Gelatin or glutin is an albuminoid which is obtained by boiling cartilage, connective tissue, skins, and other animal tissues (the so-called "collogenes") with water, skimming and straining the resulting liquid, evaporating the solution at a low temperature, and finally drying by exposure to the air. It is also official in the U.S.P. Gelatin does not pre-exist as such in the animal tissues, but is formed by the prolonged action of boiling water on collagen, which is probably an anhydride of gelatin, and is the substance of which the white fibres of connective tissue are composed. The crude glutin or bone-glue is highly coloured and odorous, but the colour and odour are removed by purification, and commercial gelatin is thus obtained. It occurs as an amorphous, more or less transparent solid; usually in thin sheets bearing the marks of the netting on which it has been dried, but also in shreds or coarse powder, colourless or with only a slight yellow tint, and almost odourless and tasteless. It is permanent in air when dry, but putrefies rapidly when moist or in solution. Although insoluble in cold water, it swells in that liquid, absorbing five or six times its weight of the water. The softened gelatin dissolves when heated, forming a viscid liquid which sets to a jelly on cooling. This property, however, which varies considerably with different samples of gelatin, is much lessened by prolonged heating of the solution, and is quickly destroyed by heating to 140° in sealed tubes, the gelatin being altered to gelatose, paragelatose, or gelatones. The gelatinising power of different samples varies considerably, so that it is found convenient to use the same brand of gelatin in the preparation of special formulae. A very pure gelatin may be prepared by soaking the best grade of the commercial substance for several days in successive quantities of water. By this treatment saline and soluble bodies are removed. The gelatin is then dissolved in hot distilled water and filtered while hot into alcohol. The white thready masses thus precipitated are redissolved in hot water, the precipitation in alcohol repeated, and the product subsequently dried. Thus prepared, the gelatin contains only 0.6 per cent. of ash. The solubility of gelatin in acetic acid distinguishes it from chondrin, a mixture of glutin with mucinoid substances, obtained from hyaline cartilage by boiling with water, and precipitated from the aqueous solution by acetic acid. An aqueous solution of gelatin gives no precipitate with acids, except tannic acid (distinction from proteins), is not affected by alum, lead acetate, ferric chloride, nor by the majority of metallic salts which precipitate the proteins, but is precipitated by chlorine or bromine water, mercuric chloride, platinic chloride, and picric acid, and is completely precipitated by saturation with ammonium sulphate, magnesium sulphate, or zinc sulphate. When potassium bichromate is added to the hot aqueous solution the jelly which forms on cooling becomes insoluble in warm water after exposure to light, while formaldehyde renders the gelatin hard and insoluble after drying. On incineration gelatin leaves a little mineral residue, but this should not exceed 2 per cent. A 2 per cent. solution in hot water should be odourless and gelatinise on cooling.
Insoluble in cold water, alcohol, ether, chloroform, benzene, carbon bisulphide, fixed oils, or volatile oils; soluble in hot water, acetic acid, or glycerin.
Constituents.—Glutin, of which gelatin chiefly consists, contains carbon, 50 per cent., hydrogen, 6.5 per cent., nitrogen, 18 per cent., oxygen, 25 per cent., and sulphur, 0.5 per cent. Its aqueous solution is neutral, indiffusible or colloidal, and strongly laevorotatory. It gives most of the protein colour tests, and, like proteins, is converted on digestion into peptone-like substances. On boiling glutin with solutions of the fixed alkali hydroxides, or with diluted sulphuric acid, it is decomposed into ammonia, glycocoll, leucine, and other amino-fatty acids. When heated with hydrochloric acid on a water-bath, it yields a glutin-peptone chlorhydrate, which is soluble in absolute alcohol, and from which glutin peptones can be obtained. Chondrin, like gelatin, is precipitated from its solution by tannic acid, but it also gives the reactions of mucin, being precipitated by acetic acid, lead acetate, and other reagents which do not affect gelatin.
Uses.—Gelatin is largely used as a nutrient; its nitrogen value is high, but its carbon value relatively low. It therefore possesses a low heat value, and its value as a foodstuff has been exaggerated. Injected hypodermically—1 or 2 per cent. in normal saline solution, sterilised—it has been used to promote the formation of clot in aneurisms, and to arrest haemorrhage from the lungs or kidneys. Stronger solutions (5 per cent.) have been used by rectal injection for purpura and haemoptysis. Gelatin has also been applied locally to bleeding surfaces. The use of gelatin as a styptic was based on the misconception that it favoured coagulation; but there is no certain evidence that it is useful in any of these conditions. Solutions of gelatin for rectal, subcutaneous, or local use must be sterilised with the most rigorous care, as tetanus has in some instances arisen from their use, and some specimens of gelatin examined have been stated to show the presence of tetanus bacilli. The heat applied should not exceed a temperature of 100°. Gelatin is largely used as a demulcent and emollient in the preparation of pastilles for the local application of medicaments to the throat (see Glycogelatinum). A similar basis containing a smaller proportion of gelatin is used in the preparation of pessaries and urethral bougies and to solidify glycerin for use as a suppository (see Gelato-glycerinum and Suppositoria Glycerini). The gelatoglycerin basis is also used for the preparation of nasal bougies. Tannin, owing to the facility with which it combines with gelatin, is better prescribed with cacao butter in bougies, suppositories, and pessaries. Pastes of gelatin with glycerin, of the type of Pasta Ichthamolis, are prepared containing zinc oxide, resorcin, or other medicaments for application to the skin in eczematous conditions. Gelatin is a valuable agent in the preparation of culture media for use in bacteriology. Certain bacteria have the power of liquefying the jelly. In pharmacy, gelatin capsules afford a suitable means of administering many, disagreeable substances in a convenient and readily soluble form. When solution is required to take place only in the intestine, the capsules may be coated with keratin or dipped in solution of formaldehyde (see Capsulae), being known as "Glutoid" capsules in the latter case. Formaldehyde gelatin (trade-name Glutol) is insoluble in water, and is used in sheets or in powder as an antiseptic dressing.
- Gelatinum Glycerinatum, U.S.P.—GLYCERINATED GELATIN.
- Gelatin, 50; glycerin, 50; distilled water, a sufficient quantity. The gelatin is allowed to stand in sufficient of the water to cover it, for one hour; the excess of water is poured off, the glycerin added, and the whole heated on a water-bath until the gelatin is dissolved and the solution weighs 100.
- Gelato-glycerinum, B.P.C.—GELATO-GLYCERIN. 1 in 3.
- Used for nasal bougies. If required for pessaries, the official glycerin suppository mass may be used, or 18 of gelatin may be mixed with 64 of glycerin and 18 of distilled water.
- Glycogelatinum, B.P.C.—GLYCOGELATIN. 1 in 8.
- Used as a basis for throat pastilles, the medicament being dissolved or suspended in the melted glycogelatin, the mixture poured into trays to solidify, and cut up into the required number of pastilles; or the melted mass may be poured into suitable pastille moulds. A firmer preparation, for stock pastilles, can be obtained by using more gelatin, but it should be noted that the glycogelatin becomes firmer on keeping, owing to loss of water by evaporation.
The British Pharmaceutical Codex, 1911, was published by direction of the Council of the Pharmaceutical Society of Great Britain.