Mould on a ferment

Telling them apart

Look at the surface, not at the brine. Cloudy brine is normal in every ferment and tells you nothing about this.

Kahm yeast — skim it

A flat, thin, white or cream film. Wrinkled or crazed like the surface of a puddle drying, sometimes faintly translucent. Lies flush against the liquid with no height to it. Smells yeasty or a bit like cheese.

Mould — throw it out

Fuzzy, raised, and three-dimensional. Grows in distinct circular colonies rather than as a continuous film. Any colour — blue, green, black, pink, grey, or white-but-furry. Smells musty rather than yeasty.

The single most useful test

Fuzz versus film. A flat film is yeast. Anything with visible fuzz, texture or height above the liquid is mould.

White is not the answer on its own

Both can be white. Kahm yeast is white and flat; some moulds are white and furry. Judge the texture, not the colour.

If it is kahm yeast

Harmless, but it tastes unpleasant and it means air is reaching the surface.

1. Skim it off with a clean spoon, taking as little brine as you can.
2. Check why the surface was exposed — something has floated up, or the liquid level has dropped.
3. Push everything back under the brine and re-weight it.
4. Move the jar somewhere cooler if the room is warm, which slows its return.
5. Taste the ferment. If it has picked up a strong yeasty flavour, it is disappointing rather than dangerous; if it tastes fine, carry on.

If it is mould

Throw the whole batch away. Not the top layer — the whole jar.

The reason is that a mould colony visible on the surface is the fruiting part of an organism whose filaments extend well beyond what you can see, into liquid that circulates freely through the whole jar. Scooping off the visible growth does not remove it, and there is no reliable way to judge by eye how far it has gone.

The second reason is that you do not know what species it is. Some moulds produce compounds that are not destroyed by cooking, and identifying a mould by looking at it is not something anyone can do. Whether a particular mould on a particular food is dangerous is a food safety judgement rather than a cooking one, and the responsible answer, in a domestic kitchen, is that a jar of vegetables costs very little and is not worth a decision made on a guess. FoodHQ does not publish salt percentages, pH figures or keeping times for home preserving. Those are safety numbers, they are set by a food safety authority rather than by a cookery writer, and the right ones depend on what you are fermenting and where you are. In the UK the Food Standards Agency is the authority — use a recipe from them or from another body with the same standing, and treat this page as the explanation of why that recipe is shaped the way it is.

Wash the jar and the weight thoroughly before reusing them.

Why it happened, and how to stop it

Almost every case comes down to air reaching the food. Mould needs oxygen; submerged food does not give it any.

• Something floated. A stray shred of cabbage or a piece of vegetable bobbing at the surface is the usual starting point. A weight or a folded outer leaf holds everything down.
• The brine level dropped, through evaporation or leakage, until the top of the food was exposed.
• The jar was too full or too empty. Too full and it overflows and loses brine; too empty and there is a large volume of air above the liquid.
• The room was too warm. Warmth speeds up everything, including the things you did not want.
• The lid was fully sealed. A ferment produces carbon dioxide, and that gas is itself a protection — it sits above the brine and excludes oxygen. A jar that cannot vent will either force brine out or be opened repeatedly, and both let air back in. An airlock, or a lid loosened daily, solves it.
• A dirty utensil went in. Use a clean one every time, and do not eat straight from the jar.

Common questions