MILK AND ITS HYGIENIC RELATIONS 



CHAPTER II 



THE GENERAL COMPOSITION OF MILK 



A. The Average Composition of Human and Cows' Milk. The 

 composition of milk is such a well-worn subject that it would 

 not be unreasonable to believe that complete information was 

 available. Definite information upon the composition of the milk 

 of numerous species is given in many text -books upon milk, and 

 this information has been gradually accepted. 



Recent investigations have shown, however, that there is no 

 precise composition which can be assigned to the milk of any 

 species. A figure giving the average composition of a number 

 of samples can be obtained, but a considerable degree of variation 

 is exhibited in the figures of the different samples. In addition 

 to the divergence in composition due to individual differences in 

 the members of the species, great variations occur in several of the 

 constituents from other causes. Such causes are the period of 

 lactation, the condition of the gland, whether full or nearly empty 

 at the time of taking the sample. Further, in cows the breed 

 exercises an important influence on the amount of certain consti- 

 tuents of the milk. 



Although the composition of milk does not show a constant 

 figure, it is useful as a foundation from which to consider the 

 variations, which will be referred to later. 



In this chapter only known chemical substances will be con- 

 sidered. The less tangible bodies, sometimes known as the ' biolo- 

 gical properties,' will be dealt with separately in Chapters V. and VI. 

 The cells and the cell content of milk will also be considered later. 



It will be convenient to consider the chemical composition of 

 human milk and cows' milk at the same time. The main differences 

 in composition of average samples of human and cows' milk respec- 

 tively as obtained by various observers for certain of the main 

 constituents of the milk are shown below : 



