490 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



and partially digested curds are indicative of bacteria that are likely to 

 be harmful in the cheese. 



An improvement over the fermentation test of foreign origin has 

 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 undesirable organisms most likely to be present in milk are 

 those of the B. coli- aero genes group; therefore the jars containing the 

 curds should be kept at temperatures, 35 to 40, that will favor their 

 development. 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 determined, 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 sur- 

 face, 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 

 temperature 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.* The methods for the determination of acidity 

 in milk have very considerable limits of error. It is not possible to 

 detect any increase in acidity until the number of acid-forming bacteria 

 has increased to hundreds of thousands per c.c. Originally it was 

 thought that no acid was produced by the growth of the acid-forming 

 bacteria during the initial stages of their development. This period 

 during which bacterial proliferation was taking place, but without an 

 apparent increase in acidity, was known as the "period of incubation." 

 It is now certain that this rests upon our inability to detect small 



* In order to illustrate the r&le 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 cheese 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. 



