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. 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. 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. 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. 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 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. hydrogen bond. The molecules in ice and liquid water are partic.i.p.ating 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. The molecules in ice and liquid water are partic.i.p.ating 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 molecules. Here are three different ways of representing a molecule of water, which is formed from one oxygen and two hydrogen atoms. Because the oxygen atom exerts a stronger pull on the electrons (small dots) it shares with the hydrogen atoms, the water molecule is electrically unsymmetrical. The separation of positive and negative centers of charge leads to the formation of weak bonds between oppositely charged centers on different molecules. These weak bonds between molecules, shown here by dashed lines, are called hydrogen bonds.

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 cl.u.s.ter 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 dissolved in the water. 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 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 more s.p.a.ce 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. s.p.a.ce 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.

Hard Water: Dissolved MineralsWater is so good at dissolving other substances that apart from distilled water, it's seldom found in anything like pure form. Tap water is quite variable in composition, depending on its ultimate source (well, lake, river) and its munic.i.p.al treatment (chlorination, fluoridation, and so on). Two common minerals in tap water are carbonate (CO3) and sulfate (SO4) salts of calcium and magnesium. Calcium and magnesium ions are troublesome because they react with soaps to form insoluble sc.u.ms, and because they leave crusty precipitates on showerheads and teapots. Such so-called hard water can also affect the color and texture of vegetables, and the consistency of bread dough (pp. 282, 535). Hard water can be softened either city-wide or in the home, usually by one of two methods: precipitating the calcium and magnesium by adding lime, or using an ion-exchange mechanism to replace the calcium and magnesium with sodium. Distilled water, which is produced by boiling ordinary water and collecting the condensed steam, is fairly free of impurities.

Liquid Water Is Slow to Heat Up Again thanks to the hydrogen bonding between water molecules, liquid water has a high Again thanks to the hydrogen bonding between water molecules, liquid water has a high specific heat, 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 1 as it does to heat an ounce of iron 1. 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 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 1 as it does to heat an ounce of iron 1. 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 free to move faster. 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 over-heated 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. 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 over-heated 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 212F/100C 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. 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 212F/100C 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 H 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 cla.s.s of chemicalcompounds that release protons into solutions, acids, acids, derives from the Latin derives from the Latin acere, acere, meaning to taste sour. We call the complementary chemical group that accepts protons and neutralizes them, meaning to taste sour. We call the complementary chemical group that accepts protons and neutralizes them, bases bases or or alkalis. 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 The standard measure of proton activity in solutions is pH, pH, a term suggested by the Danish chemist S. P. L. Srenson 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- a term suggested by the Danish chemist S. P. L. Srenson 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-accepting groups, and so a basic solution. Here's a list of common solutions and their usual pH. groups, and so a basic solution. Here's a list of common solutions and their usual pH.

Liquid

pH pH

Human gastric juice

1.33.0 1.33.0

Lemon juice

2.1 2.1.

Orange juice

3.0 3.0.

Yogurt

4.5 4.5.

Black coffee

5.0 5.0.

Milk

6.9 6.9.

Egg white

7.69.5 7.69.5

Baking soda in water

8.4 8.4.

Household ammonia

11.9 11.9.

Acids. Acids are molecules that release reactive hydrogen ions, or protons, in water, where neutral water molecules pick them up and become positively charged. The acids themselves become negatively charged. Left: Left: Water itself is a weak acid. Water itself is a weak acid. Right: Right: Acetic acid. Acetic acid.

