Fats and oils are identified by determinations of their physical properties, such as specific gravity, melting point, refractive index, etc., and by certain special color reactions for particular oils; or by measurements of certain chemical constants, such as the percentage of free fatty acids which they contain, the saponification value (i.e., the number of milligrams of KOH required to completely saponify one gram of the fat), the iodine number (percentage by weight of iodine which is absorbed by the unsaturated fatty acids present in the fat), percentage of water-insoluble fatty acids obtained after saponification and acidifying the resultant soap, etc., etc. Most of these tests must be carried out under carefully controlled conditions in order to insure reliable identifications, and need not be discussed in detail here. Full directions for making such tests, together with tables of standard values for all common fats and oils, may be found in any reference book on oil analysis.
PHYSIOLOGICAL USE OF FATS AND OILS
In animal organisms, fats are the one important form of energy storage. They also form one of the most important supplies of energy reserve material in plants. Carbohydrates commonly serve this purpose in those plants whose storage reservoirs are in the stems, tubers, etc.; but in most small seeds the reserve supply of energy is largely in the form of oil, and even in those seeds which have large endosperm storage of starch, the embryo is always supplied with oil which seems to furnish the energy necessary for the first germinative processes.
Fats are the most concentrated form of potential energy of all the different types of organic compounds which are elaborated by plants. This is because they contain more carbon and hydrogen and less oxygen in the molecule than any other group of substances of vegetable (or animal) origin. It has been pointed out that a quantity of fat capable of yielding 100 large calories of heat will occupy only about 12 cc. of space, whereas from 125 to 225 cc. of space in the same tissue would be required for the amount of starch of glycogen necessary to yield the same amount of heat, or energy, when oxidized.
The fats undoubtedly catabolize first by hydrolysis into glycerol and fatty acids, and then by oxidation possibly first into carbohydrates and then finally into the end-products of oxidation, namely, carbon dioxide and water. The following hypothetical equation to represent the oxidation of oleic acid into starch, suggested by Detmer, is interesting as a suggestion of how much oxygen is required and how much heat would be liberated by such a transformation:
C18H34O2 + 27O = 2(C6H10O5) + 6CO2 + 7H2O
Complete oxidation of oleic acid to the final end-products, carbon dioxide and water, would require much more oxygen, thus:
C18H34O2 + 51O = 18CO2 + 17H2O.
Hence, Detmer's reaction would yield only approximately one-half the total energy available in the acid; but it does indicate the possibility of redevelopment of fatty acids or fats from the unoxidized carbohydrate material which remains in the equation. Moreover, there is abundant evidence to show that, in both animal and plant tissues, energy changes are brought about chiefly by the transformation of fats into carbohydrates and vice versa.
Many different hypotheses have been put forward concerning the mode of transformation of fats into carbohydrates, and the changes which take place in oily seeds during their germination have been carefully studied by many investigators. The following seem to be fairly well established facts. First, that fats as such may be translocated from cell to cell, since cell-walls and cell protoplasm seem to be permeable to oil if it is a sufficiently fine emulsion; or they may be hydrolyzed into glycerol and fatty acids and translocated from cell to cell in these forms and recombined into fats in the new location. Second, that fats are formed from glucose in some plants, from sucrose and from starch in others, and from mannite and similar compounds in still other species. Third, that in germination the fatty acids are used up in the order of their degree of unsaturation, those which contain the largest number of double-bond linkages being used first, and the saturated acids last of all. Fourth, that the sugar produced by the oxidation of fats is derived either from the glycerol or from the fatty acids of the fat, depending upon the nature of the latter. If the fat is saturated, the glycerine is converted into sugar while the fatty acids are oxidized; but if the fat contains large proportions of unsaturated acids, these contribute to the formation of sugar.