Mold vs Kahm Yeast

Kahm yeast is a collective term for several oxidative yeast species — primarily Pichiaand related genera — that form a thin film on the surface of brine ferments. It looks like a white or cream-colored skin. It's flat. It's wrinkly, like very thin crumpled paper. It does not have texture above the surface.

Kahm forms when the brine surface is exposed to air. The yeasts are aerobic — they need oxygen. They colonize the interface between brine and air. They don't penetrate into the ferment below. The vegetables are fine. The brine is acidic enough that nothing dangerous is growing in it.

The bad news: kahm tastes terrible. It imparts a musty, yeasty, off-flavor to whatever it touches. You want to remove it. The good news: removing it is simple. Skim it off with a spoon, wipe the jar walls above the brine line with a paper towel, add a little more brine if needed, and close the jar. The ferment below is fine. Check it again in a few days — kahm can regrow if conditions are right.

Kahm yeast is more common when: salt percentage is on the lower end (under 1.5%), fermentation temperature is warm, there's a lot of airspace above the brine, or the ferment has been going for several weeks.

Mold is a filamentous fungus. It has hyphae — root-like structures that grow down into the substrate, not just across the surface. This is the critical difference from kahm yeast. When you see a fuzzy, raised, three-dimensional growth in your ferment, hyphae have already penetrated the vegetables below. You cannot scrape off the visible mold and assume the rest is safe.

Mold in fermented vegetables typically appears as green (often Penicillium or Aspergillus), black, or white fuzzy growth. Sometimes it's visible on the surface of a vegetable that has risen above the brine line. Sometimes it appears on the jar walls above brine level. If you see any of these, the jar goes in the trash — the whole thing.

Rhodotorulais technically a yeast, not a mold — it's a basidiomycete that produces pink, salmon, or orange carotenoid pigments. You'll sometimes see it called “pink mold” because of how it looks, but taxonomically it's in a different category from true molds.

That distinction doesn't matter for your ferment. Toss it. Here's why.

Rhodotorula species are opportunistic pathogens. In healthy individuals, they're unlikely to cause serious illness — your immune system handles them. But in immunocompromised individuals (chemotherapy, organ transplant, HIV, high-dose corticosteroids), Rhodotorula can cause fungemia — bloodstream infection. A 20-year epidemiological study (PMID 40626381) found it was the second most common cause of rare yeast bloodstream infection, accounting for 34.7% of cases, with universally elevated resistance to fluconazole — the most commonly prescribed antifungal.

If you're healthy, eating a small amount of Rhodotorula-contaminated food is unlikely to hurt you. But “unlikely to hurt a healthy person” is not the standard I apply to my food. The upside of eating it is zero. Toss the jar.