Fermenting vegetables: principle and safety limits

Lactic acid bacteria live naturally on vegetables. They cope well with salt and without oxygen – many putrefying and mould organisms, by contrast, do not. So salt and the exclusion of air give the lactic acid bacteria a head start. They convert the natural sugar in the vegetables into lactic acid, the pH drops and the vegetables turn sour.

The acidic environment inhibits many spoilage organisms. At the same time the salt draws juice out of the vegetables. With shredded cabbage the brine forms by itself; this is called dry salting. Whole cucumbers or larger pieces do not release enough juice, so they are covered with a salt brine.

Oxygen at the surface, on the other hand, favours yeasts and mould. So the most important step is to keep the vegetables under the liquid with a weight. The container is covered so that the fermentation gases can escape but little air and no insects can get in.

The amount of salt decides success and safety. Tested instructions give it as an amount per weight of vegetables, so vegetables and salt are weighed, not estimated. Too little salt lets unwanted germs grow before there is enough acid; too much slows the fermentation.

Temperature counts too. In a cool place with even temperature the vegetables ferment slowly and evenly. If it is too warm, the vegetables easily turn soft and unwanted organisms can take over; if it is too cold, fermentation hardly gets going. The suitable ranges are in the instructions.

Cleanliness is the foundation: clean containers with hot water, wash hands, use only fresh, sound vegetables and remove wilted or rotten parts. Label the container with contents and date, check it regularly and top up brine only as the instructions say.

Warning signs are pink, slimy or soft vegetables, an unusual putrid smell or mould growing into the vegetables. Then the whole contents are discarded – when in doubt, throw it out, do not taste it. Check briefly for these signs before each use.

The amount of salt is not reduced by feel. Anyone who must eat low-salt for a medical condition or has a weakened immune system should check with a doctor beforehand whether fermented foods are suitable. Meat, fish and eggs do not belong in experiments without specialist knowledge.

Garlic or herbs in oil are not fermentation. Under oil there is neither acid formation nor salt protection, but there is no air – at room temperature a botulism risk. Fermented vegetables also do not replace canning by tested recipe.

Fermentation is suitable for cabbage, cucumbers and other firm vegetables, especially harvest surpluses. A big advantage: preserving needs no energy for heat, only salt, a clean container, a weight and a cool place.

It does take patience – fermentation lasts weeks – and it does not always succeed; losses are part of it. Fermented food is also high in salt. For an acute emergency it is therefore less an emergency tool than a craft to learn in calm times.

Fermentation is one of the oldest preservation methods in the world, and many cuisines have their own variants. The National Center for Home Food Preservation provides tested instructions for sauerkraut and fermented pickles, with notes on salt content, temperature and signs of spoilage.

This entry explains the principle and makes no claims about health effects. Quantities and times always come from tested instructions – that is how an old craft becomes a safe one.