Is It Mold or Kahm Yeast?
I’ve been testing the limitations of open crock fermenting lately, which means I’m getting a lot more experience with a certain fermenter’s foe: Kahm yeast. Kahm yeast, despite some popular claims is not mold and it’s not at all harmful to your health. Kahm can describe a number of yeasts that will sometimes show up on the surface during fermentation.
How do you know the difference between mold and Kahm? Well, at the risk of giving a jerky answer, mold looks like mold. Mold will be raised, fluffy or maybe fuzzy looking. Kahm is flat, except where bubbles form when CO2 is trapped. Most vegetable ferment molds (which are extremely rare, anyway, with basic best practices) are white and can be scraped off. Colorful molds aren’t to be messed around with and ferments coated in bright pink, green or blue mold should be discarded. I’ve had a colorful mold exactly once in all of my fermenty experiments and I did just about everything wrong to get there.*
The bubbles you see are actually bubbles! The CO2 produced during fermentation is trying to escape, as it does, but it’s being trapped by the Kahm layer. Pretty neat looking, huh? Kinda reminds me of the view from a microscope. Or maybe the view from a telescope. If it didn’t stink up the joint, I could sit there and stare at Kahm all day.
So if Kahm isn’t bad for you, why should you care that it’s there at all?
First, it doesn’t always smell so great (the one you’ll looking at smelled like really strong nutritional yeast), and if you’re fermenting in an open container, that can be a bummer. It’s not that it smells exactly bad, but the aromas can range from yeasty to cheesy, and most people don’t like the slightly off cheese scent wafting through the air when they come home after a rough day. Or maybe that’s just me.
Second, in my experience, Kahm yeast can be an indicator that there’s been an issue with your ferment. Maybe you didn’t use enough salt, maybe the temperature was too high at the start of fermentation. There are other possibilities, but kahm can sometimes be an early warning that conditions are right for mold to form.
Third, the off flavors from kahm can impact what lies beneath. If you have a solid protective layer, the off flavors are less likely to affect the product underneath. Tasting is a quick and easy way to find out. Also, veg on the bottom will generally taste fine, even if the top layer is unfortunately Kahm-flavored. There are several ways to create such a layer: professional equipment I’ve seen, mostly in the hands of brewers, can provide an almost solid barrier between the Kahm (or whatever floaty thing) and the fermenting matter, or at least make Kahm easier to remove. Cheaper lower-tech options I’ve seen include a thick layer of cheesecloth underneath the weights or a very thick layer of cabbage leaves (the latter is my preferred method). With the batch pictured here, I actually composted the leaf layer mid-ferment and added fresh layers in, along with some saltier brine after skimming as much of the Kahm as I could. How to Avoid Kahm