Experimental cookery

Refractive index. The refractive index is the degree of deflection caused in a ray of light in passing from one transparent medium to an- other. 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 prepara- tion, 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. Texture and consistency o£ fats. The texture or consistency of a fat, and hence its properties in baked products, is not only influenced by its component glycerides, but also by the rate of cooling and mechanical agita- tion. For example, if a sample of lard is divided into two parts, and one portion is cooled slowly, i.e., kept in a warm room or an incubator just below the congealing point of the fat, the glycerides with high melting points tend to crystallize and separate from those with low melting points. As a result the crystallized glycerides form granular spots surrounded by oil. If the second portion of lard is chilled rapidly, there are fewer granules 546 FATS AND OILS