Kefir, Buttermilk - alternatively milk kefir, or bulgaros, is a fermented milk drink made with kefir "grains" (a yeast/bacterial fermentation starter) and has its origins in the north Caucasus Mountains. Kefir is a popular drink across Eastern and Northern Europe. It was consumed in Russia and Central Asian countries for centuries, but is now becoming popular in Japan, the United States and Europe.