Kahm yeast is a harmless white-to-creamy or slightly brownish film that sometimes forms on sourdough starters. While it doesn't look appealing, it is safe and poses no risks.
Let's discover what kahm yeast is, why it happens, and how you can manage it to keep your sourdough starter healthy.
What is Kahm yeast?
Kahm yeast is a harmless wild yeast that appears as a thin, white-to-beige, sometimes brown, film on the surface of fermenting foods, including sourdough starters. This yeast thrives in oxygen-rich environments and is often a sign that the starter has been exposed to air for extended periods.
Is Kahm yeast harmful?
Even though kahm yeast makes your starter look odd, it is not dangerous. While it is safe, it affects the flavor and aroma of your sourdough starter, often giving it an unusual or "cheesy" smell.
Why does Kahm yeast appear on a starter?
There are a few factors that can cause the growth of kahm yeast in your sourdough starter:
- Extended periods between feedings: If you neglect your starter for prolonged periods without regular feedings, it creates an environment that helps kahm yeast growth. A healthy, well-fed starter naturally maintains a balance that keeps harmful bacteria and yeasts, including kahm yeast, in check. But if it is left unfed for too long, that balance is disrupted, allowing kahm yeast to take over.
- Inadequate sealing: If you store your sourdough starter in the fridge in a jar without the lid tightly secured, it is exposed to excess oxygen, which promotes kahm yeast growth.
- Contaminated equipment: Using unclean utensils or jars can introduce unwanted microorganisms, including kahm yeast. Reusing the same sourdough starter jar for multiple feedings also increases the chances of kahm yeast developing.
- Cross-contamination from other fermented foods: If your starter comes into contact with other fermented foods that already contain kahm yeast, it can transfer and start growing on your starter.
How to tell Kahm yeast apart from other issues
It is important to know the difference between Kahm yeast and other problems in your sourdough starter:
Kahm yeast forms a thin, white-to-brown film. While it signals that the starter needs better care, it is not harmful like mold.
Mold appears fuzzy and can be green, blue, or black. You cannot use such a starter and should throw it away.
Hooch is a liquid that forms when your starter is hungry. It usually appears on top of the starter and can be mixed back in after feeding.
Harmful bacteria like Serratia marcescens form pink or orange streaks, which is a sign of severe contamination. Discard this starter.
How to manage Kahm yeast in your starter
If you notice kahm yeast on your sourdough starter, know that it is common and generally safe, but it affects the flavor.
First, carefully scrape off the film from the surface. Keep in mind that kahm yeast may already be present throughout the starter, so scraping alone might not prevent it from returning.
To save the healthy portion, use a clean spoon to take a tiny bit of starter from the bottom of the jar, about 2-5 grams. Transfer it to a clean jar and give it a big feed of 75-100 grams of flour and water.
This small amount of starter will re-activate and peak in about 24 hours at 86°F (28 °C), though it may take longer at room temperature.
Always wash and sanitize the old jar, and use a fresh, clean jar for each feed. This practice helps reduce the risk of kahm yeast returning.
While kahm yeast is safe, it gives an unpleasant flavor that is difficult to remove. For that reason, many bakers, myself included, choose to discard the affected starter and make a new sourdough starter. If you sell sourdough bread, it is especially not worth the risk.
That is why it is always a good idea to have a backup. Dehydrate sourdough starter so you can reactivate it if something goes wrong.
How to prevent Kahm yeast
Here are some tips to avoid Kahm yeast on your sourdough starter:
- Feed regularly: Feed your starter consistently to keep it active and balanced. Learn more about feeding sourdough starter.
- Use clean equipment: Always use clean utensils and jars when handling your starter. While some bakers reuse the same jar, using a fresh, clean jar for each feeding is a better way to avoid unwanted bacteria, mold, and kahm yeast.
- Store your starter properly: Keep it in a tightly sealed container to limit exposure to air when storing in the fridge. Learn more about how to store sourdough starter.
- Watch out for cross-contamination: If you have other fermented foods in your kitchen, like sauerkraut, kombucha, or other cultures, kahm yeast can transfer from them to your starter. Try to keep different fermented foods separate and avoid sharing utensils.
Let's talk sourdough
Have you ever seen kahm yeast on your starter? How did you handle it? Share your experiences in the comments.
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