296 SCIENCE AND PRACTICE OF CHEESE-MAKING 



therefore, they are often called "liquefiers." These 

 effects are due to enzyms which are produced by the 

 bacteria. Some of these bacteria form products that 

 are offensive in flavor ; some produce gases, and some, 

 acid. They may be a source of serious trouble in 

 cheese-making in the production of gassy curd and 

 offensive flavors in cheese. They may also cause 

 some dissolving of the curd, in which case the loss of 

 fat is unusually large. These bacteria are widely dis- 

 tributed, being found in stable filth, in soil, water and 

 floating dust. They are nearly always present to some 

 extent in milk. Fortunately, their activity is checked 

 by the presence of lactic acid, and the easiest method 

 of controlling such ferments in cheese-making is to 

 make conditions favorable for the rapid growth of 

 normal lactic acid bacteria; thi-6 is usually accom- 

 plished by the use of a pure starter. The growth of 

 digesting bacteria in milk is favored by high tempera- 

 ture; consequently, in hot weather, when the high 

 temperature favors the growth of the digesting bac- 

 teria more than it does the lactic acid organisms, the 

 undesirable forms get beyond control and seriously 

 impair the operations and results of cheese-making. 



BACTERIA PRODUCING UNDESIRABLE 

 FLAVORS 



Different bacteria are responsible for many different 

 kinds of bad flavors in milk and cheese, among which 

 are the following: Bitter (p. 119), fishy, rancid or 

 butyric acid, hydrogen sulphid (p. 116). 



YEASTS 



Yeasts are micro-organisms resembling bacteria in 

 some respects, but usually larger. They are very 



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