The short version

A successful lacto-fermentation comes down to three numbers. Salt: about 2.4% of the cabbage weight, or 24 g per kilo, based on the reference recipe from the National Center for Home Food Preservation, which specifies 3 tablespoons of salt per 5 pounds of cabbage. Temperature: 21 to 24 °C, not the 18 °C you see repeated everywhere. Time: three to four weeks at that temperature, five to six weeks if your kitchen sits at 16-18 °C, and essentially nothing below 16 °C.

Two things most articles on the subject skip entirely. First, safety does not come from the salt but from acidification: Clostridium botulinum cannot grow below pH 4.6, and a submerged ferment is precisely an oxygen-free environment, which is its favourite terrain until the acid arrives. Second, sauerkraut is a salty food: 100 g contain 661 mg of sodium according to the USDA database, or 1.65 g of salt, a third of the daily limit the WHO recommends.

I put this up front because it changes how you eat what you have made.

What is actually happening in the jar

Lacto-fermentation does not inoculate anything. The lactic acid bacteria are already there, on the cabbage leaves, alongside everything else the field put there. Salt and the absence of oxygen do one thing: they give lactic bacteria, which tolerate salt well and oxygen poorly, the advantage over putrefaction bacteria and moulds, which need air.

Those bacteria eat the vegetable’s sugars and release lactic acid. The pH drops. Once it falls below 4.6, no pathogen known in home preserving can develop, including C. botulinum. That is the threshold the World Health Organization states explicitly in its botulism fact sheet.

This mechanism has a consequence I find counter-intuitive and worth holding onto: the dangerous period is the beginning, not the end. A jar you have just packed is anaerobic but not yet acidic. Every day you shave off the pH descent is a day less spent in that window. That is why temperature matters so much, and why fermenting in a 15 °C kitchen is a poor idea: the fermentation will not start, but the anaerobic conditions are firmly in place.

The corollary is reassuring. Sauerkraut that is properly sour, that stings your nose and makes you wince, is safe sauerkraut. The acidity is not a side effect, it is the preservation mechanism itself.

Salt: how much, and which kind

The reference recipe published by the National Center for Home Food Preservation, which follows USDA guidance, works in batches of 5 pounds of cabbage (2.27 kg) with 3 tablespoons of salt, roughly 54 g. That lands at 2.4% of the vegetable’s weight.

The 2 to 3% range you see quoted is not wrong, but it is too wide at both ends. At 2% you are already at the bottom of what tested protocols accept. At 3% the ferment slows noticeably and the result turns frankly salty. Weigh rather than measure by spoon: 24 g per kilo of trimmed cabbage on a kitchen scale takes ten seconds.

The NCHFP is categorical on one point: do not cut back on the salt in sauerkraut or brined vegetables. In vinegar pickles, salt is only a flavour enhancer and can be reduced. In a ferment, it is part of the safety system.

The type of salt matters too, and here I have to correct advice you find everywhere. Unrefined sea salt and Himalayan pink salt are not the best choice. The NCHFP guidance on fermenting ingredients is specific:

• Use canning or pickling salt, with no additives.
• Avoid flake salt: its density varies too much for volume measuring to be reliable.
• Avoid iodised salt, which darkens the product.
• The anti-caking agents in table salt cloud the brine.
• Salt substitutes based on potassium chloride cause bitterness.

In Switzerland, plain non-iodised cooking salt from any supermarket works perfectly. Iodised salt is a sound habit for the rest of your cooking but not for the jar. If iodine interests you beyond this specific case, we cover it in a dedicated piece on balancing iodine on a vegetarian diet.

Sauerkraut: the detailed protocol

Equipment

Allow 4 litres of container volume per 2.3 kg of vegetable, with at least 10 to 12 cm of headspace above the cabbage. This is not excessive: the brine rises, foams and overflows more than you expect.

The NCHFP accepts stoneware, glass and food-grade plastics, and notes that fermenting in one-litre or two-litre Mason jars is acceptable but leads to greater spoilage losses, because of the small volume and unfavourable surface-to-volume ratio.

Here is the point I want to press, because it is widely misunderstood: you do not sterilise a fermentation vessel. The official protocol says to wash in hot soapy water and rinse with very hot water, nothing more. Boiling does not destroy C. botulinum spores anyway, as the WHO notes, and fermentation depends precisely on the vegetable’s natural flora. The “scald and sterilise your jars” advice belongs to canning, not to fermenting.

