on to the last, when the grains are sufficiently bruised, crushed and ground. The distance between the rollers can be easily regulated so that the seed leaving the bottom roller has the desired fineness. The comminuted mass, forming a more or less coarse meal, is either expressed in this state or subjected to a preliminary heating, according to the quality of the product to be manufactured. For the preparation of edible oils and fats the meal is expressed in the cold, after having been packed into bags and placed in hydraulic presses under a pressure of three hundred atmospheres or even more. The cakes are allowed to remain under pressure for about seven minutes. The oil exuding in the cold dissolves the smallest amount of colouring matter, &c., and hence has suffered least in its quality. Oils so obtained are known in commerce as “cold drawn oils,” “cold pressed oils,” “salad oils,” “virgin oils.”
By pressing in the cold, obviously only part of the oil or fat is recovered. A further quantity is obtained by expressing the seed meal at a somewhat elevated temperature, reached by warming the comminuted seeds or fruits either immediately after they leave the five-roller mill, or after the “cold drawn oil” has been taken off. Of course the cold pressed cakes must be first disintegrated, which may be done under an edge-runner. The same operation may be repeated once more. Thus oils of the “second expression” and of the “third expression” are obtained.
In the case of oleaginous seeds of low value (cotton-seed, linseed) it is of importance to express in one operation the largest possible quantity of oil. Hence the bruised seed is, after leaving the five-roller mill, generally warmed at once in a steam-jacketed kettle fitted with a mixing gear, by passing steam into the jacket, and sending at the same time some steam through a rose, fixed inside the kettle, into the mass while it is being agitated. This practice is a survival of the older method of moistening the seed with a little water, while the seeds were bruised under edge-runners, so as to lower the temperature and facilitate the bursting of the cells. The warm meal is then delivered through measuring boxes into closed pressbags (“scourtins” of the “Marseilles” press), or through measuring boxes, combined with an automatic moulding machine, into cloths open at two sides (Anglo-American press), so that the preliminarily pressed cakes can be put at once into the hydraulic press. In the latest constructions of cage presses, the use of bags is entirely dispensed with, a measured-out quantity of seed falling direct into the circular press cage and being separated from the material forming the next cake by a circular plate of sheet iron. The essentials of proper oil pressing are a slowly accumulating pressure, so that the liberated oil may have time to flow out and escape, a pressure that increases in proportion as the resistance of the material increases, and that maintains itself as the volume of material decreases through the escape of oil.
Numerous forms of hydraulic presses have been devised. Horizontal presses have practically ceased to be used in this branch of industry. At present vertical presses are almost exclusively in vogue; the three chief types of these have been already mentioned. Continuously working presses (compression by a conical screw) have been patented, but hitherto they have not been found practicable. Of the vertical presses the Anglo-American type of press is most in use. It represents an open press fitted with a number (usually sixteen) of iron press plates, between which the cakes are inserted by hand. A hydraulic ram then forces the table carrying the cakes against a press-head, and the exuding oil flows down the sides into a tank below. The “Marseilles press” is largely used in the south of France. There the meal is packed by hand in “scourtins,” bags made of plaited coco-nut leaves—replacing the woollen cloths used in England. The packing of the press requires more manual labour than in the case of the Anglo-American press; moreover, the Marseilles press offers inconvenience in keeping the bags straight, and the pressure cannot be raised to the same height as in the more modern hydraulic presses. 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 they are chiefly used for manufacturing processes. According to the care exercised by the manufacturer in the range of temperature to which the seed is heated, various grades of oils are obtained.
In the case of those seeds which contain more than 40% of oil, such as arachis nuts and sesame seed, the first expression in pressbags leads to difficulty, as the meal causes “spueing,” i.e. the meal exudes and escapes from the press. Hence, in modern installations, the first expression of those seeds is carried out in so-called cage (clodding) presses, consisting of hydraulic presses provided with circular boxes or cages, into which the meal is filled. These cages or boxes are either constructed of metal staves held together by a number of steel rings, or consist of one cylinder having a large number of perforations. The presses having perforated cylinders, although presenting mechanically a more perfect arrangement, are not preferable to the press cages formed by staves, as the holes become easily clogged up by the meal, when the cylinder must be carefully cleaned out. Modern improvements, with a view to cheapening of cost, effect the transport of the cages from one press battery to another on rails. In order to dispense even with the charging of the presses by hand, in some systems the cages are first charged in a preliminary press, from which they are transferred mechanically by a swinging arrangement into the final press.
