Why your kimchi stopped bubbling, and whether that's a problem

A jar of new kimchi is loud. Open it in the first week and it hisses, the brine fizzes at the edges, and lifting the lid releases a small sigh. Then, at some point, it goes quiet. This worries people enough that it is one of the most common questions we get.

The short answer: quiet is a stage, not a diagnosis. The bubbles arriving and the bubbles leaving both mean something specific, and neither of them is a health check.

Where the bubbles come from

The lactic acid bacteria on the vegetables eat the available sugars — from the cabbage itself, from the radish, from any pear or rice porridge in the seasoning paste. Some of the species involved release carbon dioxide as they work. That gas is the fizz.

So bubbling is not a sign of health in any general sense. It is a sign that a particular kind of bacteria is actively consuming a particular kind of fuel. When either of those changes, the bubbling changes with it.

Why it stops

Three reasons, usually overlapping.

The sugar ran out. Fermentation is not infinite. The easily available sugars go first, and once they are gone the gas production slows sharply. The jar is not finished — acidity keeps developing more slowly for months — but the visible, audible phase is over.

The jar got more acidic. The bacteria produce acid, the acid accumulates, and eventually the environment becomes hostile to the very organisms that made it. The population shifts toward more acid-tolerant species, which are generally less gassy. This is a normal succession and it is what turns bright young kimchi into the deeper, rounder thing people mean by 익은 김치.

You put it in the fridge. Cold does not stop fermentation, it slows it, and it slows gas production more noticeably than it slows souring. Kimchi moved from a warm kitchen to a fridge will often go quiet within a day while continuing to sour for months.

When quiet is worth investigating

Silence combined with anything below is worth a closer look. Silence on its own is not.

• Fuzzy growth. White, grey, green or black fuzz sitting on the surface is mould, and mould does not belong. A thin white film that is flat rather than fuzzy is more likely a yeast, which is harmless but signals the vegetables were exposed to air.
• Slime or stringiness. Brine that pulls into threads is a texture fault, generally traced back to insufficient salt.
• A smell that is not sour. Sour is correct. Sharp, funky, even faintly cheesy is correct. Rotten, sulphurous or solventy is not, and no amount of "it's supposed to smell strong" makes it so.

Keeping the vegetables submerged prevents most of this. The brine is the seal. Anything sticking up above the liquid is exposed to air, and air is what the problems need.

The thing people actually want to know

Underneath the question is usually a different one: is it still fermenting, or is it done?

It is still fermenting. Kimchi does not have a finish line. A jar at six months is doing less per day than a jar at six days, but it is still changing, still acidifying, still going somewhere. 묵은지 — aged kimchi that has been sitting for a year or more — is the far end of that same continuous process, and it is prized rather than tolerated.

So the absence of bubbles tells you where you are on the curve. It does not tell you the jar has stopped, and it certainly does not tell you the jar has gone bad. For that, use your eyes and your nose, which are the instruments this has always relied on.