Finally you need weight. A plate slightly smaller than the opening, weighed down with two or three water-filled one-litre jars, does the job. The variant the NCHFP describes precisely: a large sealed food-grade freezer bag holding 3 litres of water with 4.5 tablespoons of salt dissolved in it. Brine in the bag means a leak will not dilute the jar.

Steps

1. Choose and trim. Firm heads. The NCHFP recommends shredding and starting the ferment between 24 and 48 hours after harvest, which at a market means working the cabbage the day you buy it. Remove outer leaves, rinse, quarter and core.
2. Shred finely. About the thickness of a coin. Finer means more surface area and faster acidification, which serves the safety goal above.
3. Salt and knead in batches. 2.3 kg of cabbage with 54 g of salt, kneaded with clean hands until juice beads. Pack firmly into the container, then repeat with the next batch.
4. Check the immersion. The cabbage must sit 2 to 5 cm below the brine. If there is not enough juice, top up with boiled and cooled brine at 1.5 tablespoons of salt per litre of water, roughly 22 g.
5. Weigh down and cover. Plate, weights, then a clean heavy towel over the opening. The towel keeps out insects and mould spores without trapping gas.
6. Hold at 21-24 °C. Three to four weeks. At 16-18 °C, expect five to six. Below 16 °C the ferment may never start. Above 24 °C the cabbage goes soft.
7. Monitor. If you use jars as weights, the NCHFP asks for a check two to three times a week and removal of the whitish film that forms on the surface. With a sealed brine bag, you leave everything alone until bubbling stops.
8. Store. Fully fermented sauerkraut keeps for several months in the refrigerator, tightly covered.

Note the wording on that last point: fully fermented. Sauerkraut moved to the cold after ten days has not finished acidifying and does not carry the same guarantees. If you want the hands-on version, from choosing the head to packing the jars, our step-by-step sauerkraut workshop walks through the manipulation.

Airtight jars, with reservations

Plenty of people ferment in rubber-gasket jars, opening them every day or two to release gas. It works, but with two caveats. Each opening reintroduces air at exactly the moment you would rather not. And a genuinely airtight jar left for a few days can build serious pressure. The rubber gasket on a hinged jar acts as a relief valve and gives way before the glass does, which is its purpose. A screw-top jar tightened hard does not.

Kimchi: what to know before you start

Kimchi deserves a warning that sauerkraut does not: there is no laboratory-tested domestic recipe in the same sense. The sauerkraut recipes NCHFP publishes have been validated; the countless excellent Korean family recipes have not been through the same protocol. That does not make them dangerous, it means you have to apply the principles yourself rather than follow a fixed ratio.

The awkward part is the method: you salt the Chinese cabbage on the surface, let it draw out water, then rinse it. Rinsing removes an unknown fraction of the salt. By the end, the salt content is no longer controlled by a weighing. The USDA database puts commercial kimchi at 498 mg of sodium per 100 g, or 1.25 g of salt, slightly below sauerkraut but in the same territory.

Three principles, then:

• Never leave kimchi above the brine. After packing the leaves, press firmly and let liquid rise. Any dry top layer will grow mould.
• Do not leave it three days at room temperature. One to two days at 20 °C is enough to start the acidification, then the refrigerator takes over. Fermentation continues in the cold, more slowly, and that is where the flavour builds.
• Sugar is optional and modest. One tablespoon for a whole cabbage feeds the early fermentation. More gives a sweet product that takes longer to sour.

For the seasoning paste: gochugaru, garlic, ginger, soy sauce or tamari, plus julienned vegetables. Wear gloves. Korean chilli stains hands and will burn your eyes hours later.

A note on the vegetarian version: traditional kimchi almost always contains fish sauce or fermented shrimp. Swapping in tamari loses some umami depth. A splash of dried mushroom stock or a spoonful of miso compensates honestly, without pretending to reproduce the original. The same reasoning applies to homemade fermented sauces, sriracha or sambal, which run on the same chilli-and-salt logic.

Water kefir: the fastest ferment, and the most underestimated

Water kefir takes 24 to 48 hours. The grains are a consortium of lactic bacteria and yeasts held in a polysaccharide matrix. The presence of yeasts is what changes everything compared with sauerkraut.

The basic recipe

For one litre: non-chlorinated water, 2 tablespoons of grains, 3 tablespoons of sugar, two slices of organic lemon, and optionally a dried fig or a few raisins. Dissolve the sugar, add the rest, cover with a cloth, leave 24 to 48 hours at room temperature, strain, keep the grains for the next cycle.

Nothing complicated so far. It is the next two points that recipes gloss over.

