PROTEIDS OF MILK 5 



in the urine. Presumably when lactose is ingested it is changed in 

 the alimentary canal into some form of sugar, possibly dextrose or 

 galactose, which is assimilable.^ Very large amounts of dextrose 

 can be absorbed into the blood without producing glycosuria in a 

 normal animal, unless, indeed, the assimilative powers have been 

 reduced, as, for example, by starvation. The change from unas- 

 similable lactose to some assimilable form may take place rather 

 during than before the passage of the sugar through the intestinal 

 walls. This non-assimilability of lactose is certainly remarkable 

 when it is remembered that it is in this form that young animals 

 receive their carbohydrate food. 



The proteids op albuminoids of milk are now generally con- 

 sidered to be four, namely, casein, lactalbumin, lacto-globulin, and 

 Storch's mucoid. The first and by far the most abundant is casein, 

 or as it occurs in milk, according to Professor Halliburton, caseinogen.^ 

 It is this proteid which is acted upon by rennet and converted into 

 casein. The lactalbumin and lacto-globulin only exist in small 

 quantities (about one sixth to one seventh of all the proteid) and 

 of Storch's mucoid there is only a trace. For all practical purposes, 

 indeed, we may say that the albuminoids of milk are two, casein 

 and lactalbumin. During lactic acid fermentation — due to lactic 

 acid bacteria — the caseinogen of milk is precipitated, and certain 

 other fermentative bacteria are able to produce the same effect. 

 But the agency by which coagulation is most readily formed is that 

 of rennet. Rennet is an enzyme produced in the stomachs of 

 mammals, and generally prepared from the fourth stomach of the 

 calf Caseinogen is not, as we have already pointed out, in perfect 

 solution in the milk plasma, but is present in all probability along 

 with the fat in both the milk globules and the proteid particles, in 

 a state of minute division, and in partnership with phosphate of 

 lime. This opalescent solution has been credited with causing, in 

 part, the whiteness of the milk. The globules contain a maximum 

 of fat and a minimum of caseinogen. The particles contain a 

 minimum of fat and a maximum of caseinogen. In nature casein- 

 ogen is probably never seen apart from fat, and in milk the fat 

 never apart from caseinogen. The two substances appear to be 

 produced side by side by the metabolic activity of the protoplasm 

 of the cells of the acini of the mammary gland, so that caseinogen 



^ The Chemical Basis of the Animal Body, by Sheridan Lea, F.R.S., 1892 

 edition, p. 115. 



^ Caseinogen may be precipitated from milk by the addition of such acids as 

 acetic, or by saturation with salts such as sodium chlorideand magnesium sulphate. 



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