The property of the fats of being readily hydrolysed is most important, and extensive use of it is made in the arts (soap making, candle manufacture, etc., and recovery of their by products). If treated with water alone under high pressure (corre sponding to a temperature of about 220° C), or in the presence of water with caustic alkalies, alkaline earths, or basic metallic ox ides (which act as catalysers), at lower pressures, they are con verted in the first instance into free fatty acids and glycerol, e.g.: If bases sufficient in amount to combine with the fatty acids be present. the corresponding salts of these acids are formed, such as the sodium salts of fatty acids (hard soap), or potassium salts (soft soaps), soaps of the alkaline earths (lime soap) or of the metallic oxides (zinc soap), etc. For detailed descriptions of the methods employed in commercial hydrolysis, see CANDLE, SOAP.
Rendering.—The crudest method of rendering oil 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 exposing them to 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 (q.v.) and the rendering of the best (Cochin) coconut oil by boiling the fresh kernels with water. Naturally these processes can only be applied to those seeds which contain large quantities of fatty matter, such as coconuts and olives. The rendering process is, however, applied on a very large scale to the production of animal fats, such as tallow (q.v.), lard, bone fat and whale oil (q.v.). The method consists essentially in cutting up the fatty matter into small fragments, which are transferred to vessels containing water, wherein the comminuted mass is heated by steam, either under atmospheric pressure in open vats or under higher pressure in digesters. The fat gradually exudes and col lects on top of the water, whilst the membranous matter, "greaves," falls to the bottom. The fat is drawn off the aqueous (gluey) layer and strained through sieves or filters. The greaves are placed in hair or woollen bags and submitted to hydraulic pres sure, by which a further portion of fat is obtained (cf. Pressing). In the case of animal fats intended for edible purposes, such as lard, suet for margarine, etc., the greatest cleanliness must, of course, be observed, and the temperature kept as low as possible in order to obtain a perfectly sweet and pure material. To obtain a harder product, fats, such as tallow, are frequently subjected to pressing to squeeze out the more liquid portions, which are sold as oleo oil, lard oil, etc.
Pressing.—The boiling out process cannot be applied to small seeds, such as linseed, rapeseed, etc. Whilst, perhaps, the most primitive method of expression was to crush the seed in mortars until the oil should exude, in the East, where vegetable oils form an important article of food and serve also for other domestic purposes, various ingenious applications of lever and of wedge presses have been used from the remotest times. A detailed de
scription is given by Pliny of the screw presses used by the Romans for the production of olive oil. At an early stage in history the Chinese employed the same series of operations which are followed in the most advanced mills of modern times, viz., bruising and reducing the seeds to meal under edge-stones, heating the meal in open pans and pressing out the oil in a wedge press. This primitive process is still carried out in Manchuria, in the production of soya bean cake and oil, one of the staple industries of that country. The Dutch or stamper press, invented in Hol land in the I 7th century, was almost exclusively employed in Europe for pressing oil-seeds until the early years of the r9th century; it yielded place to the hydraulic press which has prac tically superseded all other appliances for expression. The sequence of operations in treating oil-seeds, oil-nuts, etc., is as follows: As an important preliminary operation the seeds, etc., are freed from dust, sand, and other impurities by sifting; in the case of seeds amongst which are found pieces of iron (hammer heads among palm-kernels, etc.), the seeds are passed over magnetic separators. The seeds and nuts are then decorticated (where requisite), the shells removed and the kernels ("meats") con verted into a meal by grinding between finely grooved rollers. The comminuted mass, forming a more or less coarse meal, is either expressed in this state, or subjected to preliminary heating accord ing to the quality of product required. For the preparation of edible oils, the meal is packed in bags and expressed in hydraulic presses in the cold, under a pressure of 30o atmospheres or more. The cakes remain under pressure for about seven minutes. Oil expressed in the cold dissolves the least amount of colouring matter, etc., and hence has suffered least in quality; oils so ob tained are known in commerce as "cold-drawn oils," "cold pressed oils," "salad oils," "virgin oils." By pressing in the cold only part of the oil is recovered. A further quantity is obtained by pressing the meal at a somewhat elevated temperature, either after the cold-drawn oil has been taken off, or in the case of oleaginous seeds of low value, e.g., cottonseed, coconut, where it is of importance to extract as much oil as possible in one operation, immediately after the meal has left the mill. The process, which is general in application to all oil-seeds is described in some detail, with illustrations of typical hydraulic presses, in the article Coco-NuT OIL AND CAKE. Oil obtained from heated meal is usually more highly coloured and harsher to the taste than cold-drawn oil, more of the extractive substances being dissolved and intermixed with the oil. Such oils are hardly suitable for edible purposes and are chiefly used in manufacturing processes. According to the care exercised by the manufacturer and the range of temperature to which the seed is heated, various grades of oil are obtained. The residual meal is used as cattle-food, or if from poisonous seeds such as castor beans, as manure.