Alcohol

The yeasts produce ethanol. That is unavoidable; it is what a yeast fermentation does. You often read “less than 1%”, but that figure circulates without a traceable source and depends entirely on starting sugar, duration, temperature and strain. No household method measures it.

In practice: the longer you ferment, the more alcohol, and a second fermentation in a sealed bottle adds more. If you serve kefir to children, to someone pregnant, or to anyone avoiding alcohol for medical or religious reasons, keep the fermentation short, skip the second fermentation, and bear in mind that the exact figure remains unknown. No reassuring number can be justified without a measurement.

Pressure

This is the real risk with water kefir, and it is physical rather than microbiological. A sealed bottle in which yeasts keep eating sugar accumulates CO₂ until pressures reach the point where glass fails. Swing-top lemonade bottles are built for that; a recycled juice bottle is not.

Simple rules: thick bottles rated for carbonation, fill to three quarters, refrigerate after 12 to 24 hours of second fermentation, and open over the sink. Sugary fruit added before bottling accelerates the pressure build-up sharply. The same principle applies to homemade kombucha, which poses exactly the same gas-under-pressure question.

What these foods actually deliver

Here are the figures from USDA FoodData Central, per 100 g:

Food	Energy	Sodium	Salt equivalent	Fibre	Vitamin C	Vitamin K
Raw white cabbage	25 kcal	18 mg	0.05 g	2.5 g	36.6 mg	76 µg
Sauerkraut (canned)	19 kcal	661 mg	1.65 g	2.9 g	14.7 mg	13 µg
Kimchi	15 kcal	498 mg	1.25 g	1.6 g	0 mg	43.6 µg

Three readings follow.

Sodium. The WHO recommends staying below 2000 mg of sodium a day, equivalent to less than 5 g of salt, and notes that the global adult mean reached 4278 mg a day in 2021, more than double. A 100 g portion of sauerkraut therefore uses a third of the daily allowance. That does not disqualify it, but it makes it a condiment rather than a freely served side vegetable. If you want to cut salt elsewhere to compensate, our notes on seasoning without salt apply directly.

Vitamin C. It falls from 36.6 to 14.7 mg between raw cabbage and the USDA sauerkraut entry. One caveat: that entry describes a canned, therefore pasteurised, product, and heat destroys part of it. Raw refrigerated sauerkraut retains more. The old story about sauerkraut protecting sailors from scurvy still holds, but it holds for the unheated product.

Vitamin K. 43.6 µg per 100 g of kimchi, 76 µg for raw cabbage. The NHS puts the requirement at roughly 1 µg of vitamin K per kilo of body weight per day, so 65 µg for a 65 kg adult. A plate of cabbage covers the day. That becomes a precise point of attention for one group of people, which I come back to below.

And the probiotics?

This is the claim you see everywhere, and it is the one that holds up worst. The NHS is sober on probiotics: there is some evidence of benefit in specific cases such as irritable bowel syndrome, but little evidence behind most of the health claims made. The NHS adds that these products are regulated as food rather than medicine, so there is no guarantee they contain the stated bacteria, in sufficient quantity, or that the bacteria survive long enough to reach the gut.

For a homemade jar those uncertainties are larger still: you do not know which strains established themselves or in what numbers. A home ferment is not a characterised probiotic, and nobody should sell it as one.

None of that takes away what is certain. It is a vegetable, it supplies fibre, it keeps for months with no energy input and no packaging, and it tastes like nothing else. Those reasons are plenty. If you specifically want documented micro-organisms, we address that in our article on choosing vegan probiotics.

Diagnosing a jar that is going wrong

The NCHFP publishes a table of causes and solutions for problem ferments. The most useful entries:

Soft or slimy vegetables. Brine too weak, temperature too high, or vegetable not fully submerged. The NCHFP adds an unambiguous instruction: if spoilage is evident, do not eat it.

Whitish film on the surface. Wild yeasts and bacteria feeding on the acid. Remove it as often as needed. It is not dangerous in itself, but letting it thicken lets the pH climb back, which is the opposite of what you want.

Fuzzy, coloured, green, black or pink mould. That is no longer a film. Throw it out. You do not “scrape the top layer” off a mouldy ferment: the mycelium penetrates the material and some moulds produce toxins that spread well beyond the visible part.

Dark or discoloured product. Minerals from hard water, iodised salt, ground spices instead of whole, or brass, copper or zinc utensils. That last cause is not cosmetic: the NCHFP states not to eat the product if brass, copper or zinc was used.

Bitter taste. Too many spices, or a potassium chloride salt substitute.

Smell. A good ferment smells acidic, of vinegar, yoghurt and cabbage, clearly and unmistakably. A smell of rot, decay or turned cheese means something else. Doubt always resolves the same way: throw it out. A cabbage costs three francs.

