Elan learned that "when I looked at conventional yogurt on the shelf... most of it isn't actually probiotic." Many yogurts pasteurize after they ferment, which "kills off all the bacteria," and the strains that make yogurt taste good aren't necessarily good for your gut.
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Elan explains two reasons conventional yogurt loses its probiotics. First: "Many yogurts will do what's called pasteurize after they ferment, and what that does is it kills off all the bacteria. So now you have this yogurt that can last a really, really long time, but it has no active cultures in it." There's a brand incentive to do this — the time from production to a fridge in someone's house is long, especially producing in New York but selling in California, so brands extend shelf life by killing off bacteria that could make it go bad. Second: "yogurt made with bacteria strains that are really, really good for making yogurt, but not necessarily really good for your gut." There are tens of thousands of strains, and the ones that are awesome at making yogurt aren't designed to benefit your microbiome. Elan started fermenting bacteria designed for the microbiome into the yogurt she was already eating every day — that became Sourmilk.