Fermenting: Sauerkraut and Kimchi from Garden Brassicas

Weigh the cabbage, use 2% of its weight in plain salt, keep every piece under the brine, and throw out the batch if it grows fuzzy or coloured mould, smells rotten or turns slimy. Cloudy brine, fizzing and a sour smell are normal. Fermenting a cabbage is a controlled process with a clear mechanism, and once you understand the chain of events it stops feeling like a gamble.

What's happening in the jar

Salt worked into shredded cabbage draws water out of the cells by osmosis. The water carries the vegetable's natural sugars with it, and together they form the brine the vegetable sits in. That brine covers the vegetable, which matters (more below), and it favours one group of microbes over the rest.

Every fresh vegetable arrives with a mixed population of microorganisms, including the lactic acid bacteria that do the preserving. Left alone, a cut cabbage would be colonised by whatever bacteria and moulds got there first, some of which spoil food and some of which can make people ill. Salt changes the odds: "salt favors the growth of desirable bacteria while inhibiting the growth of others", as the US National Center for Home Food Preservation puts it. The lactic acid bacteria tolerate salt far better than most spoilage organisms, so as the salt concentration rises the competition tips in their favour.

They then ferment the sugars in the brine and produce lactic acid, which lowers the pH. A successful sauerkraut commonly falls to around pH 3.5. That acidity is what preserves the vegetable, like vinegar in a pickle except that here the acid is made in the jar. The salt picks the right microbial team and the acid it produces finishes the job. That is why the National Center says: "Do not attempt to make sauerkraut or fermented pickles by cutting back on the salt required." Cut the salt and you don't just get a milder ferment, you remove the mechanism that keeps the wrong organisms out.

The salt percentage

Roughly 2% of the vegetable's weight in salt is the standard starting point for sauerkraut. Too low and the salt can't do its selective job, so spoilage organisms can get a foothold before the ferment acidifies. Too high and it inhibits the lactic acid bacteria too, so the ferment stalls. The 2% figure sits where the right bacteria are favoured without being suppressed. It is kitchen chemistry and works the same anywhere.

1. Weigh the trimmed, cored cabbage before shredding.
2. Multiply that weight by 0.02.
3. That's the salt: 20g for 1kg of trimmed cabbage, 50g for 2.5kg.

The National Center's published recipe works out close to this: 3 tablespoons of canning or pickling salt per 5 lb (about 2.3kg) of cabbage, which lands around 2 to 2.5% by weight. Use a kitchen scale, since a tablespoon of salt varies with how fine or coarse it is.

Use plain salt without anti-caking agents or iodine, both of which can interfere with fermentation or cloud the brine. "Canning salt" is an American product name that you won't find on a UK shelf; additive-free fine sea salt, checked on the ingredients list, is the equivalent here. If your tap water is strongly chlorinated it may slow fermentation, so leave water for any extra brine standing uncovered for a few hours, or use filtered water.

The rule that matters most: keep it under the brine

Every piece of vegetable must stay fully submerged for the whole fermentation, ideally 1 to 2 inches under the brine. Anything poking above the surface and exposed to air is at risk of mould, and once mould takes hold on an exposed piece it can spread. The protective lactic acid bacteria work without oxygen, while the moulds that spoil an exposed ferment need it.

Weight the vegetable down so it can't float up as gas bubbles form:

• A plate or follower disc that sits just inside the rim of the container, weighted with something clean and heavy. The National Center describes a dinner or pie plate slightly smaller than the container opening, held down by two or three sealed jars of water.
• A water-filled bag: a heavy, clean, food-grade plastic bag of water sits on top of the vegetable and moulds itself to the container. The National Center suggests brine in the bag (3 quarts of water with 4½ tablespoons of salt), so that a leak doesn't dilute your ferment. Never use bin bags or rubbish liners.
• Purpose-made weights, usually glass or ceramic discs, in a wide-necked jar.

Check the batch every few days. If scum forms on the surface of the brine, remove it: it is wild yeasts and bacteria feeding on the surface, and left to build up it can eat into the acidity protecting the batch. A loose cover, a clean towel or a lid set on but not sealed, keeps out dust, fruit flies and spores while letting fermentation gas escape. A fermenting lid with an airlock does the same automatically, and is a reasonable upgrade once you ferment regularly. You need about a 1-gallon (4.5 litre) container for each 5 lb (2.3kg) of vegetables.

Working or failed?

Normal, leave it alone:

• Cloudy brine. That is the lactic acid bacteria multiplying. A clear ferment often means nothing much is happening yet.
• Bubbling or fizzing. That is carbon dioxide, a by-product of fermentation. Vigorous bubbling in the first week is what you want. It slows and stops as fermentation completes, which is one sign the batch is ready.
• A sour, tangy smell that grows stronger.
• A thin, smooth white film on the brine surface, usually harmless kahm yeast. It is unattractive and can smell a little odd. Skim it off and discard any vegetable it has touched.
• Red cabbage turning the brine deep pink. Pigment leaching out tints the whole brine evenly, unlike mould, which is a distinct growth.

Spoilage, discard the batch:

• Mould: fuzzy or raised growth, or any blue, green, black, pink or orange colour, on the vegetable or brine, especially if it penetrates the food. A flat white film is the exception above; if you can't tell which you have, throw it out.
• A rotten or putrid smell, not the sharp sour smell of a working ferment.
• Sliminess on the vegetable, not just normal slight softening.

Any one of these means the batch has failed. Don't taste it to check, and don't try to rescue it. Soft or mushy results short of sliminess usually come from too little salt, brine that didn't stay above the vegetable, or fermenting too warm. They affect texture rather than necessarily safety, but check your salt weighing, your submersion and where the jar is sitting.

