204 AGRICULTURAL BACTERIOLOGY 



Control of flavor. The modern methods of butter-mak- 

 ing are designed to give the maker control over the kinds 

 of bacteria that are concerned in the fermentation of the 

 cream. The first step in this direction was to add to the 

 cream selected pure cultures of lactic bacteria that had 

 been isolated from varied sources, and tested as to their 

 favorable flavor-forming properties and their ability to 

 grow at the ripening temperatures. The organisms are 

 propagated in milk that has been heated sufficiently to de- 

 stroy all of the non-spore-forming bacteria. This is then 

 inoculated with the pure culture of lactic bacteria. If the 

 butter-maker exercises care in the prevention of contamina- 

 tion and in the control of the temperature at which it is 

 kept, this pure culture starter can be maintained for a 

 long period of time. He is thus in a position to grow the 

 desirable organism in any quantity, for addition to the 

 cream that is to be fermented or ripened. 



The lactic bacteria are quite resistant to desiccation. 

 The commercial laboratories make use of this property, and 

 market the pure cultures of the organism, not only in milk, 

 but in a dried condition. These are usually prepared by 

 adding the fermented milk to a relatively large amount of 

 some inert substance, such as milk sugar or milk powder, 

 and drying the mass at a low temperature. The organisms 

 remain alive in this condition much longer than where they 

 are exposed to their by-products, as in milk. 



Pasteurization of cream. If the starter is added to raw 

 cream, its effect will depend on the number of bacteria 

 present in the cream. If the cream is sweet and clean, the 

 number of bacteria added in the starter will greatly ex- 

 ceed those naturally found in the cream, and the fermenta- 

 tion will be largely due to the added bacteria. If, however, 

 the cream is nearly sour from the bacteria already present, 

 the fermentation may not be much influenced by the starter. 



