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I have a bottle of store bought, commercially produced kefir that seems to have undergone a second fermentation in my fridge. It's normally tart and thick, but it's changed to really tart and even thicker, and has a slight fizziness. I've never seen this happen before; normally this stuff remains unchanged in the bottle for months.
What could have happened and is this safe to eat?
It sounds like the kefir was further fermented by a second, more aggressive stain of acetic or lactic acid bacteria introduced after opening. It's likely safe, but probably not very tasty. And as with any unexpected microbial activity: When in doubt, throw it out.
