842

PROPERTIES OF CASEIN.

appears to owe its solubility to the presence of a certain proportion of free alkali. It has not been obtained free from salts. If skimmed milk be evaporated to dryness at a gentle heat, and then digested with ether to remove the fat, on treating the residue with water, the lactose and a portion of the casein are redissolved, and, on the addition of alcohol, a considerable portion of the lactose is separated. The alcoholic liquid contains casein, still contaminated with lactose and with salts.

Casein may be obtained in the coagulated form nearly pure by the following process:-Skimmed milk is to be curdled with dilute sulphuric acid. The curd, after being well washed with water, is to be dissolved in sodic carbonate, and suffered to stand undisturbed for twenty-four hours or more, to allow the oil to rise to the surface; this is to be skimmed off, and the casein again precipitated by an acid, and well washed; after repeating this operation a second time, the coagulum is to be digested in alcohol and in ether, and again dried; but it cannot be perfectly freed from the saline matters, which adhere to it with great tenacity.

Casein may also be obtained by coagulating skimmed milk by means of hydrochloric acid, and washing the curd first with water, then with water acidulated with 2 or 3 per cent. of hydrochloric acid, and finally with pure water; a glutinous mass is thus obtained, which is slowly dissolved by digestion at 110° in a large quantity of water. The solution, after being filtered, is precipitated by the cautious addition of ammonium carbonate, the coagulum is washed with water, and then digested successively with alcohol and ether.

Properties.-Casein in solution is immediately coagulated by acids. An excess of acetic or of oxalic acid dissolves the coagulum, but it is re-precipitated from its acetic solution on the addition of one of the mineral acids. According to Bouchardat, water, containing 1-2000th of its weight of hydrochloric acid, forms with casein a colourless solution, which produces lefthanded rotation of a ray of polarized light. If these acid solutions be evaporated at a high temperature, a pellicle of insoluble casein is formed over their surface. Mulder considers that when the mineral acids are employed to coagulate casein, they form definite compounds with it; the acid may, however, be entirely removed by washing. With strong nitric, sulphuric, and hydrochloric acids, casein produces the same reactions as albumin; and its acetic solution gives a similar precipitate with potassic ferrocyanide. Soluble casein is sparingly dissolved by cold alcohol, but more freely by hot alcohol. The stronger acids produce no

CASEIN PREPARATION OF CHEESE.

843

precipitate in this alcoholic solution, which, however, is immediately coagulated by infusion of galls. The solubility of casein in water is not destroyed by the action of alcohol, for the residue obtained on evaporating both the aqueous and the alcoholic solutions to dryness may be redissolved in water.

(1658) Action of Bases and of Salts.-Coagulated casein is readily dissolved by solutions of the alkalies, and of the alkaline carbonates; and if the solution be very feebly alkaline, the alkaline reaction may be completely neutralized by the casein. Solutions of common salt, of potassic nitrate, and of sal ammoniac, likewise dissolve casein with facility, and these solutions, when evaporated by the aid of heat, become covered with an insoluble pellicle. Casein also unites with the alkaline earths and forms compounds which are insoluble in water. If a piece of poor cheese, which consists principally of casein, be reduced to a paste with water, and mixed with slaked lime, it produces a tenacious lute, which sets very hard, and may be used for cementing pieces of broken earthenware. In consequence of the tendency to the formation of these insoluble compounds, a solution of casein is precipitated by salts of calcium or by magnesic sulphate, upon the application of heat to the mixture: this reaction is very characteristic of casein. Most of the metallic salts, such, for instance, as the neutral and basic acetates of lead, cupric sulphate, mercurous nitrate, and corrosive sublimate, occasion precipitates in the cold with solutions of casein.

But the most important and remarkable form under which the coagulation of casein occurs is that which is produced by the secretion from the mucous membrane of the stomach. Advantage is taken of this in the manufacture of cheese, which consists essentially of the coagulated casein of milk. The coagulation is effected by means of the substance called rennet, the inner membrane of the fourth stomach of the calf, after it has been salted and dried. When milk is maintained for a few hours at a gentle heat in contact with rennet (or with an infusion of rennet prepared at a low temperature), the casein becomes entirely coagulated, carrying with it the fatty matters which were held in suspension by the milk, and leaving a clear straw-coloured liquid known as whey. It was at one time supposed that the rennet acted by converting the sugar of milk into lactic acid, and that the lactic acid was the real agent in effecting the coagulation, but this view is no longer tenable, for it appears that the coagulation occurs equally well with milk which has been purposely rendered slightly alkaline (Selmi, Heintz), and with an aqueous

solution of casein; the action of rennet is therefore as yet unexplained. The coagulum, or curd thus obtained, when submitted to strong pressure, forms cheese. This curd is dissolved with difficulty by alkaline solutions. New cheese has but little flavour; when kept for some time, the fatty and nitrogenous substances undergo partial decomposition, and furnish ammonia, with valeric and butyric acids, and other compounds which impart pungency to old cheese.

Casein has not been detected with certainty anywhere but in the milk of animals, which contains it in very variable quantity, the amount depending greatly upon the period of lactation at which the milk is examined. Its amount also varies greatly in the milk of different species of animals. In the human female it has been found to constitute from 31 to 35 parts in 1000; in that of the cow, from 30 to 41; in that of the dog, from 80 to 146, and in that of the ass, from 19 to 23 parts in 1000.

