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I come from a Polish family and a constant of every christmas is a spicy fermented beet soup. My family has been fermenting the beets exclusively in a terracota vessel on a hot radiator when the heating is on for the winter. That's the way my mum and grandma are doing it, and the way their mothers done it before them. They claim that it's imperative that the temperature of the vessel doesn't drop, cause that risks the entire batch going bad.

Now, that is pretty much against everything I read about temperatures and fermenting (i.e. colder is tastier and safer, warmer is more spoilage-proof). I mean, especially considering that food-on-a-radiator sounds like a danger zone party for the beets.

On the other hand, that is the traditional method, nobody ever got sick in my family and it does indeed not work out when done cold. I heard of warm fermentations being done in the context of brewing beer, but that's a different kind of fermentation so I'm assuming it's unrelated. So I'm pretty puzzled about this.

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  • Not quite a vegetable fermentation, but dosa batter is fermented at 30C as well.
    – FuzzyChef
    Jan 17 at 20:34
  • related: How to make beet kvass in winter Jan 18 at 9:33
  • also see this stack of questions over on the Homebrewing SE - I've flagged this question for consideration to be migrated. Jan 18 at 9:37
  • @SteveTaylor the rule on the network is: if a question was posted on a site where it is on-topic, it should not be migrated to a different site, no matter whether it is on-topic on the other site or not. Migration is a way for salvaging questions which have to be closed.
    – rumtscho
    Jan 18 at 10:26
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    @SteveTaylor the site has historically always covered fermented food. I am not aware of a Meta discussion whether fermented foods should be included (I think everybody implicitly has always thought it should), but a few years ago I seem to have written a Meta answer about that same line: cooking.meta.stackexchange.com/questions/2087. I suppose we should edit the topic page to make it clearer.
    – rumtscho
    Jan 18 at 10:41

2 Answers 2

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Cultured Guru provides a good detailed overview of the vegetable fermentation process, including the types of bacteria that tend to dominate each phase of the process. Please see the source link for a more detailed overview of each stage.

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As for the question of temperature, the fermentation curve will vary between different vegetables and microbial cultures, but anything in the range of 20-30 degrees Celsius does seem to be something of a 'sweet spot' for most types of fermented products. Nourished Kitchen has a fermented beets recipe for 'room temperature', but I suppose that then depends on the typical temperature of your room!

Considering that this is a traditional recipe from Eastern Europe, where winters tend to be cold and bitter, it's understandable that the safest bet was to keep it on the radiator and avoid it dipping too low. In a modern thermostat-controlled house colder fermentation should be safe and possible, as Nourished Kitchen suggest.

As you suggest, higher temperatures do increase the risk of spoilage bacteria flourishing, though. Perhaps there's something else in your recipe that's inhibiting them somehow?

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While I cannot provide a full answer, I will address one specific part of your question.

Now, that is pretty much against everything I read about temperatures and fermenting (i.e. colder is tastier and safer, warmer is more spoilage-proof).

This is simply not true. Each fermentation recipe has its own optimal temperature range. If you go outside of the range - making it either too warm or too cold - it becomes unsafe. Outside of the range (and towards its fuzzy borders!) you risk breeding a different type of microorganism than usual, and the new ones may or may not be pathogenic. And you cannot recognize a bad recipe from the temperature range it prescribes, since you cannot predict what culture it was optimized for.

Bottom line, the safe thing is to follow fermentation recipes exactly, and not to second-guess them.

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  • It's not always possible to follow recipes exactly though. Like in that case, I'm not going to be able to source beets with the exact microbial flora of my family's region etc etc. But I do get the gist of what you're saying!
    – KubaFYI
    Jan 18 at 20:24

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