Mold or Yeast, How to Tell the Difference

I have six quarts of pickles in my fridge. They are the first pickles I have ever made that are as crisp as store-bought. And the flavor, well, they’re delicious. But every time my son opens the fridge, he says, “Mom, you have mold on your pickles!” Well, do I? Let’s read Kristi’s post to find out.

What is the difference between mold and yeast? The main difference is that yeast is a single-celled organism and mold is multi-cellular filaments. The yeasts found in ferments are not usually very harmful but can make a fermented food taste a little off. Mold ruins ferments, and must be thrown out. Yeast is colorless, usually it looks to be white or translucent depending on the reflection of the colors in the ferment. I have seen yeast in beet kvass that looks pink, but it is from the brine dying the yeast a pretty pink color. Mold can be many different colors. The colors most often seen in ferments are white, black and green. Below are a few facts to help you further determine if you have mold or yeast and understand what you are dealing with.

How to Tell the Difference between Mold or Yeast in Your Fermented Foods

What Mold Looks Like:

• Mold colors can be white, black, brown, grey, blue, green, yellow, pink, purple and/or orange.
• Mold grows in dark damp areas.
• Mold can become airborne (
• There are many different strands of mold. The different variations can look fluffy, fuzzy, slimy, moist and glossy. (
• Health hazards of a bad mold are allergic reactions and respiratory problems.
• Use fresh, organic ingredients.
• Mold has a very distinctive smell. You may be able to identify it by smell alone.

Foods that contain mold: Tempeh, Natto Miso, soy sauce, and cheese!

Following are several photos of mold growing on ferments.

What Yeast Looks Like:

• Yeast is usually white and described as colorless. Sometimes the brine color may make the yeast appear to have color.
• Yeast likes to grow where it is moist and has plenty of sugar and/or amino acids. (
• Yeast needs sugar and starch to grow.
• Smells funny. Yeast has some very distinct smells. You may know the smell of making a sourdough starter, or if you use nutritional yeast, and sometimes it smells downright cheesy!
• Yeast can be flat in appearance, sometimes thready. It can be flat or branching except where bubbles have formed. The bubbles form from gases that are made from the fermentation process which are trying to escape.
• Yeast may be an indicator that mold will follow. Sometimes yeast forms because of conditions that were not favorable for the ferment such as high temperatures, or maybe there was not enough salt.
• There are many types of yeasts. Kahm yeast is the one we mostly experience when fermenting. Although usually considered harmless, kahm yeast can make a ferment taste bad. There are certain vegetables that form kahm yeast in fermentation, when exposed to air.

Foods that use yeast: Wine, beer, mead and sourdough breads
Fermentation is made with bacteria, more specifically the good kind!

Following are several photos of yeast growing on ferments.

What to do with yeast on your fermented foods:

Skim off the layer of yeast. Remove the weights and followers* and replace with clean ones. Then, sample the ferment to see if the flavor has been affected by the yeast. If it has, remove the top layer of food and taste the layer under that. If it tastes good, it is best to eat it as soon as possible.

If it needs more time to ferment, you can try to add some more brine, and/or salt. Adding more salt will make it saltier. Saltiness does not lessen in ferments over time. If the entire container tastes bad, throw it out.

After finding yeast in your ferment, wash your tools and jars/vessels and weights with vinegar and very hot water, to help minimize any transfer from previous batches.

Use clean hands, don’t use anti-bacterial soap that can transfer onto the foods you chop up.

Using an airlock will help cut down on the air exposure, and will eliminate the need to “burp” the vessel, which introduces more air. Airlocks let gasses escape while keeping oxygen from getting in.

Minimize floaters* by using a follower and a weight. A follower can be a cabbage leaf for example.

Cut food into smaller pieces, the yeast will need a lot more time to form on smaller pieces. This is because there is less surface area, and it is easier for the brine to penetrate into the food.

Try to ferment at room temperature which is around 65º F to 70ºF. Keeping the temperature steady is helpful also.

Use fresh, organic food. Using the freshest possible food items will help immensely.

How to avoid mold:

First, there is a lot of controversy about whether you can scrape off mold in a ferment and then eat it. Many say this is fine. It simply is not. Let me explain. If you find mold on a hard cheese, you simply cut off an inch and eat the rest of the cheese. The moisture content is very low in hard cheeses and that is why this is acceptable. The mold has a harder time reproducing because it needs moisture to live. Now, soft cheese has to be thrown away if it has mold on it, because of the higher moisture content.

Usually, the USDA says that fruits and vegetables that have mold on them can be saved by cutting an inch off around the infected area, but this only pertains to low moisture fruits and veggies. So food submerged in liquid is, in fact, very susceptible to spreading the mold to the entire container of food.

What about the salt, doesn’t it help keep bad bacteria at bay? It does, but if you already have mold, then something in the environment of the ferment is already off. For example, the salt content might be too low, allowing mold to grow. Throw out the entire contents of the vessel, and try, try again!

Use only fresh, organic ingredients. Do not use anything that already has mold on it.

Rinse your food with water before cutting into it. This will help wash away any impurities that we don’t want in the ferment.

Wash your fermenting vessels before each use with vinegar and water.

Use clean hands. Don’t use anti-bacterial soap that can transfer onto the foods you chop up. You don’t want to use anti-bacterial soap because it affects the good bacteria too, and that is what we are trying to grow in our ferments!

Use an airlock. Airlocks are designed to keep the oxygen out. Mold can be airborne so this helps immensely to cut down on the amount that can enter the ferment.

Don’t use table salt or pickling salt. These have preservatives that may hinder the good bacteria from growing, introducing mold in its place. I use Himalayan sea salt.

Use the correct amount of salt. Different food items will need different amounts of salt. Use the handy Fermentation salt calculator as a helpful guide!

Temperature also comes into play with fermentation. It is best to keep a steady temperature no less than 65º F to no more than 70º F.

Keep food under the brine, by using a follower such as a cabbage leaf. Anytime I have experienced mold, it was when the brine was too low or from floaters. Keeping the food submerged under the brine is one of the best ways to keep mold at bay.

Even using all these wonderful tips, you may still encounter mold or yeast. It happens to the best of us. Don’t get discouraged! Keep trying. Now get out there and ferment something! Why not try to fermented carrots? Or maybe you could make some kimchi!

If you have any questions, leave us a comment!


So, after reading this, I inspected my pickles more closely. And, I am sad to say that they had mold growing on them. BUT, only one jar—the one we were eating out of. Obviously, someone introduced something to the opened jar that caused the mold growth, I think because the pickles were floating. So, if after your ferments are done and you want to keep the food submerged, don’t remove the glass fermentation weight. You can always buy extras in the Fermentools store.


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