Can Mold Hurt a Ferment?

It’s a funky and often contentious topic in the fermentation world, but at the end of the day, if you’re not working on a batch of tempeh of a craft cheese, you’re probably pretty unhappy when you find mold on your ferment.

Personally, I seem to take a more relaxed view to very small amounts of mold that may form on the surface of brine than many do. Sandor Katz says white molds on the surface aren’t a big deal, and, as a scattered phenomenon, I tend to agree. This post at Northwest Edible Life has a great photo of what I’d call the “fluffy little cloud” appearance that doesn’t bother me.

It’s worth noting that molds produce metabolites which can/may be toxic to humans. Use your own judgment in evaluating risk. My approach doesn’t have to be your approach.

Okay, so with the preliminaries out of the way, let’s talk about what happens with a huge infestation of mold that’s allowed to sit for months. In the picture at the top, that’s exactly what we’ve got.

Molds Adjust pH

Here’s the most important thing to know about a heavy infestation of mold growing on your brine: Mold changes the pH of its environment to suit it. It can adjust an alkaline environment to be more acidic, or it can adjust an acidic environment to make it alkaline. You know all the careful acidification that we expect of lactic acid bacteria to drop our ferments into a safe pH range below 4.6? Yeah, mold can – and, with enough time – will pull your perfectly acidic ferment out of that and, potentially, all the way into dangerous, pathogen-friendly territory.

The photo at the top is of a jar of okra, one of two I made on the same day, with the same batch of veggies from my garden. I knew it had gone to mold and let it sit for several months, alongside the one that didn’t grow mold. Check out this photo of a side-by-side pH test of the two. One of these things is not like the other:

So what do I do with bad surface mold?

I’ll admit that I was shocked to see that the molded ferment was so far into alkaline, and potentially unsafe, territory. Litmus paper isn’t as precise as any of us would like, but that’s easily in the 8-9 range, whereas the unmolded ferment is probably in the 3’s on the pH scale. I hate to say it, but if you haven’t peeked in on a ferment in a while and you find it covered in mold, your best and safest bet is to toss it. If you have pH strips or, even better, a pH meter, you could, I suppose, test it for safety and then make a determination. But remember, molds are making all sorts of biological byproducts that may fall into the category of “mycotoxin” as they live their lives. To me, easily-scooped away fluffy little clouds on a well-submerged brine are a category of risk I’m willing to live with. Beyond that, I’m in the compost-and-move-on camp.

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