Kefir (alternately keefir, kephir, kewra, talai, mudu kekiya, milkkefir, búlgaros) is a fermented milk drink that originated in the Caucasus region. It is prepared by inoculating cow, goat, or sheep's milk with kefir grains. Traditional kefir was made in skin bags that were hung near a doorway; the bag would be knocked by anyone passing through the doorway to help keep the milk and kefir grains well mixed.
Traditional kefir is fermented at ambient temperatures, generally overnight. Fermentation of the lactose yields a sour, carbonated, slightly alcoholic beverage, with a consistency similar to thin yogurt. Kefir fermented by small-scale dairies early in the 20th century achieved alcohol levels between 1% and 2%, but kefir made commercially with modern methods of production has less than 1% alcohol, possibly due to reduced fermentation time.
Variations that thrive in various other liquids exist. They may vary markedly from kefir in both appearance and microbial composition. Water kefir (or kefir d'acqua) is grown in water with sugar (sometimes with added dry fruit such as figs, and lemon juice) for a day or more at room temperature.
Production of traditional kefir requires kefir grains which are a gelatinous community of bacteria and yeasts. Kefir grains contain a water soluble polysaccharide known as kefiran that imparts a rope-like texture and feeling in one's mouth. Kefir grains cannot be produced from scratch, but the grains grow during fermentation, and additional grains are produced. Kefir grains can be purchased or acquired from other hobbyists, see below. Kefir grains appear white to yellow and are usually the size of a walnut, but may be as small as a grain of rice.
In addition, kefir grains will ferment milk substitutes such as soy milk, rice milk, and coconut milk, as well as other sugary liquids including fruit juice, coconut water, beer wort and ginger beer. However, the kefir grains may cease growing if the medium used does not contain all the growth factors required by the bacteria (which are all present in mammalian milk).
Milk sugar is, however, not essential for the synthesis of the polysaccharide that makes up the grains (kefiran), and studies have demonstrated that rice hydrolysate is a suitable alternative medium. Additionally, it has been shown that kefir grains will reproduce when fermenting soy milk, although they will change in appearance and size due to the differing proteins available to them.