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 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 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 Caucasians. 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 hulgaricus. If Freudenreich's descrip- 
tion is accurate, B. Caucasians differs from B. hulgaricus 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 Caucasians. 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 a lactic- acid and a mild alcoholic fer- 
mentation in milk will make kefir, although to secure the most 
desirable flavor certain organisms may be essential. 
