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Kefir is a fermented milk product that originated in southeastern Europe. It is like yogurt but is more fluid. Streptococcus and Lactobacillus bacteria convert lactose to lactic acid, giving the drink its tartness. Lactose-fermenting yeast produce a small amount of alcohol. Globules of yeast organisms float to the top and give it a grainy look. It is used as a drink and as a substitute for yogurt in quick breads, casseroles, soups, etc. Once you have kefir you may make more just as you would use yogurt to start another batch of yogurt. Kefir is available at many health food stores, both as a ready-made item and in granular form for starting your own.