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I made a vegetable soup with peas, onion, garlic, potatoes, celery, cubed chicken stock, salt, and olive oil. I put it in two containers, one open and the other airtight, and in the fridge. After 6 days, the soup in the open container had turned sour and had a vinegar taste, while the one in the airtight container had a normal taste. So I presume that oxidation plays a role in the vinegar taste.

Why does a vegetable soup turn sour?

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    What's the temperature of your fridge? Could it be due to fermentation?
    – Love Bites
    Sep 22, 2019 at 12:05
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    Sounds like it went bad. The open container would have picked up microbes floating in the air.
    – user50726
    Sep 22, 2019 at 18:09
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    @aris's comment is technically correct, but I would like to point out that a closed container would not have saved you from the soup going bad. Cooking is not sterilization, and there are enough microbes left over in your soup to spoil it even if you seal it hermetically.
    – rumtscho ♦
    Sep 23, 2019 at 7:30
  • @LoveBites An ambient temperature sensor shows 6ºC, 43ºF, but it could be lower in case it has limited sensing range.
    – miguelmorin
    Sep 23, 2019 at 15:09
  • @rumtscho If the microbes require air, the airtight container should have slowed their growth, which explains the different result with open and closed containers, right?
    – miguelmorin
    Oct 3, 2019 at 8:40

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A lot of types of bacteria (and sometimes other microbes) produce waste products that can taste "sour." And soup/stock is a good growth medium for microbes, which is why most food safety organizations recommend only keeping soup for 3-4 days in the fridge. Six days is past the point where the soup is still likely to be "fresh"; the sour taste is most likely spoilage of some sort.

As for the question of why the airtight container was different from the open one: it could be that the open container absorbed more microbes that were floating around in the air, or it could be that the open container allowed more oxygen for certain types of microbes to grow better, or both.

Bottom line: when soup turns "sour" unexpectedly, it's probably time to throw it out.

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  • Thanks! The consensus is that it was bacteria and not oxidation like I thought. The fridge has a sticker with "Anti-bacteria, natural silver". In any case, I can make my home soups last longer in an airtight container.
    – miguelmorin
    Sep 23, 2019 at 11:12
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    @miguelmorin - I'm not sure what an "anti-bacteria" fridge is, but no fridge can prevent random microbes from floating around after you open the door. Also, as rumtscho pointed out in comments, cooking is not sterilization. Some microbes could remain in spore form even after cooking, which can then spoil soup. And even small numbers of microbes present on the bowl you stored the soup in, or in the air while you were pouring the soup, could eventually grow. That the covered version stayed fresh longer could also be a coincidence. As I noted, soup/stock is generally an excellent growth medium.
    – Athanasius
    Sep 23, 2019 at 21:44
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    @miguelmorin the "anti-bacteria, natural silver" prevents microbial growth on the surface it is applied. In that case, if the bacteria stuck to the wall of the fridge they'll not grow, but fortunately for them, they fell on the soup.
    – Juliana Karasawa Souza
    Sep 24, 2019 at 16:53
  • Of course if your soup is a nice Polish barsch, fermenting in open air for a few days to make it sour is just part of the process.
    – Lee Daniel Crocker
    May 6, 2021 at 18:48
  • @LeeDanielCrocker - true, though I assume if the question was concerning such a process, the questioner would have said so. (As I noted in my final sentence: if soup turns sour unexpectedly.)
    – Athanasius
    May 18, 2021 at 15:23
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I was experiencing this, and found that I was the culprit. I had been dumping my used tofu water into the soup pot, to extend it, thinking it would at least have more flavor than just water. The fermentation that had leeched from the tofu was what did it.

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So when you are making a soup you should put in some of your veggies 30 min prior to serving like celery. What happens is when celery is over cooked it creates a acid that will ruin your soup, and sadly this is likely to happen if you have leftovers even when done properly. Now this can be countered or hidden with creams, carrots, or cabbage that add sweetness, but what I do is use celery seed which give it all the flavor without any down side. Take care though celery seed is very strong flavor so it only takes a little, and it should not be ingested by pregnant women as it is one of the herbs used for abortion just like nutmeg.

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