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I recently made preserved lemons using a recipe from Jerusalem: The Cookbook. Having poked around a variety of recipes online, it's pretty much the same as all the rest: Lots of salt, squeeze out the juice, fill up the container with more fresh juice, add some rosemary and a red pepper. Float a little oil on top. Let sit in a cool, dark place for 30 days.

I opened the lemons last night, and a considerable amount of gas escaped from the lemon juice, which bubbled for about 2-3 minutes before calming down. The cap on the jar wasn't distended, and the mixture wasn't malodorous in any way. My instinct is to say, "This is bad; throw this away", but I haven't canned anything before so I'm not sure what to expect. What little information I can find on google is either contradictory or tongue-in-cheek and suggest it's fermentation.

What's up with my lemons? Are the gas bubbles escaping from the mixture a sure sign of spoilage?

EDIT: After a week, I'm still not not sure which answer to accept, as this, too, contains contradictory information... I suppose time and votes will tell.

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This is totally not a problem.

This style of lemon preservation relies on fermentation. The salt is not intended to halt all fermentation- it just restricts it to the tasty kind. Fermented pickles are a common and traditional way to preserve food because the salt and acid and thriving tasty bacteria make a very inhospitable environment for bad bugs.

The production of gas is just a symptom of the fermentation and not a problem. I regularly make Indian lemon pickle, which sounds like a similar recipe, and it produces some amount of gas. Recipes will typically call for the lid to be gently closed to allow some of the gas to escape- just to prevent the bottle from exploding.

This is not your typical canning. Typical canning uses acid or high temps to stop all bacterial growth. In that kind of canning gas production would be a very bad sign and you would have to throw it out.

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It's probably fermentation, and as that requires microbial activity it's a sign your food has not been preserved. Unless the recipe specifically says this is expected and desired I would not eat it.

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The recipe you describe is a "wild fermentation" recipe.

So, as @Sobachatina said, the bubbles are a good sign, not a bad sign.

The strategy with "wild fermentation" is to create an environment that gives "good microbes" an edge over "bad" ones, in such a way that their advantage continues to increase over time. Salt gives the good ones an initial edge--most bad microbes are halophobic. Beyond that, most good microbes digest carbohydrates and produce CO2 plus acids and/or alcohols, and most good microbes tolerate acidic environments and up to moderate amounts of alcohol better than the bad microbes. At some point, the baddies have zero chance of making any progress.

Canning is a very different strategy: create a hermetic seal, then kill them all, good and bad, with heat. The challenge is that if you fail to seal it tight, or if you fail to kill them all, then what you have is a tabla rasa for whatever microbes wind up in there. And some of the microbes that enjoy a mid-pH high-moisture anaerobic environment can cause serious problems, like botulism!

So for fermented foods, bubbly = good. For canned foods, bubbly = bad.

PS I would discourage you from putting a float of oil on top of any ferment. Little bits of stuff suspended in oil can create just the sort of environment that botulism microbes enjoy. Lemons are acidic enough that it may not be a problem in this case, but it's a bad practice.

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You didn't "can" anything this time, either. You made "SaeurLemon" ( a salt-fermented "cure" like sauerkraut, only with lemons, not cabbage) - Gas is perfectly normal.

If you HAD "canned" it, bubbles would be bad. There, you're sterilizing (or trying to) the food with high heat and vacuum sealing it. It should remain pretty much as canned, and there should be a vacuum seal until you open the jar. Anything making gas in "canned" food is highly suspect. That is not what you did...

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This sounds to me like a wild fermentation. Yeast will happily consume the sugars in your jar and create carbon dioxide (the gas you noted) and perhaps some ethanol and various flavor esters. This in itself isn't necessarily a problem, but where yeast can thrive so can many other organisms. The key to avoiding this in the future is to make sure everything that touches the ingredients is sanitized (the jar, spoons, juicer, etc). What you have many not be too bad (depends on what else is growing in there) but it probably won't taste good either. I'd toss it and try again.

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    The problem with a wild fermentation is you don't know what did it. It might not even be yeast at all, but some other spoilage organism that can make you very very ill.
    – Escoce
    Commented Mar 9, 2015 at 20:02
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    Actually, wild fermentation is incredibly safe. It's one of the oldest preservation methods we have. By packing that jar with salt and lemon juice he's created a selective environment that keeps bad bacteria out. There is no oxygen in contact with the lemons and they're in a low pH, high salinity solution. The biggest concern is mold growing on the surface, where oxygen is in contact with the solution. We may not know which yeast caused the bubbles, but we can assume it's a good one. C. Botulinum is also not a concern. Although, it is heat tolerant, it is vulnerable to the low pH.
    – Joe Flynn
    Commented Mar 9, 2016 at 14:51

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