Another widespread form of fermentation is the metabolism of sugars into lactic acid by lactic acid bacteria. Many varied foods and beverages are products of this process: sauerkraut, kimchi, and other fermented pickles are lactic acid fermentations, in which plant sugars are metabolized into lactic acid by bacteria found on all plant life. Yogurt, kefir, and many cheeses also rely on lactic acid fermentation of milk sugar lactose. Yogurt and kefir are cultured by specific microbial communities that define the ferments, but lactic acid bacteria are present in all raw milk and will spontaneously clabber (sour) the milk if it is left unrefrigerated. Grains, too, will yield lactic acid after spontaneous fermentation.