Fermented foods: the science of sauerkraut

Fermented foods are very much “en vogue” at the moment, so let's make some!
10 May 2021

Interview with 

Ljiljana Fruk, University of Cambridge


Cabbage, the main ingredient in sauerkraut


Fermented foods are very much “en vogue” at the moment. Some are dubbing the sector a “mega trend in the making”. But biochemically speaking, fermentation is a metabolic process where microorganisms like yeast turn sugars in foods into alcohols and acids. That’s how we make wine and beer of course. The same processes can produce delicious and nutritious foods too - lactic acid bacteria breaking down the sugars in milk will make cheese and yoghurt, for example. And sauerkraut - a tangy german side dish made from fermented cabbage, is also made from lactic acid bacteria. Eva Higginbotham’s been trying her hand at making some...

Eva - I love to cook, but I’ve never fermented anything myself before, and so sauerkraut, a relatively quick fermentation process that requires only 2 ingredients - cabbage, and salt, seemed like a good place to start.

So, I lightly rinsed the outside of my cabbage, chopped it up thinly, and then added some salt in a big bowl. Then, I massaged the salt into the chopped cabbage, and after a few minutes of TLC the cabbage became a bit limp and released a lot of water. I grabbed handfuls of my wet cabbage and loaded it into a clean jam jar before pouring over the remaining salty liquid in the bowl. Finally, I took the core of the cabbage and used it as a weight to hold the cabbage under the salty, cabbage-ey brine, before I covered the jar with a clean paper towel and rubber band to leave at room temperature to ferment.

I’d been following a few different recipes I found online, but, being a Naked Scientist, I wanted to understand what the science was behind the different steps. So I called up University of Cambridge chemist Ljiljana Fruk…The first thing is that I had to wash everything that was going to be touching the cabbage really carefully. Why is that?

Ljiljana - Because on every surface that we have and use there are microorganisms living, and some of them might be really beneficial to the microorganisms involved in fermentation and others could be having destructive action, which means that you need to wash as many microorganisms off of your equipment, so that the microorganisms that you use in fermentation come mainly from the cabbage leaf.

Eva - The first thing I had to do was chop the cabbage very thinly, and then I added salt to a big bowl. What does the salt do? And does it matter how much you add?

Ljiljana - Salt is very beneficial for us, but if we take too much of it we will ultimately die. And so it happens to microorganisms as well. So a certain amount of salt will be damaging to the bad microorganisms, which you don't want to have in fermentation, but it will be tolerated by bacteria, which you need for fermentation of the cabbage. But the balance is very fine. So lactic acid bacteria, which you need for the cabbage fermentation, will tolerate up to 8% of salt for weight, which is good. But if you add a little bit more, you will kill it as well so you will not get anything. So by carefully controlling the amount of the salt, you are promoting the health and the growth of fermentation bacteria, but you are removing the microorganisms that might cause it to rot, or they might cause some other effects, which you don't want to have. So you are just allowing the salt to act as a kind of distinguisher between the bad and the good.

Eva - When I put the cabbage into the jar I then had to pour the brine - so all the salty liquid that had come out of the cabbage - on top of the chopped up cabbage to make sure it was all covered. Why is it important to make sure the chopped up cabbage is covered in brine?

Ljiljana - It's important because you are preventing the access of oxygen closer to the areas of fermentation. So fermentation will happen within the leaf and on the leaf of the cabbage. So you want to keep this area, where fermentation is happening, oxygen free and rich in salt. And so you need to make sure that you're covering the area of your cabbage so the oxygen from the air cannot get in.

Eva - The other thing is, from what I saw, I had to use a breathable cloth to cover the cabbage. I'm wondering why I should need to use a breathable cloth if we're trying to keep oxygen out?

Ljiljana - In the process of fermentation you might have different processes, chemical reactions happening. And in some of those reactions you produce gasses. And one of the gasses is carbon dioxide, and you wouldn't like to accumulate the carbon dioxide in a large amount within the whole fermentation vessel because carbon dioxide could dissolve a little bit in the liquid and it could increase the acidity of the whole system. And then again, if you increase the acidity, this might be detrimental to some of the microorganisms.

Eva - What can I expect to see through the glass? Are there going to be any visual changes?

Ljiljana - Yeah. So you might see that the liquid which you might have noticed becomes a little bit turbid, which is not as transparent. And you might see the change in the texture of the cabbage. If you see something black or brown, probably it's about the time to remove everything and throw it away. But as long as you just see slight color changes in terms of the liquid, or maybe changes in the texture of the leaf, that should be still okay.

Eva - Most importantly, what should I plan on eating it with?

Ljiljana - Well, depends if you are looking for the vegetarian version or non-vegetarian version! But what I like the most is if you ferment your cabbage correctly, it can have such a wonderful flavour. And you can just add a little bit of garlic to it, maybe a tiny bit of pepper, and have a wonderful salad. I love it. Particularly this time of the year to give me a boost of C vitamin as well.


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