The handbook of soap manufacture - W. H. Simmons; H. A. Appleton

Stearin (Tristearin) can be prepared from tallow by crystallisation from a solution in ether, forming small crystals which have a bright pearly lustre. The melting point of stearin appears to undergo changes and suggests the existence of distinct modifications. When heated to 55° C. stearin liquefies; with increase of temperature it becomes solid, and again becomes liquid at 71.6° C. If this liquid be further heated to 76° C., and allowed to cool, it will not solidify until 55° C. is reached, but if the liquid at 71.6° C. be allowed to cool, solidification will occur at 70° C.

Palmitin (Tripalmitin) may be obtained by heating together palmitic acid and glycerol, repeatedly boiling the resulting product with strong alcohol, and allowing it to crystallise. Palmitin exists in scales, which have a peculiar pearly appearance, and are greasy to the touch. After melting and solidifying, palmitin shows no crystalline fracture; when heated to 46° C. it melts to a liquid which becomes solid on further heating, again liquefying when 61.7° C. is reached, and becoming cloudy, with separation of crystalline particles. At 63° C. it is quite clear, and this temperature is taken as the true melting point. It has been suggested that the different changes at the temperatures mentioned are due to varying manipulation, such as rate at which the temperature is raised, and the initial temperature of the mass when previously cool.

Olein (Triolein) is an odourless, colourless, tasteless oil, which rapidly absorbs oxygen and becomes rancid. It has been prepared synthetically by heating glycerol and oleic acid together, and may be obtained by submitting olive oil to a low temperature for several days, when the liquid portion may be further deprived of any traces of stearin and palmitin by dissolving in alcohol. Olein may be distilled in vacuo without decomposition taking place.

Laurin (Trilaurin) may be prepared synthetically from glycerol and lauric acid. It crystallises in needles, melting at 45°-46° C., which are readily soluble in ether, but only slightly so in cold absolute alcohol. Scheij gives its specific gravity, d60°/4° = 0.8944. Laurin is the chief constituent of palm-kernel oil, and also one of the principal components of cocoa-nut oil.

Fatty Acids.—When a fat or oil is saponified with soda or potash, the resulting soap dissolved in hot water, and sufficient dilute sulphuric acid added to decompose the soap, an oily layer gradually rises to the surface of the liquid, which, after clarifying by warming and washing free from mineral acid, is soluble in alcohol and reddens blue litmus paper. This oily layer consists of the "fatty acids" or rather those insoluble in water, acids like acetic, propionic, butyric, caproic, caprylic and capric, which are all more or less readily soluble in water, remaining for the most part dissolved in the aqueous portion. All the acids naturally present in oils and fats, whether free or combined, are monobasic in character, that is to say, contain only one carboxyl—CO.OH group. The more important fatty acids may be classified according to their chemical constitution into five homologous series, having the general formulæ:—

I. Stearic series C_nH_2n+1COOH
II. Oleic series C_nH_2n-1COOH
III. Linolic series C_nH_2n-3COOH
IV. Linolenic series C_nH_2n-5COOH
V. Ricinoleic series C_nH_2n-7COOH

I. Stearic Series.—The principal acids of this series, together with their melting points and chief sources, are given in the following table:—

Acid.	Formula.	Melting Point, °C.	Found in
Acetic	CH₃COOH	17	Macassar oil.
Butyric	C₃H₇COOH	...	Butter, Macassar oil.
Isovaleric	C₄H₉COOH	...	Porpoise and dolphin oils.
Caproic	C₅H₁₁COOH	...	Butter, cocoa-nut oil.
Caprylic	C₇H₁₅COOH	15	Butter, cocoa-nut oil, Limburg cheese.
Capric	C₉H₁₉COOH	30	Butter, cocoa-nut oil.
Lauric	C₁₁H₂₃COOH	44	Cocoa-nut oil, palm-kernel oil.
Ficocerylic	C₁₂H₂₅COOH	...	Pisang wax.
Myristic	C₁₃H₂₇COOH	54	Nutmeg butter, liver fat, cocoa-nut oil, dika fat, croton oil.
Palmitic	C₁₅H₃₁COOH	62.5	Palm oil, most animal fats.
Daturic	C₁₆H₃₃COOH	...	Oil of Datura Stramonium.
Stearic	C₁₇H₃₅COOH	69	Tallow, lard, most solid animal fats.
Arachidic	C₁₉H₃₉COOH	75	Arachis or earth-nut oil, rape and mustard-seed oils.
Behenic	C₂₁H₄₃COOH	...	Ben oil, black mustard-seed oil, rape oil.
Lignoceric	C₂₃H₄₇COOH	80.5	Arachis oil.
Carnaubic	C₂₃H₄₇COOH	...	Carnauba wax.
Pisangcerylic	C₂₃H₄₇COOH	...	Pisang wax.
Hyænic	C₂₄H₄₉COOH	...	Hyæna fat.
Cerotic	C₂₅H₅₁COOH	78	Beeswax, China wax, spermaceti.
Melissic	C₂₉H₅₉COOH	89	Beeswax.
Psyllostearylic	C₃₂H₆₅COOH	...	Psylla wax.
Theobromic	C₆₃H₁₂₇COOH	...	Cacao butter

Medullic and margaric acids, which were formerly included in this series, have now been shown to consist of mixtures of stearic and palmitic, and stearic palmitic and oleic acids respectively.

The acids of this group are saturated compounds, and will not combine directly with iodine or bromine. The two first are liquid at ordinary temperatures, distil without decomposition, and are miscible with water in all proportions; the next four are more or less soluble in water and distil unchanged in the presence of water, as does also lauric acid, which is almost insoluble in cold water, and only slightly dissolved by boiling water. The higher acids of the series are solid, and are completely insoluble in water. All these acids are soluble in warm alcohol, and on being heated with solid caustic alkali undergo no change.