GENERAL CHEMISTRY OF MILK 105 



normal alkali for neutralization when phenolphthalein was the 

 indicator, while human milk required but 2 c.c. per liter. 



Acidity is higher at the beginning of lactation than at later' 

 periods. Milk from diseased cows is often more alkaline than 

 normal milk. 



VARIABILITY IN THE COMPOSITION OF MILK 



It has been mentioned that milks from different species of 

 mammals contain practically the same constituents, but that the 

 proportion of these substances varies within wide limits. Varia- 

 tion in composition is by no means confined to difference in species; 

 in fact, there are conditions which cause considerable variability 

 in the composition of milk within the same species. Further- 

 more, there are differences in composition of milk according to 

 breed, individual character, and even in the milk from the same 

 individual. The limits of variation in the composition of milk 

 from different species may be seen from a table by Leach: 



LIMITS OF VARIATION IN COMPOSITION OF MILK FROM DIFFERENT MAMMALS 



This table shows that the protein, fat, and sugar are subject 

 to considerable variation, and that consequently the total solids 

 and specific gravity also vary in proportion. It is obvious that 

 the taste and food value must be different in the milks according 

 to the prevalence of one or the other constituent. 



Cow's milk has been studied more intensively than any other 

 milk, and there are many observations referring to variations in 

 the composition of cow's milk. As a general rule it can be stated 

 that the fat is the most variable character; the protein follows 

 next in importance, while the sugar is less variable, and the ash 

 relatively constant. 



It is clear that the variability in composition of milk is of 

 economic importance. The food value of milk is proportionate 



