340 CREAM AND CREAM RIPENING 



75 F. in winter, for twenty-four to forty-eight hours, 

 until it has become sour but not curdled. 



The development of more than .6 to .7 per cent, of 

 acid should be avoided as likely to weaken the useful 

 lactic bacteria, and give an opportunity for the butyric 

 and putrefactive types to grow. 



After removing the cream from the surface the skim- 

 milk may be used to inoculate the cream to be ripened. 

 If a larger quantity of starter is needed, that already 

 prepared in the above manner should be mixed with the 

 requisite amount of separated milk, which has been pas- 

 teurized at a temperature of 180 F. for half an hour, 

 and the whole then allowed to sour. Some prefer not 

 to use the spontaneously soured milk directly for in- 

 oculation, but the second or third subcultures of it in 

 pasteurized milk. 



In any case starters of this kind are the result of 

 chance, and are not always satisfactory ; bad taints 

 difficult to get rid of may be propagated by them. 

 Trial alone will settle whether a sample prepared in this 

 way is good or bad. 



Ex. 154. Prepare a home-made starter by the method just 

 described. 



Plate out and determine the kinds of organisms in it. 

 Try their action upon milk separately. 



Pure Culture Starters. With a view of reducing 

 failures to a minimum many attempts have been made 

 to prepare pure cultures of bacteria which, when intro- 

 duced in sufficient numbers into cream will lead to good 

 uniform ripening. 



Very many kinds have been isolated from milk, cream, 

 and other dairy products and tested from this point of 



