FATTY OILS AND FATS The fixed oils and fats form a well-defined and homogeneous group of substances passing through all gradations of consistency, from oils which are fluid even below the freezing point of water, up to the hardest fats which melt at about 50° C. Therefore, no sharp distinction can be made between fatty oils and fats. Never theless it is convenient to apply the term "oil" to those glycerides which are fluid below 20° C, and the word "fat" to those which are solid above this temperature. As a general inclusive term the expression "fat" is preferable, since this avoids confusion with mineral and essential oils.
Chemical Composition.—No oil or fat is found in nature consisting of a single chemical individual. Following on Chevreul's work, the fats were considered to consist in the main of mixtures of "simple triglycerides," this is, of glycerides in which each molecule of glycerol is combined with three molecules of the same acid, e.g., tristearin, the glyceryl ester of stearic acid, triolein, (O. CO.C171133 )3. Recently, however, it has been shown that "mixed glycerides," i.e., compounds wherein one molecule of glycerol is united with two or three different fatty acid radicles, preponderate, e.g., dipalmito-stearin, ; palmito oleo-stearin, C3H5(0.CO•C15H31) (0.CO•C17H33) (0.CO-C17H35). The natural fats, therefore, may differ, firstly in the fatty acids which they contain, and, secondly, by the different arrangement of the acids in simple and mixed glycerides. Fatty diglycerides only occur occasionally as products of partial hydrolysis of the oil by ferment action.
Fatty Acids.—The most important and widespread of the fatty acids are palmitic, C15H31COOH and stearic, CI7H35COOH, acids (of the saturated series), and the unsaturated oleic acid, C The fatty acids are classified into related (homologous) series according to their degree of unsaturation, i.e., the number of un saturated or ethenoid bonds > C = C < , in the molecule of the acid. This classification is amplified in the list of examples which here follows.
Straight chain fatty acids of the saturated (acetic, or stearic) series, general formula include among the lower members butyric acid, characteristic of but ter fat (q.v.), caproic, caprylic, capric, lauric, and myristic acids, present in butter and fats of the coconut class; stearic and palmitic acids are higher in the series. Acids of the oleic series, (one pair of doubly-linked carbon atoms), of which oleic acid present in nearly all oils and fats, and erucic acid characteristic of rape and fish oils, are the most important. Acids of the linoleic series, (two ethenoid bonds), include linoleic acid, CI7H3ICOOH, present to a large extent in maize and cottonseed oils. The linolenic series
acids, CnH2„-5COOH (three double bonds) of which linolenic acid, the characteristic acid of linseed oil is very impor tant, as the high degree of unsaturation of this acid is responsible for the "drying" power or oxidisability upon which the commer cial application of the oil in the paint industry depends. Still more unsaturated acids occur, such as clupanodonic acid, C2lR33•COOH (five ethenoid bonds), typical of fish, liver and blubber oils. The oxidation products of these oils, however, do not form such tenacious films as linseed oil, to which they are much inferior as paint oils. The hydroxylated acids of the ricinoleic se ries, are typified by ricinoleic acid, the occurrence of which is responsible for the solubility of castor oil in alcohol.
The occurrence of cyclic (closed chain) acids is confined to oils of the chaulmoograc and hydnocarpus family. Acids with more than twenty-two carbon atoms in the molecule are rare among the fats, but are of fairly frequent occurrence among the waxes.