The last-mentioned process, the dissociation of the casein com pounds into casein and its base, is also the cause of the formation of the skin which begins at about 50° C. (122° F.). At 60° C. (140° F.) the destruction of the ferments begins (including even the fibrin fer ment of cow's milk and the salolase), and it is completed at 80° C. (176° F. ). The same is true of the alexins; but typhoid agglutinins, for their complete destruction, require a heat of 120° C. (248° F.) during 15 minutes.
By superheating, the milk-sugar, and with it the milk, are browned and lactocaramel is developed. As in the ease of casein, the decompo sition is accompanied by the formation of acid products. The acidity is again increased (in human milk this occurs at body temperature). The milk now contains newly dissolved particles of phosphoric and nitrogenous decomposition products of the casein, and the rennin coag ulation is strongly retarded, finally ceasing altogether. The resulting proteids coagulate at 140° C. (284° F.) A serum is then formed, and from this some of the products of decomposition may be precipitated by acidification, others only by phosphotungstic acid. The lecith albumins are decomposed at body temperature.
The milk globules become partially fused by prolonged heating. It has not yet. been shown that even a slight generation of fatty acids takes place, either from lecithin or otherwise. The specific gravity of boiled milk, after restoring the water lost in boiling, is lower by 0.004. Its transparency has been diminished by the dissociation of the casein compounds and the coagulation of the lactalbumin. Its freezing-point and the electrical resistance have been raised by the precipitation of the salts which were formerly carried in solution, but may be lowered again by superheating. The viscosity, after the lost water has been restored, begins to decrease above G0° C. (140° F.). Coagulation, whether effected by acids or by rennin, produces finer flakes in boiled milk, especially if this has been superheated.
The addition of water naturally lowers the specific gravity of milk and makes it more transparent. Owing to the hydrolytic dissociation of the salts, the freezing-point and the electrical resistance do not rise in proportion to the dilution. Dissociation of phosphates and of casein compounds decreases the combining power of bases (for example, pure milk, with an acidity of 6, when diluted with water 1 to 2, has an acidity of 4.4), but it likewise decreases that of acids. A part of the calcium
salts is precipit ated by reason of the dilution of the solvent and no longer passes through the clay in filtration. The viscosity falls and rennin coagulation is retarded greatly.
Addition of alcohol increases the acidity by reducing dissociation. In larger quantities it precipitates all the proteids and a part of the calcium phosphates.
Acids which have the power of transforming phosphates into mono phosphates separate the casein from its solvent and precipitate it. Where special conditions of temperature and pressure arc necessary in the one case they will be requisite in the other. Carbonic acid, there fore, causes precipitation only at pressures above the normal; at normal pressure it releases only a part of the suspended calcium phosphates. At the beginning and at the completion of the precipitation of casein we do not need molecularly equal quantities of the different acids, but more of those that are dissociated in a weaker state. The slighter the affinity of the acid employed the longer the casein remains in combina tion with its base. When concentration has reached a certain stage, sodium chloride and other neutral salts retard the precipitation by acids. The time of the appearance of precipitation depends on the acid employed; while with the same acid it depends on the rest or motion of the fluid, the concentration of the acids, and the temperature. The warmer the milk the less acid is necessary for precipitation, but the coagultun formed in acidified milk at boiling heat is a calcium com pound of casein. In precipitation by acids, the fat, the cellular constit uents, the superoxidases and reductases, the lactomucins, and the lacto globulins are precipitated along with the casein. The sour whey, on the other hand, retains the lactalbumins, the milk-sugar, all the salts, the citric acid, and probably all the other ferments and the transuded alexins; but this implies a thorough washing of the filter residues. Excess of acid dissolves the newly precipitated casein in the milk again. Indeed, with cow's milk, for complete solution as much hydrochloric acid is necessary as during the whole process of precipitation. Concen trated acids precipitate the casein once more as acidified casein.