336 CREAM AND CREAM RIPENING 



ment, from 500,000,000 to 1,500,000,000 are present ; 

 few other natural liquid media contain so many. 



After reaching this point the numbers decline with the 

 age of the cream ; in four or five days a few millions 

 only are found, and at the end of a week nearly all 

 living organisms have disappeared except the fungus 

 Oospora lactis. 



There is thus a periodic rise and fall in the number of 

 bacteria in cream similar to that which has been observed 

 in milk when it is kept. 



In freshly-raised cream there is a miscellaneous collec- 

 tion of different kinds of bacteria. Liquefying varieties 

 are common, as well as others which have little apparent 

 action on milk or gelatine media ; lactic acid forms are 

 comparatively rare. 



During the first twenty-four hours after the milk is 

 drawn from the cow the growth of all the kinds goes on 

 simultaneously, and large numbers are carried by the 

 upward stream of fat globules into the cream layer 

 after the milk has been " set " for a time. About 2 to 

 20 per cent are liquefiers ; 10 to 75 per cent, neither 

 produce acid nor liquefy gelatine ; the rest are lactic 

 acid varieties. 



As ripening proceeds the latter multiply enormously, 

 until when the cream is thirty-six to forty-eight hours old 

 they form 95 to 99 per cent, of the many millions present ; 

 all other varieties decrease to very small proportions, or 

 die out altogether. 



Of the lactic acid bacteria found in ripened cream, 

 St. lacticus and a nearly allied form are far the most 

 abundant. Bact. lactis aerogenes is also invariably met 

 with at all stages of the process, but is never found in 

 large numbers when ripening runs its normal course, 



