108 EXPERIMENTAL DAIRY BACTERIOLOGY 



curd, would be more evident. The curd mass in the Wiscon- 

 sin Curd Test can also be removed from the jar, its texture 

 determined, the nature of the surface of the curd noted, and 

 thus the presence of undesirable bacteria detected. 



This test for the quality of milk is also used in the exami- 

 nation of market milks. It should be remembered that a milk 

 of very high grade may give poor results when tested in this 

 manner, especially if the results are interpreted from the 

 usual standpoint. A curd of fine texture is only possible 

 when the milk contains a considerable number of lactic 

 organisms, since the acid they produce changes the curd so 

 that the small pieces gradually fuse into one mass and show 

 the flaky texture desired by the cheese maker. With a milk 

 produced under such conditions as to exclude the lactic bac- 

 teria (certified and sanitary milk), the matting of curd will 

 be largely prevented and the texture will be faulty. On the 

 other hand, a milk of poor quality from the standpoint of 

 the milk consumer, on account of its high bacterial content 

 and consequently poor keeping qualities, may produce a firm, 

 solid curd, free from gas holes. 



Each student should make curd tests from various samples 

 of milk. In case milks illustrating the various types cannot 

 be obtained, they may be prepared by inoculating fresh milk 

 with pure cultures of lactic-acid organisms ; gas-forming and 

 liquefying bacterial mixtures may also be used. Gas-form- 

 ing types are readily secured by adding a small quantity .of 

 manure to milk. The milk should be inoculated and allowed 

 to stand over night, in order to give a chance for the inocu- 

 lated organisms to develop before the curd tests are made. 

 If facilities permit, the instructor can have a small cheese (10 

 pounds or so) made from milk contaminated with gas-form- 

 ing organisms, so that the course of changes in the cheese 



