Sometimes ferments fail. It happens to all of us, even the most seasoned fermentation gurus can have a ferment fail.
A common way for ferments to fail is by getting contaminated with mold or kahm yeast. As much as we like our free range yeasts and bacteria, we don’t really like to invite the wrong type of free range critters to our fermentation party.
Mold is pretty obvious. It’s usually blue, green, or white, fuzzy looking and floats on top of a ferment. When you see mold on top of a ferment it’s a sporing body of mold, and actually means that your entire ferment is contaminated. Like mushrooms and other fungus, most of it lives below the surface of the sporing bodies.
While some molds (think cheese and tempeh) are fine to eat, the majority of the molds are NOT good to eat. They can be toxic in all sorts of different ways, so DON’T EAT MOLDY FERMENTS, even if you can scrape the mold off the top, DON’T EAT IT.
Kahm yeast is just another free range yeast, but unlike sourdough yeasts, it’s not one that we like to encourage. Basically, it doesn’t taste great and doesn’t make for happy ferments. However, unlike mold, it is not toxic. If you find a layer of kahm yeast floating on top of your ferment, you can just skim it off. (I recommend skimming it off then eating it pretty quickly to prevent the yeast from growing back again).
Kahm yeast looks fairly different from mold. It forms a layer that completely covers the top of a ferment, and you usually can see trapped air bubbles below the surface. In the case of kahm yeast a picture is worth more than 1000 words, so I have a picture of the glass measuring cup with kahm growing over top of a millet ferment above.
It is impossible to have a perfect ferment every time, but there are number of things we can do to ensure that our ferments don’t get contaminated.
1. Cleanliness: Always clean everything thoroughly between ferments. You can reuse brine, etc. but if you’ve had any kahm yeast growth, then don’t reuse it as the brine will be contaminated. If you’ve had mold/kahm in your kitchen then sterilize everything (boiling water should be sufficient for most things).
2. Temperature: Try to ferment a little below room temperature (unless the recipe requires warmth).
4. Follow the rules: Use the right amount of salt in your brine, keep your cultures well-fed and happy, and the good bacteria/yeast/fungus will prevent the unwanted bugs from contaminating your ferment.