The yeast which is used in the preparation of kefir is found in grains of various sizes, and has an appearance something like cauliflower. Their color, when dry, is yellowish or reddish, while, when moist, they are almost white. This yeast is composed chiefly of the ordinary yeast used in the preparation of beer, namely: saccharomyces cerevisiœ.

Formerly this yeast was difficult to secure, it being carefully guarded as of sacred origin, and could rarely be bought. During the last 20 years it has come gradually into commerce, until now it can be obtained in large quantities in the markets of the Caucasus.

In the country of its origin it was customary to give a quantity of this yeast as a dower to a daughter of the house upon her marriage. The yeast itself is supposed to be derived originally from the same yeast which produces the koumys. It is thought by growing a koumys yeast in cow's milk it has gradually been transformed into its present form of kefir grains.