(1659) Legumin.-The seeds of most leguminous plants, as well as the sweet and the bitter almond, contain a substance which presents a close analogy in properties with the casein of milk. Liebig indeed considers it to be identical with this substance, but the analyses of Dumas and Cahours indicate a difference in composition between the two bodies. There is, however, some doubt whether the chemists last named operated upon a homogeneous substance.

Legumin is usually extracted from peas or from almonds, by digesting the pulp of the crushed seeds in warm water for two or three hours. The undissolved portion is strained off by means of linen, and the turbid liquid is allowed to deposit the starch which it holds in suspension: it is next filtered and mixed with dilute acetic acid. A white flocculent precipitate is thus formed, which must be collected on a filter and washed. It is then dried, powdered, and digested, first in alcohol and afterwards in ether. In this form it was analysed by Dumas and Cahours. Rochleder however considers that it is not perfectly pure, since this substance is not entirely soluble in a cold concentrated solution of potash, which leaves a flocculent matter. The alkaline solution may be decanted from the undissolved flocculi, and again precipitated by the addition of acetic acid. The legumin thus obtained is regarded by Rochleder as pure, and yielded on analysis the same results as were furnished by casein.

According to Dumas and Cahours, legumin is precipitated from its concentrated solutions in pearly flocculi by the addition of acetic acid. It is redissolved by acetic acid in excess, and by

GELATIGENOUS SUBSTANCES.

845

many other vegetable acids; it is also soluble in ammonia and in dilute solutions of the alkalies. It is insoluble in boiling water, and in cold alcohol and ether. Cold water dissolves it in considerable quantity, and the liquid, when heated to ebullition, deposits it in flocculi resembling those of coagulated albumin. The aqueous solution of legumin obtained by digesting peas in water is not coagulated by heat, according to Liebig, but on evaporation it forms insoluble pellicles resembling those yielded by solutions of casein. Legumin is associated in the seed with considerable quantities of potassic, calcic, and magnesic phosphates. It is coagulated by rennet like the casein of milk, and the Chinese make a kind of cheese from peas and beans. Dried peas contain about one-fourth of their weight of legumin.

§ II. GELATIGENOUS SUBSTANCES.

(1660) The substances contained in the second group of azotised bodies have hitherto been found only in animals. They contain a smaller amount of carbon and a larger quantity of nitrogen than those of the albuminoid group. The proportion of sulphur is also considerably less. The gelatigenous bodies consist of two principal varieties; viz., those which yield gelatin, and those which yield chondrin; neither gelatin nor chondrin appears to exist in the soluble form in the animal body, but it is in all cases the result of the prolonged action of boiling water upon the gelatigenous or chondrin-producing tissues..

According to the researches of Mulder, of Scherer, of Fremy, and of other chemists, the composition of gelatin is identical with that of the tissue which yields it by boiling. Chevreul found that a given quantity of tendinous matter when converted into gelatin by boiling it with water was not altered in weight. The following table exhibits the composition of gelatin, chondrin, and some gelatigenous tissues :

The quantity of nitrogen given in Fremy's analysis of ossein is probably too low, for the same chemist found in it upon another occasion as much as 17.86 per cent. of nitrogen. Schlieper has shown that gelatin and the gelatigenous tissues contain a small quantity of sulphur, amounting in isinglass to 0.6 per cent. It will be observed that the composition of chondrin differs from that of gelatin in containing more oxygen and less nitrogen; whilst the per centage of carbon and of hydrogen is the same in both substances.

(1661) GELATIN is an azotised principle which hitherto has been obtained from animal substances only. Certain structures (the principal of which are the white fibrous tissue, the cellular tissue, the skin, the serous membranes, and the organic constituent of bone), when boiled with water, yield a compound, which if the solution contain more than 1 per cent. of it, causes the liquid to gelatinize on cooling; hence its name of gelatin. This substance does not appear to exist in a free state, but is developed by the action of boiling water upon the tissues above mentioned; its solution, when evaporated to dryness, leaves a brownish-yellow transparent mass. Isinglass which is prepared from the inner membrane of the floating bladder of sturgeons and other fishes, when boiled with water, furnishes gelatin in a state of considerable purity. Glue is a variety of gelatin of inferior quality. Size, as prepared from parchment, is another form of gelatin which is met with in commerce.

Gelatin softens and swells up in cold water, but is not dissolved until the liquid is warmed; it is insoluble in alcohol and in ether. It may be precipitated from its aqueous solution by the addition of an excess of alcohol, when it falls in white flocculi, which are nearly free from saline compounds. Its solution is without taste or smell; it is abundantly precipitated by solutions of corrosive sublimate, and of platinic chloride, as well as by the infusion of galls. The compound which gelatin forms with tannic acid is the basis of leather; it is of a buff colour, and is formed even in very dilute solutions. Albumin is also precipitated by the same solutions, but gelatin may be distinguished from albumin, since it is not rendered turbid as albumin is by the addition of potassic ferrocyanide to its solution in acetic acid. Salts of aluminum, of lead, of copper, and of iron, do not produce any precipitates in a solution of gelatin; but if a solution of potash be added to the mixture of gelatin with alum, or with ferric sulphate, the basic aluminic or ferric sulphate which is formed carries down a large proportion of gelatin. In a dissolved