Guide · Kahm or mould

Kahm yeast or mould? How to tell

A thin, flat, white-to-cream skin across the whole surface is kahm yeast: harmless, skim it off. Raised, fuzzy or coloured spots (green, blue, black, pink, or fluffy white) are mould: throw the whole batch out. If you honestly can't tell, treat it as mould.

Tell them apart in four questions

1. Look at the top. Clear brine, a skin over the whole surface, patches of fuzz, or vegetable sticking up out of the brine? Patches of fuzz in any colour: that is mould.
2. Get close to the skin. Flat and a bit wrinkled, like the skin on warm milk, is kahm. Raised, fuzzy or velvety, with a texture of its own, is mould.
3. Smell it. Sour like sauerkraut or yoghurt, or yeasty like beer, is normal. Rotten, putrid or cheesy means the wrong microbes won.
4. Tip the jar. Brine that runs like water, clear or cloudy with sediment, is fine. Brine that pulls into slimy threads like egg white is not.

If it's kahm yeast

Kahm is a wild yeast, not a mould. It makes no toxins and doesn't harm what's underneath, but it tastes musty if you leave it, so:

• Skim it off within a day or two and wipe the rim of the jar.
• Make sure every piece is back under the brine.
• Move the jar somewhere cooler. Kahm loves warmth, low salt and exposed vegetable; fix one of those and it usually stops coming back. If it keeps returning, the ferment is probably far enough along for the fridge.

If it's mould

Don't scoop it off and save the rest. A jar of brine isn't a block of hard cheese: mould threads reach down into the liquid and any toxins spread through it. Empty the jar, wash it in hot soapy water, and start again.

• Mould needs air, so it starts on whatever pokes above the brine. Weigh everything down next time.
• Leave 2 to 3 cm of headspace and keep the rim clean.
• Check the salt was in range for the vegetable. Weak brine lets mould get ahead of the lactic bacteria.

Things that look worrying but are fine

• Cloudy brine and sediment in a ferment: the lactic bacteria at work.
• Garlic turning blue or green: a harmless reaction with the acid.
• Bubbling over: a lively ferment, common with carrots, beets and warm rooms. Burp the jar daily.
• Vegetable above the brine gone dull or dry, with no fuzz: lift those pieces out, top up with brine at the same strength (never plain water), and weigh the rest down.

More guides

• How much salt for sauerkraut
• Kimchi salt ratio and recipe

For long-term, shelf-stable canning, follow a lab-tested recipe for your altitude and equipment. These guides are for fridge pickles and fermenting in a jar.