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How to Prevent Mold When Fermenting Foods

Hadar Iron

Ceramic Artist & Fermentation Educator

Many people are worried about mold appearance in their fermented food, and this anxiety delays the joy of making that healthy and tasty food. On this page, I want to build your confidence to make fermented food free of worries. I’d like to emphasize that although the worrying is understandable, since mold is harmful, it can’t grow in a water-seal crock if you follow the rules below.

What Happens to the Vegetables During the Fermentation?

Naturally occurring yeasts, bacteria, enzymes, or fungi convert carbohydrates into carbon dioxide, alcohols, and organic acids during the fermentation. Fermentation is a decomposing process, however we control the process in our crock’s sealed, safe environment and then we stop it in its tasty, healthy phase.

Not harmful growth of yeast floating on the water above the stone-weights, while fermented vegetables are kept safe and healthy under the brine.

How to Keep Mold Away From the Fermentation

The good bacteria, which exists in the vegetables and the air (hence the name Wild Fermentation), can overtake harmful pathogens if oxygen is eliminated from the environment. The lactic acid bacteria are anaerobic, which do not require oxygen for growth. Mold, on the other hand, requires oxygen, water, and food to grow. Thus, if we keep vegetables inside the crock under the water, separated from air, mold will not develop inside the crock.

Two colors of fermented red-cabbage sauerkraut; on the right we see pale purple sauerkraut that was below the stones, and on the left we see vivid purple sauerkraut that was underneath and represents the color and quality of the whole batch. The pink cabbage is not harmful, alas less tasty.

5 Rules to Keep Fermented Food Mold Free

  1. Clean the vegetables and have clean tools and surfaces before cutting the vegetables. Don’t use vegetables that already have mold, and clear off parts that got rotten.
  2. Pack them tightly or compress the mix to avoid air pockets.
  3. Submerge the vegetables under the brine, which is salty water or water with whey.
  4. Press the stone-weights on top of the brine and make sure there is a brine on top of the weights. That way, you guarantee the anaerobic environment for the lactic acid bacteria to flourish.
  5. Keep the lid on the crock’s trough and occasionally add tap water; do not let the trough dry off, which will compromise the seal. The water doesn’t let oxygen enter the vessel but allows the bubbles produced during the fermentation escape.


Should You Wash Vegetables Before Fermenting?

Yes. You should wash vegetables before fermenting them.

Can You Ferment Vegetables Too Long?

No. The fermented food will not be harmful if it was fermented for a long time. However, the taste will be different, similar to the aging process of cheese and curing of meat. In Korea, kimchi is known for its endless variations; almost every family has its expression of kimchi that is also defined by the time-length it is fermented, young kimchi, or aged one.

How Do You Know if Fermented Food is bad?

Suppose you find other material in greens, pink or black, with the fuzzy texture that is probably mold. I have never had these in a water seal crock, but it has occurred in glass jars and my kombucha jars. Discard the moldy layers as I do with my kombucha, and keep fermenting (See Picture Above). Still, if your fermented food smells terrible and has a terrible taste, it is recommended not to consume it. Discard the batch and clean the container.

Is Soft Sauerkraut bad?

If your sauerkraut is not crunchy, it does not mean it is harmful. I have added soft sauerkraut in dishes like pastes, soups or smoothies, and it only enhanced their flavor and filled them with probiotics. A full cup of soft sauerkraut added in the last minute of cooked chicken-soup made the soup delicious.

Why is My Ferment Cloudy?

If you see a Slimy white-gray film on top of the stones, do not worry, it is not mold. It is the decomposed sugars that were exposed to the air above the water and produced yeast. It doesn’t have the mold’s fuzzy substance, only a slimy,  thin layer, which can be rinsed off the stones and the vegetables’ upper layer. The slimy film is also called “bloom”.

Cloudy brine above the stones. The green cabbage sauerkraut under the stones is delicious, as well as the cloudy brine that I keep with the fermented vegetables.

White film on the stone’s surface.