Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/801

OILS 741 Another, the oleic series, contains two atoms less of hydrogen than the allied stearic scries, having the general formula C n H 2n _ 2 2 . To it belong the following : Boiling and Melting point. Substances in which chiefly found. C 4 H,; ()., Boil. pt. 72" Croton oil. Hypogseic or Physetoleic Oleic Ci 6 H 30 Oj Ci 8 H ;}4 0, C->..H 4 .>O., Melt, pt. 34 "." 33 Earth-nut oil and whale oils. Most fats and fluid oils. Rape and mustard oils. To these there remain to be added, not members of either series, two important fatty acids, linoleic acid, C 16 H., 8 0.,, characteristic of linseed oil and other drying oils, and ricinoleic acid C^H^Oj, the chief product of the saponification of castor oil. Among saponifiable bodies the true waxes are distinguished from other solid fats by containing no glycerin. They are principally ethers of the higher monatomic solid alcohols cetylic and cerylic alcohol, &c. Thus spermaceti, the solid wax obtained from the head matter of the O TT r sperm whale, is a cetyl palmitate p 16 Tj 31 r 0. Certain of the vegetable waxes e.g., Japanese wax contain some proportion of glycerides. Extraction of Oil. The ordinary method for separating vegetable oils and fats from the nuts, seeds, &c., of which they form con stituent parts is by pressure, with or without the assistance of heat. They are also obtained by the agency of solvents, principally by the use of bisulphide of carbon and the light petroleum spirit benzin, these being methods of pro duction of comparatively recent introduction. Animal oils and fats are principally isolated by simple melting or " rendering " by heat. The degrees of heat and pressure necessary for separating the several fats vary very much with the fluidity of the oils themselves, the proportion in which they are present in the substances, and the nature and consistence of the associated materials. Sperm oil is indeed obtained direct in its fluid condition from the head of the sperm whale. Virgin olive oil is obtained with the gentlest pressure, and palm oil and several other vegetable fats and waxes are liberated by the agency of boiling water. Vegetable Oil Pressing. Pliny describes in detail the apparatus and processes used for obtaining olive oil among his Roman contemporaries, by which it appears that they derived a knowledge of the screw press from the Greeks, and applied it to the pressure of oil from pulped olives. In the East, where vegetable oil forms a most important article for food and for other personal and domestic purposes, various ingenious applications of lever presses and of combined lever and wedge presses have been in use from the earliest times. The Chinese employ the same series of operations which are followed in the most advanced oil mills of modern times, viz., bruising and reducing the seeds to meal under an edge-stone, heating the meal in an open pan, and pressing out the oil in a wedge press in which the wedges are driven home by hand hammers. The apparatus used in Europe in modern times for the extrac tion of oil by pressure consists of forms of the screw press, the Dutch or stamper press, and the hydraulic press. With the screw press, even of the most improved form, the amount of pressure practically obtainable is limited from the failure of its parts under the severe inelastic strain which can be put on it. It is on this account only used in the pressure of olives and of animal fats, where the power necessary is not great. The Dutch or stamper press, which has played an important part in the oil industry, was invented, as its name indicates, in Holland in the 17th century. The invention of the hydraulic press in 1795 effected the greatest revolution in the oil industry, bringing a new, easily controlled, and almost unlimited source of power into play, and on the great scale that apparatus has practically superseded all other means of pressing. The sequence of operations in treating oil-seeds for the separation of their contained oils is ordinarily as follows : (1) the crushing and grinding of the seed or other substance, (2) heating the oleaginous meal so prepared, and (3) expression of the oil by mechanical power. Grinding. As a preliminary operation oil-seeds are freed from dust, sand, and other impurities by sifting in an inclined revolving cylinder or screening machine, covered with woven wire having meshes varying according to the size and nature of the seed operated upon. In earlier times the seeds were pounded to meal by means of stamper mills. These consisted of a series of heavy wooden stampers or pestles made to rise and fall by the action of cams or wypers fixed on a revolving shaft, a pair of such stampers falling alternately with heavy force into an egg-shaped mortar about two- thirds filled with seed. As the process proceeded the material became heated, and