164 



DAIRY BACTERIOLOGY 



water bath. Failing a proper thermostat, the temperature may 

 be kept fairly constant by means of a spirit lamp or gas burner, 

 provided that a large well-insulated water bath is used, and that 

 the temperature of the room does not vary too much. The 

 samples are examined after twelve and twenty to twenty-four 

 hours. If quite fresh, the milk will remain liquid for twelve hours. 

 The sooner visible alteration occurs, the greater the importance 

 to be attached to the results of the test. On the other hand, milk 



01 02 03 



FIG. 66. Gelatinous Types. 



&1 &2 &a 



FIG. 67. Blown Types. 



containing relatively few bacteria gives unreliable indications, 

 duplicate tests often showing different types of fermentation. 



According to Peter's classification, we may distinguish between 

 the following types in the fermentation test : Fluid (/), gela- 

 tinous (g), blown (b), spongy (s) and cheesy (c) (see his illustrations). 

 In the gelatinous type the true lactic acid bacteria predominate ; 

 a perfectly homogeneous coagulum is denoted by g v a coagulum 

 with very few streaks and bubbles by g 2 , and one with few streaks 

 and bubbles by g 3 . In the blown (gassy) milk the pseudo lactic 



