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I've made fermented garlic paste many times before.

It's essentially a bunch of raw garlic cloves blended with 2% salt by weight, then left in a container to ferment at room temperature for 2-3 weeks before being stored in the fridge. The paste turns a deep amber colour and should have a deep, savoury garlic smell.

However... my latest batch had a problem. I made it exactly as before except this time I made more than usual and stored it in a large, deep, bucket-like container.

I used it for a few weeks and it looked/smelled/tasted totally fine, until one day I went to use it and I noticed that the paste beneath the surface in the middle of the container looked a lot lighter in colour (white instead of amber) and it had a really strong, acrid smell.

It wasn't a "rotten" or mouldy smell; it just smelled extremely bitter, like if you've ever put a pill in your mouth without water.

There was no sign of mould but I discarded the entire container anyway (3KG of garlic wasted!).

What might have happened, and how can I avoid it happening in future? Is it possible the garlic under the surface didn't ferment at all? That would explain the difference in colour. Or was there some other chemical reaction?

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It must have fermented with a different strain than usual.

Fermentation is a process in which you start out with a food contaminated with dozens of species of bacteria, all competing with each other. You provide conditions known to be perfect for the development of a desired strain (or multiple strains) and that strain outmultiplies all other bacteria, taking away the resources they'd need for a living. In result, your preferred strain colonizes the food completely, and you can eat it.

This tends to work, but there are ways for it to fail, by either having the wrong conditions, or the wrong bacteria present. The conditions can change even when you don't realize they are different - maybe the ambient temperature is different, or the garlic heads were drier or wetter than usual, changing the water activity, or they were from a batch that was invisibly different from typical batches (just normal cultivar variation) and hadn't produced something that inhibits a given bacterial strain or had a slightly different pH. Even if the change was minimal, it might have brought the system over a tipping point

The other possibility is simply a random contamination. The conditions prescribed for a ferment are supposed to favor a given strain above others typically present on garlic. But it is possible that an unexpected bacteria or yeast strain was present, which likes the fermentation conditions just as much, or even more than, the desired one. Then it was the one to outcompete all others, including the one you want.

Because now your garlic is thoroughly colonized by an unknown bacterial strain, it is not safe to consume. It doesn't matter if the new smell is off-putting or not; you just cannot know what lives in there and what it will do to a human. Only successfully fermented products are safe to eat.

There are no feasible ways to prevent this in the future, as a normal person who does a few batches of fermented food at home. You just have to live with the fact that a few of your batches will be duds. If this was happening too frequently, it would be a sign of a recipe not suited to your environment, and you should be looking for different recipes. But if it is only once in a while, everything is as it should be.

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    “Fermentation is a process in which you start out with a food contaminated with dozens of species of bacteria, all competing with each other.” It does have a bit of an eeewww! ring, doesn’t it? LOL! And nevertheless we so much enjoy the results if everything turns out right.
    – Stephie
    Jan 5, 2023 at 15:17
  • @Stephie indeed, it does have an ewww ring. And I can shock even more users by mentioning that all food we eat is contaminated with bacteria, mold, and other microscopic stuff. Maybe we should rethink the didactic value of telling small children that bacteria equal illness.
    – rumtscho
    Jan 5, 2023 at 15:47
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    If it happens more than once, it's time to scrub all of your equipment and your whole kitchen with vinegar and/or bleach. That means that the bad bac is present in the environment.
    – FuzzyChef
    Jan 5, 2023 at 19:15
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    @WackGet commercial producers would find out what all relevant environmental variables are, and create a process where they are kept constant, at their optimal levels. Also, in some cases, commercial producers process their products in a sterile and/or oxygen-poor atmosphere, although this is more common in fermenting animal products, such as making yogurt. I wouldn't try an innoculation - 1) since the fermentation works in most cases, it is unlikely that your problem is the absence of the desired organism, 2) if such an obvious and easy step was helpful, it would have been incorporated...
    – rumtscho
    Jan 8, 2023 at 13:38
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    ... in the original recipe, and 3) any change to a fermentation recipe makes it unsafe. For example, you cannot know what is going to grow when the moisture coming out of the garlic cloves is not immediately absorbed by dry salt, because you'll have introduced a wet component. Your fear is valid: there is a risk that next time, the fermentation will fail again. There is no way to do anything about reducing that risk. If you don't want to carry that risk, the only thing you can do is to give up on garlic fermentation altogether.
    – rumtscho
    Jan 8, 2023 at 13:43

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