Fermenting is safe when it is done by a proven method, and not otherwise. The recipes on this site follow one named authority’s process each, and give its numbers exactly. This page explains why those numbers matter, what means a batch has gone wrong, and the few things never to do.
What the UK says
The Food Standards Agency’s advice on preserving food at home names fermenting, and sauerkraut, as one of the methods it covers. Its central instruction is short: Consumers should always:
understand the preservation method they are using follow trusted instructions exactly avoid taking risks if they are unsure whether a food has been preserved safely
The FSA’s main concern is Clostridium botulinum, the bacterium behind botulism: “Botulism is a rare but serious illness that affects the nervous system and can cause paralysis or death.”
The FSA does not publish salt levels, temperatures or times for fermenting, and as far as we can find no UK authority does. So the figures on this page come from the food authorities of the places these foods come from, Germany, Sweden and Korea, and from the US Department of Agriculture and US university extension services, which publish the most detailed home guidance. They broadly agree. Where they differ, we say so.
How fermenting makes vegetables safe
Lactic acid bacteria, which live naturally on vegetables, turn the vegetables’ sugars into lactic acid and carbon dioxide. Harmful bacteria do not survive well in the salty, acidic, low-oxygen conditions that result, and the acid has to reach pH 4.6 or below to prevent botulism (UC Davis).
Speed matters. The Swedish Food Agency stresses that the acid has to start building quickly, because slow souring at room temperature could favour C. botulinum. In proven recipes, it says, the salt is set so that the pH falls below 4 within about two days (Livsmedelsverket, 2017). Its 2024 risk assessment adds that some pathogens can grow at the start of fermentation, so a ferment should be fully fermented before anyone eats it (Livsmedelsverket, 2024).
Salt is what lets the right bacteria win, but it does not stop botulism by itself: an extension food-safety scientist explains that salt gives the lactic acid bacteria a head start so that they produce acid faster, and that a ferment should get its pH below 4.6 as fast as possible. She is also candid that there has not been much research on exactly which conditions control botulism in fermented foods. That is the strongest reason to follow a proven recipe rather than improvise one.
The rules
Weigh the salt, and never cut it. Every authority treats the amount of salt as critical to safety, not just flavour. The usual figure for sauerkraut is about 2% of the weight of the cabbage: a German district agriculture office gives it as 20 g of salt per kilo of vegetables, and Colorado State University as 2 to 2.5% by weight. The Swedish Food Agency’s range for home lactic fermentation is about 0.5 to 3%, and the Codex standard for kimchi gives 1 to 4% salt in the finished product (Codex). USDA warns against making sauerkraut or fermented pickles with less salt than the recipe requires, and does not recommend reduced-sodium salt substitutes (USDA Guide 6). If finished kraut tastes too salty, Oregon State’s advice is to rinse it in cold water before serving, not to use less salt.
Use plain salt. Colorado State recommends non-iodised salt with no anti-caking agents. USDA notes that anti-caking agents can make the brine cloudy and that flake salt varies too much in density to measure by the spoon. Weighing it avoids the problem.
Keep it at the right temperature. The authorities give slightly different ranges for sauerkraut:
- Germany: 18 to 22°C, for at least two weeks, which the Bodenseekreis agriculture office says is how long the bacteria take to bring the pH below 4.6.
- US: 21 to 24°C (70 to 75°F). Cooler, at 16 to 18°C, it takes longer; below about 16°C it may not ferment at all, and above 24°C it may go soft (Oregon State). Penn State puts the upper limit at 27°C (80°F), above which kraut may spoil as well as soften (Purdue, from Penn State).
Too cold is a risk too, not only too slow: the Swedish Food Agency notes that high salt and low fermenting temperatures hold back the lactic acid bacteria, so the pH falls more slowly. In a van, a summer afternoon can easily pass the top of these ranges and a winter night the bottom, so pick the steadiest spot you have and check the batch often.
Keep everything under the brine. USDA says fermenting cabbage must stay 1 to 2 inches (2.5 to 5 cm) under the brine; the Bodenseekreis office says at least 2 cm. Hold it down with a plate and weight, a glass fermenting weight, or a sealed food-grade bag of brine. If there is not enough brine to cover the cabbage, Colorado State’s advice is to stop fermenting at room temperature and refrigerate it.
Use the right container, clean. Food-grade glass or plastic, washed in hot soapy water and rinsed in very hot water before use (USDA). Never a bin bag or any plastic that is not food-grade, and Colorado State says not to use metal containers or anything cracked or chipped. Jars work, though USDA notes they can mean more spoiled batches than a crock. Leave a way for the gas to get out: a fermenting lid or airlock, or a lid left loose.
Wash the vegetables. See the outbreak below.
When to throw it away
Throw the whole batch away if you see mould, if it feels slimy, or if it smells bad. Do not taste it to check. Purdue’s guidance, from Penn State, says not to taste kraut at all if any of those is true.
