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 the Wrath of Kahm
Use small pieces – The more surface area you expose, the better access the microbes have to the vegetables natural sugars. Better access means quicker acidity. With rapid acidification, kahm isn’t a problem.
Ferment at cooler temperatures – Especially at the start of fermentation, temperatures should be what is commonly described as “room.” Something around 70 F (22 C) is best.
Ensure that your equipment is thoroughly cleaned between batches – Sometimes stray yeast can jump over from the last batch. If you do get kahm, that’s a time to use white, distilled vinegar and boiling water to clean your crock, jar, spoon, whatever after each batch.
Use sufficient salt, especially for larger pieces and longer fermentation times – Salt can be adjusted to compensate for other variables. If you’re using bigger pieces, add a bit more salt. If you know you’ll be fermenting something for a long time, use more salt. Salt does not diminish during fermentation so prepare yourself for a saltier end product.
Make sure you’ve submerged everything – I can’t overstate this. Don’t let anything float to the surface and your risk of getting Kahm (or mold) is very, very low.
What to do if Kahm somehow ends up on your sauerkraut (or pickles or kimchi or whatever) despite your best efforts
With packed vegetable ferments like kimchi and kraut, the answer is pretty easy. Skim it off, along with a layer of vegetables. Maybe sprinkle a bit of salt around the edges of the crock. With brined ferments, like pickles, either remove the barrier you included or skim as much Kahm as you can. I have dipped a very clean kitchen cloth into an area that has lots of small bits. They tend to cling when you remove the cloth, so just rinse and wash that cloth thoroughly afterwards.
Once you’ve skimmed, there are a few options. You’ll never get it all through skimming, although the cheesecloth method above does quite a good job. No need to do all of these, just try a couple and see how it goes.
- Add a little topper of brine with a higher salt concentration than what you used originally. (Yes, this will make your ferment saltier.)
- Sprinkle a bit of salt over the top.
- Remove weights and soak them in a distilled white vinegar solution for 15 minutes, then rinse thoroughly in very hot water. Allow them to cool before returning them to the crock (or rinse them under cool water after hot).
- Press saran wrap to the top or sprinkle salt. Make sure you’re checking back to see that it hasn’t started spreading again.
- None of the above. Just return frequently to skim some more.
If you’re fermenting in jars, this is much less of a problem. Just skim if it’s there (extremely rare!) and skim more if you see it again after a few days, until you’re ready for the fridge. The couple times this has happened to me, I’ve moved the ferment from the jar into a clean jar with fresh brine at a much lower salt percentage before refrigerating. That generally removes any off flavors that may have come from the Kahm.
So, Kahm isn’t all that bad, but it can be annoying. And, by the way, the pictured Kahm is far and away the worst I’ve ever had. I started this batch of whole head kraut right before leaving for 9 days and without sufficient salinity. So it was giant “pieces” in high heat (we turned the AC off before we left town) and not enough salt. This is how I felt when we got home from vacation and I saw the surface:
*It was a whole vegetable ferment and I didn’t use extra salt. I started it in the middle of an in-home heat wave. I didn’t submerge the vegetables properly. I didn’t monitor the ferment at all during its months of fermentation.