

MILK FERMENTATIONS 65 



determined. The important questions are those that admit 

 of a positive or negative answer. Those which involve de- 

 tailed descriptions are of less importance to the beginner; 

 hence, with a few exceptions, not so much emphasis is laid 

 on the cultural characteristics as on the morphology and 

 the biochemistry of the organism. 



Preliminary cultivation. Whenever an organism is to be 

 studied and an attempt made to determine to what partic- 

 ular group it belongs, it is important that it be in a state of 

 high vitality, as otherwise uniform results cannot be obtained. 

 In nearly all cases the optimum conditions for the growth 

 of an organism are not known. It has been determined that 

 by a preliminary cultivation the constancy of the results 

 obtained may be materially increased. The standard proce- 

 dure of preliminary cultivation is as follows : From an agar 

 culture of the organism a broth culture is made, and incu- 

 bated for twenty-four hours at 20 C. Transfer from this 

 broth culture to a second tube of broth, and incubate for 

 twenty-four hours at 20 C. ; repeat a third time. In order 

 to be certain that the final trials are made with a pure 

 culture, from the third broth tube gelatin plates are pre- 

 pared. From one of the colonies on the gelatin plates in- 

 oculate a tube of sloped agar and incubate for forty-eight 

 hours at 20 C. From this the subsequent inoculations are 

 to be made into various media. 



This method of restoring the vitality of the organism pre- 

 supposes that it will find favorable conditions for growth on 

 the ordinary nutrient media and at a temperature of 20 C. 

 With some groups of bacteria, important, in the dairy, this 

 method of preliminary cultivation should be modified. The 

 great group of lactic-acid bacteria grow very poorly, often 

 not at all, in the ordinary nutrient broth. If, however, this 



