What makes yogurt - well, yogurt? The words "live and active cultures" refer to the living organisms, Lactobacillus bulgaricus and Streptococcus thermaophilus, which convert pasteurized milk to yogurt during fermentation. Note that the milk is pasteurized before culturing to remove any harmful bacteria. The process is very similar to that used when making beer, wine or cheese, in that beneficial organisms ferment and transform the basic food. This fermentation process is what creates yogurt, with its unique taste, texture and healthful attributes. Yogurt is a fermented milk product which originated in Turkey in which a mixed culture of Lactobacillus bulgaricus (or occasionally L. acidophilus ) and Streptococcus thermophilus produce lactic acid during fermentation of lactose. The lactic acid lowers the pH and makes it tart and causes the milk protein to thicken. The partial digestion of the milk when these bacteria ferment milk makes yogurt easily digestible. In addition, these bacteria will help settle GI upset including that which follows oral antibiotic therapy by replenishing non-pathogenic flora of the gastrointestinal tract.