Plain kefir is a cultured milk beverage traditionally made by fermenting cow's milk with kefir grains, a community of lactic acid bacteria, acetic acid bacteria, and yeasts held together in a natural polysaccharide matrix. Goat's milk can also be used to make kefir. During fermentation, these microorganisms consume some of the milk's lactose and produce lactic acid, carbon dioxide, small amounts of alcohol, and other fermentation compounds, giving kefir its characteristic tart flavor and lightly effervescent texture. Plain kefir provides protein, calcium, phosphorus, vitamin B12, riboflavin, and other nutrients found in milk, along with live microorganisms and compounds created during fermentation.