The handbook of soap manufacture

by W. H. Simmons; H. A. Appleton

Practically all the oils and fats used in soap-making consist of mixtures of these compounds of glycerol with fatty acids, which invariably occur in nature in the form of triglycerides.

It was formerly considered that the three acid radicles in any naturally occurring glyceride were identical, corresponding to the formula—

CH₂OR
|
CHOR
|
CH₂OR

where R denotes the acid radicle. Recent work, however, has shown the existence of several so-called mixed glycerides, in which the hydroxyls of the same molecule of glycerol are displaced by two or sometimes three different acid radicles.

The first mixed glyceride to be discovered was oleodistearin, C₃H₅(OC₁₈H₃₅O)(OC₁₈H₃₅O)₂, obtained by Heise in 1896 from Mkani fat. Hansen has since found that tallow contains oleodipalmitin, C₃H₅(OC₁₈H₃₅O)(OC₁₆H₃₁O), stearodipalmitin, C₃H₅(OC₁₈H₃₅O)(OC₁₆H₃₁O), oleopalmitostearin, C₃H₅(OC₁₈H₃₃O)(OC₁₆H₃₁O) (OC₁₈H₃₅O) and palmitodistearin, CH(OC₁₆H₃₁O)(OC₁₈H₃₅O)₂, the latter of which has also been obtained by Kreis and Hafner from lard, while Holde and Stange have shown that olive oil contains from 1 to 2 per cent. of oleodidaturin, C₃H₅(OC₁₈H₃₃O)(OC₁₇H₃₃O)₂, and Hehner and Mitchell have obtained indications of mixed glycerides in linseed oil (which they consider contains a compound of glycerol with two radicles of linolenic acid and one radicle of oleic acid), also in cod-liver, cod, whale and shark oils.

In some cases the fatty acids are combined with other bases than glycerol. As examples may be cited beeswax, containing myricin or myricyl palmitate, and spermaceti, consisting chiefly of cetin or cetyl palmitate, and herein lies the essential difference between fats and waxes, but as these substances are not soap-making materials, though sometimes admixed with soap to accomplish some special object, they do not require further consideration.

The principal pure triglycerides, with their formulæ and chief constants, are given in the following table:—

Glyceride.	Formula.	Chief Occurrence.	Melting Point, °C.	Refractive Index at 60° C.	Saponification Equivalent.
Butyrin	C3H5(O.C4H7O)3	Butter fat	Liquid at -60	1.42015	100.7
Isovalerin	C3H5(O.C5H9O)3	Porpoise, dolphin	...	...	114.7
Caproin	C3H5(O.C6H11O)3	Cocoa-nut and palm-nut oils	-25	1.42715	128.7
Caprylin	C3H5(O.C8H15O)3	Do. do.	-8.3	1.43316	156.7
Caprin	C3H5(O.C10H19O)3	Do. do.	31.1	1.43697	184.7
Laurin	C3H5(O.C12H23O)3	Do. do.	45	1.44039	212.7
Myristin	C3H5(O.C14H27O)3	Nutmeg butter	56.5	1.44285	240.7
Palmitin	C3H5(O.C16H31O)3	Palm oil, lard	63-64	...	268.7
Stearin	C3H5(O.C18H35O)3	Tallow, lard, cacao butter	71.6	...	296.7
Olein	C3H5(O.C18H33O)3	Olive and almond oils	Solidifies at -6	...	294.7
Ricinolein	C3H5(O.C18H33O2)3	Castor oil	...	...	310.7

Of the above the most important from a soap-maker's point of view are stearin, palmitin, olein and laurin, as these predominate in the fats and oils generally used in that industry. The presence of stearin and palmitin, which are solid at the ordinary temperature, gives firmness to a fat; the greater the percentage present, the harder the fat and the higher will be the melting point, hence tallows and palm oils are solid, firm fats. Where olein, which is liquid, is the chief constituent, we have softer fats, such as lard, and liquid oils, as almond, olive and cotton-seed.