MILK AND ITS PRODUCTS 149 



patrons are expected to have milk as free as possible 

 from these germs so that it may arrive at the creamery 

 in a sweet condition. They are therefore expected to 

 thoroughly cool and care for it, not alone to suppress 

 the action of the lactic acid bacteria but also that of the 

 abnormal species that might have gained access to the 

 milk. 



While the acid bacteria are objectionable in milk, in 

 cream made into butter they are indispensable. The 

 highly desirable aroma in butter is the result of the 

 growth of these organisms in the process of cream 

 ripening. There are a number of different species of 

 bacteria that have the power of producing lactic acid. 



2. CURDLING AND DIGESTING FERMENTATION. 



In point of numbers this class of bacteria ranks perhaps 

 next to the lactic acid type. Indeed it is very difficult to 

 obtain milk that does not contain them. It is not often, 

 however, that their presence is noticeable owing to their 

 inability to thrive in an acid medium. 



According to bacteriologists most of these bacteria 

 secrete two enzymes, one of which has the power of 

 curdling milk, the other of digesting it. The former 

 has the power of rennet, the latter of trypsin. "As a 

 rule," says Russell, "any organism that possesses the 

 digestive power, first causes a coagulation of the casein 

 in a manner comparable to rennet." 



It is only occasionally when the lactic acid organisms 

 ,ire in a great minority, or when for some reason their 

 action has been suppressed, that this class of bacteria 

 manfests itself by curdling milk while sweet. The curd 

 thus formed differs from that produced by lactic acid in 

 being soft and slimy. 



