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I had a leftover block of commercially bought frozen yeast. I kept it it the freezer, well below 0 celsius. I'll admit I didn't check on it for years.

I opened the container today. It was stinking like nothing I've ever seen. But it was also... liquid ? What on earth happened ? It stank so much, I just threw it all away without taking pictures or anything. But I now wonder, is this alive enough to generate enough heat to thaw itself ? What processes were going on ? Is this a typical/known issue ?

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TIL that there are organisms which not only survive temperatures under 0 Celsius, but are able to actively live in that range, as opposed to just hibernating. Nevertheless, I doubt that baker's yeast falls under them - it is the kind of factoid one would expect to read in popular books on food science. Also, it is unlikely that something adapted for life under such conditions would be most vigorous at +35ish, as yeast is.

My guess is that it thawed in much more prosaic manner. You either had a power outage that you were not aware of, or the cumulative effect of years of self-defrosting cycles was sufficient to enable several generations of proliferation.

You describe it as being both liquid and stinky - this sounds like they produced stuff and they suffocated in it, just like in overfermented dough. It would have been mostly ethanol, but mixed with other waste compounds. A pure water-ethanol mixture created by yeast won't freeze (it shouldn't go above the alcohol content of wine), but it is not outlandish that, with enough other molecules swimming around, the whole mixture was not frozen solid at slightly below zero. So once their "civilization" had multiplied enough to drown in its own waste, it may have stayed liquid, even though it was in the freezer. Also, for such a long period of time, some enzymatic reactions might have happened even at those unlikely temperatures, leading to further decay.

Bottom line, it seems that the yeast had an opportunity to multiply (at above zero) and took it.

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  • From personal experience, I find that yeast quality decreases considerably after three months in the freezers.
    – user141592
    Commented Jan 18, 2022 at 19:40
  • There's actually a whole bunch of organisms that live below 0C, but none that are relevant for this question.
    – FuzzyChef
    Commented Jan 18, 2022 at 19:43
  • @FuzzyChef : but if there was contamination (it was a "leftover block"), those other organisms may have been present and eaten the yeast or the medium in there to support it
    – Joe
    Commented Jan 18, 2022 at 21:24
  • Joe: the organisms that can live below freezing are generally speaking from the deep sea and the antarctic. While contamination of yeast with those might be possible, it's not likely.
    – FuzzyChef
    Commented Jan 19, 2022 at 1:52
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    @ChrisH indeed, now that I looked the exact numbers up, a pure water-ethanol mixture at 10-20% should freeze between -4 and -9 C. I don't know what exact compounds would have been produced by a dying yeast colony, but I think your intuition and mine are aligned - it is not surprising that, once the fermentation had happened, the result stayed liquid while the freezer was working normally and keeping other food frozen.
    – rumtscho
    Commented Jan 19, 2022 at 14:47

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