350 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



been devised by Babcock and Russell and is known as the Wisconsin 

 Curd Test. It has for its basis the same principle as the simple fermen- 

 tation test; however, a modification is introduced; the milk is curdled by 

 the addition of rennet and the curd is cut and drained to free it from the 

 whey as completely as possible. The jars employed in testing are kept 

 at temperatures favorable for the kinds of bacteria sought; those for the 

 colon group, requiring a higher optimum temperature than the lactic 

 bacteria, should have a temperature of incubation from 35 to 40, those 

 for the lactic group from 30 to 35. The great advantage of the Wisconsin 

 Curd Test is its greater delicacy, since the bacteria are concentrated in a 

 small volume, and thus their presence is more evident than would be the 

 case in the larger mass of curd obtained when no rennet is added. The 

 curd can also be removed from the jar, cut, tasted, and its texture deter- 

 mined, all of which aid in judging the quality of the milk. The curd 

 should have a clean acid odor and taste; it should be free from sliminess 

 on the surface, and possess a uniform texture. Such a curd can be 

 obtained only in the presence of a considerable number of lactic bacteria. 

 Very clean, fresh milk is likely to give an undesirable result, since milk 

 always contains microorganisms which will grow rapidly at the high tem- 

 perature in the absence of the acid-forming bacteria and which will 

 usually produce undesirable flavors in the curd. This fact should be kept 

 in mind in the testing of market milk. 



RIPENING OF MILK.* It has been shown that the growth of acid- 

 forming bacteria in milk is not followed by a parallel increase in the acidity 

 of the milk. Indeed milk may contain hundreds of thousands of acid- 

 forming bacteria per c.c. and yet the acidity be no greater than when the 

 milk was drawn from the cow. Ultimately the acidity begins to increase, 

 and for some time increase in bacteria and acidity run parallel. The 

 period during which bacterial proliferation is taking place but without 

 corresponding increase in acidity is known as the "period of incubation. "f 

 Its length is determined by the temperature at which the milk is kept and 

 by the amount of initial seeding with acid-forming bacteria. 



* In order to illustrate the role of microorganisms in the making and ripening of cheeses, a 

 somewhat detailed summary of the present knowledge concerning their action in Cheddar cheese 

 will be given. Many of the factors concerned in the ripening of this kind of cheeses also function 

 in the ripening of other rennet cheeses. In their description only such additional factors need be 

 considered as are not active in Cheddar cheese. 



j" This should not be construed to mean that no acid is produced during this "so-called" 

 period of incubation, but that such acid does not respond to the usual tests and may be found in 

 combination with or neutralized by the constituents of the milk, such as casein. 



