SOURING OF MILK. 447 



From these results lie draws conclusions as under : 



Milk held at 20 (7. (68 J.). In the unheated milk the lactic 

 bacteria increased rapidly, and the milk became acid in about 

 twelve hours. The peptonising bacteria increased in six hours 

 to about 5,000,000 per cubic centimetre, and then decreased 

 slowly. 



In the heated milk the peptonising bacteria increased rapidly 

 after twelve hours, and the milk was usually curdled in forty- 

 eight hours, with a disagreeable taste and odour. Occasionally 

 lactic bacteria survived pasteurisation and multiplied rapidly 

 after twenty-four hours, completely inhibiting the peptonising 

 bacteria. 



Milk held at 10 C. (50 F.).ln unheated milk the growth 

 of the bacteria and the consequent curdling of the milk was 

 much retarded. The average milk did not contain sufficient 

 acid to affect the taste until it was over forty-eight hours old. 

 The proportion of peptonising to lactic bacteria was greater than 

 at the higher temperature, and the taste of the milk occasionally 

 showed the influence of the former. 



In the pasteurised milk the bacteria increased very slowly, 

 and in nearly every case the milk was unchanged in taste and 

 appearance ninety-six hours after pasteurisation. In only two 

 of fourteen cases was there a marked increase of peptonising 

 bacteria. The predominating bacteria were species having little 

 or no effect on milk. 



The lactic bacteria inhibited the development of the pepton- 

 ising bacteria only when they had developed sufficient acid to 

 render the milk unfit for use. 



It seems probable that the acid had a distinct inhibitory action 

 on the proteolytic enzymes of the peptonising bacteria. 



Distribution of Micro-organisms on Separating. When 

 milk is separated by centrifugal force, both the cream and 

 the separator slime contain a larger proportion of organisms 

 than the original milk ; this is partly due to the organisms 

 being carried down mechanically, but partly also to an actual 

 separation taking place, as the following experiment will 

 show : 



A sample of separated milk was run for fifteen hours in a 

 centrifugal machine at the rate of 1,000 revolutions per minute. 

 Cultivations on gelatine were made from the top portion, the 

 middle, and the bottom. The results were, after eight days 

 at 22 : 



Colonies per r ^ 7 c.c. Remarks. 



Top, . .197 Growth rapid, about 20 per cent, liquefied. 



Middle, . 5 



Bottom, . 194 Growth slow, none liquefied. 



