Gelatine, in Chemistry. Little is yet definitely known of the chemical nature of gelatine. It consists approximately of carbon 49.6, oxygen 25.4, nitrogen 18.3, and sulphur about 0.1 per cent. It is soluble in hot water, in acetic acid, and in cold sulphuric acid, and is insoluble in alcohol, ether, and other organic liquids; the aqueous solution is precipitated by tannic acid, chrome alum, and corrosive sublimate, but not by most acids, salts, or alkalies in dilute solution. Gelatine may be purified by dissolving it in water and pouring the solution into a large bulk of alcohol; the clot which forms consists of nearly pure gelatine, containing only a trace of ash. By dry distillation gelatine yields a quantity of carbonate of ammonia, and a foul smelling brown oil containing carbonate, sulphide and cyanide of ammonia, aniline, methylamine, picaline, and a number of pyridine bases. Gelatine solution dissolves lime and calcium phosphate much more freely than cold water, forming with the latter a definite compound, which probably forms part of the tissue of bones.
In Technology, the term gelatine, although usually applied to only one variety of the substance obtained by dissolving the soluble portion of the gelatinous tissues of animals, nevertheless properly belongs also to Isinglass (q.v.) and Glue (q.v.), which are modifications of the same material. Vegetable jelly is also analogous. Gelatine and glue signify the more or less pure and carefully prepared jelly of mammalian animals; but the term isinglass is only applied to certain gelatinous parts of fishes, which from their exceeding richness in gelatine, are usually merely dried and used without any other preparation than that of minute division for the purpose of facilitating their action.
Gelatine proper is prepared for commercial purposes from a variety of animal substances, but chiefly from the softer parts of the hides of oxen and calves and the skins of sheep, such as the thin portion which covers the belly, the ears, &c.; also from bones and other parts of animals. One of the best, if not the best of the varieties of gelatine manufactured in Great Britain, is the 'sparkling gelatine' of Messrs Cox of Gorgie, near Edinburgh, which is remarkable for its great purity and strength, or gelatinising power, and is purified by processes patented by them. The materials they use are carefully selected portions of ox only imported from South America. Another preparation, made by Mackay of Edinburgh from calves' feet, is deserving of special mention.
The general method adopted with skin-parings or hide-clippings is first to wash the pieces very carefully; they are then cut into small pieces and placed in a weak solution of caustic soda for a week or ten days. When this process of digestion has been sufficiently carried on, the pieces of skin are then transferred to revolving cylinders supplied with an abundance of clean cold water, and afterwards are placed still wet in another chamber lined with wood, in which they are bleached and purified by exposure to the fumes of burning sulphur; they next receive their final washing with cold water, which removes the sulphurous acid. The next operation is to transfer them to the gelatinising pots. Water is poured in with the pieces, and kept at a high temperature by means of the steam in the cases surrounding the pots.
By this means the gelatine is quite dissolved out of the skin, and is strained off whilst still hot; it is poured out in thin layers, which as soon as they are sufficiently cooled and consolidated are cut into small plates, usually oblong, and laid on nets, stretched horizontally, to dry. It is then cut into shreds and is ready for market.
Another process, introduced by Mr Swineburne, consists in treating pieces of calfskin by water alone, without the soda and sulphur processes; the pieces, after simple washing, being transferred at once to the pots to be acted upon by the steam. Inferior gelatine is made from bones and other parts of animals; and it is understood that the enormous number of rats killed in the sewers and abattoirs of Paris are used by the gelatine-makers. The French manufacturers succeed better than any others in clarifying these inferior gelatinates, and they rarely make any others; they run their plates out very thin, which gives them greater transparency; and they colour them with most brilliant colours, and form very fine-rolled sheets, tempting the eye with an appearance of great delicacy and purity.
Gelatine should never be judged by the eye alone. Its purity may be very easily tested thus: soak it in cold water, and then pour upon it a small quantity of boiling water; if pure it will form a thickish, clear, straw-coloured solution, free from smell, but if made of impure materials it will give off a very offensive odour, and have a yellow gluey consistency. No article manufactured requires such careful selection of material and such nice and cleanly manipulation to ensure a good marketable character; and those anxious for purity should avoid all artificially coloured varieties, however temptingly got up, unless they are required for merely decorative purposes and not for food. Of late years the commercial uses have greatly increased. Gelatine is the foundation of the dry-plate system of photography, and by its means the science has been revolutionised and its capabilities extended to an extraordinary degree. To the printing process as employed by Messrs Goupil of Paris and others the world is indebted for cheap and at the same time highly artistic copies of many admirable pictures. It is further very extensively used by druggists for coating pills and nauseous drugs; and by confectioners for some kinds of sweetmeats. Chondrin, closely akin in composition and properties to gelatine, is obtained by the action of boiling water on cartilage. For gelatine as food and in picture work, see DIET, ILLUSTRATION, PHOTOGRAPHY. See also GLUTEN, ISINGLASS.
One of the qualities of gelatine is its power to form chemical combinations with certain organic matters; hence, when it is mixed and dissolved in a fluid containing such matters, it combines, and the compound is precipitated. It would appear that this combination, however, is threadlike in its arrangement, and that the crossing threads form a fine network through the fluid, which, in falling, carries down all floating substances that by their presence render the liquid cloudy; hence its great value in clarifying beer and other liquids. For this reason isinglass, which has been found the best gelatine for the purpose, is very largely consumed by brewers.
Various kinds of animal food are valued for the abundance of gelatine they contain, as the Trepang and Bêche-de-Mer (species of Holothuria), sharks' fins, fish-maws, ray-skins, elephant hide, rhinoceros hide, and the softer parts, all of which are luxuries amongst the Chinese, Japanese, Siamese, Malays, &c. Turtle-shells, or the upper and lower parts of the shield (carapace and plastron), constitute the callipash and callipée of the epicure, and form, in the hands of the experienced cook, a rich gelatinous soup. The fleshy parts of the turtle, calves' head and feet, and many other things might be enumerated as valuable chiefly in consequence of their richness in this material.