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I love to add Greek yogurt to soups and chili as a garnish on top. When I plunk it into steaming food like that, am I effectively destroying the beneficial bacteria?
Yogurt is made by incubating bacteria in milk. The bacteria, which include Lactobacillus, streptococcus, bifidobacteria, and other species, ferment lactose in warm conditions and the lactic acid that is produced not only gives the yogurt its tart flavor but also acts on the proteins to create a silky texture.
When you purchase good quality yogurts they have at least some degree of live cultures. You can, in fact, make your own yogurt from other yogurt. Simply heat a quart of milk to 185°F and allow it to cool to around 110°F. Mix in a couple of tablespoons of Dannon or Stoneyfield or another yogurt that contains live cultures, then keep in a warm place at about 113°F. There are a lot of yogurt machines that will maintain that heat precisely.
Don't forget to save a couple of tablespoons of your new yogurt to act as a starter.
Sadly, if you heat the yogurt to over about 140°F, as you would with a soup, it is likely that you are killing all that good bacteria. That's not to say that you should stop. A couple of tablespoons of tart yogurt in your chili goes a long way to adding both flavor and a creamy texture to your dish, so it's not all wasted.
Timothy S. Harlan, MD, FACP