Preventing Kahm Yeast and Mold in Fermentation

It happens to all of us. You open a jar of sauerkraut or fermented pickles and there’s something unwanted growing on the top. Don’t worry! Here is everything you need to know about kahm yeast and mold.

• Is it still safe to eat?
• What is the difference between kahm yeast and mold?
• How can you prevent it from happening?

Kahm Yeast

Kahm yeast is a wild-yeast strain that forms a layer that completely covers the top of a ferment. If it’s been left to ferment for a while, it may have trapped air bubbles below the surface.

The picture below is of a glass measuring cup with kahm yeast growing over the top of fermenting millet. You can really see how it creates a complete layer.

What to do about kahm yeast

While kahm yeast is not ideal, it is not actually harmful. So it is fine to eat a ferment contaminated with kahm yeast.

I most often get it on my ginger beer, hot sauce, and apple cider vinegar.

Here’s how to deal with kahm yeast:

1. If you find a layer floating on top of your ferment, just skim it off. It is usually pretty easy to remove.
2. However, the ferment will still be contaminated and the yeast will grow back again and again. So I recommend finishing up the ferment quickly so you don’t have to keep removing yeast.

Mold

Mold is a much bigger concern. It should be pretty obvious when a ferment has mold. It can be blue, green, black, brown or white. It’s usually fuzzy looking, and unless it’s been there for a long time, it doesn’t cover the entire ferment.

When you see mold on top of a ferment it is just the sporing body. Like mushrooms and other fungi, most of the mold filaments (called hyphae) are unseen, below the surface.

There are certain kinds of mold that are perfectly safe to eat. These include cheese, miso, and tempeh. However, the majority of the molds are NOT safe to eat.

Here’s how to deal with mold on a ferment:

1. If the mold is on sourdough starter, a fermented beverage, or a condiment, then throw it out. The hyphae will have contaminated the entire ferment. It can’t just be scraped off.
2. The only type of fermentation that is safe to eat after mold contamination is salt-brined ferments. And there are still a few rules:
   • The brine must be at least 2% salt to prevent the hyphae from growing in the liquid. So mold on a lower salt ferment isn’t safe.
   • Remove the mold completely, it usually comes off easily enough. And remove any of the vegetables/fruit/etc. that was floating above the brine as it will be contaminated by the mold.
   • The salt-brine fermented food that remained completely below the brine is safe to eat. I recommend eating the ferment quickly, as the mold usually comes back within a few days of being removed.
   • If eating a mold-contaminated ferment isn’t your thing, it’s fine to dump it into the compost. Never eat a moldy ferment that you are uncertain about. It’s always better to be safe.

Prevention

It is impossible to have a perfect ferment every time, but there is a number of things we can do to ensure that our ferments don’t get contaminated.

1. Cleanliness: If you’ve had any mold or kahm yeast in your kitchen then make sure that you’ve sanitized all the affected jars and equipment. And be extra clean for the next few months.
2. Temperature: Mold and yeast both thrive in warm temperatures, so try to ferment at less than 68 F (20 C).
3. Air-tight ferments: If you don’t have a pickle pipe or fido jar then make sure your vegetables are completely submerged below the brine.
4. Follow the recipe: Use the right amount of salt in your brine. Keep your cultures well-fed and vigorous. And the good bacteria/yeast/fungus will prevent contamination.