Fatty Oils and Fats

The amount of unsaponifiable matter in an oil or fat does not, as a rule, exceed 2%, but in the case of some shark-liver oils the proportion is abnormally high, as large quantities (up to 85%) of highly unsaturated hydrocarbons, such as spinacene and squalene are present ; the relationship of these bodies to the glycerides normally occurring in oils, including fish-body oils, is still a matter for research.

Classification.

Following subdivision into animal and vege table oils and fats, a second guiding principle to classification is afforded by a consideration of the amount of iodine the various fats are capable of absorbing. This is a measure of the unsat urated acids present, since iodine is absorbed (under suitable conditions) at the ethenoid linkages of the molecule of fatty acid.

About 8o of the natural fats are commonly utilised to a greater or less extent ; the annual world consumption of each of about 20 of these approaches or exceeds i00,000 tons, and in the case of some half dozen is in the neighbourhood of i,000,000 tons. They are classified as follows : Solid Vegetable Fats.—These are found mainly in the kernels and seeds of tropical fruits. They melt at from 2o-35° C, and contain in general but little combined oleic acid, the bulk of the component fatty acids being lauric, myristic and palmitic. Fruits of many of the palm family, notably coconut oil and palm kernel oil, contain very large amounts of combined lauric acid. Palm oil contains chiefly palmitic and oleic glycerides. All these fats, when suitably refined, are used for edible purposes, and many are used in the manufacture of the higher grades of toilet and other soaps.

Solid Animal Fats.

A most important group of animal fats is that of milk fats, which are used universally as butter. All milk fats are characterized by the presence of definite, if relatively small, quantities of fatty acids which are known respectively as butyric, caproic and caprylic acids.

Physical Properties.

The specific gravities of oils and fats range from 0.913 (rape oil) to 0.975 ( Japan wax, myrtle wax); for the bulk of the fats the value is between 0.915 and 0.945. Some oils, notably those of the castor oil and chaulmoogra groups, rotate the plane of polarisation of light.

The oils and fats are practically insoluble in water; with the exception of oils of the castor oil group they are insoluble in cold, and only sparingly soluble in boiling, alcohol. They are com pletely soluble in ether, carbon disulphide, chloroform, carbon tetrachloride, petroleum ether and benzene. Oils and fats have no distinct melting or solidifying point ; this is not solely due to the fact that they are mixtures of several glycerides, for even pure glycerides exhibit the phenomenon of a "double melting point." The freezing points of the oils range from a few degrees

above zero down to —28° C (linseed oil). At low temperatures (e.g., at 12° C in the case of cottonseed oil), solid portions, usually termed "stearine" separate out from many oils. These solid portions can be filtered off; the filtrates constitute the commercial "demarginerated oils" or "winter oils" which will remain limpid at low temperatures.

Oils and fats can be heated to a temperature of 20o° to 250° C without undergoing any material change, provided prolonged con tact with air is avoided. On being heated above 250° up to 30o° C some oils, e.g., linseed oil, safflower oil, tung oil (Chinese or Japanese wood oil, q.v.) and even castor oil, undergo a change, probably due to polymerisation, resulting in the formation of semi-solid or solid products. Above 300° C fats are decom posed; this is evidenced by the evolution of acrolein (a decom position product of glycerin), which possesses the pungent odour of burning fat. Hydrocarbons are formed at the same time (see PETROLEUM).

On exposure to the air oils and fats gradually undergo certain changes. The drying oils absorb oxygen ("dry") somewhat rap idly, thin layers forming a skin or film. Extensive use of this property is made in the paint and varnish trades. The semi-drying oils absorb oxygen more slowly than the drying oils and are, therefore, useless as paint oils; still, in course of time, sufficient oxygen is absorbed to produce distinct thickening. The oxidation of the semi-drying oils is accelerated by spreading such oils over a large surface, notably over woollen or cotton fibres, when oxygen absorption may proceed so rapidly that spontaneous inflammation ensues. Many fires in cotton and woollen mills have been caused in this way. The non-drying oils, of which olive oil is typical, do not become oxidised readily on exposure to the air, although gradually changes take place, including slow hydrolysis (splitting to fatty acids and glycerol) and subsequent oxidation; the oils thicken slightly and acquire the peculiar disagreeable smell and acrid taste defined by the term "rancidity." The chemical reac tions involved in the development of rancidity have not yet been fully explained. If the action of the air and moisture is allowed free play, the hydrolysis of the oils and fats may become so com plete that only the insoluble fatty acids remain, the glycerol being washed away. This is exemplified by adipocere (corpse fat) and also by Irish bog butter, which consists chiefly of free fatty acids.