1 88 ECONOMIC BACTERIA IN MILK AND MILK PRODUCTS 



organisms per c.c. unripened, and 350,000,000 organisms per c.c. 

 ripened. Other examples of unripened cream averaged more than 

 90,000,000, but when ripened averaged over 300,000,000. Normally 

 ripened cream probably averages four or five hundred millions of 

 bacteria per c.c. which is greatly in excess of any other natural 

 media. The number of organisms in unripened cream varies 

 much more widely than in ripened cream, which is due to the fact 

 that " unripened " cream is examined at different stages of the 

 ripening process. What we have already stated as characteristic 

 of souring milk is also true of cream. In both we have detected 

 the " tides " of organisms. At first the ordinary miscellaneous and 

 extraneous bacteria ; and secondly, after some thirty hours, the 

 acid producing organisms. Conn and Esten have carried out a 

 series of experiments on cream of a parallel nature to our own 

 experiments on milk, and the results are full of interest. They find 

 that the number of acid-producing bacteria in unripened cream 

 varies from 50 to 97 per cent, whereas in the ripened cream the 

 number varies from 97 to 100 per cent. The total number of 

 bacteria present at the maximum is independent of the number 

 present at the minimum. The maximum is commonly reached 

 within forty-eight hours, after which there is a decline, as occurs 

 in milk. After a week has elapsed a few acid organisms remain, 

 and an abundance of Oidiuni lactis. 



The most characteristic feature of cream ripening is the growth 

 of the acid producing organisms, chiefly B. acidi lactici, and the 

 decline of the liquefying and extraneous organisms. B. acidi 

 lactici is found in very small numbers in fresh milk as we have 

 already pointed out, and also in cream. But as the ripening process 

 proceeds, with uniform regularity, the numbers of this organism 

 rapidly increase, until ultimately it reaches its maximum, when it 

 furnishes 90 per cent, of all the millions of organisms present. It 

 has been suggested that this bacillus is the cause of the ripening 

 process. Against that suggestion it has to be remembered that 

 cream is ripening within the first twelve hours, when the B. acidi 

 lactici and other acid producing bacteria are present in small 

 numbers, and the liquefying and miscellaneous organisms pre- 

 dominate. It would, therefore, appear that other bacteria in addition 

 to the acid bacteria have some effect upon the cream ripening. Conn 

 and Esten think that the process consists of two phases, the first 

 comprising the first twelve hours or more of ripening, and due to 

 the growth of miscellaneous bacteria ; the second beginning after 

 twelve hours, and due almost wholly to the growth of lactic acid 



