BUTTER 203 



Acid-fermentation and flavor of butter. As earlier set 

 forth, two groups of acid-forming bacteria are concerned in 

 the souring of milk. The lactic group tends to predominate 

 in milk that is produced under clean conditions, since it 

 grows more rapidly at ordinary temperatures than does the 

 colon group, which, by reason of the products formed, in- 

 jures the taste and odor of the milk, and hence the flavor of 

 the butter. In tin- lactic fermentation not only is lactic 

 acid produced, but also other acids, alcohols, and esters that 

 impart to the sour milk and cream an agreeable odor and 

 taste. Butter-fat has the power of absorbing some of these 

 volatile substances, just as it will absorb the odors of fruits 

 when kept in the same container with them. The difference 

 in flavor between the sweet and ripened cream butter is, 

 therefore, due to the absorption of some of these products 

 during the ripening of the cream and the churning process. 

 Cream that is allowed to sour spontaneously will usually 

 contain many more lactic than colon organisms, and the 

 butter made from it may be of very excellent quality. 



As long as butter was made on the individual farm, no 

 great need of control was felt ; but as its manufacture be- 

 came centralized in creameries, it became necessary to con- 

 trol not only the kind of organisms growing in the cream, 

 but also the rate at which the fermentation should go on. 



The bacterial content of cream, both qualitatively and 

 quantitatively, will depend on the method by which the 

 milk has been creamed. If it has been allowed to stand in 

 shallow vessels at the fluctuating air temperatures and ex- 

 posed to contamination from the air, it will contain greater 

 numbers and more varied kinds of bacteria than if the 

 creaming takes place at low temperatures and in covered 

 containers. If the centrifugal separator is employed, the 

 bacterial content of the cream will be similar in kind to 

 that of milk. 



