Why Is My Sauerkraut Fizzy? And How Long Does It Last?
Short answer: Fizzy sauerkraut is usually a good sign. During natural fermentation, the bacteria that make sauerkraut sour also release carbon dioxide. That gas creates bubbles in the brine, pressure inside the jar, and sometimes pushes a little brine out of the lid. In a raw, unpasteurised sauerkraut, gentle fizz means the ferment was alive and active.
If you have opened a jar of sauerkraut and heard a hiss, seen bubbles rising through the brine, or found a little leaked liquid in the bag, here is what is going on.
Why sauerkraut gets fizzy
Traditional sauerkraut begins with cabbage and salt. Salt draws moisture from the cabbage to create a brine, and naturally present lactic-acid bacteria ferment sugars in the cabbage into acids. That process also releases carbon dioxide gas.
The gas has to go somewhere. Some of it dissolves into the brine, like a very gentle sparkling drink, and some collects at the top of the jar as pressure. When you open the lid, that pressure escapes with a hiss, and the dissolved gas can rise through the brine as bubbles. That is the fizz.
Fermentation slows right down in the fridge, but it does not stop completely. A raw sauerkraut can keep quietly working in the jar, which is why fizz can show up days or weeks after packing.
Normal signs of active fermentation
In a naturally fermented sauerkraut, all of these are normal:
- small bubbles rising through the brine
- brine that turns cloudy over time
- a soft hiss or pressure when you first open the jar
- brine pushed up toward the lid, or a little leakage
- a clean, sour, fresh aroma
- a little white sediment at the bottom of the jar
Fizz is often strongest in a fresh jar or a warm room, and much quieter once the jar is well chilled. If your sauerkraut is not fizzy at all, that is fine too. Cooler temperatures and slower batches are part of how a living food behaves. We answer more questions like this in our FAQ.
Why the brine sometimes leaks
Brine leakage is usually just physics. Gas pressure builds inside the jar, and liquid expands as temperatures change, for example during delivery on a warm day. The pressure pushes brine past the lid seal and a spoonful or two escapes.
A little leaked brine on the outside of the jar does not mean the sauerkraut inside is off. Pop the jar in the fridge when it arrives, wipe the rim, and keep the cabbage covered by the remaining brine where you can.
How to open a fizzy jar
If you suspect a jar is lively:
- Chill it in the fridge first. Cold sauerkraut fizzes less.
- Open it slowly over the sink, with the lid pointed away from you.
- Press the cabbage back under the brine with a clean fork.
- Keep the jar refrigerated between serves.
How long does sauerkraut last?
Keep mine in the fridge. Unopened, it keeps for up to 3 months (check the best-before date on your tub). Once it's open, enjoy it within 1 month. Three habits keep it at its best:
- Keep it cold. The fridge slows fermentation right down.
- Keep the cabbage under the brine. Press it back down after every serve.
- Use a clean fork every time, never one that's been in your mouth or in other food.
While it's in your fridge it will keep slowly souring, because it's still alive. That's normal. If you ever see the warning signs below, throw it out.
Fizzy versus spoiled: when it is not normal
Fizz itself is not a warning sign. Genuine spoilage looks and smells different. Throw sauerkraut out, or at least the affected portion, if you see:
- visible mould in larger patches, especially fuzzy or coloured growth
- a slimy texture through the kraut
- a rotten, putrid or otherwise off smell rather than a clean sour one
- pink or discoloured kraut that is also slimy
A thin white film or scum on the surface is a different thing. Oregon State University Extension notes that white scum can form when cabbage is not fully covered by brine, and it can simply be skimmed off. Keeping the cabbage submerged is the best prevention, which is why we always suggest pressing it back under the brine after serving.
Trust your senses. A good sauerkraut smells fresh and sour. If a jar smells rotten or looks wrong in a way this list does not cover, do not eat it.
Why shelf-stable sauerkraut does not fizz
Sauerkraut sold on the supermarket shelf has usually been pasteurised, which means heat-treated so it can sit at room temperature. Heating stops the fermentation, so there is no gas, no pressure and no fizz.
Fizz is one of the honest signs that a sauerkraut is raw and unpasteurised, with fermentation still gently ticking along. It is the same living process we talk about in Does Real Sauerkraut Need Vinegar? Fermentation vs Pickling. At Cabbage Mama we ferment in small batches and keep our sauerkraut raw, which you can read more about in our Sydney sauerkraut guide.
Ready to try a jar that still has some life in it? Explore Cabbage Mama sauerkraut.
Frequently asked questions
Is fizzy sauerkraut safe to eat?
Usually, yes. Fizz, bubbles and pressure are normal results of live fermentation. Check for the real warning signs instead: mould in larger amounts, slime, or a rotten smell. If none of those are present and it smells clean and sour, it is fine.
Why did my jar leak in transit?
Gas pressure from active fermentation, plus liquid expanding on a warm day, can push a little brine past the lid. The sauerkraut inside is normally unaffected. Refrigerate it when it arrives and keep the cabbage covered by brine.
Should I let the gas out of the jar?
You do not need to do anything special. Opening the jar normally, slowly and over the sink, releases the pressure. Refrigeration keeps fermentation slow and calm.
Does more fizz mean more live cultures?
Not necessarily. Fizz tells you fermentation was active, not how much of anything is in the jar. What fizz does tell you is that the sauerkraut is raw and unpasteurised. If you're curious about real numbers, here's what an independent lab found.
How long does sauerkraut last in the fridge?
Unopened, Cabbage Mama sauerkraut keeps for up to 3 months in the fridge (check the best-before date on your tub). Once open, enjoy it within 1 month. Keep it cold, under the brine, and use a clean fork.
My sauerkraut is not fizzy. Is something wrong?
No. Fermentation is slower at cold temperatures and varies from batch to batch, just like bread or wine. A quiet jar can be every bit as good as a lively one.
Sources: National Center for Home Food Preservation, "Sauerkraut", and Oregon State University Extension, "Sauerkraut: Problems and solutions".