ocess of
koumiss-making to keep the milk in a state of agitation during the
period of fermentation, a process which is intended to permit of oxygen
being taken up by the fermenting fluid, while, at the same time, the
casein is broken up into a state of fine division. The casein also, or
at least a portion of it, becomes very soluble, and after twelve hours
of fermentation the taste of the product is only slightly sour, and the
milk taste still remains. This taste, however, disappears in
twenty-four hours, owing to the rapid development of the lactic acid
organisms. After this lapse of time the sugar is entirely destroyed, and
the strong koumiss which results is a thin sour fluid which effervesces
briskly, and in this condition will keep for an indefinite period. "The
net change which has taken place in the original milk may be summed up
by saying that the sugar of the milk has been replaced by lactic acid,
alcohol, and carbon dioxide, the casein has been partly precipitated in
a state of very fine division, and partly pre-digested and dissolved,
while the fat and salts have been left much as they were."[24]

Violent stirring or agitation of the cultures does not seem to work so
much by supplying oxygen to the fermenting liquid, as by ensuring a
thorough distribution of the micro-organisms throughout the liquid, and
thus dividing the casein.

The greater number of the organisms are facultative anaerobes and oxygen
is not necessary. Again, koumiss put up in bottles on the first day is
regularly shaken although air is excluded.

_Keffir._--Keffir is a kind of fermented milk which has been in use in
the Caucasus for quite a long time, as koumiss has been in the steppes.
It differs from koumiss, however, in this respect, that it is prepared
from either sheep's, goats', or cows' milk. The process is started by
the addition of keffir grains to the milk, which is contained in
leathern bottles. These keffir grains are small solid kernels which are
kept in families and handed on from one generation to another.[25] The
grains are the origin of the ferment, as they disseminate in the milk
micro-organisms of a lactic yeast (_Saccharomyces kefir_ Beyerinck and
Freudenreich) and also the bacillus _Bacterium caucasicum_, which
develop rapidly and split up the milk sugar into carbon dioxide,
alcohol, and lactic acid. Small quantities of glycerine, acetic,
succinic, and butyric acids are also formed, the casein and albumen
being partl