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What you need

Large jar
Cabbage (any variety will do)
Spring onion
Salt
Garlic
Ginger
Chilli

USUALLY we strive to avoid letting microbes colonise our food. But not all of them are bad for us, and many cuisines preserve food by encouraging the growth of non-harmful microbes and discouraging the bad ones. We call this fermentation. Kimchi, a staple food in Korea, is a prime example.

In this case, the microbes we want to cultivate are principally lactic acid bacteria (LAB). These are hardy – able to tolerate acidic, salty and low-oxygen conditions. They make lactic acid as a result of their metabolism and this, along with the salt used to make kimchi, kills most other bacteria and allows LAB to dominate.

As fermentation progresses, some bacteria flourish and then are supplanted by others. Lactic acid levels rise, so the most acid-tolerant microbes come to dominate. The most abundant in a mature ferment is typically Lactobacillus plantarum. Once this species has become established, the kimchi should also be acidic enough to keep pathogens at bay.

LAB are found in very low numbers on growing vegetables under normal conditions and make up less than 1 per cent of bacteria in soil or on farms. When cabbages are grown in sterile conditions and inoculated with LAB, the bacteria won’t persist on them, according to microbiologist Ben Wolfe at Tufts University in Massachusetts.

So where do they come from? Wolfe told Gastropod, a US food science podcast, that his team are testing the idea that insects bring them to cabbages, and they may reside mainly in insects’ guts.

In whatever way they get there, the cast of microbial characters will be different in every jar of kimchi, depending on the vegetables, where they were grown and the kitchen. Along with lactic acid, the bacteria produce carbon dioxide, acetic acid and a range of flavour compounds that give kimchi its complex, tangy character. You can make kimchi with a variety of vegetables, but cabbage kimchi is the best known.

To make your own, chop a cabbage into small, evenly sized pieces and put in a large bowl. For every 400 grams of cabbage, add a teaspoon of finely ground salt and mix well, pressing and squeezing the cabbage until water leaches out. Chop a few spring onions, some cloves of garlic and a piece of ginger and add these, along with some Korean chilli flakes, known as gochugaru, or chilli powder.

Pack the mix tightly into a large jar. The vegetables should be fully submerged – if not, add more water and press down on them. Leave some space at the top for the CO₂ that will build up, and release the pressure every couple of days.

Leave the jar somewhere cool. After a week, have a taste. If you like the taste, move it to the fridge, or leave it for a few more days for the flavour to develop further. It will keep for a couple of months in the fridge, but will become more sour over time.

For next week

White and wholemeal flour

Water

Salt

Glass jar, casserole pot

Next in the series

1 The scientific shortcuts to cooking delicious caramelised onions

2 How to make halloumi and ricotta cheese using ancient biotechnology

3 How to cook perfect chips: Learn the science of crispiness

4 Here’s how to make your own tofu for Chinese New Year

5 Use the science of curing to turn salmon into gravlax at home

6 How tempering chocolate hacks its crystalline structure

7 Umami: How to maximise the savoury taste that makes food so satisfying

8 Pancake day 2020: Here’s a scientific recipe for better batter

9 Kimchi and fermentation

10 Sourdough bread

All projects are posted at Email: cooking@newscientist.com