GLYCERIDES are one of Nature's usual modes of storing up reserves, in the vegetable or animal realms, in the form of stable compounds containing carbon, hydrogen, and oxygen. They constitute, in fact, over 90% of the substances present in vege table and animal oils, fats, and waxes (qq.v.), and the terms glyceride and fat are frequently, but not quite correctly, used interchangeably. All naturally occurring glycerides appear to be associated with a small amount (4-2%) of non-glyceridic sub stances called non-saponifiable or unsaponifiable matter, which have little direct industrial importance, but probably a very pro found physiological significance, since they include the sterols and the fat-soluble vitamins (q.v.).
In order to understand the modern uses of fats (see OILS AND FATS ; WAX) it is necessary to have some knowledge of the chemi cal structure of the glycerides. This is fortunately very simple in general outline, although complicated in detail. Glycerides belong to the class of compounds called "esters" by the chemist. In simple terms, an ester is the salt of an acid with an alcohol, just as sodium chloride is the salt formed by the union of sodium hydrox ide and hydrochloric acid; a simple example of an ester is ethyl acetate, from ordinary alcohol and acetic acid, a volatile liquid with a fruity smell. The glycerides are built up from the alcohol, glycerol and fatty acids (q.v.) of which for the moment we may take the most common, oleic acid, C17H33•COOH, as an example. The essential nature of a glyceride glycerol ester may therefore be grasped by the following comparison: Complications set in when it is recollected (a) that glycerol has three points at which an acid group may enter (just as, for example, iron hydroxide unites with three acidic groups whilst sodium hydroxide NaOH only combines with one, i.e., is "monobasic") and (b) that at least 20 different fatty acids are common in nature, of which from about five to 15 are generally present in any single natural fat. Even if, in a hypothetical case, we assume the presence of only three acids X, Y, Z in a glyceride, we can envisage the possibility of the formation of no less than 18 distinct triglycerides, of which the following are examples: When, as in actual fact, the number of acids present is greater, the number of possible variations becomes enormous.
Very little is, indeed, yet known definitely as to the actual struc ture of different fats, although the occurrence of any one simple triglyceride, e.g., tri-olein, or tri-palmitin, is now known to be much less common than was formerly supposed. The properties (consistency, melting-point, "drying" or "non-drying" character, etc.) of any fat are determined partly by the manner in which the different fatty acids are linked up with the glycerol, and partly, indeed mainly, by the nature of the fatty acids (q.v.) (see also