is driven off and recovered by condensing the vapours in a cooling coil, to be used again. The last remnant of volatile solvent in the oil is driven off by a current of open steam blown through the oil in the warm state. The extracting process in the hot is carried out in apparatus, the principle of which is exemplified by the well-known Soxhlet extractor. The comminuted seed is placed inside a vessel connected with an upright refrigerator on trays or baskets, and is surrounded there by the volatile solvent. On heating the solvent with steam through a coil or jacket, the vapours rise through and around the meal. They pass into the refrigerator, where they are condensed and fall back as a condensed liquid through the meal, percolating it as they pass downwards, and reaching to the bottom of the vessel as a more or less saturated solution of oil in the solvent. The solvent is again evaporated, leaving the oil at the bottom of the vessel until the extraction is deemed finished. The solution of fat is then run off into a still, as described already, and the last traces of solvent are driven out. The solvent is recovered and used again.
With regard to the merits and demerits of the last two mentioned processes—expression and extraction—the adoption of either will largely depend on local conditions and the objects for which the products are intended. Wherever the cake is the main product, expression will commend itself as the most advantageous process. Where, however, the fatty material forms the main product, as in the case of palm kernel oil, or sesame and coco-nut oils from damaged seeds (which would no longer yield proper cattle food), the process of extraction will be preferred, especially when the price of oils is high. In some cases the combination of the two processes commends itself, as in the case of the production of olive oil. The fruits are expressed, and after the edible qualities and best class of oils for technical purposes have been taken off by expression, the remaining pulp is extracted by means of solvents. This process is known under the name of mixed process (huilerie mixte).
Refining and Bleaching.—The oils and fats prepared by any of the methods detailed above are in their fresh state, and, if got from perfectly fresh (“sweet”) material, practically neutral. If care be exercised in the process of rendering animal oils and fats or expressing oils in the cold, the products are, as a rule, sufficiently pure to be delivered to the consumer, after a preliminary settling has allowed any mucilaginous matter, such as animal or vegetable fibres or other impurities, and also traces of moisture, to separate out. This spontaneous clarification was at one time the only method in vogue. This process is now shortened by filtering oils through filter presses, or otherwise brightening them, e.g. by blowing with air. In many cases these methods still suffice for the production of commercial oils and fats.
In special cases, such as the preparation of edible oils and fats, a further improvement in colour and greater purity is obtained by filtering the oils over charcoal, or over natural absorbent earths, such as fuller’s earth. Where this process does not suffice, as in the case of coco-nut oil or palm kernel oil, a preliminary purification in a current of steam must be resorted to before the final purification, described above, is carried out. Oils intended for use on the table which deposit “stearine” in winter must be freed from such solid fats. This is done by allowing the oil to cool down to a low temperature and pressing it through cloths in a press, when a limpid oil exudes, which remains proof against cold—“winter oil.” Most olive oils are naturally non-congealing oils, whereas the Tunisian and Algerian olive oils deposit so much “stearine” that they must be “demargarinated.” Similar methods are employed in the production of lard oil, edible cotton-seed oil, &c. For refining oils and fats intended for edible purposes only the foregoing methods, which may be summarized by the name of physical methods, can be used; the only chemicals permissible are alkalis or alkaline earths to remove free fatty acids present. Treatment with other chemicals renders the oils and fats unfit for consumption. Therefore all bleaching and refining processes involving other means than those enumerated can only be used for technical oils and fats, such as lubricating oils, burning oils, paint oils, soap-making oils, &c.
Bleaching by the aid of chemicals requires great circumspection. There is no universal method of oil-refining applicable to any and every oil or fat. Not only must each kind of oil or fat be considered as a special problem, but frequently even varieties of one and the same oil or fat are apt to cause the same difficulties as would a new individual. In many cases the purification by means of sulphuric acid, invented and patented by Charles Gower in 1792 (frequently ascribed to Thénard), is still usefully applied. It consists in treating the oil with a small percentage of a more or less concentrated sulphuric acid, according to the nature of the oil or fat. The acid not only takes up water, but it acts on the suspended impurities, carbonizing them to some extent, and thus causing them to coagulate and fall down in the form of a flocculent mass, which carries with it mechanically other impurities which have not been acted upon. This method is chiefly used in the refining of linseed and rape oils. Purification by means of strong caustic soda was first recommended as a general process by Louis C. Arthur Barreswil, his suggestion being to heat the oil and add 2% to 3% of caustic soda. In most cases the purification consisted in removing the free fatty acids from rancid oils and fats, the caustic soda forming a soap with the fatty acids, which would either rise as a scum and lift up with it impurities, or fall to the bottom and carry down impurities. This process is a useful one in the case of cotton-seed oil. As a rule, however, it is a very precarious one, since emulsions are formed which prevent in many cases the separation of oil altogether. After the treatment with sulphuric acid or caustic soda, the oils must be washed to remove the last traces of chemicals. The water is then allowed to settle out, and the oils are finally filtered. The number of chemicals which have been proposed from time to time for the purification of oils and fats is almost legion, and so long as the nature of oils and fats was little understood, a secret trade in oil-purifying chemicals flourished. With our present knowledge most of these chemicals may be removed into the limbo of useless things. The general methods of bleaching besides those mentioned already as physical methods, viz. filtration over charcoal or bleaching earth, are chiefly methods based on bleaching by means of oxygen or by chlorine. The methods of bleaching by oxygen include all those which aim at the bleaching by exposure to the air and to sunlight (as in the case of artists’ linseed-oil), or where oxygen or ozone is introduced in the form of gas or is evolved by chemicals, as manganese dioxide, potassium bichromate or potassium permanganate and sulphuric acid. In the process of bleaching by means of chlorine either bleaching powder or bichromates and hydrochloric acid are used. It must again be emphasized that no general rule can be laid down as to which process should be employed in each given case. There is still a wide field open for the application of proper processes for the removal of impurities and colouring matters without running the risk of attacking the oil or fat itself.
Oil Testing.—Reliable scientific methods for testing oils and fats date back only to the end of the ’seventies of the 19th century. Before that time it was believed that not only could individual oils and fats be distinguished from each other by colour reactions, but it was also maintained that falsification could be detected thereby. With one or two exceptions (detection of sesame oil and perhaps also of cotton-seed oil) all colour reactions are entirely useless. The modern methods of oil testing rest chiefly on so-called “quantitative” reactions, a number of characteristic “values” being determined which, being based on the special nature of the fatty acids contained in each individual oil or fat, assist in identifying them and also in revealing adulteration. These “values,” together with other useful methods, are enumerated in the order of their utility for the purposes of testing.
The saponification value (saponification number) denotes the number of milligrams which one gramme of an oil or fat requires for saponification, or, in other words, for the neutralization of the total fatty acids contained in an oil or fat. We thus measure the alkali absorption value of all fatty acids contained in an oil or fat. The saponification values of most oils and fats lie in the neighbourhood of 195. But the oils belonging to the rape oil group are characterized by considerably lower saponification values, viz. about 175 on account of their containing notable quantities of erucic acid, C22H42O2. In the case of those oils which do not belong to the rape oils and yet show abnormally low saponification values, the suspicion is raised at once that a certain amount of mineral oils (which do not absorb
