4 Signs your kraut is fermenting normally (and what to do if it’s abnormal)

People new to making sauerkraut in an Airlock Fermenter may wonder, is my kraut advancing normally and is it safe to eat? Some aspects of making sauerkraut in a crock or mason jar apply to making sauerkraut in an Airlock Fermenter, because the basic process is the same – choose, shred, salt, toss, and pack. However, there are also differences to note. This article will highlight four signs that that can be expected – and what to in the absence of these.


Whether you use a crock, mason jar, plastic bucket, or an airlock-adapted Fido jar, brine should fill the headspace in the first 1-4 days during the room-temperature phase. This should submerge cabbage shreds or topper leaves by one inch or more. From the first sprinkle of salt onto freshly shredded cabbage, salt begins drawing water from cabbage shreds into brine (salt water). 

Brine fills the headspace during the first 2-5 days at room temperature.

Sometimes brine doesn’t develop as described above (as was my experience for the first time this past fall). This is a problem, because vegetables above the brine will not ferment. Experience taught me that cabbage variety, thickness of shreds, and rupturing the cell walls can affect brining. Winter cabbage, with its thick leaves and high moisture content, is the best for making crispy sauerkraut that stores well from harvest to harvest. This cabbage, however, depends on thin shredding to allow salt to draw moisture from inside the shreds. Paper-thin shreds or about 1-millimeter thick (about the thickness of a quarter or dime) release moisture easier than thicker shreds. Conversely, shredded summer cabbage brines well when shredded thinly or thicker, because the cell walls are thin. 

If you can’t shred winter cabbage thinly, rupturing cell walls by hand will assist in drawing out moisture locked inside thick winter cabbage leaves. Normally, I recommend gently tossing cabbage shreds with salt in a large bowl to distribute salt throughout the shreds while preventing any damage to the cabbage cells. In the case of thick shreds, however, I recommend gently squeezing cabbage with your hands while mixing cabbage and salt. This slight pressure will rupture some cell walls, and encourage moisture inside the cabbage leaves to migrate into the brine. Assess the brine level to determine if you need more aggressive pressure.

Recognizing the difference between cabbage varieties to predict how it will brine takes experience. If you didn’t recognize and respond with different mixing techniques, don’t panic. If, after the first 5 days at room temperature, topper leaves are still exposed, open the lid. Remove the glass weights and discard the topper leaves that are still green. With a clean hand or with a flat, non-metallic utensil, such as a silicone spatula, gently push down the shreds, working your way, multiple times, around the outer walls. Brine should rise as the shreds compress. If the brine level doesn’t stay above the shreds, make ½ cup of premade 2% brine. Only add what is needed to bring the brine level flush with the top shreds (no more than ½ cup to a 3-Litre Airlock Fermenter). Don’t expect to cover the topper leaves with this amount of added brine. Use this as a last resort as adding too much of a premade brine may cause discolouration and changes in texture and flavour. Re-clamp the lid and allow it to finish at room-temperature, if needed, before transferring to cold storage.

Do not, in any case, open your lid before the first 72 hours. Lactic acid bacteria need 72 hours to gain a competitive advantage over spoilage bacteria and mould. Winning this edge requires an anaerobic environment.

Brine contained inside the vessel

Some batches of kraut brine perfectly – not too much that it overflows out of the jar and not too little that topper leaves are exposed above the brine. Cabbage below brine will ferment, while preserving all the probiotic-rich brine inside the jar.

When brine exceeds the headspace between the top layer of cabbage and the bottom stem of the airlock, brine will flow into the airlock. Without an airlock, a Fido jar or mason jar may lose brine through the gasket (seam of the lid and jar). Overpacking the jar is one cause of overflow. To correct this, pack shreds one inch below the shoulder of the Fido jar, leaving ample headspace to collect brine. High-brining cabbage is the other cause of overflowing brine. Cabbage varieties with thin cell walls, for example, summer cabbage, release moisture easily with the addition of salt. Once packed, brine quickly surfaces to the top and continues to fill the headspace – much more than with winter cabbage. The challenge will be slowing brine production, which is tempered by reducing room temperature to 16-18 degrees C.

By following the steps for sauerkraut in the Introduction to Vegetable Fermentation Manual and the tips above, brine still may overflow. If this happens (it has happened to me numerous times), wait until brine production stops. Before transferring your jar to cold storage for 10-12 weeks, clean the airlock: Remove the dirty airlock and insert the plug into the grommet hole to prevent air from entering the jar. Clean the airlock with soap and hot water. Refill it with clean, non-chlorinated water; cap it with the perforated lid; remove the plug; and reinsert the airlock. Alternatively, if you have a spare airlock on hand, fill it with water and quickly swap out the dirty one for the clean one. Transfer the Airlock Fermenter to cold storage. Brine shouldn’t draw into the vessel during cold storage, because microbial activity reduces in cooler temperatures.

Brine reabsorbing into the shreds

Ideally, during the room-temperature phase, brine forms to cover the topper leaves by an inch or more. Great! Now when kraut is transferred to cold storage for 10-12 weeks, brine suddenly disappears. Darn! What happened?

This is completely normal. Brine is not lost; it reabsorbs into the shreds. As microbial activity decreases, as CO2 production reduces, and as fermentation transitions to another stage, brine settles into pockets between cabbage shreds. Where once CO2 displaced brine between the shreds and pushed it into the headspace, now, the brine falls back into the gaseous pockets and CO2 rises to the headspace.

If you need proof of that brine is just hiding among the shreds, bring your jar of kraut back to room-temperature for 20 minutes or so. As it warms, microbes move at a faster speed, and brine will surface to the top. After you’re convinced, transfer it back to cold storage for the remaining weeks. 

Brine level below the top layer of cabbage

Read any fermentation book, and in big, bold letters it will emphasize the need to submerge vegetables under a brine at all times. While this is true for ferments made in a crock, mason jar, or any other fermenting vessel that does not seal completely, it is not 100% true for sauerkraut made in an Airlock Fermenter. 

This jar just came from cold storage. Note the bine level: It is below the shreds underneath the topper leaves. The top layer isn’t brown or discoloured.

In fermentation vessels without a perfect seal, oxygen enters. Any vegetable above the brine is open for mould growth and spoilage yeast. Submerging under a brine decreases the risk for mould growth. A perfect seal, as in an Airlock Fermenter, completely restricts oxygen. As long as the lid isn’t open and the airlock stays filled, shreds above the brine won’t mould. They shouldn’t even discolour. Storing kraut in an Airlock Fermenter with brine that’s below the cabbage shreds isn’t a concern. Without oxygen, mould can’t growth. Left unopened and kept in cold storage, kraut will last over a year, from harvest to harvest, ready to be enjoyed by all for function and flavour. (If the top layer of your kraut, made in an Airlock Fermenter, turned brown or grey, contact me. I’ll send you a new grommet for free.) 

In summary, brine is a love-hate relationship for new fermenters to understand. It pays to understand the differences and similarities between making kraut in an Airlock Fermenter versus other fermenting vessels. It saves you getting concerned when it doesn’t align with what general blog articles tell you to expect. The main difference between fermenting in an Airlock Fermenter and other vessels is oxygen. The main similarity is brine. In Airlock Fermenters it should be above the topper leaves at room temperature but often goes below the leaves during cold storage. You don’t’ have to worry about the latter, because there is no oxygen in the vessel.

If you would like to learn more about making sauerkraut and other healthy ferments for targeted health benefits, consider taking the Online Introduction to Fermented Vegetables Course. Learn at your own pace and maximize benefits from homemade fermented foods. Click HERE to learn more about the online course

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