Salt in fermentation

Selecting the microbes

A cabbage leaf carries a mixed population of organisms. Some of them would rot it; a few of them are lactic acid bacteria, which will sour it instead. Fermenting is the business of making sure the second group wins.

Salt is how that is arranged. Lactic acid bacteria tolerate salt considerably better than most of the organisms competing with them, so a salted environment handicaps the competition without handicapping them. They get a head start, they produce acid, and the acid finishes the job — because once the pH has dropped far enough, almost nothing else can establish at all.

So salt does not preserve the food directly. It buys time for the acid to be made. That is the whole mechanism, and it explains why the salt matters most at the beginning and matters less and less as the ferment gets going.

Everything else salt is doing

• Drawing out water. Osmosis pulls liquid out of the vegetable, which is what creates the brine in a dry-salted ferment like sauerkraut and what makes the shreds go limp and packable.
• Keeping the texture firm. Salt helps hold pectin in the cell walls together, so a properly salted ferment stays crunchy and an under-salted one goes soft. This is the commonest home-fermenting disappointment and it is usually a salt problem.
• Slowing the ferment down. More salt means slower acid production. A slower ferment develops more complex flavour, which is why some traditional recipes are saltier than the minimum would require.
• Restraining the enzymes in the vegetable itself that would otherwise soften it.
• Seasoning, last. The finished food does taste salty, but that is a consequence of the quantity rather than the reason for it.

Why the quantity is not yours to adjust

The temptation, reasonably, is to treat the salt in a fermentation recipe as a taste preference and reduce it. It is not one.

Below a certain concentration the handicap on the competing organisms disappears, the lactic acid bacteria no longer get their head start, and what happens next is not a milder ferment — it is a different and unpredictable one. Too much salt has the opposite failure: the lactic acid bacteria themselves are slowed to the point where the ferment stalls, sits without acidifying, and is vulnerable for far longer than it should be.

There is a working range, it is narrow-ish, it differs by vegetable, and it is a food safety figure rather than a culinary one. FoodHQ does not publish salt percentages, pH figures or keeping times for home preserving. Those are safety numbers, they are set by a food safety authority rather than by a cookery writer, and the right ones depend on what you are fermenting and where you are. In the UK the Food Standards Agency is the authority — use a recipe from them or from another body with the same standing, and treat this page as the explanation of why that recipe is shaped the way it is.

The salt itself

Which salt matters less than people think, with two real exceptions.

Weight, not volume

Salt crystals vary enormously in density. A tablespoon of flaky sea salt and a tablespoon of fine salt are not the same quantity of salt, and a recipe given by volume is much less reliable than one given by weight. This is the single most common practical error.

Anti-caking agents

Present in some table salt. Generally harmless to a ferment but can make brine cloudy in a way that is hard to distinguish from a problem.

Iodised salt

Widely avoided in fermenting on the grounds that iodine inhibits bacteria. The effect at the concentrations involved appears small, but plain salt removes the question and costs nothing.

Sea salt, rock salt, kosher salt

All fine. Use whichever, weighed.

Salt substitutes

Potassium chloride based products do not behave the same way and should not be swapped in.

Common questions