With the exception of long-life yogurt, all yogurts contain active but friendly bacteria and are therefore 'live'. Yogurt is a cultured product, produced by fermenting milk with bacteria. Several kinds of bacteria can be used, but two of the most common are Lactobacillus bulgaricus
and Streptococcus thermophilus
To make yogurt, the milk is first heated to destroy any unwanted micro-organisms, then a yogurt starter containing the chosen bacteria is added to set and sour the milk. This produces a refreshing, slightly acidic flavor. The bacteria remain active even while the yogurt is refrigerated, so the level of acidity gradually increases. This limits the storage life of yogurt, which will eventually come to taste unpleasantly tart.