Oils vary much in their tendency to “dry,” or become converted into solid or sticky resin-like substances. This tendency is greatest in some of the seed oils, and least in olive oil, and the oily part of animal fats (tallow oil, neatsfoot oil). Sperm oil, a “liquid wax,” is also very free from this tendency, but all fish oils possess it in a greater or less degree. It is not due to evaporation, but to the absorption of oxygen by the fatty acid. The tendency to oxygen-absorption, and consequently to drying (and, in the case of leather-oils, to “spueing”), is measured analytically by the “iodine-value,” the absorption of iodine being proportional to that of oxygen, while it is much more easily measured.
There are no simple tests by which the purity of oils can be determined, though in a few cases the presence of particular oils can be detected. The mixing and adulteration of oils is now a science, and those who practise it are well acquainted with the customary tests, and take care to adjust their mixtures so as to meet them. Taste and smell however, with practice, often furnish useful indications.
Natural oils and fats are invariably mixtures of the glycerides of several fatty acids, and their qualities depend simply on the character of these glycerides and the proportions in which they are mixed. The fatty acids form several groups, differing in their degree of “saturation,”[161] or, inversely, in their power of taking up oxygen, on which their tendency to drying depends. The members of any one of these groups resemble each other strongly, differing principally in melting points, density, and other physical characteristics.
[161] A “saturated” compound is one, the constituents of which are present in such proportions that all the combining affinities of each are satisfied by the others. Iodine value, see L.I.L.B., p. 176, and Jour. Soc. Ch. Ind., 1902, p. 454.
Saturated Fatty Acids.—Stearic acid, C18H35O.OH, and palmitic acid, C16H31O.OH, are the most important. At ordinary temperatures they are hard, white, crystalline bodies, and melt at 69° and 62° C. respectively. They do not, under ordinary circumstances, absorb any oxygen, nor iodine, and are very little liable to chemical change. Together with oleic acid, they are the principal acids of tallow and other animal fats, while palmitic acid and some lower members of the same group are more common in vegetable oils. Free stearic acid is an important constituent of the “distilled stearines” used in currying; while “oleostearine” consists mainly of the neutral fats or glycerides of stearic and palmitic acids.
Liquid Fatty Acids, Non-drying.—Of these, oleic acid is much the most common and important; its glyceride, olein, forming the liquid part of animal fats, and being the principal constituent of vegetable non-drying oils. Olive oil consists almost entirely of olein, with a little palmitin. The formula of oleic acid is C18H33O.OH, thus differing from stearic acid in having two less atoms of hydrogen. The “bonds” or affinities corresponding to these two atoms are linked together, but can separate, and attach two atoms of iodine, bromine, or chlorine, or one of oxygen. The iodine-value of pure olein is 83·9 (that is, 100 grm. absorb 83·9 grm. iodine); and that of olive oil about 83. Any oil with a higher “iodine-value” than olein must contain drying oils, though a lower value does not necessarily indicate their absence, if palmitin or other saturated acids are also present.
Unsaturated Liquid Fatty Acids.—Of these there are several groups, differing in their degree of saturation, and also probably in their structure. Their glycerides, together with olein, and sometimes palmitin, are the constituents of the seed oils, the drying tendency of which depends on their proportion of unsaturated acids, and the particular group to which they belong. The fish oils contain a peculiar group of unsaturated acids, together with olein, and usually stearin and palmitin, like the other animal fats. Linolenic acid, C18H29O.OH, one of the acids of linseed oil, has six hydrogen atoms less than stearic acid, and therefore three double linkings, and will take up six atoms of iodine. Its theoretical iodine-value is 274, while linseed oil itself often has an iodine-value exceeding 180. The iodine-value of cod-liver oil is sometimes nearly as high. Both oils therefore contain other acids less unsaturated than linolenic.
The “spueing” of leather is due to the absorption of oxygen and consequent resinification of the oils, and therefore all drying oils, however pure, are capable of producing it, though some are more liable to do so than others (cp. [pp. 363], [365], [366], [368], [390]).
Linolenic acid, and probably other allied acids, become converted by absorption of oxygen into solid varnish-like substances, which are important to the tanner, as furnishing the principal constituents of japans for leather. The unsaturated acids of fish oils seldom give hard varnishes, though menhaden oil ([page 367]) is sometimes used as paint-oil for outside work.
Most fats are liable to become rancid by exposure to the air, acquiring a disagreeable taste and smell, and an acid reaction from the liberation of the fatty acids. The changes which take place are somewhat complex.