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CHEMISTRY OF FOOD FATS Most separated natural fats are made up of about 98 to 99 percent triglycerides, of which 92 to 95 percent is fatty acid and the remainder glycerol. The remaining 1 or 2 percent of separated natural fats includes traces of monoglycerides, diglycerides, free fatty acids, phospholipids, and unsaponifiable matter containing sterols. Fatty Acid and Glyceride Structures Most fats are mixtures of triglycerides containing four or five major fatty acids and many more minor, or trace, constituents. In butterfat, at least 60 different fatty acids have been identified. The individual glyceride molecules in most food fats contain both saturated and unsaturated fatty acids. Fully saturated glycerides are a rarity in natural fats, appearing in only a few, such as beef tallow and coconut oil. Thus "saturated" and "unsaturated" are not truly definitive terms when applied to food fats. In foods for human consumption, myristic, palmitic, and stearic acids are the most abundant of the saturated fatty acids. Of the unsaturated acids, oleic acid with one double bond is the most abundant, and of the polyunsaturated acids, linoleic acid is the most abundant and constitutes a high percentage of the com- monly used vegetable oils. Several of the polyunsaturated acids cannot be synthesized in the animal body and must be provided in the diet. These have been termed "essential fatty acids" and are mainly represented by linoleic acid. Linolenic acid, which may partially relieve some of the evidence of essential fatty acid deficiency in experimental animals but which is not an essential fatty acid in the strict sense, occurs in several seed fats, such as linseed and soybean oils. Arachidonic acid does not occur in vegetable oils in appreciable amounts, but is synthesized from linoleic acid in the animal body and comprises about 1 percent of most animal fats. In the naturally occurring polyunsaturated fatty acids, the double bonds are not conjugated (contiguous), but
Chemistry of Food Fats are almost always separated by a methylene group. In addition, the configuration is of the cis type in which the hydrogens are on the same side of the chain and a 120Â° bend is introduced into the chain itself. Fatty acids containing trans bonds, in which the hydrogens are on opposite sides, are found in small amounts in natural fats and in greater amounts after processing involving catalytic hydrogenation (98). Fatty Acid Composition of Fats The fatty acid composition of common food fats is extremely variable (Table 1). The unqualified terms "animal fat" and "vegetable fat" do not indicate fatty acid composition or nutri- tional value. Cow's milk, in contrast to human milk, contains appreciable quantities of fatty acids with fewer than ten carbon atoms (55). The linoleic acid content of human milk fat is usually considerably higher than that of butterfat. Many animal fats, such as beef and mutton tallow, are rela- tively rich in saturated fatty acids, but some fats of animal origin, such as poultry fat and fish oils (94), have a high degree of unsaturation. The fatty acid composition of nonruminant animal fat is markedly influenced by diet. It has long been known that linoleic acid may vary from a low percentage to as much as one third of the fatty acids in pork fat, depending on the feed of the pig (52). The fatty acid composition of egg yolk from commercially available eggs is relatively saturated, owing to the low-fat rations used in feeding the laying hen. A considerable shift in fatty acid pattern can be effected, however, by incorporating unsaturated fat in the rations. Unprocessed vegetable fats are generally liquid at ordinary temperatures because of their high percentage of unsaturated fatty acids. Especially abundant in these fats are oleic and linoleic acids. Oleic acid may make up four fifths of the fatty acid content of olive oil. In corn, peanut, cottonseed, safflower, and soybean oilsâused for cooking, salads, margarines, and shorteningsâ12 to 25 percent of the fatty acids are saturated, wheareas 21 to 53 percent may be linoleic acid. Even among the vegetable fats, however, there are exceptions. Coconut oil is one of the most saturated (about 90 percent) of the food fats. The intake of essential fatty acids in the American diet is mostly from vegetable sources. Chicken fat and the fats of other
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Chemistry of Food Fats poultry may contain as much as 25 percent linoleic acid, but these fats are not used extensively. Fish oils are rich in polyunsaturated fatty acids but are poor sources of the essential fatty acids, arachidonic and linoleic acids being minor components (188). Monoglycerides and Diglycerides In addition to the triglycerides, glycerol esters of the fatty acids in which one or two of the hydroxyl groups remain unesterified may be encountered in food fats. Trace amounts on the mono- and diglycerides and of free fatty acids are present in natural fats. Within the body, these lipids are found during digestion and absorption (2) and are present in the circulating lipids of the plasma (31). The fat used to provide the physical character desired in shortening for such baked goods as cakes may contain 5 to 20 percent of added mono- and diglycerides. Phospholipids Phospholipids form a group of complex lipids having in common in their molecules a phosphate radical, an esterified fatty acid, and usually a nitrogenous base. Lecithin, or phosphatidyl choline, is the most widely distributed of the phospholipids. Raw vegetable oils, such as corn oil, contain traces of lecithin, and many animal lipids, notably egg yolk and liver (but not depot fats), are rich in this phospholipid. Assessing the role of food phospholipids in human health is complicated because of the diverse chemical nature of this group of lipids. The complex mixture of the phospholipids in common food products is illustrated by the phospholipids of egg yolk. A typical analysis of egg yolk reveals 72.8 moles percent phos- phatidyl choline, 14.8 phosphatidyl ethanolamine, 5.8 sphingomye- lin, 2.1 lysophosphatidyl ethanolamine, and 0.9 mole percent plasmalogen, 0.6 inositol phospholipid, and 0.2 phosphatidyl amino acids (170). Phospholipid is one third of the lipid of egg yolk; one egg contributes approximately 2 gm of phospholipid to the diet. Invisible fat of both plant and animal tissues contains apprecia- ble amounts of phospholipid, but the visible separated fats are generally poor in these lipids, except when lecithin has been deliberately added to take advantage of its emulsifying and
