Tests. Its aqueous solution is recognised as follows:—1. It gelatinises on cooling. 2. It is precipitated by alcohol. 3. Bichloride of mercury gives a whitish flocculent precipitate. 4. Tannic acid or infusion of galls gives a copious yellowish-white, curdy precipitate, which, on being stirred, coheres into an elastic mass, insoluble in water, and incapable of putrefaction, and which, when dried, assumes the appearance of over-tanned leather. 5. The gelatinising property is destroyed by nitric acid. 6. It is not affected by either alum or acetate of lead. In this respect it differs from chondrin.
Qual. The goodness of commercial gelatin intended for food is readily proved by pouring boiling water over it, and digesting the two together for a short time. If it is pure and wholesome, its colour remains unaltered, and during its solution it continues entirely free from smell. The resulting solution and jelly are also odourless, neutral to test-paper, free from unpleasant taste, and perfectly transparent. If it forms a yellow gluey-looking mass, and evolves an offensive odour, it should be rejected as of inferior quality, and unfit for culinary purposes.
Uses, &c. Gelatin is largely employed as an article of food, as in soups, jellies, &c.; but its value in this respect has been, perhaps, overrated.[329] Animals fed exclusively on gelatin die of starvation. But when mixed with other food, especially with substances abounding in albumen, casein, or fibrin, gelatin may be useful as an aliment, and serve directly to nourish the gelatinous tissues. (Liebig.) Hence gelatin is a fitting substance to form part (but only a part) of the diet of convalescents, as it conveys nutrition directly to these tissues, without tasking the diminished powers of life for its conversion; but its use should be accompanied by a proper quantity of azotised animal food to supply the elements to the blood, for the support and increase of the muscular tissue, or fleshy portion of the body. In France gelatin obtained from bones is employed as a part of the diet in hospitals with the best effect, materially abridging the period of convalescence; but when given alone, all animals soon become disgusted with it, and die if not supplied with other food. (D’Arcet.) See Glue, Isinglass, and below.
[329] The reader interested in this subject should consult a paper by Carl Voit in the ‘Zeitschrift für Biologie,’ viii, 297-388.
Gelatin, Bone. Obtained from crushed bones by boiling with water, or by the action of steam and water successively, either with or without pressure; or by maceration in dilute hydrochloric acid, to extract the phosphate of lime, the remaining gelatinous mass being well washed in cold water, and afterwards dissolved in boiling water in the usual manner. A little carbonate of soda is commonly added to the last water. Gelatin has even been extracted from fossil bones. “A soup was prepared from one of the bones of the great mastodon by the préfet of one of the departments of France.” (Pereira.) Butchers’ meat contains, on an average, 24% of dry flesh, 56% of water, and 20% of bone. The last will yield, by proper treatment, nearly 1-3rd of its weight of dry gelatin, or a quantity equal to about 6% of the meat from which it is cut. This, as well as other varieties of gelatin, is frequently blanched by sulphurous acid or animal charcoal, and tinged of various colours with the ordinary vegetable dyes. Thus, blue is given with sulphate of indigo or the juice of blue berries; green, with the juice of spinach; and red, with juice of red-beet.
Gelatin, French. Syn. Cake gelatin. Gelatin made up into small thin cakes, like the finer sorts of glue. A good deal of it is prepared in Paris from the cuttings of the skins used in making kid gloves and slippers.
Gelatin, Patent. Various qualities of gelatin are manufactured from glue pieces, or cuttings of the hides of beasts and skins of calves, and from inferior isinglass. According to Mr Nelson’s specification, the crude materials, freed from hair, wool, flesh, and fat, after being thoroughly washed and ‘scored,’ are macerated for 10 days in a lye of caustic soda, and are then placed in covered vessels at a temperature varying from 60° to 70° Fahr., until they become tender; they are next washed to free them from alkali, and are then exposed to the vapour of burning sulphur until they acquire a sensibly acid reaction; they are now dissolved in water contained in earthen vessels heated to 150° Fahr., and the solution, after being strained, is put into ‘settling vessels,’ and heated to 100° to 120° Fahr., for 8 or 9 hours; at the end of this time the clear liquor is drawn off, and poured on the ‘cooling slabs,’ to the depth of about 1⁄2 an inch. As soon as the jelly is cold, it is cut into pieces, and washed in water until perfectly free from acid. It is then redissolved in water at about 85°, the solution poured out on slabs as before, and when cold, it is cut up, and, lastly, dried on nets.
According to another specification (Rattray’s
Patent) glue-pieces are steeped in water until they begin to putrefy, then washed with water, drained, and put from 12 to 24 hours into water strongly soured with sulphurous acid; they are afterwards washed first with cold water, and then in water at 120° Fahr., and are lastly converted into size by digestion for 24 hours in water at 120° Fahr., the resulting solution being filtered through bags of double woollen-cloth.
Patent gelatins are often sold cut up in imitation of ‘picked isinglass,’ to which, for the preparation of jellies, soups, and blancmanges, they are not much inferior.