Gelatin

GELATIN, a familiar protein (q.v.) ; it is derived from sub stances in the supporting structures of vertebrate animals by boil ing with water or dilute acids. These precursors are known as "col lagens," and belong to the class of proteins called "Scleropro teins" or "Albuminoids." They are characteristically deposited in long fibres in the tendons, cartilages, bones, skin and white con nective tissue, for the purpose of supporting and padding the tis sue. They are insoluble in water or salt solution, hot or cold, and swell in dilute acids and alkalis, but do not appear to dissolve. If hydrolysed gently by boiling in water, by means of high-pressure steam or by boiling with dilute acids, the collagen is converted to gelatin, some of which is further hydrolysed to products inter mediate between gelatin and the amino-acids. It is probable that the conversion of collagen to gelatin is not merely an hydration, since ammonia is evolved on heating collagen with water, but not on boiling gelatin.

Preparation.

Gelatin occurs in commerce in varying degrees of purity; the purer form obtained from skins and bones (to which this article is restricted) is named gelatin; a preparation of great purity is "patent isinglass," while isinglass (q.v.) itself is a fish-gelatin; less pure forms constitute glue (q.v.), and an aqueous solution appears in commerce as size.

In the manufacture of gelatin the bones are degreased and steeped in dilute hydrochloric acid to remove the mineral matter, which together with the acid is removed by repeated washings with water. The product is known as osseine. Skin gelatin is limed to remove the albuminous and mutinous constituents. This process has to be carefully controlled to minimize bacterial decomposition. This is done by frequently turning the stock and also by replacing the liquor with fresh milk of lime. The stock is then thoroughly washed with water to remove the lime, and subsequently treated with dilute hydrochloric or sulphurous acid to cause the maximum swelling; the latter acid is often used for its bleaching properties though seldom for food or photographic gelatin. Sulphites are sub sequently removed (in the preparation of edible gelatin) by oxida tion to sulphates by means of hydrogen peroxide. The hydro chloric acid used must be free from iron, which has an effect on the colour of the final product, and from arsenic.

The "boiling" process has to be conducted with great care, otherwise the gelatin itself is decomposed, and distilled water is therefore used. The heating is best carried out in aluminium vessels, since copper and zinc offer danger of metallic contamina tion. Generally the first heating is at about 6o° C for some hours, or until the liquor has dissolved about 5% of its weight of gelatin. This is drawn off and the second heating with a fresh volume of water is carried out at about 7o° C. The runnings are clarified, concentrated in vacuum pans, chilled and cut into slices which are dried by hot air. The dried sheets can be further purified by soaking in dilute hydrochloric acid, to convert insoluble calcium phosphate to soluble calcium chloride, and then dialysing against a stream of distilled water till free from chlorides. Methods designed to produce a specially pure product for scientific work are described by S. B. Schryver (1923-27).

The percentage composition of pure gelatin is very similar to that of the other proteins (q.v.). On the whole the nitrogen is rather higher (18%), and the sulphur very low (0.2% to o.6%). The amino-acids obtained by complete hydrolysis are char acterized by the high amounts of glycine, proline, oxyproline and the absence of tryptophane and tyrosine.

Properties.

Gelatin is a nearly colourless, transparent, amor phous substance, flexible and horny when in the normal dry con dition, in which state, however, it retains about 17% of water. It swells to many times its normal volume when immersed in cold water, dilute alkalis and salt solutions. The amount of swelling depends on the acidity, being least at the isoelectric point which is about 4.7. (See HYDROGEN-ION CONCENTRATION.) The swell ing increases on either side of this reaction, reaching a maximum at about 2.6 and then decreasing to I•I. In solutions more acid than this the gelatin passes into solution. On the alkaline side the swelling increases to about 9.8 and then decreases slightly. In solutions more alkaline than 11.6 the gelatin dissolves. The swelling at maximum acidity is about double that at maximum alkalinity. Inorganic salts generally depress the swelling, the per centage reduction being greater with increasing strength of salt.

