Updated on 3/21/2005
Most yogurts on supermarket shelves probably don't have the beneficial "live" or "active" cultures typically found in yogurt in countries such as Greece, Israel, Lebanon, or India. In fact, much of the custard-textured products sold in this country bear little resemblance to traditional yogurt containing live cultures, which I much prefer. Traditional is healthier, because it doesn't contain the sugars found in flavored yogurts, and because the live cultures help maintain "friendly" bacteria in the digestive system.
No matter how it ends up, all yogurt starts out by adding two types of bacteria, Lactobacillus bulgaricus and Streptococcus thermophilus, to milk (collectively, these bacteria are known as acidophilus). The combination is warmed for a few hours, during which the bacteria convert milk sugar (lactose) to lactic acid, which curdles the milk and gives it a distinctive, pleasant, tangy flavor. What happens next determines the fate of the bacterial cultures.
To give their products a longer shelf life, manufacturers often heat-treat yogurt after fermentation. This kills off the live cultures. The Food and Drug Administration (FDA) requires that these products be labeled "heat-treated after culturing." To determine whether the yogurt you buy contains living bacteria check the labels for the words "active yogurt cultures," "living yogurt cultures," or "contains active cultures." Don't be fooled by the words "made with active cultures." All yogurts are made with live cultures, but no live cultures survive heat-treatment. You can keep yogurt containing live cultures in the refrigerator for about two weeks. If mold and gas bubbles form, throw it out. But don't worry about greenish or clear liquid floating on top, this is the whey or liquid component of yogurt, it's harmless and can be stirred in before eating.
Andrew Weil, M.D.