24 SCIENCE AND PRACTICE OF DAIRYING. 



larger globules appears to be finer in flavour, and to possess a more oily 

 appearance. 



Bouchardat and Quevenne drew attention, as early as 1857, to the fact 

 that the average size of the fatty globules in human milk was different 

 from that found in the milk of cows or of sheep. It is probable that the 

 average size of the fatty globules of cows' milk, in the same cow, is not at 

 all times, and under all conditions, the same; and that in the case of 

 different cows, perhaps also in the case of different breeds of cows, even 

 under similar circumstances, the size varies. On this subject we know as 

 yet very little. The methods, according to which the numbers and the 

 determination of the average size of the fatty globules have been made, 

 are the same as have been applied for the purpose of counting the number 

 of yeast cells, blood corpuscles, &c., and consist of utilizing very fine capil- 

 lary tubes of glass. 



Milk -fat is soluble in ethyl -ether, chloroform, carbon bisulphide, 

 benzine, &c. The common solvent is ethyl-ether. 



7. Milk-sugar. Milk-sugar occurs in solution in the milk of all 

 mammals, but does not elsewhere occur in nature. It is a carbo- 

 hydrate, and is one of the sugars capable of being converted directly 

 into alcohol by means of fermentation. In German milk the per- 

 centage of milk-sugar ranges between 3 and 6 per cent, and is on 

 an average 4 '6 per cent. 1 



In a state of solution, as it is in milk, the milk-sugar quickly 

 and easily undergoes decomposition, and is converted into lactic 

 acid. This is effected by a large number of different kinds of 

 bacteria, the so-called lactic bacteria. The transformation of milk- 

 sugar into lactic acid may, or may not, be accompanied by the 

 formation of small quantities of carbonic acid, with or without 

 alcohol. As the bacteria which give rise to the formation of lactic 

 acid are to be invariably found more or less abundantly on the 

 cow's udder or in the byre, in the dairies or in the vessels contain- 

 ing the milk, and have therefore easy access to the milk, it follows 

 that milk, on keeping, becomes sooner or later subject to lactic 

 fermentation. As soon as a sufficient quantity of lactic acid is 

 produced, milk sours and becomes unsuitable for its chief uses, both 

 in the house and the dairy. In milk which has become spontane- 

 ously sour, several kinds of lactic bacteria may be identified. With 

 regard to one kind of bacteria, viz. the bacillus acidi lactici, I. 



1 The same holds good for English milk. American milk ranges from 4 to 5'5, with an 

 average of 4'95. (See Aikman's Milk: its Nature and Composition, p. 11.) English Editors, 



