230 MILK HYGIENE 



if the coagulum is beginning to dissolve, or if the milk 

 remains in a fluid state but has a bad odor and taste, 

 these are signs that the milk is contaminated with bac- 

 teria of putrefaction. In the application of this test, it 

 is important not to allow the milk sample to stand too 

 long before the examination is begun, and precautions 

 must be taken to prevent contamination while gathering 

 the sample and during the test. 



In pasteurized milk, the fermentation test may give 

 general information concerning the bacterial content, 

 through noting the time that passes until appreciable 

 changes take place. Usually such milk does not 

 ' ' sour. ' ' But no precipitate conclusion should be drawn 

 from the results of this test. 



By boiling the milk for a short time and then incu- 

 bating the samples, a serviceable guide can be obtained 

 concerning the quantity of the spore bearing bacteria 

 in the milk. These will survive the heating, and, as they 

 are not checked in their growth by the lactic acid form- 

 ing bacteria, they increase rapidly and cause the milk to 

 curdle, by the action of ferments. 



[Eussell ®^ describes a curd fermentation test used by 

 cheese makers, as follows: " When the milk is 95° F., 

 about 10 drops of rennet extract are added to each sam- 

 ple and mixed thoroughly with the milk. The jar should 

 then remain undisturbed until the milk is completely 

 curdled; then the curd is cut into small pieces with a 

 case knife and stirred, to expel the whey. The whey 

 should be poured off at frequent intervals until the curd 

 mats. If the sample be kept at blood heat (98° F.), for 

 six to eight hours, it will be ready to examine. 



" The curd from a good milk has a firm, solid tex- 



" H. L. Russell, Dairy Bacteriology, Sixth Edition. Madison, 

 1905. 



