Kefir is a thick, fermented milk originating from the Caucasian mountains, made with kefir grains; a combination of bacteria, yeast and polysaccharides. Kefir is often referred to as a 'probiotic' as it is thought to contain bacteria and yeasts that are beneficial to the body. Officially a probiotic or not, we would say that it is certainly a rich source of natural live cultures.