Kahm yeast and mold, reading the surface
Day three of a sauerkraut run and the brine has grown a pale skin. Day seven of a kombucha and the pellicle's edge looks rough. Every fermenter meets a patch of something on the surface, and the whole question is one identification: is that film working with the batch, or eating it? The fermentation community's answer is stable enough to teach, and it starts with touchable differences rather than panic.
Two organisms, two signatures
Kahm yeast shows up as a thin, flat, whitish film, a skin laid across the surface. It sits level, it sheets off when skimmed, and the batch beneath it usually smells exactly as a healthy ferment should: sour, perhaps faintly bready. The plan cards on this site treat it as maintenance, which is why the wizard's sauerkraut and brined-vegetable checkpoints both say the same thing at day three and day seven, check it's under the brine, and skim any film. Skimming kahm is routine; NCHFP's plan presumes it.
Mold announces itself differently. It has texture and structure: fuzzy patches that grow upward, white or colored (green, black, pink, blue), anchored where the surface touches air instead of floating evenly across the brine. The kombucha guide's fixes table draws the same line: fuzzy dry patches means compost the culture, clean everything, and restart from strong starter at the correct temperature. Mold is not a maintenance event. The batch ends; the jar becomes the next batch's clean start.
Reading it in the jar
- Surface position: kahm spreads level across the whole surface; mold anchors in patches and climbs above the liquid line.
- Texture: kahm flakes and sheets when skimmed; mold holds together and pulls up as a mat.
- Color: don't rely on it. Mold is often white or gray, especially early on. Fuzzy, raised, or dry patches, or growth climbing above the liquid, mean mold whatever the color. When in doubt, treat it as mold.
- Smell: the batch under kahm smells sour and alive; under mold it smells off in the ways fermentation isn't.
- Everything else: cloudiness, sediment, bubbles, and strands under the surface are the fermentation working, the guide calls the strings yeast, and the app's plan says to check them and move on.
What each reading means next
Kahm's routine: skim it, press the veg back under the brine, and keep the salt where the plan put it, a mist of vinegar on the surface is the community's usual second move, because the acid rebalances the surface the yeast liked. Mold's routine: end the batch, clean the vessel hot, and restart with strong starter liquid at the plan's temperature. And the plan that sits over both: the vinegar guide's checkpoint for its own film, check the film, without disturbing it, because in an acetobacter batch the film is doing exactly what it should.
Read the surface, write the row. Label the batch. — the cellar crew