The Definition of pHThe pH of a solution is defined as "the negative logarithm of the hydrogen ion concentration expressed in moles per liter." The logarithm of a number is the exponent, or power, to which 10 must be raised in order to obtain the number. For example, the hydrogen ion concentration in pure water is 10-7 moles per liter, so the pH of pure water is 7. Larger concentrations are described by smaller negative exponents, so a more acidic solution will have a pH lower than 7, and a less acidic, more basic solution will have a pH higher than 7. Each increment of 1 in pH signifies an increase or decrease in proton concentration by a factor of 10; so there are 1,000 times the number of hydrogen ions in a solution of pH 5 as there are in a solution of pH 8. moles per liter, so the pH of pure water is 7. Larger concentrations are described by smaller negative exponents, so a more acidic solution will have a pH lower than 7, and a less acidic, more basic solution will have a pH higher than 7. Each increment of 1 in pH signifies an increase or decrease in proton concentration by a factor of 10; so there are 1,000 times the number of hydrogen ions in a solution of pH 5 as there are in a solution of pH 8.

Fats, Oils, and Relatives: Lipids Lipids Don't Mix with Water Fats and oils are members of a large chemical family called the lipids, 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. 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 betacarotene 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 betacarotene 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 cla.s.s of chemical compounds, the triglycerides. triglycerides. They differ from each other only in their melting points: oils are liquid at room temperature, fats solid. Rather than use the technical They differ from each other only in their melting points: oils are liquid at room temperature, fats solid. Rather than use the technical triglyceride triglyceride to denote these compounds, I'll use to denote these compounds, I'll use fats 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 (p. 778). 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 (p. 778).

Glycerol and Fatty Acids Though they contain traces of other lipids, natural fats and oils are triglycerides, a combination of three Though they contain traces of other lipids, natural fats and oils are triglycerides, a combination of three fatty acid fatty acid molecules with one molecule of molecules with one molecule of glycerol. glycerol. Glycerol is a short 3-carbon 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. Glycerol is a short 3-carbon 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 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 saturated lipid is one whose carbon chain is saturated - 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 lipid is one whose carbon chain is saturated - 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 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 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. polyunsaturated.

Fats and fatty acids. Fatty acids are mainly chains of carbon atoms, shown here as black dots. (Each carbon atom has two hydrogen atoms projecting from it; the hydrogen atoms are not shown.) A fat molecule is a triglyceride, triglyceride, which is formed from one molecule of glycerol and three fatty acids. The acidic heads of the fatty acids are capped and neutralized by the glycerol, so the triglyceride as a whole no longer has a polar, water-compatible end. The fatty-acid chains can rotate around the glycerol head to form chair-like arrangements which is formed from one molecule of glycerol and three fatty acids. The acidic heads of the fatty acids are capped and neutralized by the glycerol, so the triglyceride as a whole no longer has a polar, water-compatible end. The fatty-acid chains can rotate around the glycerol head to form chair-like arrangements (bottom). (bottom).

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. 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 "zippering" - 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.

Saturated and unsaturated fatty acids. An unsaturated fatty acid has one or more double bonds along its carbon chain, and a rigid kink at that point in the chain. The structural irregularity caused by the double bond makes it more difficult for these molecules to solidify into compact crystals, so at a given temperature, unsaturated fats are softer than saturated fats. In the hydrogenation of vegetable oils to make them harder, some cis-unsaturated fatty acids are converted to trans-unsaturated fatty acids, which are less kinked and behave more like a saturated fatty acid, both in cooking and in the body.

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 s.p.a.ce 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 fragmentfats and cause rancidity. The more unsaturated the fat, the more p.r.o.ne 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. Saturated fats are also more stable, slower to become rancid than unsaturated fats. The double bond of an unsaturated fat opens a s.p.a.ce 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 fragmentfats and cause rancidity. The more unsaturated the fat, the more p.r.o.ne 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 cuc.u.mber 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.

Saturated and Unsaturated Fatty Acids in Foods and Cooking FatsProportions of fatty acids are given as a percentage of the total fatty-acid content.

Fat or Oil Saturated Fatty Acids Saturated Fatty Acids Monounsaturated Fatty Acids Monounsaturated Fatty Acids Polyunsaturated Fatty Acids Polyunsaturated Fatty Acids

b.u.t.ter 62 62.

29 29.

4 4.

Beef 50 50.

42 42.

4 4.

Lamb 47 47.

42 42.

4 4.

Pork 40 40.

45 45.

11 11.

Chicken 30 30.