The belief is prevalent among the Mohammedan tribes of the Caucasus that keffir grains were, in the first instance, presented by Allah, as a sign of immortality, to one preferred tribe. Others hold that, in past ages, they were found by shepherds growing on a shrub in the Caucasian highlands; while, according to Skolotowski,[47] they were originally found adhering to the walls of an oaken vessel used for the preparation of airam. This is a soured milk beverage similar to keffir, but possessing a weaker alcoholic fermentation, and prepared from goats' milk by the addition of pieces of calf's stomach. This would undoubtedly serve to introduce various species of lactic acid bacteria, and will be referred to in the portion dealing with soured milks. Keffir is prepared by the Caucasians from cows', sheep's, or goats' milk, and the operation is carried on in large leathern tubes or bottles. After the addition of the grains or seeds to the milk the vessel is placed in a cool chamber, and the fermentation is allowed to proceed for one or two days, by the end of which time the normal fermentation is at an end. During this period the keffir grains have increased enormously in size, assume a bright yellow colour, and lose their sour buttery smell. Previous to the removal of the fermented liquid, a portion of the bottle is firmly bound from the rest by a stout cord, and the greater portion of the remaining keffir is quickly removed for use, thus avoiding, as far as practicable, any outside infection. After the addition of fresh warm milk the cord round the end of the bottle is removed, and the old and new milk thoroughly mixed for a time in order to ensure uniform inoculation of the new milk for the next fermentation. During the winter months the leathern vessels are often placed in the sunshine, so that the temperature remains at 61° to 65° F. The necessary agitation of the vessel is said to be supplied in the form of kicks by passers-by or by the children during their play. The beverage prepared in this way is so gaseous in character that it is often blown forcibly from the vessel during removal, and possesses, according to Podowyssozki,[48] a very acid taste. During any interruption in the preparation of keffir in the above manner, the grains are taken out, and after having been well washed in clean water, are spread out on a clean cloth to dry in the sunshine. They thereby assume a characteristic cheesy or buttery odour and become rather darker in colour. Thorough desiccation is essential in order to prevent subsequent mouldiness or disease of the grain.