GENERAL CHEMISTRY OF MILK 75 



COMPOSITION OF, BUTTERMILK 



Sour Sweet Separated 



SpeciBc gravity. 



Water 



Fat 



Sugar 



Lactic acid. 



Protein 



Ash... 



THE PHYSIOLOGIC ORIGIN OF MILK CONSTITUENTS 



Casein, fat, and lactose are substances characteristic of milk, 

 inasmuch as they do not occur in any .other place. Their origin, 

 therefore, must be sought in the tissue of the maternal system or 

 in the mammary glands. Considerable study has been made of 

 this subject, the results of which are briefly summarized in the 

 following. 



Casein, Lactalbumin, and Globulin. Casein occurs exclu- 

 sively in the mammary secretions of mammals. In small quan- 

 tity it has been found in the tallow glands of animals and in some 

 rump oil glands of birds. The body of the lactating mammal 

 contains no casein except in the mammary glands. Two hy- 

 potheses have been developed to account for the appearance of 

 casein in milk, namely, the enzymic theory, which assumes that 

 blood proteins are transformed into casein by a special enzym, 

 characteristic of the milk gland, and the pairing theory, accord- 

 ing to which blood proteins combine with substances derived 

 from the milk glands. 



EnzymeTheory. This theory originated before it was known 

 that casein was a phosphoprotein. Without this knowledge it 

 was not possible to arrive at conclusions which could stand the 

 test of later experiments. Later Hammarsten (1894) ascribed the 

 origin of both casein and milk-sugar to a protein "nucleogly co- 

 protein" which he isolated from milk glands. According to this 

 idea, casein and milk-sugar are formed by breaking down of this 

 complex protein which is then regenerated by the milk glands. 

 Hildebrand in 1904 observed that even after autolysis of a cow's 

 udder there was no casein formation from the substance of the 

 milk glands. He thought that a ferment was active in the forma- 

 tion of casein from gland cells and blood albumin. The blood 

 proteins are supposed to break down into simple compounds, 

 building stones, which are then synthesized into casein. 



Pairing Theory. Basch in 1889 considered the nucleic acid of 

 the milk glands to be the mother substance of casein. He thought 

 that nucleic acid combined with serum albumin in the alveoli of 

 the milk glands forming casein. However, chemists soon showed 

 that casein does not yield the decomposition products which are 



