Fermentation safety: mould vs kahm yeast, pH, botulism and when to throw it out

TUREFF• •0

Home fermentation has a good safety record when it is done properly, because the acid that the bacteria produce protects the food. Most problems come from a fermentation that never gets going, from vegetables left above the brine, or from mixing fermentation up with other kinds of preserving that carry different risks. This guide covers how to read what you see in the jar and which situations are genuinely dangerous.

Why a good ferment is safe

In a lacto-ferment, salt holds back spoilage microbes in the first few days while lactic acid bacteria multiply. They produce acid, and the pH falls. Food safety authorities use pH 4.6 as the key line: below it, the spores of Clostridium botulinum, the bacterium that causes botulism, cannot germinate and produce toxin. A healthy vegetable ferment drops well below that. The risk comes when the fermentation fails: too little salt, too warm, too cold, or a recipe that never develops enough acid. That is why a tested recipe and correct salt by weight matter more than anything else.

Kahm yeast: usually harmless

Kahm yeast is a flat, thin film on the brine surface. It is white or cream, matte, sometimes wrinkled like skin, and may smell yeasty or slightly off.

It is not dangerous, but it can make the ferment taste flat or musty.
It tends to form when there is air in the jar, too little salt, too little acid early on, or warm conditions.
Skim it off, check that what is underneath smells cleanly sour, and make sure everything is under the brine.

Mould: throw it out

Mould looks different: raised, fuzzy or velvety, usually in round spots, often green, blue, black, pink or furry white. Some fermenters scrape mould off and carry on. Most food safety guidance is stricter, because mould threads can reach below what you can see, and some moulds produce toxins. The cautious and recommended approach is to discard the whole batch if you see fuzzy or coloured mould.

Throw it out if

There is fuzzy or coloured mould
The vegetables are slimy or have gone mushy all the way through
It smells rotten or putrid rather than sour
It is discoloured throughout, not just in the top layer
A jar or lid is bulging, leaking or spurting when you would not expect it, with an unpleasant smell
It never soured at all after the time the recipe gives

And the rule that overrides all of these: when in doubt, throw it out.

Testing pH

You do not need to test pH for well-established recipes like sauerkraut, but it is a useful check if you are experimenting.

pH test strips covering the acidic range are cheap and adequate for a rough reading; a calibrated pH meter is more accurate.
A finished vegetable ferment should read well below 4.6. Many home fermenters aim for around 4 or lower before refrigerating.
Test the brine with a clean spoon or cup, never by dipping into the jar with something that has touched other food.

Botulism: where the real risks are

Botulism is rare but serious and can be fatal. Its toxin has no reliable smell, taste or look, so you cannot rely on your senses to rule it out. The situations that carry real risk at home are:

Garlic or herbs in oil. Oil shuts out air, and raw garlic is a low-acid food, which is exactly what C. botulinum needs. Extension services advise that home-made garlic or herb oils must be refrigerated and used within 4 days, or frozen. They are unsafe at room temperature unless the garlic or herbs have been properly acidified first using a tested method.
Low-acid foods that fail to sour. A fermentation that stalls, or a recipe with too little salt, may never reach a safe pH.
Fermenting under oil, or adding oil to a ferment. Do not do it.
Home canning of low-acid foods without a pressure canner. That is a separate subject, but often confused with fermentation.

Signs of botulism

Symptoms can include blurred or double vision, drooping eyelids, slurred speech, difficulty swallowing, and muscle weakness spreading downwards. Botulism is a medical emergency: call emergency services straight away if you suspect it.

Good habits

Use tested recipes and weigh the salt.
Keep vegetables submerged.
Ferment at cool room temperature.
Wash hands, jars and tools well.
Label jars with the date and salt percentage.
Refrigerate when it tastes right.

Have you had a batch go wrong and worked out why? Share it, or post a description of something you are unsure about, in the comments.