FERMENTED MILKS. 15 



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 (45) in 1881, but, owing 

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

 erroneous. 



Freudenreich (25) describes four organisms that he isolated from 

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

 charomyces he-fir; 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 organ- 

 isms 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 connec- 

 tion with yogurt, as Bacillus bulgaricus. If Freudenreich's descrip- 

 tion 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 (64), three organ- 

 isms are always present in the fermented milk. One of these, Bac- 

 terium caucasicum, which forms the filament of the grain, is evidently 

 identical with Freudenreich's Bacillus caucasicus. This investigator 

 considers this bacterium, with a torula yeast fermenting lactose, dex- 

 trose, and cane sugar, as essential to the production of kefir. Other 

 bacteria and yeasts are found in the grains and the fermented milk, 

 but they are looked upon as contamination. 



It is probable that kefir is produced under different circumstances 

 by different organisms. Any combination of bacteria or of bacteria 

 and yeasts that will produce ;i bictic-acid and a mild alcoholic fer- 

 mentation in milk will make kefir, although to secure the most 

 desirable flavor certain organisms may be essential. 