Temperature and speed

Fermentation is far more sensitive to temperature than jam-making or chutney. Published US guidance for sauerkraut shows how much:

These come from a US extension source, so treat the weeks as a rough guide. The point is that cooler means slower. Most Scottish homes probably aren't heated to 21 to 24°C on a worktop outside summer, so many ferments here will land at the cooler, slower end of the table. That isn't a disadvantage: a slower ferment is easier to watch and less prone to softening. It means more patience than a recipe written for a warm climate suggests, and judging progress by the signs above, not a fixed number of days. If a batch shows no bubbling after a week or so, the likeliest cause is a cold spot such as an unheated porch or outbuilding. Move it somewhere gently warmer, such as a kitchen worktop or near the boiler, aiming for 21 to 24°C and not above.

Sauerkraut

Dense, tight-headed cabbages ferment better than loose, leafy ones. Firm heads shred into pieces that pack down with little trapped air and hold their texture. A firm white or red storage cabbage such as January King or Tundra, or a Savoy-type winter cabbage, is a better choice than loose spring greens, and it is the glut moment anyway when these are being harvested. For growing it see the cabbage growing guide, and storing cabbage for winter keeps what the ferment pot can't take. Before shredding a homegrown cabbage, check between the leaves for slugs, aphids and caterpillars and cut away damaged or slime-trailed outer leaves; fermenting doesn't remove them.

What you'll need

A clean glass, food-grade plastic or stoneware container with a wide opening; something to weight the cabbage; a kitchen scale; and plain additive-free salt. Wash everything in hot soapy water and rinse with very hot water first. Keep bare, uncoated metal away from the brine, since the acid reacts with it: stick to glass, food-grade plastic, stoneware or stainless steel.

Method

1. Trim and core the cabbage, remove damaged outer leaves, and weigh what remains.
2. Calculate 2% of that weight in salt.
3. Shred finely, about the thickness of a coin.
4. In a large, scrupulously clean bowl, work the salt in with clean hands, squeezing and massaging. Within ten to twenty minutes liquid should pool in the bowl, which is the osmosis drawing brine out.
5. Pack tightly into the container, pressing down firmly to force out air pockets and bring the brine over the cabbage. If its own liquid doesn't cover it, top up with a spare brine at about 2% (roughly 1½ tablespoons of salt per litre of water).
6. Weight the cabbage under the brine and cover loosely.
7. Leave at room temperature, checking every few days for the normal signs and skimming surface scum.
8. Fermentation is complete once bubbling has stopped, typically after three to six weeks depending on temperature. Move it to the fridge, where it keeps for several months, or eat it fresh. It is a salty food, so check with your doctor if you are on a salt-restricted diet. Making it shelf-stable for the cupboard is a separate process, proper water-bath canning with its own safety rules, so treat the fridge as the default unless you follow dedicated canning guidance.

Kimchi

Kimchi runs on the same chemistry: salt favours lactic acid bacteria, they make lactic acid, the pH drops, and submersion matters just as much. What differs is the build. It usually involves more than one vegetable, a separate pre-salting stage for the cabbage, and a seasoning paste added before fermentation. Traditional kimchi uses napa (Chinese) cabbage, which isn't widely grown in Scottish gardens. A firm, dense winter cabbage, as for sauerkraut, is a reasonable substitute with the same steps below; only the leaf texture changes. The proportions below are typical home-recipe quantities, so adjust to taste.

The cabbage is salted first. Cut into quarters or large pieces, it is rubbed with salt in layers or soaked in a strong brine for anywhere from one to several hours, until the leaves are soft and bend without snapping. Same osmosis, applied as a separate step. It is then rinsed to remove surface salt and drained well.

A paste carries flavour and extra vegetables. A typical paste combines Korean chilli flakes (gochugaru), garlic, ginger and often fish sauce or salted shrimp paste, sometimes with a little rice flour porridge or sweetener to help it cling. Daikon, spring onion and carrot are cut and added at this stage. Each leaf is coated in paste and packed in layers, not shredded and mixed.

1. Quarter or thickly slice about 2kg of napa (or firm winter cabbage). Layer it in a large bowl with salt between the leaves, about 2% by weight as for sauerkraut, slightly more if using a plain saltwater soak since some salt stays in the water. Leave 1 to 2 hours, turning occasionally, until the leaves bend without snapping.
2. Rinse two or three times in cold water, drain well and squeeze out pooled water.
3. Cut daikon and carrot into matchsticks and slice spring onion.
4. Make the paste: around 4 to 6 tablespoons of gochugaru, several cloves of garlic and a thumb of ginger (finely grated) and a tablespoon or two of fish sauce (or soy sauce for a vegetarian version), loosened with a little water or soaking liquid into a thick, spreadable paste. Start light on the chilli if unsure: it is easy to add more but not to take away.
5. Wearing gloves, massage the paste into the cabbage and other vegetables until every piece is coated.
6. Pack tightly into a clean container, pressing so the liquid rises above the surface, and weight down so everything stays submerged.
7. Leave at room temperature for a day or two to get going, then move somewhere cooler to continue slowly, checking every few days.
8. Once it tastes pleasantly sour, move it to the fridge, where it keeps for months and continues to develop flavour slowly.

Gochugaru and fish sauce are usually easiest to find in an Asian or international supermarket, or online. The same salt discipline, submersion rule and spoilage signs apply as for sauerkraut. The one timing difference is a much shorter initial room-temperature stage, a day or two against sauerkraut's three to six weeks.

Common questions

Sources