FERMENTED MILK 497 



threads. In the outer layer yeast cells are found mingled with 

 the bacteria. When the grains are added to milk, they swell 

 and increase in size by forming new grains. At the beginning 

 of the fermentation they settle to the bottom, but in a short 

 time they are carried to the surface by attached bubbles of 

 gas. If the fermentation is active, a thick layer will be formed 

 on the surface, but on shaking or stirring this layer settles 

 again to the bottom. 



The biology of kefir was studied by Kern in 1881, but, 

 owing to the faulty technique of that day, his descriptions are 

 evidently erroneous. 



Freudenreich describes four organisms that he isolated from 

 kefir grains. One of these was a yeast which he designated 

 Saccharomyces kefir; this he found to grow best at 22 C. 

 (72 F.), but not at all at 35 C. (95 F.). It ferments maltose 

 and cane-sugar, but not lactose. It gives a peculiar flavor to 

 milk, but causes no fermentation. The cells are oval, 3 to 5 

 microns by 2 to 3 microns. It is not identical with the ordinary 

 beer yeasts. Two of the organisms were of the lactic acid 

 bacteria type, but differed from them in forming gas in lactose 

 media. The most interesting of the organisms described is a 

 long, slender bacillus corresponding to one described by Kern 

 as Dispora caucasica and to which Freudenreich gave the name 

 Bacillus caucasicus. In morphology, failure to grow on ordi- 

 nary laboratory media, and in high-acid production in milk, 

 this bacillus resembles very closely the bacillus mentioned 

 later, in connection with yogurt, as Bacillus bulgaricus. If 

 Freudenreich's description is accurate, B. caucasicus differs 

 from B. bulgaricus by forming gas from lactose and in being 

 feebly motile. Gas was formed slowly at 35 C. and still more 

 slowly at 22 C. (72 F.). No one of these organisms alone 

 produced kefir, but when the four together were grown in milk 

 typical kefir was produced on the first or second transfer. 



According to the investigations of Nikolaiewa, three or- 



2K 



