Kefir.. really healthy but why is it sour, carbonated and slightly alcoholic?!
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Kefir is made by a process of fermentation, which is the same process used for yogurt, bread dough or for making alcoholic beverages like beer and wine. Specialized bacteria are introduced to milk and they begin to convert the natural sugars it contains. As the bacteria work their way through the milk, they produce several by-products, including carbon dioxide (which is the carbonation), lactic acid (which gives it sourness), and alcohol.
EDIT TO ADD: Where I got it from? I dunno, it's mostly common knowledge. Have you ever made bread before? Try just adding a package of dried yeast to some warm-ish water or milk and then watch it for about 20 minutes. You'll be able to see all of this happen right in front of your eyes. It will poof up in a foamy mass and have that yeasty sour smell.
If you need sources for an essay, try looking up some articles about "fermentation." Wikipedia can explain the basics:
THe sourness comes from lactobacillus. I never thought of it as alcoholic, but if it is, the carbonation and alcohol must come from sacchromyces, or Yeast.