Sauerkraut means sour cabbage, so when a jar you have made tastes of salty cabbage and not much else, something has held the ferment back. It is usually one of three things, and none of them means the batch is lost.
My babcia kept a barrel of it, and it still goes into a lot of what I cook, bigos above all. Here is what is happening in the jar, and how to get it where it should be.
The short answer
Sauerkraut that is not sour is usually too cold, has not had long enough, or was made with too much salt. It wants a room at around 18 to 24°C, anything from one to six weeks, and salt at roughly 2% of the cabbage’s weight. Give it warmth and time and it gets there.
What makes it sour
The sourness does not come from anything you add. The cabbage leaves carry their own lactic acid bacteria, and once the cabbage is packed down under its brine, away from the air, those bacteria get to work. They eat the sugars in the cabbage and turn them into lactic acid, along with some carbon dioxide and a little alcohol. The acid is what makes it sour, and it is also what keeps other microbes out.
The salt is there to draw water and sugar out of the cabbage, which makes the brine, and to give you a liquid that covers the shreds and keeps the air off them.
Too cold
Temperature decides which bacteria take over and how fast they work. In a cool jar, the bacteria that do best make a mild mixture of acids and flavours. In a warmer one, the ones that take over make almost nothing but lactic acid, and that is where the proper sourness comes from. Most ferments go through both, the mild ones first and the sour ones taking over as the acid builds.
So a jar in a cold kitchen stays mild for longer. Sauerkraut is usually fermented at about 18 to 24°C. Korean kimchi, by contrast, is fermented much colder on purpose, at around 5 to 14°C, and it finishes noticeably less acidic than sauerkraut, which shows you what cold does. If your jar is somewhere cool, move it somewhere a little warmer, out of direct sun.
Not long enough
Sauerkraut is slow. It takes anywhere from one to six weeks, and a finished one carries around 1 to 1.5% lactic acid. A jar that has only had a few days has not got there yet. Taste it every few days rather than every few hours, and it will keep getting sourer until it reaches the point you like, which is when it goes in the fridge.
Too much salt
Salt keeps the ferment safe, but it also slows it down. Finished sauerkraut carries only about 1 to 2% salt, which is less than a lot of people use when they are guessing. Too much and the bacteria are held back, so it stays salty and mild.
Weigh it rather than measuring it by the spoon, because a spoonful of fine salt weighs far more than a spoonful of flaky salt. My fermentation brine calculator does the arithmetic in its salt by weight mode.
Not bubbling
The bubbles are carbon dioxide from the bacteria, so a lively jar bubbles in its first days. If yours is not bubbling at all, it is usually too cold, and a warmer spot will often wake it up. A jar that bubbled and then went quiet is not a problem on its own: taste it rather than watching the bubbles.
Soft, brown or mouldy on top
Everything has to stay under the brine. The ferment depends on keeping the air out, and any shreds sticking up into the air are where it goes wrong: the top turns brown and soft first. Press it back down, top it up with a little brine at the same strength if the level has dropped, and keep a weight on it.
A flat white film on the surface is usually a yeast growing where the brine meets the air. Skim it off. Anything fuzzy, raised or coloured, or anything that smells rotten rather than sour, is mould, and that batch goes in the bin.
For crunch, the salt matters as well. Unrefined sea salt carries calcium and magnesium, which help firm up the cabbage’s cell walls, so it tends to give a crisper kraut than refined salt.
Where to go from here
The same rules run through every ferment I make. Ogórki kiszone are cucumbers in a garlic and dill brine, and my fermented radish, carrot and beetroot is a jar of radishes, carrots and beetroot that turns pink as it sours. Once one of them works for you, the rest follow the same way.
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