MILK AS A FOOD 43 



secretion of the milk glands of all mammalian animals. It 

 is insoluble in water, but forms soluble salts with alkalies. 

 A solution of casein has a milky appearance. In milk, 

 casein is found dissolved in the form of a neutral calcium 

 salt which aids in giving to milk its white, opalescent ap- 

 pearance. The casein in milk exists in combination with 

 calcium phosphate. It is not coagulated by heat, but is 

 precipitated by acids, which combine with the calcium 

 phosphate and throw the casein out of solution as a curd. 

 This flaky or lumpy precipitate may again be dissolved in 

 hme water and dilute alkalies. Casein is also thrown out 

 of solution by rennin. The ferment in rennin throws out 

 the casein from human milk and donkey milk as a flaky 

 precipitate, but from cow's milk as a firm gelatinous 

 mass. The difference in the character of the curds does 

 not come from differences in the casein, but is due to the 

 different salt content of the different kinds of milk. The 

 casein appears to show no chemical difference in dif- 

 ferent kinds of milk. The percentage of casein, however, 

 varies considerably. I For example, the protein in cow's 

 milk consists chiefly of casein, — 3.02 per cent, — while 

 the protein in woman's milk contains only 1.03 per cent of 

 casein. 



It is important to have a clear understanding of the 

 casein as well as the other proteins found in milk, for 

 the reason that these substances are the ones which are the 

 most obviously acted upon when milk spoils, when milk 

 is heated, or when milk undergoes putrefaction. 



Lactalbumin. The lactalbumin in milk is very similar to 

 the serum albumin of the blood and corresponds to the al- 

 bumin found in the white of an egg. Lactalbumin is coag- 

 ulated by heat at 70° C, but not with dilute acids, while, 

 on the other hand, casein is not coagulated by heat, but is 

 readily precipitated from its solution by dilute acids.| Lact- 

 albumin contains sulphur, but not phosphorus; casein 

 contains phosphorus. There is very little lactalbumin in 



