LACTIC ACID FERMENTATION 201 



that the yeasts present are those capable of producing the 

 enzyme lactase, therefore, of fermenting lactose. 



Sour Curd Cheese. — When milk that has been soured 

 is heated, the curd contracts, the whey is expelled and may 

 readily be drained away. The curd may then be worked or 

 molded, or mixed with butter or various flavoring materials, 

 and converted into a sour milk cheese usually termed Dutch 

 cheese or cottage cheese. This type of cheese is not usually 

 subjected to a ripening process. 



The Use of Starter. — While cream may be churned into 

 butter without any preliminary fermentation and while 

 butter from this sweet cream is preferred in certain locali- 

 ties and for certain purposes, nevertheless, most of the 

 butter sold in the United States is churned from cream 

 which has been allowed to ripen, that is, it has been allowed 

 to undergo a process of acid fermentation. The flavor and 

 aroma of the butter will depend very largely upon the type 

 of organisms which have been growing in the cream. The 

 butter fat absorbs considerable amounts of dissolved sub- 

 stances present in the fermenting cream, it is, therefore, 

 highly desirable that these should be of the proper char- 

 acter. If batches of cream are allowed to ripen spontane- 

 ously without the addition of any starter, there is apt to be 

 a marked lack of uniformity in the product. A much better 

 and more uniform result is secured by inoculating the 

 cream which is to be ripened with a considerable amount of 

 desirable lactic acid bacteria. Such organisms are sold 

 under the name of commercial starters. It is generally 

 assumed that they are pure cultures, or practically pure 

 cultures, of Streptococcus lacticus or some very closely 

 related organism. This is introduced into pasteurized milk 

 and allowed to grow until the milk has reached a satisfac- 

 tory state of acidity. With this a larger bulk of milk is 

 inoculated and finally the whole mass used as a starter for 

 the cream which is to be churned. A heavy inoculation with 



