How to ferment vegetables at home: lacto-fermentation basics, salt and brine

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Lacto-fermentation is one of the oldest ways of keeping vegetables, and it needs nothing more than vegetables, salt, water and a jar. Bacteria that already live on the vegetables, mainly lactic acid bacteria, turn their sugars into lactic acid. The acid sours the vegetables, gives them their tang and, once it has built up, makes the jar a poor place for the microbes that spoil food or make people ill. This guide covers the basics that apply to almost any vegetable. For specific recipes, see the sauerkraut guide in this room, or the kimchi recipe in Korean Food.

What you need

Vegetables that are fresh and firm. Bruised or soft produce ferments badly.
Salt without iodine or anti-caking agents. Plain sea salt, kosher salt or pickling salt all work; iodine can hold back the fermentation.
A clean glass jar or a ceramic crock that is sold for food use.
A weight to keep the vegetables under the brine: a small glass weight, a smaller jar filled with water, or a clean food bag filled with brine.
Kitchen scales. Measuring salt by the spoon is unreliable, because different salts have very different grain sizes.

How much salt

Salt is what keeps the ferment safe in its first days, before enough acid has formed. Work it out by weight, not by volume.

Dry-salting (shredded vegetables that make their own brine, like cabbage): weigh the vegetables and use about 2 to 3 per cent of that weight in salt. For 1 kg of shredded cabbage, that is 20 to 30 g.
Brining (whole or chunky vegetables covered in salted water): university extension guidance commonly gives around 3 per cent for shredded vegetables and around 5 per cent for large pieces such as whole cucumbers and carrots. A 3 per cent brine is roughly 30 g of salt per litre of water.

More salt slows the fermentation and keeps vegetables crisper; less salt speeds it up. Do not cut the salt below what a tested recipe gives. Food safety sources are clear that reducing salt in a lacto-ferment is not safe, because it lets unwanted microbes get ahead before the acid takes over.

Step by step

Wash your hands, the jar, the weight and your tools in hot soapy water and rinse well.
Prepare the vegetables: shred, slice or leave whole.
Weigh them and calculate the salt, or mix your brine to the right strength.
Pack the vegetables into the jar firmly, leaving a few centimetres of space at the top.
Cover completely with brine and put the weight on top.
Cover the jar loosely, or use an airlock lid, so gas can escape.
Keep it at cool room temperature, out of direct sun, on a plate to catch any overflow.
Taste after a few days with a clean spoon. When it is sour enough for you, put a lid on and move it to the fridge.

The one rule that matters most: keep it under the brine

Anything below the surface is in an oxygen-poor, salty, increasingly acidic environment where lactic acid bacteria thrive. Anything poking out is exposed to air, which is where mould grows. Check the jar every day or two, press the vegetables back down, and top up with brine of the same strength if the level drops.

What is normal

Bubbles and a fizzing sound when you press down: the bacteria are working.
Cloudy brine: normal, and a sign of active fermentation.
A sour, pickled smell: exactly what you want.
A thin, flat white film on the surface: usually kahm yeast. It is not dangerous, but it can affect the flavour. Skim it off.

When to throw it out

Discard the whole batch if you see fuzzy or coloured mould (green, black, blue, pink), if the vegetables are slimy or mushy, or if it smells rotten rather than sour. If you are unsure, throw it away. Vegetables are cheap; food poisoning is not. The fermentation safety guide in this room goes into more detail.

Food safety basics

Use a tested recipe and follow its salt amount.
Keep everything clean, but there is no need to sterilise the vegetables: you are relying on the bacteria on them.
Do not add raw garlic or herbs in oil to the jar. Oil does not belong in a lacto-ferment.
Once it tastes right, keep it in the fridge, which slows the fermentation almost to a stop.

What are you fermenting first? Share your recipes, your salt ratios and your questions in the comments.