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slight contraction of total volume accompanies swelling, and also a liberation of heat. From this it follows that low tern peratures favour the absorption of water by gelatin, Whilst high temperatures favour the drying-out process. When the swollen gelatin is heated to 35° C it goes into solution. If this solution contains more than 1% of gelatin it sets to a firm jelly when allowed to stand at 10° C. This is the most characteristic and important property of gelatin, and on this depends its great use as an adhesive, in the form of glue.

Gelatin is insoluble in the usual organic solvents, such as ether, chloroform, benzene, and absolute alcohol. The amount of alcohol required to precipitate gelatin from aqueous solution depends on the hydrogen ion concentration of the fluid and the temperature. It is most readily precipitated at the isoelectric point. Gelatin is completely precipitated from aqueous solution by the addition of an equal volume of an aqueous saturated solution of ammonium sulphate. Potassium dichromate reacts with gelatin in the presence of light to produce a jelly which, on drying out, is insoluble. This property is made use of in photography and photo-lithography. Formaldehyde produces an insoluble condensation product with gelatin.

Gelatin, like all other proteins and amino-acids, is an amphoteric substance, acting as a weak base in solutions more acid than the isoelectric point, and as a weak acid in solutions more alkaline. Thus it will form gelatine chloride when treated with dilute hydro chloric acid. This is more soluble and more ionized than gelatin itself. Also these acid salts carry a positive electric charge. In solutions made alkaline to the isoelectric point by the addition of caustic soda or milk of lime, sodium or calcium gelatinates are formed, which are more soluble than gelatin and carry a negative electric charge. It has been shown that at the isoelectric point, osmotic pressure, electrical conductivity and the lowering of the surface tension are at a minimum; whereas the viscosity, turbidity and precipitability by alcohol are at a maximum.

Observations on the specific rotatory power of gelatin solutions at various temperatures by C. R. Smith (1919) have led him to the view that there are at least two substances in gelatin or two forms of the same substance. One, "sol form A," is stable at 35° C and above ; the other, the "gel form B," is stable at 15° and below. Apparently 0.55 to i •o% of the gel form B must be present for gelation to occur; below 15°, the gelatin being all in this form, a 0.55% solution will set. At 3o°, at least 10 grams of gelatin must be present in ioo c.c. in order that there may be 0.55-1.0 gram of the gel form present. Above 35° all the gelatin is in the sol state and gelation does not occur at any concentration.

Schryver and his colleagues have described experiments on purified gelatin which also indicate that at least two substances are present ; previous to this a great deal of work was done by Field, Sheppard and Smith. They find further that on heating gela tin with water intramolecular changes take place which result in a profound modification of the physical properties, but which are not accompanied by the degradation into lower products as with commercial gelatin. It would seem, therefore, that gelatin behaves as a mixture of substances which may undergo reversible changes on heating and cooling, and also irreversible changes on prolonged heating in water.

Uses of Gelatin.

As a food, in jellies and soups, it has con siderable value, being readily digested and absorbed; but owing to the lack of certain essential amino-acids it can only partially replace other proteins in the diet. As an emulsoid colloid, it has a protective action, inhibiting the precipitation of salts and the flocculation of other colloids. For this reason it is sometimes added to cow's milk when used for infant feeding, to inhibit the forma tion of large curds of casein in the stomach. It is also exten sively used in the manufacture of ice-cream, to prevent the forma tion of large ice crystals, to maintain the permanency of the emulsion of milk and to give "body" to the material. As an emul sifying agent it is used in the confectionery trade, in the prepara tion of such sweets as "marsh mallows." For the same reason it is employed in making spraying emulsions of insecticides and fungicides. Pure gelatin is used extensively in the preparation of photographic plates, films and papers, for obtaining the precipi tated silver salts in an extremely fine-grained suspension. In pharmaceutical preparations it is used for coating pills and making capsules. It is also used for making culture media for bacterio logical work. A new use for gelatin that is rapidly assuming importance is the adding of one quarter of 1% in cultural butter milk to prevent wheying off, i.e., the separation of the casein from the liquid.

See R. H. Bogue, The Chemistry and Technology of Gelatin and Glue (1922) . (S. W. C.)