In human milk the addition of acids usually causes a coagulum which is visible only with the microscope. liy a slight excess of acid this is again dissolved; but, on the other hand, it coagulates more firmly by a heat of 40° C. (104° F.) and bacterial acidification. The fineness of the coagulum depends on the reaction between the concen tration of the casein compounds and the abundance of chlorine alkalies, on the poverty in calcium salts, the relative amount of lactalliumin, and the character of the casein compounds.
Acid salts, among them alum, act like acids. When mineral or metallic salts are used, however, casein is precipitated as a compound with the corresponding base. Here, too, an excess of acid brings about solution.
Alkalies and alkaline salts precipitate calcium phosphates. especially when heated, and carry the casein compounds with them. When milk is heated with alkalies it is browned by Oxidation of the milk-sugar and the casein is thoroughly decomposed. If ammonia is used, the milk-sugar turns red and the presence of citric acid evokes a violet hue (the Umikoff reaction). This, however, is true only with human milk for, when cow's milk is heated with ammonia, becoming slightly yellow in the process, the citric acid is precipitated as calcium citrate.
Of the neutral salts, saturation with ammonium sulphate* precip itates all the proteids, and the ferments and alexins adhering to them and to the milk globules. Half saturation with ammonium sulphate, or saturation with precipitates the casein compounds and the lactoglobulins; here, too, naturally, the milk globules are precipitated, together with the ferments that adhere to them. Saturation with so dium chloride precipitates nearly all the casein compounds. The salts of the alkaline earths (e.g., Ca precipitate a part of the phosphates, and in this way increase the acid reaction and the acidity. With heat they too cause the milk to coagulate.
Rennin ferment causes the milk to coagulate at body temperature. In milk which (like that of women) naturally gives an alkaline reaction, coagulation will take place only when the alkaline matter which de stroys all the rennin has been removed. Whether the coagulum is formed in one continuous mass or in several smaller ones depends on the concen tration and character of the casein solution, as well as the concentra tion of the dissolved calcium salts and the strength of the ferment. In the first case the whey is forced out by the contraction of the cheese. The whey contains the lactalbumins, all dissolved substances, and per haps a new proteid, whey albumin. It has not yet been determined
whether the action of the rennin upon the casein solution results in a decomposition accompanied by the formation of what is called whey albumin, in association on a higher plane, or in an inner redistribution of the molecules. The transformation of the casein may even take place when it is cold and when no alkaline earths are present. The production of cheese arises from the fact that the paracasein calcium, as we may call the transformed casein calcium salts, is far more easily precipitated by soluble calcium salts than when in its original form. The more concentrated the solution of calcium salts, within definite limits, and the more free casein the casein solution contains, the more quickly the rennin coagulation follows. In diluted, boiled, or alkaline milk it is correspondingly slow. If the coagulation is proceeding slowly it can be hastened by a higher temperature (metacasein reaction).
In the stomach, rennin always coagulates the milk within a few minutes. In a test-tube both the peptic and the tryptic digestion of the milk-albumins (though not of egg-albumin) are retarded by rennin. We must here remark that it is not yet known whether rennin coagula tion and peptic or tryptic digestion are only different activities of one and the same ferment, or whether they are produced by two different enzymes which are always found together. The less casein in the milk the more quickly it leaves the stomach. A large proportion of fat seems to a-ct in the opposite way. It has not yet been decided to what extent pepsin decomposes the proteids in the stomach. According to some experiments, the milk fat is found to be almost half-decomposed in the stomach itself. On the other hand, where milk is the only nutriment., the greater part of the fat seems to be reabsorbed before it is decom posed. Sugar and salt, like water, are absorbed in the beginning of the small intestine, the former after it has been hydrolysed by lactase into galactose and dextrose. The calcium phosphates also, in limited quan tities, can be taken up undissolved. By tryptic digestion the proteids are ordinarily separated into polypeptids, that is, into complex com pounds of the amino-acids. The easeins yield products of decomposi tion more strongly acid than those from egg-albumin. Boiled milk is more quickly decomposed. Erepsin, a ferment of the mucous membrane of the intestines, decomposes casein. It attacks only peptones and never albumins in their native state.