I started making kefir two months ago; the grains had grown ten times. I use organic whole milk, sometimes 2%. Today the smell went from sour to a chemical smell, like Acetone. What's going on? Is it still good?
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It sounds like the odor is pretty common when the kefir starts to ferment.
This link has the following advice:
Why does my kefir smell like nailpolish remover or vomit?
Acetone which gives off the 'nail polish remover smell' is a normal process of fermentation and is present in very small amounts (and is not dangerous). But once your brew smells strongly of this, it is best to continue to ferment as usual, but not drink the kefir. Most times it will go away on its own (as it is usually a temporary imbalance in the yeast and bacteria). If not, it is best to discard the grains and start anew. This can happen when over-active yeast uses the dissolved oxygen too rapidly and are not completely fermenting all the sugar (which can allow it to be too available to other invading bacteria and yeast). Most of the invading bacteria that can cause problems do not tolerate too low of an acid environment (ph 2-4) High Butyric acid levels (also present in small amounts naturally) smell like vomit. These can found quite often on dirt or tea leaves (generally a quite common bacteria) and will most likely pass once you get your ferment in a safe ph level for a couple ferments.