Ten mistakes that keep coming back

1. Measuring salt with a tablespoon instead of a scale.
2. Fermenting in a 17 °C room and expecting a result in a week.
3. Sterilising the jars, which achieves nothing here and wastes time.
4. Using iodised salt, flake salt or a potassium substitute.
5. Filling the jar to the rim and finding brine on the shelf.
6. Leaving vegetable poking above the brine.
7. Refrigerating in week one, assuming the flavour will keep developing at the same pace.
8. Scraping off fuzzy mould and eating the rest.
9. Bottling kefir in a bottle not rated for pressure.
10. Treating a homemade jar as a probiotic supplement, with expectations it will not meet.

Depending on your situation

On warfarin or another vitamin K antagonist. The NHS states that foods high in vitamin K, including green vegetables, affect how warfarin works. It does not ask you to cut them out but to talk to your anticoagulation clinic before any significant dietary change. Starting to eat 150 g of kimchi a day is a significant change. The NHS also advises against vitamin K supplements, cranberry products and grapefruit juice.

High blood pressure or a low-salt diet. Count sauerkraut and kimchi as salty foods, not neutral vegetables. A third of the WHO limit per 100 g is a portion, not a free side.

Weakened immunity. The NHS recommends speaking to a doctor before taking probiotic supplements if your immune system is weakened. The reasoning applies at least as much to a home ferment whose flora you do not know.

Pregnancy. Properly acidic homemade sauerkraut kept cold poses no particular problem, but the same caveats hold: correctly acidified, refrigerated, and no kefir with an extended second fermentation because of the ethanol.

Infants under one year. No honey in flavoured kefir intended for a baby. The WHO explicitly links spore-contaminated honey to cases of infant botulism and advises against giving it before the age of one.

Balanced meals. The Swiss federal food safety office recommends five portions of fruit and vegetables a day, three of them vegetables, at 120 g per portion. Fermented vegetables count, but their salt load places them closer to the condiment side than to a full portion.

Frequently asked questions

How long does homemade sauerkraut keep? Several months in the refrigerator, tightly covered, provided it was fully fermented before going cold. The NCHFP is clear about that precondition.

Can you ferment without salt? Not sauerkraut or brined vegetables. The NCHFP explicitly instructs against cutting the salt in these preparations, because there it serves safety and texture, unlike vinegar pickles where it is only a flavour enhancer.

Do I need a special jar with an airlock? No. The official protocol describes a plate, weights and a clean towel. An airlock simplifies monitoring and reduces surface film, but it is not a condition of success.

My sauerkraut is foaming and overflowing, is that bad? It is normal in the first days; that is the CO₂. Stand the container in a deep dish and next time allow 10 to 12 cm of headspace above the vegetable.

Can I add apples or sugar? The NCHFP recommends against altering the proportions of a tested recipe. Adding fruit brings extra fermentable sugars, which changes the timing and the outcome. Keep quantities small and shorten the storage time.

Can I reuse brine from a successful jar to start the next one? It does speed up the start. But it does not exempt you from salting the new batch properly: the brine you add is acidic, the cabbage you have just shredded is not.

Is water kefir suitable for children? With reservations, because it contains ethanol in an unknown quantity. Short fermentation, no second fermentation in a sealed bottle, small servings.

Why has my kefir stopped fizzing? Usually a lack of minerals or chlorinated water. Swiss tap water is high quality but chlorinated in some municipalities; leave it standing in the open for a few hours. One dried fig per litre supplies the missing minerals.

Going further

Fermentation is also a remarkable anti-waste tool: a whole cabbage lasts months in a jar with no electricity. That is the angle we develop in our pieces on anti-waste practices and on storing fruit, vegetables and pulses properly. For a first attempt without waiting a month, 24-hour quick pickles give immediate feedback. And if you want to dig further into safety than this article goes, our guide to homemade lacto-fermentation is devoted to it.

One last remark, because it cost me a few jars. The difficulty of fermenting is not technical, it is psychological: you have to leave a jar alone for three weeks without opening it every twelve hours to check. The cabbage manages perfectly well unsupervised.

Sources

• National Center for Home Food Preservation, University of Georgia: sauerkraut recipe, general information on fermenting, containers, covers and weights, causes and solutions for problems
• World Health Organization: botulism, sodium reduction
• NHS: probiotics, vitamin K, warfarin
• Swiss Federal Food Safety and Veterinary Office: dietary recommendations for adults
• USDA FoodData Central for nutritional values