carry out its particular job. The weak, temporary nature of the hydrophobic bonds allows it to change its shape as it works. The overall shape of a protein can range from a long, extended, mostly helical molecule with a few kinks or loops, to compact, elaborately folded molecules that are called “globular” proteins. Collagen is an example of a helical protein, and the various proteins in eggs are mainly globular.

PROTEINS IN WATER

In living systems and in most foods, protein molecules are surrounded by water. Because all proteins are capable to some extent of hydrogen bonding, they absorb and hold at least some water, although the amounts vary greatly according to the kinds of side groups present and the overall structure of the molecule. Water molecules can be held “inside” the protein, along the backbone, and “outside,” on polar side groups.

Whether or not a protein is soluble in water depends on the strength of the bonds between molecules, and on whether water can separate the molecules from each other by hydrogen bonding. The wheat proteins that form gluten when flour is mixed with water are a kind of protein that absorbs considerable amounts of water but doesn’t dissolve, because many fat-like groups along their molecules bond with each other, hold the proteins together, and exclude water. Similarly, the proteins that make up the contracting muscle fibers in meat are held together by ionic and other bonds. On the other hand, many of the proteins in milk and eggs are quite soluble.

PROTEIN DENATURATION

A very important characteristic of proteins is their susceptibility to denaturation, or the undoing of their natural structure by chemical or physical means. This change involves breaking the bonds that maintain the molecule’s folded shape. (The strong backbone bonds are broken only in extreme conditions or with the help of enzymes.) Denaturation is not a change in composition, only a change in structure. But structure determines behavior, and denatured proteins behave very differently from their originals.

Proteins can be denatured in many ways: by exposing them to heat—usually to somewhere between 140–180ºF/60–80ºC—or to high acidity, or to air bubbles, or to a combination of these. In each case, the unusual chemical or physical conditions— increased molecular agitation, or lots of reactive protons, or the drastic difference between the air bubble and the liquid wall that surrounds it—breaks many of the bonds between amino acid side groups that hold the protein molecule in its specific folded shape. The long proteins therefore unfold, exposing many more of their reactive side groups to the watery environment.

Protein Coagulation 

There are several general consequences of denaturation that follow for most food proteins. Because the molecules have been extended in length, they’re more likely to bump into each other. And because their side groups are now exposed and available for forming bonds, denatured proteins begin to bond with each other, or coagulate. This happens throughout the food, and results in the development of a continuous network of proteins, with water held in the pockets between protein strands. The food therefore develops a kind of thickness or density that can be delicate and delightful, as in a barely set custard or perfectly cooked piece of fish. However, if cooking or other denaturing conditions continue, given the extreme physical or chemical environment that caused the proteins to denature in the first place, only the stronger bonds can form and survive, which means that the proteins bond together more and more tightly, densely, and irreversibly. And as they do so, they squeeze the pockets of water out from between them. The custard gets dense and a watery fluid separates from the solid portion; the fish gets tough and dry.

The details of protein denaturation and coagulation in any given food are intricate and fascinating. For example, acidity and salts can cause egg proteins to cluster together even before they begin to unfold, and thus affect the consistency of scrambled eggs and custards. Such details are noted in the descriptions of particular foods.

ENZYMES

There’s a particular group of proteins that are important to the cook not so much for their direct contribution to food texture and consistency, but for the way they change the other components of the food they’re in. These proteins are the enzymes. Enzymes are biological catalysts: that is, they increase the rate of specific chemical reactions that otherwise would occur only very slowly, if at all. Enzymes thus cause chemical change. Some enzymes build molecules up, or modify them; some break molecules down. Human digestive enzymes, for example, break proteins into individual amino acids, and starch into individual glucose units. A singe enzyme molecule can catalyze as many as a million reactions per second.

Enzymes matter to the cook because foods contain enzymes that once did important work for the plant or animal when it was alive, but that can now harm the food by changing its color, texture, taste, or nutritiousness. Enzymes help turn green chlorophyll in vegetables dull olive, cause cut fruits to turn brown and oxidize their vitamin C, and turn fish flesh mushy. And bacterial spoilage is largely a matter of bacterial enzymes breaking the food down for the bacteria’s own use. With a few excep-tions—the tenderizing of meat by its own internal enzymes, the firming of some vegetables before further cooking, and fermentations in general—the cook wants to prevent enzymatic activity in food. Storing foods at low temperatures delays spoilage in part because it slows the growth of spoilage microbes, but also because it slows the activity of the food’s own enzymes.

Cooking Accelerates Enzyme Action Before Stopping It 

Because the activity of an enzyme depends on its structure, any change in that structure will destroy its effectiveness. So cooking foods sufficiently will denature and inactivate any enzymes they may contain. One vivid example of this principle is the behavior of raw and cooked pineapple in gelatin. Pineapples and certain other fruits contain an enzyme that breaks proteins down into small fragments. If raw pineapple is combined with gelatin to make a jelly, the enzyme digests the gelatin molecules and liquifies the jelly. But canned pineapple has been heated enough to denature the enzyme, and makes a firm gelatin jelly.

There’s a complication, though. The reactivity of most chemicals increases with increasing temperature. The rule of thumb is that reactivity doubles with each rise of 20ºF/10ºC. The same tendency goes for enzymes, up to a range in which they begin to denature, become less effective, and finally become completely inactive. This means that cooking gives enzymes a chance to do their damage more and more quickly as the temperature rises, and only stops them once they reach their denaturation temperature. In general, the best rule is to heat foods as rapidly as possible, thereby minimizing the period during which the enzymes are at their optimum temperatures, and to get them all the way to the boiling point. Conversely, desirable enzyme action—meat tenderizing, for example— can be maximized by slow, gradual heating to denaturing temperatures.

By Harold McGee in "On Food and Cooking", Scribner, New York, USA, 2004, excerpts pp. 792-809. Adapted and illustrated to be posted by Leopoldo Costa.