Page:EB1911 - Volume 20.djvu/67

 origin; the latter is characteristic of all oils and fats of animal origin. This important difference furnishes a method of distinguishing by chemical means vegetable oils and fats from animal oils and fats. This distinction will be made use of in the classification of the oils and fats. A second guiding principle is afforded by the different amounts of iodine (see Oil Testing below) the various oils and fats are capable of absorbing. Since this capacity runs parallel with one of the best-known properties of oils and fats, viz. the power of absorbing larger or smaller quantities of oxygen on exposure to the air, we arrive at the following classification:—

On exposure to the atmosphere, oils and fats gradually undergo certain changes. The drying oils absorb oxygen somewhat rapidly and dry to a film or skin, especially if exposed in a thin layer. 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, they absorb oxygen distinctly enough to become thickened. The property of the semi-drying oils to absorb oxygen is accelerated by spreading such oils over a large surface, notably over woollen or cotton fibres, when absorption proceeds so rapidly that frequently spontaneous combustion will ensue. Many fires in cotton and woollen mills have been caused thereby. The non-drying oils, the type of which is olive oil, do not become oxidized readily on exposure to the air, although gradually a change takes place, the oils thickening slightly and acquiring that peculiar disagreeable smell and acrid taste, which are defined by the term “rancid.” The changes conditioning rancidity, although not yet fully understood in all details, must be ascribed in the first instance to slow hydrolysis (“saponification”) of the oils and fats by the moisture of the air, especially if favoured by insolation, when water is taken up by the oils and fats, and free fatty acids are formed. The fatty acids so set free are then more readily attacked by the oxygen of the air, and oxygenated products are formed, which impart to the oils and fats the rancid smell and taste. The products of oxidation are not yet fully known; most likely they consist of lower fatty acids, such as formic and acetic acids, and perhaps also of aldehydes and ketones. If the fats and oils are well protected from air and light, they can be kept indefinitely. In fact C. Friedel has found unchanged triglycerides in the fat which had been buried several thousand years ago in the tombs of Abydos. If the action of air and moisture is allowed free play, the hydrolysis of the oils and fats may become so complete that only the insoluble fatty acids remain behind, the glycerin being washed away. This is exemplified by adipocere, and also by Irish bog butter, which consist chiefly of free fatty acids.

The oils and fats used in the industries are not drawn from any very great number of sources. The tables on the following pages contain chiefly the most important oils and fats together with their sources, yields and principal uses, arranged according to the above classification, and according to the magnitude of the iodine value. It should be added that many other oils and fats are only waiting improved conditions of transport to enter into successful competition with some of those that are already on the market.

Extraction.—Since the oils and fats have always served the human race as one of the most important articles of food, the oil and fat industry may well be considered to be as old as the human race itself. The methods of preparing oils and fats range themselves under three heads: (1) Extraction of oil by “rendering,” i.e. boiling out with water; (2) Extraction of oil by expression; (3) Extraction of oil by means of solvents.

Rendering.—The crudest method of rendering oils from seeds, still practised in Central Africa, in Indo-China and on some of the South Sea Islands, consists in heaping up oleaginous fruits and allowing them to melt by the heat of the sun, when the exuding oil runs off and is collected. In a somewhat improved form this process of rendering is practised in the preparation of palm oil, and the rendering the best (Cochin) coco-nut oil by boiling the fresh kernels with water. Since hardly any machinery, or only the simplest machinery, is required for these processes, this method has some fascination for