Safety
White Film on Your Ferment: Kahm Yeast or Mould?
How to tell harmless kahm yeast from mould on sauerkraut and other brined vegetables, and what each one means for the jar.
7 min read ·
Why does my sauerkraut have a white film floating on top?
Almost always kahm yeast, and almost always because something is touching air. Kahm is a catch-all name for several aerobic yeasts that colonise the surface of a brine once they have oxygen to work with. It is not a sign that the ferment has failed, and it is not a sign that you did something dirty. It is a sign that the top of the jar is exposed.
It looks like a thin skin: flat against the surface, matte white or faintly cream, often wrinkled or crazed like dried paint, sometimes patchy rather than continuous. It has no height. Tilt the jar and it moves as a sheet, tending to break up rather than hold its shape.
The kraut underneath is usually fine. What kahm costs you is flavour — a lingering mustiness that gets stronger the longer the film sits — not safety.
How do I tell kahm yeast from mould?
Look at height first, colour second. Kahm is flat and white; mould stands up off the surface and is fuzzy. Texture is the reliable signal, because a young mould colony can start out white before it develops any pigment, and colour alone will mislead you in the first day or two.
Once mould has established, colour settles it: green, blue-green, black, grey and pink are all mould. Pink deserves particular caution, because pink discoloration in a brine can also come from bacterial spoilage rather than mould, and neither belongs in food.
The third signal is pattern. Kahm spreads as a continuous or patchy sheet across the whole surface. Mould arrives as discrete circular colonies with a defined edge, often several at once, sometimes with a paler halo around a darker centre.
Is kahm yeast safe to eat?
It is generally treated as harmless, and the standard advice is to skim it off and carry on. Skim rather than stir: mixing the film into the brine spreads the musty flavour through the whole batch instead of removing it.
The judgement is about taste, not risk. A thin film caught early costs you nothing. A thick one that has been sitting for two weeks has already flavoured everything below it, and at that point the honest question is whether you want to eat it, not whether you can.
One qualification. This applies to a ferment that has otherwise behaved. If the jar has a white film and also never turned sour, the film is not the main problem — see the last section.
What do I do if it is mould?
Discard the batch. Not the top inch — the batch. On a liquid ferment, mould hyphae extend below the visible growth into the brine, and some species produce mycotoxins that diffuse into the liquid. There is no reliable depth to scoop to, because the thing you are removing is not confined to the part you can see.
This is stricter than the advice you will read for a hard cheese or a dense jam, and the difference is the medium. A firm block resists penetration; a brine does not resist it at all. The rule for a jar of kraut is not the rule for a wedge of cheddar.
Wash the jar and any weights in hot soapy water before reusing them. Vegetable fermentation does not need sterilised equipment, but it does need equipment that is not carrying what just grew on it.
How do I stop the film coming back?
Keep everything under the brine and cut the headspace. A glass weight, a folded outer cabbage leaf tucked under it, a smaller jar for the same amount of kraut — all work by the same mechanism, which is denying the surface its air. More salt does not help, and past roughly 3% it slows the lactic acid bacteria you want.
An airlock or a loosely closed lid helps too, because the carbon dioxide the ferment produces displaces oxygen when it is allowed to accumulate rather than escape freely. Warm rooms make it worse: kahm is more common above roughly 22 °C, which is also where vegetable ferments start to soften.
And check the acidity if you are unsure. A vegetable ferment that has crossed pH 4.6 within a couple of days is in the state that keeps it safe, whatever is on the surface. One that has not is a batch to start again, film or no film.