Whilst the meal is under pressure the oil works its way to the edge of the cake, whence it exudes. For this reason an oblong form is the most favourable one for the easy separation of the oil. The edges of the cakes invariably retain a considerable portion of oil; hence the soft edges are pared off, in the case of the oblong cake in a cake-paring machine, and the parings are returned to edge-runners, to be ground up and again pressed with fresh meal. Through the introduction of the cage (clodding) presses circular cakes have become fashionable, and as the material of these presses can be made much stronger and therefore higher pressure can be employed, more oil is expressed from the meal than in open presses. The oil flowing from the presses is caught in reservoirs placed under the level of the floor, from which it is pumped into storage tanks for settling and clarifying.
Extraction by Solvents.—The cakes obtained in the foregoing process still retain considerable proportions of oil, not less than 4 to 5%—usually, however, about 10%. If it be desired to obtain larger quantities than are yielded by the above-described methods, processes having for their object the extraction of the seeds by volatile solvents must be resorted to. Extraction by means of carbon bisulphide was first introduced in 1843 by Jesse Fisher of Birmingham. Thirteen years later E. Deiss of Brunswick again patented the extraction by means of carbon bisulphide (Eng. Pat. No. 390, 1856), and added “chloroform, ether, essences, or benzine or benzole” to the list of solvents. For several years afterwards the process made little advance, for the colour of the oils produced was higher and the taste much sharper. The oil retained traces of sulphur, which showed themselves disagreeably in the smell of soaps made from it, and in the blackening of substances with which it was used. Of course, the meal left by the process was so tainted with carbon bisulphide that it was absolutely out of the question to use the extracted meal as cattle food. With the improvement in the manufacture of carbon bisulphide, these drawbacks have been surmounted to a large extent, and the process of extracting with carbon bisulphide has specially gained much extension in the extraction of expressed olive marc in the south of France, in Italy and in Spain. Yet even now traces of carbon bisulphide are retained by the extracted meal, so that it is impossible to feed cattle with it. Carbon bisulphide is comparatively cheap, and it is heavier than water, hence there are certain advantages in storing so volatile and inflammable a liquid. But owing to the physiological effect carbon bisulphide has on the workmen, coupled with the chemical action of impure carbon bisulphide on iron which has frequently led to conflagrations, the employment of carbon bisulphide must remain restricted. In 1863 Richardson, Lundy and Irvine secured a patent (Eng. Pat. No. 2315) for obtaining oil from crushed seeds, or from refuse cake, by the solvent action of volatile hydrocarbons from “petroleum, earth oils, asphaltum oil, coal oil or shale oil, such hydrocarbons being required to be volatile under 212° F.” Since that time the development of the petroleum industry in all parts of the world and the large quantities of low boiling-point hydrocarbons—naphtha—obtained from the petroleum fields, and also the improvements in the apparatus employed, have raised this system of extraction to the rank of a competing practical method of oil production. Of the other proposed volatile solvents ordinary ether has found no practical application, as it is far too volatile and hence far too dangerous. Carbon tetrachloride, chloroform, acetone and benzene are far too expensive. Carbon tetrachloride would be an ideal solvent, as it is non-inflammable and shares with carbon bisulphide the advantage of being heavier than water. Efforts have been made during the last few years to introduce this solvent on a large scale, but its high price and its physiological effect on the workmen have hitherto militated against it. At the present time the choice lies practically only between the two solvents, carbon bisulphide and naphtha (petroleum ether). Naphtha is preferable for oil seeds, as it extracts neither resins nor gummy matters from the oil seeds, and takes up less colouring matter than carbon bisulphide. Yet even with naphtha traces of the solvents remain, so that the meal obtained cannot be used for cattle feeding, notwithstanding the many statements by interested parties to the contrary. It is true that on the continent extracted meal, especially rape meal from good Indian seed and palm kernel meal, are somewhat largely used as focd for cattle in admixture with press cakes, but in England no extracted meal is used for feeding cattle, but finds its proper use in manuring the land.
The apparatus employed on a large scale depends on the temperature at which the extraction is carried out. In the main two types of extracting apparatus are differentiated, viz. for extraction in the cold and for extraction in the hot. The seed is prepared in a similar manner as for pressing, except that it is not reduced to a fine meal, so as not to impede the percolation of the solvent through the mass. In the case of cold extraction the seed is placed in a series of closed vessels, through which the solvent percolates by displacement, on the “counter-current” system. A battery of vessels is so arranged that one vessel can always be made the last of the series to discharge finished meal and to be recharged with fresh meal, so that the process is practically a continuous one. The solution of the extracted oil or fat is then transferred to a steam-heated still, where the solvent