from time to time had to be sprinkled with water. Stampers are now seen only in small old-fashioned establish ments. In a modern oil-mill the screened seed is passed through crushing rollers to bruise or open the husk. The crushing rollers consist of a pair of cast-iron rollers horizontally mounted, commonly of unequal size, the larger being 4 feet in diameter, while the smaller is about 1 foot. The larger roller is revolved by power and the smaller moves by simple friction against the other. Between these rollers the seed is fed by a hopper, and in passing through it is bruised and broken and so prepared for the thorough grinding it receives under the revolving edge-stones to which it next passes. These are a pair of circular stones having a diameter of about 7 feet with a thickness at their running edge of 16 inches, each weighing from 2J to 3 tons. They are made of very compact limestone, granite, or fine-grained sandstone, and are mounted on a vertical driving shaft, to which they are attached by a horizontal axle pass ing through their centre. They revolve on a bed of similar hard compact stone, and the compound rubbing and bruising effect of their rotary motion quickly reduces the bruised seeds to a fine meal. The stones are provided with sweepers, which in their revolution bring the material pressed out towards the side again into their path, and there is a separate sweeper for clearing out the finished meal from the bed of the machine by way of a slide or door provided in the side. The edge-stones revolve about twenty times per minute, and a charge of seed which is slightly moistened during the process is sufficiently ground on an average in about twenty-five minutes. Heating. In dealing with certain oils which are easily separated, and especially with oils used in cookery and otherwise consumed, where it is desirable to preserve the pleasant, bland, and faint fruity or nutty taste, the ground oleaginous meal is taken direct to the press and pressed for cold-drawn or virgin oil. The cake from such cold pressing, as it still retains a large proportion of oil, is subse quently broken lip, reduced to meal, and heated ; after which it is again subjected to pressure to obtain a further flow of oil. Ordin arily, however, the meal is artificially heated previous to any pressure, and it depends greatly on the nature of the seed and the individual manufacturer s method of working whether the material is fully pressed at first or twice submitted to the operation. The warming of the seed meal renders the contained oil more fluid and conse quently more readily separable with moderate pressure. It also enables the oil-presser to obtain a larger proportion of the contained oil, coagulates and holds back the albuminous constituents of the seeds, and similarly dries and retains mucilaginous matter. On the other hand, oil from heated meal usually is more highly coloured and harsher to the taste than cold -drawn oil ; and the quality is seriously deteriorated if by chance the heat applied should exceed at most 80 C. The heat is applied either in open shallow iron kettles or pans heated over a direct fire or through a sand - bath ; but preferably, and now generally, the meal is heated by steam circulating freely between the casings of a jacketed or double-walled pan or pans. Mechanical stirrers are kept in continuous rotation within the pan, to ensure a uniform warming effect throughout the mass. A highly-approved and convenient form of heating apparatus consists of a double steam kettle, one pan being placed above the other, each steam-jacketed and provided with mechanical stirrers. In this machine the heating action is continuous. The meal is first heated for ten or fifteen minutes in the upper_ pan, which is closed over with a sheet-iron cover, after which a slide in the bottom of the pan is opened and the charge is shot down into the lower pan, where it is raised to the full heat, while the upper pan is again re charged and worked up. When fully heated in the lower pan the charge is swept out at a door in the side of the pan by the action of the mechanical stirrers, and falling into a funnel is passed in measured quantities direct into bags, and without delay prepared for and placed in the press. The kettles are of a capacity sufficient to heat seed for charging three single presses at each operation. A form of heating kettle is also now in use in which the object is effected by direct injection of steam into the mass, whereby the meal is not only heated but a beneficial amount of moisture is distributed throughout the material. In mills of the most recent construction such steam kettles are used in connexion with an im proved form of crushing rollers, employment of edge-mills being dispensed with. These rollers consist of aperies of four or five chilled iron or steel cvlinders mounted in vertical order like the bowls of