Funky Fungi: Dealing with Mold and Yeasts on Vegetable Ferments

To the fermenting newbie, the presence of funky white or other colored stuff on the top surface of a vegetable ferment can be quite off-putting and cause panic. I’ve heard more than one story about someone throwing out an entire batch of otherwise fine sauerkraut or pickles because they found something fuzzy or weird looking on top.

Know this: It’s normal and there is nothing to worry about! We’ll take you through it.

It’s Fine Under the Brine

When fermenting vegetables, the portion of the container that is exposed to air (i.e. the top surface) can attract airborne mold and yeasts. These microscopic critters are present in both the air and on the vegetables we ferment.

Lactic acid fermentation happens without air (anaerobically) under the brine. As long as your vegetables are submerged under the brine, then they are fine. The mold that forms on the surface cannot grow/thrive in the brine due to its acidic environment.

The mold and yeasts that form naturally are not harmful, but they don’t taste good and can throw off the flavor of the finished ferment.

Tools and Techniques

Al Fresco (open air) method

Personally I prefer a simpler approach which I picked up from my “fer-mentor”, the amazing Sandor Katz. Everyone can start out like this because it requires no special equipment, instead using items you already have in your house. Also, these (marginally) higher maintenance methods encourage you to check in on, look, smell and taste your ferment more regularly. That way, you can learn h how ferments behave over time.

It entails keeping the contents under the brine (e.g. a plate with a wine bottle) and then covering and securing with a breathable cloth to keep flies out. Here are some crocks and a jar that use this method. I tied cabbage twist-ties together to create a secure band.

Covered ferments

Pro Fermentation Tools

There is a plethora of clever gadgets and equipment out there that can address mold formation on ferments. These devices work by keeping a pocket of carbon dioxide in the airspace at the top of the vessel. This discourages air-breathing molds and yeasts.

Airlock systems can be retrofitted into lids and usually fit on top of standard sized jars like wide-mouth mason jars. Here are some examples of airlock systems:

Check out our “Fermentation Device Smackdown” YouTube series where we test these and various other fermentation tools side-by-side.

Harsch-style crocks are built with a lip which you fill with water. The lid interlocks inside this lip to create a seal.

Harsch-style crock
Harsch-style crock

Keeping it Clean

  1. Check your fermented veggies every few days, and more often when the weather is warmer. Microorganisms are like us– they’re more active in the summer!
  2. Remove the cover, lid and weight. Clean them with warm soapy water and then dry them.
  3. With a spoon, paper towel, or your hands, scoop, wipe or pick out the top layer of mold or yeast. Do the best you can; you won’t get every single spore.
  4. There may be a top layer of veggies that has dried out or gotten discolored. Scoop that layer out too and compost it. Note that the layer immediately underneath should be fine.
  5. Give a taste. If you like it, transfer it to the refrigerator.
  6. If not, replace the lid, weight, and cover.

I have had great luck with kimchi (that is, not forming mold). The combination of the spiciness of the peppers, along with onions, garlic and ginger (all naturally antimicrobial) seem to keep the mold at bay. Also, kimchi typically ferments for a shorter time than something like sauerkraut.

all clean!
all clean!

What about Slimy Brine?

Sometimes, your brine may appear “slimy”. Symptoms are that it is thicker than normal water and maybe a bit cloudy. However it doesn’t affect the taste or texture.

During the normal course of vegetable fermentation, there is actually a kind of bacterial “relay race” which occurs. First a coliform bacteria such as Enterobacter and Klebsiella colony starts; as it digests carbohydrates in the veggies, it creates various acids and lowers the pH. Eventually it’s enough to kill themselves off. At that time, Leuconostoc species take over, as they prefer a slightly more acidic environment. They repeat the process, lowering the pH even further, and doing themselves in. Finally, Lactobacillus takes the baton and remains the predominant species for the duration of the fermentation. [1]

The “slime” is simply viscous polysaccharides (dextrans) produced by the Leuconostoc species during fermentation. These substances are quite useful in the biochemical and pharmaceutical industries, but don’t exactly produce a desirable mouth feel in your sauerkraut. But don’t fret! As the Leuconostoc species subsides and are overtaken by Lactobacillus, the slime will naturally go away as it is digested by Lactobacillus. {BURP!} [2]

Here’s the recipe for Fennel Ginger Sauerkraut we made in the video.