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Fermenting
Fermenting — kept by acid, made. How the method works, the ratios and temperatures that make it safe, the steps in order, what it costs in nutrients, and the classic Nordic practice.
How it works
Vegetables arrive covered in a mixed population of microbes, some of which spoil and one family of which — the lactic acid bacteria — tolerates salt better than the rest. Salt at the right concentration suppresses the competition; the lactic bacteria then eat the sugars in the vegetable and excrete lactic acid. Within a few days the pH falls through 4.6, the line below which Clostridium botulinum cannot grow, and it keeps falling to about 3.4–3.8. From there the jar is preserved by its own acidity, exactly as a vinegar pickle is preserved by acid you added. Everything must stay under the brine, because the process is anaerobic and the moulds that take hold on an exposed cabbage leaf are not.
How long it keeps
A finished vegetable ferment kept under its brine at 0–4 °C keeps six months and often through to the next season; it goes on souring, slowly.
The numbers
| Figure | Value |
|---|---|
| Dry-salted, shredded (kraut style) | 2–2.5 % of the vegetable weight — 20–25 g per kg |
| Brined, whole vegetables (cucumber style) | 3.5–5 % — 35–50 g of salt per litre of water |
| Fermentation temperature | 18–22 °C is the comfortable range |
| Sauerkraut, fully fermented | about 3–4 weeks at 21–24 °C; 5–6 weeks at 16–18 °C |
| Finished pH | about 3.4–3.8 |
| The safety line | pH 4.6 |
| Salt type | plain, non-iodised, no anti-caking agent |
| Storage after fermenting | 0–4 °C |
How to do it
- Weigh. Ferment salt is a percentage, and coarse and fine salt differ so much by volume that a spoon measure is a guess — use a scale.
- For shredded vegetables, salt them dry, work them with your hands until they give up enough liquid to cover themselves, and pack down hard.
- For whole vegetables, make the brine to weight and pour it over. Leaves rich in tannin — blackcurrant, oak, cherry, horseradish — go in for crispness.
- Weight everything under the surface. A glass weight, a water-filled bag, or a clean cabbage leaf and a stone.
- Leave it at room temperature with a way for gas to escape, on a plate, because an active ferment overflows.
- Taste from day four onward. When it is sour enough for you, move it to the cold and it nearly stops.
What makes it safe
- Under the brine is the whole safety story: exposed vegetable grows mould, submerged vegetable does not.
- Do not cut the salt below about 1.5 %. Salt is not seasoning here, it is what selects for the right bacteria.
- A white film on the surface is kahm yeast — a nuisance you skim. Fuzzy growth in blue, green, black or pink is mould, and that jar is finished.
- Trust the smell. A ferment smells sharp, sour and clean; anything putrid, cheesy or slimy is not a ferment that went right.
- A properly soured ferment sits well below pH 4.6, which is why fermented pickles can be water-bath processed and fresh vegetables cannot.
Where it goes wrong
- Warm rooms above about 24 °C, where the ferment goes soft and yeasty instead of crisp and sour.
- Iodised or anti-caking salt, which clouds the brine and can darken the vegetables.
- Topping up with fresh water part-way, which dilutes the acid at exactly the wrong moment.
- Sealing the jar hard with no gas escape. Fermentation makes carbon dioxide, and jars do burst.
What it costs in nutrients
| Nutrient | Survives the method |
|---|---|
| Vitamin C | High |
| B vitamins | High |
| Vitamin A and carotenoids | High |
| Minerals and fibre | High |
Fermenting is the gentlest method in this table because nothing is heated. Vitamin C largely survives — sauerkraut was a ship's anti-scurvy ration for exactly that reason — and lactic bacteria synthesise some B vitamins as they work. The cost is sodium: a 2 % ferment is a salty food, and the brine carries some of the water-soluble vitamins with it, so the liquid is worth using.
The Nordic tradition
Sauerkraut and brined cucumbers are the two pillars of the Baltic winter and are still made at household scale across Estonia, Finland, Latvia and the Russian-speaking region. Hapukapsas and hapankaali are the same thing: shredded white cabbage, salt, and often caraway, grated carrot or a handful of lingonberries. Hapukurgid, suolakurkut and solyonye ogurtsy are cucumbers in brine with dill crowns, garlic, horseradish leaf and blackcurrant leaf — never vinegar, which would stop the fermentation before it started. Salted mushrooms belong to the same tradition: a lactic ferment under salt rather than a vinegar pickle.