On mould, this page follows the UK regulator and the German federal authorities. The FSA says do not eat preserved food if it “shows signs of mould or spoilage”.
Germany’s Federal Institute for Risk Assessment says food with mould on it should not be eaten but thrown away at once, because moulds can produce toxins. Some US guidance, including USDA’s, says surface mould can be skimmed off during fermentation. We do not recommend that.
Kahm yeast. German guidance describes one surface growth that is not mould: kahm yeast, a thin grey skin that can form on the brine, which the Federal Centre for Nutrition says should be skimmed off regularly to stop it spoiling the taste. But no authority we have found explains how to tell kahm yeast from mould. So: if you are not sure what is growing on your ferment, treat it as mould and throw the batch away.
Bubbles while it ferments are normal: they are the carbon dioxide the bacteria make (UC Davis).
And the FSA adds the line that matters most:
Preserved foods that have not been prepared correctly may not look, smell or taste unusual. Do not rely on appearance, smell or taste to decide whether food is safe to eat.
That is the final reason the method matters more than any check you make at the end.
Never do these
- Do not ferment garlic in honey. An extension food-safety scientist writes that there is no data to support the safety of the process: garlic is well above pH 4.6, the acidity of honey varies widely, and honey commonly carries botulism spores. Her conclusion is that she is not comfortable with its safety.
- Do not put a ferment under oil, or add oil to one. Oil keeps air out, which is exactly what C. botulinum needs. See garlic and herbs in oil.
- Do not cut the salt, or use a salt substitute. See above.
- Do not improvise a recipe. No authority we have found, in the US, Europe or Asia, publishes a home process for fermented chillies or fermented hot sauce. Asked for one, Ask Extension’s answer was that there is no specific recipe (Ask Extension). Taiwan’s Food and Drug Administration goes further and, because of botulism, urges people to avoid making pickled and canned products at home at all (Taiwan FDA). Where there is no proven process, this site does not publish one.
Acid is not a guarantee
In early 2022, a shop-bought kimchi caused an outbreak of E. coli O157 in Canada: 14 laboratory-confirmed cases in Alberta and Saskatchewan, with no deaths or hospital admissions reported (Public Health Agency of Canada).
The investigators’ paper is the useful part. The contaminated sample had a pH of 4.1, well into the acid range. The most likely source was the napa cabbage, which was the only raw vegetable in the product that had not been through a sanitising wash. The authors note that kimchi’s acidity often leads people to assume it is an unlikely source of outbreaks, and that this outbreak and earlier ones in East Asia called that assumption into question (Smith and others, 2023). The Public Health Agency of Canada lists pregnant women, people with weakened immune systems, young children and older adults as most at risk from this kind of E. coli.
Korea’s Ministry of Food and Drug Safety gives practical advice for exactly this step. In its tests, washing salted cabbage about three times cut the bacteria on it by 95% and coliform bacteria by 93%. It recommends soaking the cabbage, rubbing gently between the leaves and at the root end, then rinsing twice, and not washing more than three times, which damages the cabbage. It also advises using bought salted cabbage on the day you buy it, because coliforms can increase if it is kept at room temperature for a day or more (MFDS, via korea.kr).
For a home ferment: start with fresh vegetables with no spoilage, wash them, and work clean. Fermenting reduces risk. It does not undo contamination you started with.
Histamine
Fermenting produces histamine. Germany’s Federal Centre for Nutrition says fermented vegetables are off-limits for people with histamine intolerance.
Storing it, and the van problem
A finished ferment belongs in the fridge. Germany’s Federal Centre for Nutrition says a ferment kept closed in the fridge lasts eight weeks to six months; USDA and Oregon State say sauerkraut keeps for several months refrigerated.
Kimchi is traditionally a cold ferment. Korea’s Rural Development Administration advises making winter kimchi once the daily average temperature stays at or below 4°C, and says it takes about two to three weeks at 3 to 5°C to ripen (RDA). University of Georgia Extension explains that in the early stages the pH may not yet be low enough to stop harmful bacteria, recommends keeping kimchi at 5°C (41°F) or below, and says it takes about two weeks to become acidic enough, below pH 4.6, to inhibit most foodborne pathogens (UGA Extension).
No authority we have found supports keeping a finished ferment at room temperature. A Bavarian state food and agriculture office says that stored cool and dark, fermented foods keep for several months, even without electricity (AELF Kaufbeuren, 2026). That is the traditional cellar, and it may be true of a cold cellar. But it gives no temperature, and neither does any other authority, so it cannot be relied on in a van that warms up during the day. The only shelf-stable route the US guidance offers is canning in a boiling-water canner by a tested process, which this site does not cover (see home canning).
For a trip without a fridge, that means one of two things: make the ferment at home and take it with you in a cool box, eating it while it stays cold; or make a small batch and eat it as soon as it is ready. A jar of shop-bought sauerkraut or kimchi, unopened, is the easy alternative; follow the storage instructions on its label.