This process, since it yields soap as one of its products, is called "saponification." All fats, when saponified, yield soaps and either glycerol or (more rarely) some of the other alcohols which are described below.
Glycerine is also prepared from fats by hydrolysis with superheated steam. Using olein, a glyceride which is present in olive oil and many common fats, as the example in this case, the equation for the reaction is:
| C3H5(C17H33·COO)3 | + | 3H2O | = | 3C17H33·COOH | + | C3H5(OH)3 |
| Olein | Steam | Oleic acid | Glycerol |
In this case the free fatty acid, instead of a soap, is the product which is obtained in addition to glycerol.
In the equations presented above, a single glyceride has been used as the example in each case. In the saponification, or hydrolysis, of natural fats and oils which, as has been shown, are mixtures of many glycerides, the resultant soaps, or fatty acids, are mixtures of as many compounds as there were individual glycerides of the original fat, but the glycerol is identical in every case.
When glycerol is heated with dehydrating agents, it is easily converted into acrolein, an unsaturated aldehyde having a peculiar characteristic pungent odor. Hence, the presence of glycerol, or glycerides, in any substance may usually be detected by mixing the material with anhydrous acid potassium sulfate and heating the mixture in a test tube, when the characteristic odor of acrolein will appear.
Glycerol possesses all the characteristic properties of an alcohol, forming alcoholates with alkalies, esters with acids, etc. It is an active reducing agent, being itself easily oxidized to a variety of different products depending upon the strength of the oxidizing agent used and the conditions of the experiment. Microorganisms affect it in a variety of ways, either converting it into simple fatty acids, or condensing it into longer-chain compounds.
Open-chain monohydric alcohols, higher members of the ethyl alcohol series, such as cetyl, C16H33OH, carnaubyl, C24H49OH, ceryl, C26H53OH, and melissyl, C30H61OH, are found in the esters which constitute the major proportion of the common waxes.
Cholesterol and phytosterol are empirical names for certain closed-ring, monohydric alcohols which are found in relatively small amounts in all fats, the former term designating those found in animal fats and the latter those of plant origin. Their composition has not yet been definitely established. They are known to contain two, or three, closed rings, probably of the phenanthrene type; to form dichlor- and dibrom- addition products, showing that they contain one side-chain double linkage; and to yield ketones when oxidized, indicating that they are secondary alcohols. They form acetyl esters, or acetates, which can be separated from each other and identified by their crystal forms and melting points. Because of this fact and of the further fact that they are present in detectable quantities in practically all fats and oils, they afford a qualitative means of distinguishing between fats of animal and of plant origin. This possibility is the most interesting fact known concerning these complex alcohols; although their presence as esters in all plant and animal fats indicates that they must have some biological function.
Phytosterol is not a single alcohol, but a mixture of at least two, which have been separated and studied as sitosterol, C27H43OH, and stigmasterol, C30H49OH. As has been said, these are found in small proportions in all vegetable fats, being present in largest amounts in oily seeds, especially those of the legumes.