The fatty acid of castor oil is of peculiar constitution, being an oleic acid in which one of the hydrogen-atoms is replaced by a “hydroxyl” or OH group. The solubility of castor oil in alcohol has already been alluded to. It does not dry, and is an excellent oil for lubricating heavy machinery. It is sometimes adulterated with “blown” oils, which are made from non-drying, or slightly drying seed oils, like cotton-seed or rape, by blowing air through them in a warmed condition. Under this treatment they increase greatly in viscosity and density and in their solubility in alcohol, but do not acquire the other valuable properties of genuine castor oil.
The “foots” or sediments which oils deposit on standing, sometimes consist of animal or vegetable fibres, or mucilage combined with water, but often are simply the harder fats, stearin, palmitin, etc., which crystallise from the oil on cooling. In this case they are re-dissolved on warming the oil. Such oils, which like neatsfoot and tallow oils become turbid in cold weather, are styled “tender.”
Non-Drying Fats and Oils.
Tallow (Fr. Suif; Ger. Talg) is the fat of various mammalia, principally of the ox and sheep, but occasionally also of the goat. The mixed fat obtained from all parts of the carcass is known as “rendered tallow,” while that obtained from the region of the kidneys (suet) is harder. A substance commonly referred to as “pressed tallow” or “oleo-stearine” is obtained by pressing ordinary tallow, in cloths, in the hydraulic press. The more liquid portion which is expressed is tallow-oil, the finer qualities of which are used in making margarine. Oleo-stearine must not be confounded with the “distilled stearine,” obtained from Yorkshire grease by distillation and pressure ([page 359]), nor with candlemakers’ “stearine,” which is a mixture of free stearic and palmitic acids.
Pure tallow is white and tasteless, but much of that sold is yellowish and of a disagreeable, slightly rancid flavour. Mutton tallow is usually harder and whiter than that of beef. Goat tallow has a characteristic odour, as have the recovered stearines and other waste greases from glue-works. Buck tallow, which is particularly hard, has now been largely replaced by oleo-stearine.
Beef tallow melts at about 40° C.; mutton tallow at 45°.
In chemical composition, tallow consists chiefly of a mixture of the tri-glycerides of palmitic, stearic and oleic acids; its hardness diminishing with the increase of the last.
Tallow should, when melted, be perfectly clear, turbidity indicating the presence of water or other foreign matters, due either to carelessness in the manufacture or, possibly, adulteration. Traces of phosphate of lime, or fragments of animal tissue, may be present as accidental impurities; lime, on the other hand, is sometimes added to thicken the tallow and enable it to retain more water; starch, china clay, whiting, heavy spar, etc., are also occasionally employed. Tallow has been not infrequently adulterated with the distilled fatty acids from wool grease. When this is the case, crystals of cholesterol (see L.I.L.B., p. 181) may be detected by examination of the unsaponifiable matter of the mixture under a microscope. It would also give the tallow an unusually high “acid-value.”
Methods for the proximate analysis of tallow are given in the ‘Laboratory Book,’ pp. 189 et seq.
The fats produced by the boiling of fleshings for glue, and by the pressing of sheep-skins, are of the nature of soft tallows. If the fleshings are delimed with acid, and boiled fresh, the grease is generally of good colour, and with little unpleasant odour, but contains traces of free fatty acids derived from the decomposition of the lime-soaps. If the fleshings have been dried and the lime carbonated, the grease will generally be brown, and more or less rancid; but the lime-soaps are not decomposed, unless the “scutch” or refuse be treated with acid, when a further yield of grease is obtained. The grease from sheep-skins is generally somewhat brown, and often smells of the volatile acids and other constituents of the tan-liquors, especially if larch bark has been used. These greases are usually much improved in appearance and odour, if well washed by boiling or steaming on water, or by blowing a mixture of air and steam through them, or sometimes even by mere heating to a sufficient temperature to evaporate the water and drive off the volatile matters. By allowing the grease to cool slowly, so as to favour crystallisation, till it is of a soupy consistency, and then pumping through a filter press with woollen cloths, the more liquid is separated easily from a more solid portion, and both may in many cases be used in leather manufacture, the tallow for currying, and the oil in place of neatsfoot oil.