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So I left some grape juice out overnight, I forget how long, but now it has developed a "fizzy" taste. Also when you open the bottle you can hear the fizz escape. I've put it back in the refrigerator, is it still safe to drink? It tastes okay.

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    grapes are actually one of the items used for starting your own sourdough. I assume it's from whatever wild yeast might've fallen on them as they grew ... but I'd have expected bottled grape juice to be pasteurized, killing whatever yeast it might've gotten from the farm. – Joe Apr 2 '15 at 8:37
  • Without knowing what exactly colonized your juice it's impossible for any of us to know if it's safe or not. – Mr. Mascaro Apr 2 '15 at 13:53
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    @zanlok - This is the process that makes wine, so if you think wine is safe to drink, then this should be to. It might be really, really bad wine, but that's basically what is happening. – PoloHoleSet Oct 9 '17 at 15:22
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I'd be very wary of this, if only because it seems to have fermented remarkably quickly. The fizz is likely the result of carbon dioxide being produced by yeast eating the sugars in the grape juice; this is the same process that carbonates beer or sparkling wines.

The thing is, most of these yeasts are introduced deliberately, and they take a while to do their job. In my experience with home brewing, it would generally take at least a few days for the yeast to begin producing significant carbonation. If you truly did just leave this out overnight and it's already noticeably carbonated, that seems to be a very aggressive or efficient strain of yeast. Since it's impossible to know for sure, the safe thing to do would be to throw it out.

Wild fermentation of this kind is used to produce certain beer styles, and other fermented goods such as sauerkraut, so it's not unsafe by definition. I would exercise caution here only because that level of carbonation seems very unusual to me, and grape juice is unlikely to be precious enough to justify the possible hazard.

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    Some kind of reference to natural or wild yeasts that are dangerous might be in order. Otherwise, I'm not seeing the need for wariness. – PoloHoleSet Oct 9 '17 at 15:24
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It's probably safe to drink. If it's fizzy, but doesn't smell bad it's likely a natural grape yeast from the white bloom on the skin of the grapes.

Wine makers test their brew by smell and taste and only use chemistry to find out how much a shift of chemistry is predicted to get it to taste the way you want.

  • I suppose depending on the origin, but if it's from a cartoon wouldn't it be pasteurised? – ctokelly Apr 2 '15 at 18:30
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    Probably, but some yeasts are rather heat hardy, champagne yeast comes to mind. It only takes one survivor. – Escoce Apr 2 '15 at 18:34

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