Since glycerol is a trihydric alcohol, i.e., contains three hydroxyl (OH) groups, the hydrogen atoms of which are displaceable by acid radicles, the above reaction may be supposed to take place in three stages. Thus, we may have:—

There are two possible forms of monoglyceride and diglyceride, according to the relative position of the acid radicle, these being termed alpha and beta respectively, and represented by the following formulæ, where R denotes the acid radicle:—

Monoglyceride:—

Diglyceride:—

According to the relative proportions of fatty acid and glycerol used, and the temperature to which they were heated, Berthelot succeeded in preparing mono-, di- and triglycerides of various fatty acids.

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:—