Kind of fat

Unsaturated glycerides

Investigator

Percentage

Coconut oil

6 to 8

Lewkowitsch

Butter

40

Lewkowitsch

Lard

60 to 65

Lewkowitsch

Chicken fat1

77

Kerr

Cottonseed oil

75

Jamieson and Baugham

Peanut oil

From Spanish-type peanuts

75

Jamieson and Baugham

From Virginia-type peanuts

79

Jamieson and Baugham

Corn oil

85

Jamieson and Baugham

1 Personal communication from R. H. Kerr, Bureau of Animal Industry.

Since the above figures are approximate, it follows that individual samples of each oil or fat vary slightly, some having a higher and some a lower percentage of unsaturated glycerides. Lard obtained from leaf fat has a lower percentage of unsaturated glycerides than that obtained from the back fat of the same animal. In addition, the composition of such fats as butter, lard, tallow, and chicken fat may be influenced by the diet. Pork fed soybeans or peanuts in sufficient quantity produces the so-called "soft pork," because the body fat contains a higher percentage of unsaturated glycerides, hence is softer at room and refrigerator temperatures.

Melting point of fats. Because of the mixture of glycerides the melting points of fats are not constant but vary. The melting points of fats are not only affected by the percentage of their component unsaturated fatty acids but also by the length of the saturated acid chains. The shorter saturated fatty acids have lower melting points than the longer ones, and, in general, the fats composed of glycerides containing a high percentage of the higher or longer chained saturated fatty acids have the highest melting points; those with a high percentage of the shorter saturated fatty acids have intermediate melting points; and those with a high percentage of unsaturated acids have the lowest melting point. As the percentage of the shorter fatty acids or the unsaturated fatty acids is increased the melting point of the fat is lowered. Butter produced in summer has a lower melting point than butter produced in the winter months because the percentage of unsaturated fatty acids is higher in the summer. The oils have a higher percentage of oleic acid and consequently have a low melting point. Hard fats like mutton tallow contain high percentages of palmitic and stearic acids and little oleic and have a high melting point. Soft fats like lard contain more oleic acid than tallow but not so much as the oils.

Congealing point. The solidifying point of a glyceride or a mixture of glycerides is lower than the melting point, the difference in the two often being great. For example, Bloor cites the melting point of a sample of tristearin as 71.5°C, whereas the solidifying point was 52.5°C. A sample of butter melted at 34.5° but congealed at 22.7°C. This wide spread in melting and congealing points has some advantages and some disadvantages. When melted fats are added to a batter or other product they do not solidify as rapidly as they would if the melting and congealing points were the same. On the other hand, butter once melted requires a lower temperature to congeal.

Iodine number (or value). The iodine number is defined as the number of grams of iodine absorbed by 100 grams of fat or other substance. The iodine is absorbed by the unsaturated carbons or at the double bonds. The class to which an oil belongs is indicated by its iodine number. The non-drying oils, such as peanut and coconut, have iodine values less than 100. Cacao butter and the fats such as lard, beef, and mutton tallows, and butter also, have iodine values below 100. The semi-drying oils, which include corn, cottonseed, sesame, wheat, oat, rice, rye, Brazilnut, raisin seed, peach, cherry, and apricot kernels, and many others, have iodine values between 100 and 130. Drying oils, which include linseed and soybean, have iodine values above 130.

Acid value. Acid value is defined as the number of milligrams of KOH required to neutralize the free fatty acids in one gram of substance. It is a measure of the quantity of free fatty acids or those not combined in glycerides and is usually reported as "free fatty acids as oleic."

Refractive index. The refractive index is the degree of deflection caused in a ray of light in passing from one transparent medium to another. For some fats the iodine value may be calculated with a fair degree of accuracy from the refractive index. Since the determination can be made rapidly, the refractive index is used to determine the extent of hydrogena-tion during the hydrogenation process.

Peroxide value. The peroxide value is the amount of peroxide present per 1000 grams of fat. It is expressed as milli-equivalents or as milli-moles, 1 milli-mole equaling 2 milli-equivalents. The peroxide is determined in accelerated tests to learn the relative induction periods or relative keeping quality of fats. King, Roschem, and Irwin describe the principle of this test as follows: By aerating a sample of fat in a test tube held at constant temperature in a bath, the aging of the fat is greatly accelerated. They state that the temperature at which the fat is held, the surface exposed, the amount of oxygen available, the amount of agitation, and the presence or absence of light, all have a bearing on the peroxide value. When these conditions are controlled, an accelerated test may be completed in a short time. The aging of the fat under these conditions is as rapid in a few hours as in a few weeks or months when stored at lower temperatures with exclusion of light and air.

Hydrogenation. Hydrogenation is the process by which hydrogen is added to the unsaturated carbon bonds of oils, thus forming hard fats. By using a metal catalyst, such as iron, copper, nickel, or platinum, at an elevated temperature in the presence of an atmosphere of hydrogen the oils take up hydrogen at their unsaturated bonds, becoming plastic, or hard, brittle fats, depending upon the completeness of hydrogenation. Crisco and Snowdrift, hydrogenated fats in common use in food preparation, are not completely hydrogenated. They contain about 25 per cent of the saturated fatty acids. If they were completely hydrogenated the product would be too hard to use in food preparation. In addition to oils or mixtures of oils some soft lards are also hydrogenated for edible purposes.