What happens during kefir fermentation process?
Well essentially the added Lactobacillus species ferment lactose and other sugars to lactic acid, thus lowering the pH of the product (they take up lactose use it as an energy source under anaerobic conditions and secrete lactic acid). This gives the kefir and other fermented milk products their sourness. Also the lower pH prevents many other bacteria from growing in the product so this is also serves as a natural preservation method. Further more the low pH can denature some of the protein in the milk/kefir resulting in a more solid form. Many of these Lactobacillus species or their relatives are natural part of our gut flora.