When it goes wrong · Ferment
White film on the surface.
Can you save it?
Reviewed for this symptom
Skim it off with a clean spoon, wipe the sides of the jar, push everything back under the brine and carry on; kahm yeast is harmless. If the film is fuzzy, raised or coloured, it is mould — discard the batch.
Safety boundary: Fuzzy, raised or coloured growth is mould, not kahm yeast; on a wet ferment the batch is discarded.
How to confirm the diagnosis
Observable cues, not inferred user state
- A flat, white, wrinkled film
- Not fuzzy and not coloured
- Some vegetables or brine were exposed to air
Not this?
What actually happened
Only this symptom
Kahm yeast — harmless, caused by oxygen contact, and skimmed off. Fuzzy or coloured growth is mould and is different.
The full mechanism
Shared by every ferment failure
Fermentation is controlled spoilage, and the control comes from making conditions that one organism tolerates and others do not. In vegetables, salt at roughly 2% by weight is enough to inhibit most spoilage bacteria while lactic acid bacteria — already present on the vegetable — carry on. They convert sugars to lactic acid, the pH falls, and below about 4.6 almost nothing dangerous can grow, so the ferment protects itself as it proceeds. Submersion matters for the same reason: the useful bacteria work without oxygen, and the moulds and yeasts that spoil a crock work only with it, so anything above the brine line is at risk while everything below is safe. In dairy, the same acidification makes casein clump, which is the thickening in yogurt and crema. In bread, wild yeast and lactobacilli produce gas and acid together.
The full Ferment lesson — how to tell, where to practise it, what it carries to.
Next time
Prevent this specific failure
Keep everything submerged under a weight, fill the jar high to limit air, and keep it cool.
What right looks like
Shared by every ferment failure
- Bubbles appear within a day or two and the brine turns cloudy — both are signs it is working, not spoiling.
- The smell is sharply sour and clean. Anything yeasty, cheesy or putrid is a different organism.
- Vegetables lose their bright colour and go translucent as the acid works into them.
- Taste is the real gauge, and it should be sour enough to make you flinch slightly before it is done.