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, 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.
When polar water and non-polar 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), 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 carbonhydrogen 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 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 carbons long.
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 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 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 “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. Shortchain 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. Manufacturers are 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 baking 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 500ºF - 750ºF / 260ºC - 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 capable of 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 example—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.
