The saponification of esters of cholesterol and phytosterol is a difficult and unsatisfactory process; but since this affords the only known means to distinguish between fats of plants and of animal origin, its technique has been fairly well worked out, and the process used in the study of the changes which take place in plant fats when they are used by animals as food.
HYDROLYSIS AND SYNTHESIS OF FATS
The reaction for the hydrolysis of fats has been discussed in connection with the process for the manufacture of glycerine. This reaction takes place very slowly with cold water alone, can be easily brought about by the action of superheated steam, and much more easily and rapidly in the presence of some catalyst (sulfuric acid is an especially effective catalyst for this purpose).
Fats can be artificially synthetized by heating mixtures of glycerol and fatty acids, under considerable pressure, for some time at temperatures of 200° to 240° C.; or by heating a mixture of the disulfuric ester of glycerol with a fatty acid dissolved in sulfuric acid. Recently, fatty acids have been prepared from carbohydrates, by first breaking the hexoses down into three-carbon compounds, then carefully oxidizing these to pyruvic acid, CH3·CO·COOH, which can then be condensed into acids having longer chains. The violent reagents and long-continued processes which must be employed for the artificial hydrolysis or synthesis of the fats are in sharp contrast with the easy and rapid transition of carbohydrates to fats, and vice versa, which take place in both plant and animal nutrition.
THE EXTRACTION OF OILS FROM PLANT TISSUES
There are three types of methods which are employed for the extraction of oil from oil-bearing seeds, etc., either as a commercial industry or for the purposes of scientific study. These are (1) by pressure; (2) extraction with volatile solvents; and (3) boiling the crushed seeds or fruits with water.
By the first method, the seeds are first cleaned, then "decorticated" (hulls removed), crushed or ground, then subjected to intense pressure in an hydraulic press. In the commercial process, the ground seeds are first pressed at ordinary temperature, which yields "cold-drawn" oil, then the press cake is heated and pressed again, whereby "hot-drawn" oil is obtained. The crude oil is refined by heating it to coagulate any albumin which it may contain, and is sometimes bleached by different processes before it is marketed. The press cake from many seeds, such as flaxseed (linseed), cottonseed, etc., is ground up and sold for use as stock feed.
In the second method, the finely crushed seeds are treated with solvents such as gasoline or carbon bisulfide, in an apparatus which is so arranged that the fresh material is treated first with solvent which has already passed through various successive lots of material and has become highly charged with the oil, followed by other portions which contain less oil, and finally by fresh solvent, whereby the last traces of oil are removed from the material. The saturated solvent is transferred to suitable boilers and the solvent distilled off and condensed for repeated use, leaving the oil in the boiler in very pure form.
Extraction by boiling with water is sometimes used in the preparation of castor oil and olive oil. In such cases, the crushed seeds are boiled with water and the oil skimmed off as fast as it rises to the surface.