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:—
| I. Fatty Oils or Liquid Fats | |||||
| | A. Vegetable oils. | | B. Animal oils. | | |
| 1. Drying oils. 2. Semi-drying oils. 3. Non-drying oils. | 1. Marine animal oils. (a) Fish oils. (b) Liver oils. (c) Blubber oils. 2. Terrestrial animal oils. | ||||
| II. Solid Fats | |||||
| A. Vegetable fats. | B. Animal fats. | | |||
| 1. Drying fats. 2. Semi-drying fats. 3. Non-drying fats | |||||
Physical Properties.—The specific gravities of oils and fats vary between the limits of 0·910 and 0·975. The lowest specific gravity is owned by the oils belonging to the rape oil group—from 0·913 to 0·916. The specific gravities of most non-drying oils lie between 0·916 and 0·920, and of most semi-drying oils between 0·920 and 0·925, whereas the drying oils have specific gravities of about 0·930. The animal and vegetable fats possess somewhat higher specific gravities, up to 0·930. The high specific gravity, 0·970, is owned by castor oil and cacao butter, and the highest specific gravity observed hitherto, 0·975, by Japan wax and myrtle wax.
In their liquid state oils and fats easily penetrate into the pores of dry substances; on paper they leave a translucent spot—“grease spot”—which cannot be removed by washing with water and subsequent drying. A curious fact, which may be used for the detection of the minutest quantity of oils and fats, is that camphor crushed between layers of paper without having been touched with the fingers rotates when thrown on clean water, the rotation ceasing immediately when a trace of oil or fat is added, such as introduced by touching the water with a needle which has been passed previously through the hair.
The oils and fats are practically insoluble in water. With the exception of castor oil they are insoluble in cold alcohol; in boiling alcohol somewhat larger quantities dissolve. They are completely soluble in ether, carbon bisulphide, chloroform, carbon tetrachloride, petroleum ether, and benzene. Oils and fats have no distinct melting or solidifying point. This is not only due to the fact that they are mixtures of several glycerides, but also that even pure glycerides, such as tristearin, exhibit two melting-points, a so-called “double melting-point,” the triglycerides melting at a certain temperature, then solidifying at a higher temperature to melt again on further heating. This curious behaviour was looked upon by Duffy as being due to the existence of two isomeric modifications, the actual occurrence of which has been proved (1907) in the case of several mixed glycerides.
The freezing-points of those oils which are fluid at the ordinary temperature range from a few degrees above zero down to –28° C. (linseed oil). At low temperatures solid portions—usually termed “stearine”—separate out from many oils; in the case of cotton-seed oil the separation takes place at 12° C. These solid portions can be filtered off, and thus are obtained the commercial “demargarinated oils” or “winter oils.”
Oils and fats can be heated to a temperature of 200° to 250° C. without undergoing any material change, provided prolonged contact with air is avoided. On being heated above 250° up to 300° some oils, like linseed oil, safflower oil, tung oil (Chinese or Japanese wood oil) and even castor oil, undergo a change which is most likely due to polymerization. In the case of castor oil solid products are formed. Above 300° C. all oils and fats are decomposed; this is evidenced by the evolution of acrolein, which possesses the well-known pungent odour of burning fat. At the same time hydrocarbons are formed (see Petroleum).
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 property of oils and fats of being readily hydrolysed is a most important one, and very extensive use of it is made in the arts (soap-making, candle-making and recovery of their by-products). If oils and fats are treated with water alone under high pressure (corresponding to a temperature of about 220° C.), or in the presence of water with caustic alkalis or alkaline earths or basic metallic oxides (which bodies act as “catalysers”) at lower pressures, they are converted in the first instance into free fatty acids and glycerin. If an amount of the bases sufficient to combine subsequently with the fatty acids be present, then the corresponding salts of these fatty acids are formed, such as sodium salts of fatty acids (hard soap) or potassium salts of the fatty acids (soft soap), soaps of the alkaline earth (lime soap), or soaps of the metallic oxides (zinc soap, &c.). The conversion of the glycerides (triglycerides) into fatty acids and glycerin must be looked upon as a reaction which takes place in stages, one molecule of a triglyceride being converted first into diglyceride and one molecule of fatty acid, the diglyceride then being changed into monoglyceride, and a second molecule of fatty acid, and finally the monoglyceride being converted into one molecule of fatty acid and glycerin. All these reactions take place concurrently, so that one molecule of a diglyceride may still retain its ephemeral existence, whilst another molecule is already broken up completely into free fatty acids and glycerin.
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 machineny, is required for these processes, this method has some fascination for
