ADVANCED CHEMICAL PHYSIOLOGY 



329 



In everyday life the two milks of the greatest importance are cow's 

 milk and human milk. These two milks vary in composition : 



Milk. 

 Cow. 

 Human 



Water. 

 87-8 

 87-6 



Protein. 

 3-4 

 20 



Fat. 

 3-7 

 3-2 



Carbohydrate. 

 4-7 

 6-4 



Salts. 

 0-7 

 0-3 



The milks also differ in that (1) the proportion between the amount 

 of the kinds of protein in cow's and human milk is different. 



Cow's milk, 2-7 per cent, caseinogen, 

 Human , -80 



50 per cent, lact-albumin. 

 1*1 



It will be seen that human milk contains far more lact-albumin than 

 does cow's milk, so that even when cow's milk is diluted there is the 

 discrepancy between the relative amount of the proteins to be taken 

 into account. 



(2) The caseinogens of the two milks are not of the same composition 

 either in percentage or actual composition. 



(3) The percentage of the salts present differs in the two milks 



ASH CONTENT. 



In order to study the chemistry of milk, we usually employ cow's 

 milk, because it is easily obtainable. 



Cow's Milk. This is an opalescent solution, possessing a character- 

 istic taste, and of amphoteric reaction. 



EXPERIMENT I. Place a drop of fresh milk on a piece of glazed red 

 litmus paper, and wash it off with distilled water ; a blue stain is left : 

 if the drop be placed on blue litmus, a red stain is left. This peculiar 

 reaction is due to the fact that milk contains a mixture of acid and 

 alkaline salts. By ascertaining how much decinormal acid or alkali 

 are required to produce neutralisation with the aid of different indicators 

 the amount of each of these kinds of salt can be determined. 



The specific gravity of fresh milk varies between 1-028 and 1-0345. 

 The more fat (i.e. cream) the milk contains the lower is the specific 

 gravity. 



EXPERIMENT II. Estimate by a hydrometer the specific gravity 

 (a) in skimmed milk and (b) in fresh milk. In the former it is about 

 1-0345, in the latter 1-028. By adding water to (a) the specific gravity 

 obviously falls, and by removing the cream from (b) it rises. 



Fresh milk does not coagulate on boiling, but a skin forms on its 

 surface. A similar skin is produced when any emulsion containing 

 protein is boiled, and in the case of milk it is composed in part of 