Which foods suit it
| Food | Verdict | Why |
|---|---|---|
| Cabbage | Best | Shred, salt at 2–2.5 % of the weight, submerge, and leave 3–6 weeks at cellar-cool room temperature. |
| Cucumber | Best | Whole in a 3.5–5 % brine — 35–50 g salt per litre of water — with dill crowns, garlic and tannin leaves. |
| Tomato | Good | Firm green or barely ripe tomatoes in a 3.5–5 % brine, exactly like cucumbers. |
| Beetroot | Good | Sliced raw in a 2–3 % brine; the sweet earthy ferment used for beet kvass and for a sour borscht base. |
| Carrot | Good | Sticks or coins in a 2–3 % brine with dill and garlic. One of the easiest first ferments there is. |
| Garlic | Good | Peeled cloves in a 3 % brine. They mellow and turn sweet; blue-green colouring is a harmless pigment reaction. |
| Green beans | Good | Whole beans in a 3.5 % brine with dill and garlic; they stay firmer than most fermented vegetables. |
| Apple | Good | Into cider and then into cider vinegar — the oldest use for a bad-keeping apple crop. |
| Onion | OK | Ferments well in a 3 % brine and makes a good condiment, though it is rarely the main event. |
| Pumpkin and winter squash | OK | Ferments in a 3 % brine; a niche preparation but a good one. |
| Peas | OK | Whole mangetout or sugar-snap pods ferment in a 3 % brine and stay surprisingly crisp. |
| Wild mushrooms | OK | Salted mushrooms are a lactic ferment under salt, which is the traditional route. |
| Plum | OK | Possible, and mostly a route to plum wine or a sour condiment rather than a staple. |
| Rhubarb | OK | Rhubarb ferments into a sharp soda or wine; not a staple preserve. |
| Raspberry | OK | Wine and shrub vinegars rather than a food preserve. |
| Blackcurrant | OK | Wine and country vinegars; the leaves matter more to the fermenting tradition than the fruit does. |
| Lingonberry | OK | Water-covered lingonberries are the traditional cold-store preparation and rely on the berry's own acidity. |
| Sea buckthorn | OK | A base for country wines and vinegars. |
| Bilberry and blueberry | OK | Wine and country vinegar rather than a food preserve. |
| Herbs | OK | Herbs go into vegetable ferments as an ingredient rather than being fermented themselves. |
| Strawberry | Poor | Not a traditional ferment beyond wine. |
| Baltic herring | No | Fermented fish preparations are a genuine botulism hazard and are not a home project. |
Safety rules, temperatures, blanching times, headspace and the pH 4.6 line: USDA Complete Guide to Home Canning and the National Center for Home Food Preservation guidance (US Government / land-grant publications, public domain). Acidity classes: the FDA's published approximate-pH tables. Jam composition: Directive 2001/113/EC. Fish freezing before raw or lightly cured use: Regulation (EC) No 853/2004, Annex III. Checked 2026-09-06.
Sources for this method
Frequently asked questions
How much salt for sauerkraut?
Between 2 and 2.5 % of the weight of the shredded cabbage — 20 to 25 g of plain salt per kilo. The published USDA recipe works out to about the same figure. Weigh the cabbage after shredding and before salting.
How long does sauerkraut take?
About three to four weeks at 21–24 °C and five to six at 16–18 °C. Cooler is slower and generally better: the texture stays crisper and the flavour is cleaner.
Is the white film on top dangerous?
A flat, white, slightly wrinkled film is kahm yeast. Skim it, make sure everything is under the brine again, and carry on. Fuzzy, coloured or hairy growth is mould, and the jar goes.
Can I ferment without salt?
Not reliably, and not as a beginner. Salt is what gives the lactic bacteria their head start over everything else on the vegetable. Reducing it below about 1.5 % puts the outcome in doubt.
The other methods
All methods compared
The ten hurdles, and the foods each one suits.
Freezing
The default method in a northern kitchen: fast, forgiving, and the one that keeps the most of what was in the food. It pauses spoilage rather than ending it.
Drying
The oldest method, and the only one that needs no cold, no jars and no electricity once it is done. It works by taking the water away rather than by making the food hostile.
Pickling in vinegar
Fast, repeatable and completely different from fermenting: here you add the acid instead of growing it, so the jar is as safe as the vinegar you used.
A pantry you can see
TempoLife tracks the food you logged fresh and the jars you put down in autumn, so the batch numbers on the lids match the calendar.