2l8 BACTERIA 



be members of the acid-producing class; but probably the 

 flavour is not dependent upon the acid. Moreover, the 

 aroma of good ripening is also probably independent of 

 the acid production. 



Of all the methods of ripening — natural ripening, the 

 addition of natural starters, the addition of pure cultures 

 with or without pasteurisation — there can be no doubt that 

 pure culture after pasteurisation is the most accurate and 

 dependable. The use of natural starters is a method in 

 the right direction; yet it is, after all, a mixed culture, 

 and therefore not uniform in action. In order to obtain 

 the best results with the addition of pure cultures, Professor 

 Russell has made the following recommendations: 



1. The dry powder of the pure culture must be added to 

 a small amount of milk that has been first pasteurised, in 

 order to develop an active growth from the dried material. 



2. The cream to be ripened must first be pasteurised, in 

 order to destroy the developing organisms already in it, and 

 thus be prepared for the addition of the pure culture. 



3. The addition of the developing starter to the pas- 

 teurised cream and the holding of the cream at such a 

 temperature as will readily induce the best development of 

 flavour. 



4. The propagation of the starter from day to day. A 

 fresh lot of pasteurised milk should be inoculated daily 

 with some of the pure culture of the previous day, not the 

 ripening cream containing the culture. In this way the 

 purity of the starter is maintained for a considerable length 

 of time. Those starters are best which grow rapidly at a 

 comparatively low temperature (60-75° F.), which produce 

 a good flavour, and which increase the keeping qualities of 

 the butter. Now, whilst it is true that the practice of using 

 pure cultures in this way is becoming more general, very 

 few species have been isolated which fulfil all the desirable 

 qualities above mentioned. In America starters are pre- 



