16 MILK HYGIENE 



great differences are shown by quantitative analyses of 

 the milk of the different species. 



Water. The quantity of water usually amounts to 

 80 to 90 per cent, of the weight of the milk ; under certain 

 circumstances and with some animals it is even consider- 

 ably less. With the whale, the quantity of water is 

 scarcely 50 per cent. 



Proteids. Of proteids, the three following are always 

 found : casein, lactalbumin and lactoglobulin. A kind of 

 albumose-like compound is sometimes found in insignifi- 

 cant quantity — the so-called animal gum — ^which is also 

 found in certain tissue and in the saliva, and probably 

 comes from the breaking down of glycoproteids (mucin, 

 etc.). Moreover, other proteids are often described as 

 normal constituents of milk (albumins, albumoses, meat 

 acids, etc.), but it is safe to conclude that these do not 

 occur in fresh milk, while such compounds are readily 

 formed by bacteria and ferments and can be formed by 

 chemical means. 



Casein is a nucleo-albumin and, as such, it contains 

 phosphorus. It is insoluble in water, but, by virtue of 

 its property as an acid, it forms soluble salts with alka- 

 lies. There are two series of casein salts, basic and 

 neutral ; the solutions of the latter have a milky appear- 

 ance. Casein is found dissolved in milk in the form of 

 a neutral lime salt, which aids in giving to milk its 

 white, opaque appearance. When boiled, a solution of 

 casein is overcast with a thin membrane of coagulated 

 casein ; but a real coagulation, as it occurs in albumins 

 and globulins, does not take place. When the fluid has 

 reached a certain degree of acidity casein coagulates at 

 75° C. and the precipitated casein is not again soluble. 

 After the addition of diluted mineral acids or of acetic 

 acid, casein is precipitated as flakes or lumps, but under- 

 goes no chemical change and is again easily soluble in 