Dietary Fat and Human Health antioxidant properties. The amount of phospholipid associated with the glycerides of seeds is usually small, and expression, purification, and refining of the oil remove most of this amount. In animal tissues, important amounts of phospholipid are found in liver and other edible parts, but the phospholipid concentra- tion in most adipose tissues is low. In the preparation of sepa- rated fat, such as lard or butter, much of the phospholipid is removed. Unsaponifiable constituents also are decreased during preparation of visible fats. Unsaponifiable Constituents The small amount of Unsaponifiable matter in food fats consists of sterols, fatty alcohols, hydrocarbons, pigments, glycerol ethers, and various other compounds. One of the principal dif- ferences between animal and vegetable fats is the nature of the sterols present. Cholesterol, found almost exclusively in animal tissue, differs from phytosterols (plant sterols) in that phyto- sterols have more highly branched side chains and may have a second double bond in the nucleus. Cholesterol occurs in all animal fats. It is a normal constituent of every animal tissue and a major component of brain and nerve tissues. Whole eggs contain about 0.5 percent cholesterol on a fresh weight basis (all in yolks). Butter, organ meats, and shellfish contain between 0.2 and 0.3 percent, and meats and animal fat contain less than 0.1 percent cholesterol. Hydrogenation The process of hydrogenation converts oils from a liquid to a plastic state, a property necessary for margarines and shortenings. The process also imparts increased stability. This is of particular importance for soybean oil in which the presence of up to 10 percent linolenic acid renders it very susceptible to oxidation. Hydrogenation reduces linolenic acid to a level at which oxidative rancidity is not a problem. If hydrogenation were complete, a fully saturated hard fat of simple composition would result. Such a fat would be unacceptable as a food item. The partial hydrogenation process that is employed results in four types of chemical change.
Chemistry of Food Fats 1. The main change is a conversion of some of the polyunsatu- rated fatty acids to monounsaturated fatty acids, as indicated by lowering of the iodine value. The extent of this conversion depends on the hydrogenation conditions selected. The melting point of the fat is raised but, as employed, this process results in the formation of only small amounts of additional saturated fatty acids. 2. The double bond may shift position along the carbon chain, producing isomeric acids. These new unsaturated acids have the same iodine number, but may differ from the original in melting point. 3. The predominantly occurring cis configuration may change to the trans configuration. This isomerization also leaves the iodine number unchanged, but leads to a significant rise in the melting point. For example, oleic acid melts at 13Â° C and is liquid at room temperatures; its trans isomer melts at 44Â°C and thus is solid at room temperatures. 4. During hydrogenation of linoleic, linolenic, or arachidonic acids, double bonds may become conjugated, in which system they are no longer separated by a methylene group. These con- jugated systems are relatively rare in natural food fats and the amount of conjugated acids found in hydrogenated fats is quite small. It is apparent that the complexity of the chemical composition of fats is increased by partial hydrogenation. To a great extent, these changes can be controlled by varying the processing con- ditions. Trans fatty acids are not, strictly speaking, unnatural. They occur in measurable amounts in the depot fats of ruminants (86), apparently arising through the activity of rumen bacteria. Never- theless, these trans isomers of unsaturated fatty acids are only minor constituents of natural fats. Summer butterfat may contain 9 or 10 percent trans acids (40), and human milk fat may contain 5 or 6 percent. Amounts of several percent have been reported in human depot fat. In the latter two instances, the trans acids may be derived from the diet. Margarines and shortenings with a wide range of fatty acid composition and increased levels of polyunsaturated fatty acids have been made available. These fats have a range in unsaturation between those of olive and cottonseed oils, but they are solid. Their higher melting points are due to a slight increase in satu- rated fatty acids and to the presence of trans acids. The derived
Dietary Fat and Human Health properties of the margarines or shortenings are obtained by the proper blending of partially hydrogenated, lightly hydrogenated, or unhydrogenated oils. Trans acids may account for from 15 to 40 percent of the total. A portion of the polyunsaturated acids escaping hydrogenation are converted to isomeric acids. Polyunsaturated fatty acids that have undergone any of the four chemical changes resulting from the partial hydrogenation process lose their essential fatty acid activity; indeed, these acids may be preferentially converted to monounsaturated acids by hydrogenation. Whereas highly hydrogenated types of marga- rines and shortenings are comparable to natural fats of similar firmness as sources of essential fatty acids, the newer products, although indistinguishable as far as appearance is concerned, contain much higher levels of essential acids. All of these types of margarines and shortenings are equally well utilized as energy sources. Trans and positional isomers of the natural fatty acids appear to be metabolized by the body with the same ease as the more common cis forms (146). Neither of the trans isomers of oleic and linoleic acid has essential fatty acid activity. Evidence of interference with the activity of cis linoleic acid is conflicting (96, 140). Cooking Common methods of cooking have no appreciable effect on the essential fatty acids in such foods as beef, lamb, poultry, and pork (34). Similarly, the fatty acid makeup of lard (33) is not altered in baked products. Changes have been observed in heated corn oil used in deep fat frying. It is possible that the polyunsatu- rated fatty acids that make up a high percentage of such oils, under conditions of elevated temperature in the presence of air, can absorb oxygen to form peroxide or polymer products which, in large amounts, may prove to be toxic. However, under the usual conditions of commercial use, such products are not formed in appreciable amounts (44, 145), and the nutritive value, measured by growth rate of animals, is not altered. Rancid fat has a toxic effect upon rats on a low-fat diet, the toxic agent apparently being peroxidized fatty acids or their esters (160) rather than aldehydes or low polymers. 10