What is the white film on top of my ferment?

This is the single most common question in home fermenting, and the answer is usually reassuring. Kahm yeast is an aerobic yeast that colonises the surface of a ferment when it gets air, warmth or a slow start. It is not mould, it is not dangerous, and its arrival is a message about your setup rather than a verdict on your kraut.

Safety

Step by step

1. Look at whether it is flat or raised.Kahm sits on the surface like a skin — flat, sometimes wrinkled into ridges, sometimes powdery or chalky, always white or cream. Mould stands up off the surface in fuzzy circular colonies you can see texture in from across the room.
2. Look for colour.Kahm has none. Any blue, green, black, grey, pink or orange means mould, and the decision is made. White fuzzy circles are also mould — the fuzziness matters more than the colour.
3. Smell it, at a distance.Kahm smells faintly yeasty or of nothing much over the ordinary sour smell. Mould brings a musty, earthy, cellar smell with it. If the jar smells putrid or rotten rather than sour, throw it away whatever the film looks like.
4. Skim the kahm off with a clean spoon.Take as much as you can, then wipe the inside of the jar above the brine line with clean kitchen paper. It will come back if the conditions do not change, and that is not a problem — skim it again.
5. Fix the brine level.The usual cause is food or debris sitting above the liquid. Push everything under, reposition the weight, add 2% brine if the level has dropped, and clear any stray shreds off the glass above the line.
6. Move it somewhere cooler.Kahm thrives above about 22°C. A jar on a sunny worktop or near a radiator will grow it repeatedly. 16 to 20°C is where vegetable ferments behave best.
7. Get it into the fridge sooner.A ferment that has reached the flavour you want and is still sitting at room temperature is just giving kahm more time. Cold stops it.
8. Accept a small flavour cost.A heavy kahm layer can leave a slightly yeasty or musty taste. The ferment underneath is safe and usually fine, but a jar that has grown kahm three times running is worth eating sooner rather than keeping for months.

Tips

• An airlock lid largely prevents kahm, because it stops fresh air reaching the surface while letting gas out.
• A ferment that starts slowly is more prone to kahm, because the acid that suppresses it takes longer to build. Warmth for the first two days and cool afterwards is a good pattern.
• Kahm on a kombucha or a vinegar mother is common and behaves the same way — skim and carry on.

Common mistakes

• Throwing away a perfectly good ferment because of a white film — Kahm looks alarming and is harmless. Learning to recognise it saves a lot of good food. The distinction is flat and featureless versus raised and fuzzy.
• Skimming mould and calling it kahm because you want it to be — The rule is not negotiable and it exists because mould in a wet soft food is not confined to the surface. If it is fuzzy or coloured, the jar goes.

Questions people ask