How do I know if my ferment has gone bad?

Fermenting worries people because a lot of what a healthy ferment does looks alarming — it goes cloudy, it bubbles, it smells strong, the colours change. Knowing which of those signs are normal and which are not is the whole skill, and it is a short list on both sides.

Safety

Step by step

1. Start with the smell, at arm's length.A good vegetable ferment smells sour, sharp, briny and clean — recognisably like sauerkraut or a pickle. A bad one smells putrid, cheesy, of rot, of bins or of nail varnish. The distinction is usually immediate and you do not need experience to make it.
2. Learn what is normal and looks worrying.Cloudy brine is normal and expected. Bubbles are normal. Colours dulling from bright green to olive, or red cabbage going magenta then pink, are normal. A brine that has thickened slightly in the first week often clears again. A strong smell in the first three days is normal.
3. Learn the white film in the middle: kahm yeast.A flat, white, wrinkled or powdery film on the surface with no raised fuzz and no colour is kahm yeast. It is harmless. It means too much air contact or too warm a spot. Skim it off, push everything back under the brine, move the jar somewhere cooler, and carry on.
4. Learn the absolute stop: fuzz.Anything raised, fluffy or hairy — blue, green, black, grey, white-and-fuzzy, pink or orange — is mould. The whole jar goes in the bin. Do not skim it and continue, because moulds send hyphae down through the food and some produce toxins that are not confined to the visible growth.
5. Treat pink and orange as a stop as well.Pink or orange slime, or pink streaks through a kraut, generally means the wrong bacteria have taken over, usually because the salt was too high or the ferment too warm. Bin it.
6. Check the texture of the brine.Slimy, ropy or stringy brine that pulls into threads sometimes appears early in a low-salt ferment and resolves as the acid builds. If it is still ropy after a week, or it smells wrong at the same time, throw it away.
7. Look at the brine line before anything else.Almost every ferment that spoils spoils because something was sitting above the liquid. If you find mould, look for a floater — that is where it started, and it is what to fix next time.
8. Apply the rule when unsure.If you cannot decide, throw it away. A jar of cabbage costs about a pound. There is no version of this decision where gambling makes sense, and 'have a small taste to see' is not a test — it is how a bad ferment gets eaten.

Tips

• Take a photo on day one. Comparing appearance over time is far easier than remembering what it looked like.
• Ferments smell strongest in the first week and settle afterwards. A ferocious smell on day three that is turning pleasantly sour by day six is normal.
• A ferment kept at 16 to 20°C behaves far more predictably than one on a warm worktop. Most of the alarming outcomes are temperature problems.
• Once it is in the fridge, always use a clean utensil. A used fork is how an otherwise perfect jar picks up mould in week three.

Common mistakes

• Skimming mould off and eating what is underneath — The visible growth is the fruiting body. The threads are already through the liquid and the food below, and some moulds produce toxins that heat does not destroy. Fermented food is soft and wet, which is the worst case for this.
• Tasting a suspect jar to decide — Tasting is not a test — many spoilage organisms and toxins do not announce themselves on the tongue, and by the time you have tasted it you have already eaten some. Smell and appearance are the tests.
• Confusing kahm yeast with mould and throwing away good ferments — The mistake runs both ways. Kahm is flat, white and featureless; mould is raised and fuzzy and usually coloured. Learning the difference saves a lot of perfectly good kraut.

Questions people ask