Why Your Vegetable Ferment Grew Mold, What to Do about It, and How to Prevent It


If you’ve been fermenting vegetables for a while then you’ve probably experienced it at some point. You’ve cut or shredded your vegetables, added salt or brine, and left them to ferment.

Then a day or more later you opened your jar to find, much to your horror, a white film of mold. You may have been sorely disappointed, assuming all that time and money was wasted.

Don't despair, however. Mold may not be the ruin of your batch of fermented vegetables.

Is it Mold?

The first thing to know is that the white film often present on the surface of vegetable ferments is often something called kahm yeast.

Kahm yeast is not harmful, although it may be unattractive or even smell a little odd. It should be removed from the ferment so it doesn’t impart a bad odor, but a little bit left in the jar won’t hurt the vegetables, and won’t hurt you.

Kahm yeast is likely to develop if a fermentation solution is insufficiently acid, especially when you start it, or if there is not enough salt in the brine. Kahm can also develop if the culturing temperature is too warm, or if the brew is over-exposed to oxygen. Poor hygiene can be another cause.

If kahm yeast develops in the ferment, skim it off the surface of the liquid. Discard any solid matter that has mold or has discolored. As usual, your senses are the test: if it smells and tastes okay, it probably is.

True molds are usually colorful and the deposits are round and often fuzzy or fluffy. They might even be white, but there is a distinctive difference between the smooth film that is kahm, and the puffy growth that is mold.

What to Do with a Moldy Ferment

If you opened your jar or vessel and did find a layer of mold on top, don’t panic. You may be able to easily dispose of this and have perfectly good fermented vegetables below the brine.

Lacto-fermentation is an anaerobic process, meaning it must be in an oxygen-free environment for it to happen properly and fully. Any oxygen put into the mix can cause unwelcome microorganisms such as mold or yeasts.

So while your vegetables may be happily fermenting under the brine, the surface of the brine can still be exposed to some oxygen.

As with yeast, simply scrape off the top layer, give the container a few seconds to air out, then test aroma and flavor. If it smells and tastes ok, it's probably ok to consume. 

Factors That May Contribute to Mold Growth

If you are experiencing a mold epidemic of sorts in your ferments then their maybe something more at play. You may want to investigate the following possibilities:

Fermentation Temperature

Vegetable ferments, in general, prefer a cooler temperature, which can make preserving at the peak of the growing season difficult.

Try to find a cool place to ferment your vegetables. A root cellar is ideal, and traditional, but a cool basement or garage is also helpful. Or just find the coolest place in your home. A temperature of 65° to 70°F is your best bet in avoiding mold. Consult our article on Keeping Cultures Cool in Warm Weather for tips. 

Vegetable Submersion

Probably the most important factor in lactic acid fermentation is the submersion of the vegetables underneath the brine. Lacto-fermentation is an anaerobic process, meaning it requires a no-oxygen environment.

If you are experiencing mold problems check to make sure your vegetables are well-submerged under the brine. If not, find some means to Keep Vegetables Submerged During Fermentation.

Salt Content

The purpose of salt in a lacto-ferment is to inhibit the growth of undesirable pathogens including molds and other microorganisms. Too much salt won’t allow lactic acid fermentation to occur fully, but too little salt can result in off flavors or mushy vegetables.

Likewise, too little salt will not preserve the food between the start of fermentation and when the lactic acid bacteria begin to proliferate and create an acidic environment on their own. This can lead to mold more readily taking hold of your ferment.

Quality of Vegetables

Finally, consider the vegetables themselves. Are they in a state of decay? Did you wait too long to ferment them? Were they sprayed with chemicals that could interfere with the natural fermentation process?

Use only fresh produce in your ferments. Unsprayed, homegrown, or organic vegetables are best, if you have access to them.


Why Your Vegetable Ferment Grew Mold, What to Do about It, and How to Prevent It

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