i MILK 7 



foods. It is true that by the use of more nutritious food the 

 yield of milk may be increased, but the proportions in which the 

 components of the milk stand to one another cannot be changed. 



The protein substances in the milk are of three kinds : the 

 casein, the lacto-albumin, and the lacto-globulin. Casein is 

 the most important of these, and is present in the largest 

 quantity, there being 85 per cent, of the protein of milk in 

 this form, 15 per cent, as lacto-albumin, and only traces of 

 lacto-globulin. In other words, of the 3'5 per cent, of protein 

 in the milk, 2'9 per cent, are casein, and 0*6 per cent, are albumin 

 and globulin. 



Casein is present in milk not in a dissolved, but in a sus- 

 pended or colloidal condition, which can easily be proved by 

 allowing milk to pass through a filter of unglazed porcelain. 1 

 After this operation a white mass is found on the surface of the 

 filter, which consists of almost the whole of the casein of the milk 

 together with the fat which was present. As has already been 

 mentioned, the casein also assists in giving the opaque, porcelain- 

 like appearance to milk. According to its chemical composition, 

 casein is a nucleo-albumin and is distinguished from other 

 albumins, above all by its phosphorus content and its behaviour 

 towards rennet. Casein has the following chemical com- 

 position : 2 



C 53-0 per cent. 



H 7-0 



N 15-7 



S 0-8 



P 0-85 



.. 22-65 



100-00 per cent. 



Casein behaves like a fairly strong acid which dissolves in 

 water on the addition of a small quantity of alkali ; it also 

 dissolves in water in the presence of calcium carbonate, from 

 which it drives out carbon dioxide. In pure water casein, how- 

 ever, is not soluble. 



Probably casein is present in milk in combination with lime 

 as a lime-casein compound. Sb'ldner has prepared two lime- 

 casein compounds (di- and tri-calciumcasein) which contain 

 1'55 and 2 36 per cent. CaO respectively. 



1 Duclaux, Le Lait, Etudes chimiques et microbiologigues, Paris, 1887. 



2 Hamniarsten, Lehrbuch der Physiologischen Chtmie. 



