The vegetable fats and oils have many important industrial uses. Some of them, such as olive oil, cottonseed oil, cocoanut oil, etc., are largely used as human food. Others, as castor oil, are used as lubricants. The so-called "drying oils" (see [page 132]), such as linseed oil, etc., are used in the manufacture of paints and varnishes. Some cheap vegetable oils are used as the basis for the manufacture of soaps, etc. Hence, industrial plants and processes for the extraction of oils from plant tissues are of very great economic importance.
CHEMICAL CONSTITUTION
The fats (of either plant or animal origin) are glycerides, i.e., glycerol esters of organic acids. As has been pointed out, esters are derived from organic acids and alcohols in exactly the same way that mineral salts are derived from inorganic acids and metallic bases.
Glycerol is, however, a trihydric alcohol, i.e., it contains three replaceable (OH) groups. Its formula is C3H5(OH)3, or CH2OH·CHOH·CH2OH. Hence, three molecules of a monobasic acid are required to replace all of its (OH) groups.
For example,
It is theoretically possible, of course, to replace either one, two, or three of the (OH) groups in the glycerol with acid radicals, thus producing either mono-, di-, or triglycerides. If the primary alcohol groups in the glycerine molecule are designated by (1)
and the secondary one by (2), thus, CH2(1)OH·CH(2)OH·CH2(3)OH, it is conceivable that there may be either (1) or (2) monoglycerides, either (1, 1) or (1, 2) diglycerides, or a triglyceride, depending upon which of the (OH) groups are replaced. Compounds of all of these types have been produced by combinations of glycerol with varying proportions of organic acids under carefully controlled conditions; and all of them found to possess fat-like properties. All natural fats are triglycerides, however. Most natural fats are mixtures of several different triglycerides in each of which the three (OH) groups of the glycerol has been replaced by the same organic acid radical, as in the example of stearin shown above. But recent investigations have shown that some of the common animal fats, and perhaps some plant oils, may be made up of mixed glycerides, i.e., those in which the different (OH) groups have been replaced by different acid groups, as oleo-stearin, oleo-stearo-palmitin, etc.