Fermentation is as old as food itself. It is the method for making, among other things, miso, hot sauce, beer, salami, yoghurt, sourdough, cheese and coffee. Many chefs love the hypercharged flavour of fermented foods, but it is their health benefits that are increasingly taking centre stage.

Sandor Katz, the author of The Art of Fermentation, has been coaxing readers into fermenting food since the 1990s. More recently, the epidemiologist Tim Spector (the co-founder of the Zoe health app), has homed in on what he calls “the four Ks” – kefir, kombucha, kimchi and kraut (sauer) – as being vital to keeping our microbiome healthy.

Fermentation involves using microbes (such as bacteria and yeast) to break down carbohydrates in food under conditions in which there is no oxygen. During the process, these beneficial microbes break down sugars and starches into alcohols and acids, preserving the food so it can be stored for longer – and making it tasty. Fermented foods contain enzymes necessary for digestion, leading Spector to call them “fertiliser for our resident gut bacteria”.

Keen to give making the four Ks a go? Here’s how to get started.

Kombucha

This fermented tea is Chinese in origin, dating back thousands of years. The food writer Betty Liu has recalled that, when she discovered kombucha, she excitedly told her father, who promptly asked: “But isn’t this hong cha jun?” When he grew up in Shanghai, the whole neighbourhood would join in brewing sessions.

You will need two tall, wide-mouth jars and a muslin cap (to ward off fruit flies). You will also need a scoby – symbiotic culture of bacteria and yeast – which is a rubbery, slimy round disc. I got one from a friend’s “hotel” (every time you brew a batch of kombucha, a new layer of scoby forms, which you can remove to a separate jar, hence the name “hotel” for the extra ones). A scoby can be bought online if you don’t know anyone else brewing. You will also need some starter liquid – aged, healthy kombucha that will help to increase the acidity of your tea and improve the fermentation process.

First, brew some strong tea. Black or green teas are recommended, although Liu writes that oolong and pu-er work, too. Sweeten at a ratio of one part sugar to 10 parts water, stirring to dissolve. Allow to cool completely. Strain off the tea leaves, or remove the bags, and add the liquid to your jar with the starter (at a ratio of one part starter to 10 parts tea) and the scoby. Cover and leave at room temperature, out of direct sunlight, for at least a week.

The scoby feeds on the sugar, souring the tea. It gradually thickens into a pale, rubbery disc. There will be darker deposits floating about: this is just the yeast at work. The fermentation expert Kenji Morimoto says you should trust your senses: “Smell it, taste it; it’s not going to hurt you. If it’s still very sweet and you want it to be more sour, let it continue to ferment. If it tastes delicious, then it’s done.”

When you are happy with the balance, you can drink it as it is, or do a second fermentation, to flavour and carbonate it. To do this, remove the scoby to the stack in your hotel (I add a little kombucha to keep my scobies in liquid and store the jar, covered with a cloth cap, next to my fermenting kombuchas). Then, strain the kombucha into sealable bottles; old-school flip-top glass bottles are best. Don’t overfill the jars – too little space for expansion and the buildup of CO2 risks an explosion.

For flavouring, Morimoto suggests “fruit, syrups, aromatics and jam”. He has made a ruby-hued blueberry, ginger and rosemary batch and a date and sultana one, as well as strawberry and mint, and pineapple and lemongrass. If you prefer, you can strain off the liquid – and eat the fruit – before drinking the tea.

Kefir

Consumed across eastern Europe, Russia and the Caucasus for millennia, this fermented milk is incredibly easy to make. All you need is milk and kefir grains. If you don’t know someone who can give you any grains, you can buy some online. Place the grains in a glass jar and cover with a cup of milk. Some methods specify whole milk, but I have found semi-skimmed works just as well.

Cover with muslin secured by an elastic band, or a loose lid, and leave at room temperature until the milk thickens to a yoghurt-like consistency. As with all the Ks, taste as you go. As the milk sours, gauge your preferred level of pungency. When you are happy with it, pour out the kefir through a small strainer, to catch the grains; put them in a fermenting jar ready to reuse.

The polish food writer Zuza Zak, in her book Amber and Rye, recommends an Estonian way with kefir that involves mixing in a cupful of roasted and ground grain flakes (typically oat, rye and barley). I make some by toasting and blitzing a muesli base. It is malty and wholesome. You can have it savoury, or sweetened with berries and honey.

Benjamin Bourrie and Ben Willing, Canadian food scientists at the University of Alberta, are conducting studies on the potential health benefits of kefir for humans. The problem is, says Bourrie: “When you read about health benefits, especially in journal papers, a lot of this work is done in animals first, which are good models, but they don’t necessarily carry over to what we see in humans.”

Initial results are promising regarding kefir’s impact on cardiometabolic health – which includes heart disease risk factors and type 2 diabetes – specifically around inflammation and “bad” LDL cholesterol. Willing cautions, though: “There is minimal evidence to really broaden out these potential benefits we’re investigating in kefir to all fermented foods.”

Bourrie and Willing say regular consumption is important for any benefits to be noticeable, so a cup of kefir a day would be a good start. Also, they recommend making it yourself. “In Canada, the kefir that you buy in the store often doesn’t have the active yeast and bacteria found in kefir made with grains. Industrially mass-produced kefir is often made with the bacterial cultures used to produce yoghurt.”

Kimchi

With more than 200 officially recognised variations, the Korean government declared kimchi a national treasure in 1984. And, as Junghyun Park and Jungyoon Choi point out in The Korean Cookbook, Unesco added kimjang, the practice of making it, to its World Intangible Cultural Heritage list in 2013. Kimchi’s seasoning culture, write Park and Choi, dates back to the 12th century – which is to say, there is much to learn.

Songsoo Kim, a chef based in London, writes in the food newsletter Vittles: “The crunchiness of a kimchi is integral.” She says it is essential to salt the vegetables before fermenting, to draw out the water and “avoid making vegetable mush”. The flavours are built on top of this.

I consult the Korean-American YouTuber Maangchi’s 2014 classic kimchi recipe. She has made dozens of other kimchi videos, some experimental, but this baechu-kimchi (made with napa cabbage or Chinese leaf) has had more than 28m views. As she says: “It is the best. I can make 50 different recipes with it: stir-fried rice, string beans with pork, soup with pork or tofu, fluffy kimchi pancakes … I cannot live without it.”

Maangchi makes a 4.5kg (10lb) batch every few months. To do this, she takes three large heads of Chinese leaf, washes them and trims the bases. Then, she slices each cabbage in half, cuts a slit in the base of each half and adds salt (Kim recommends 2% to 3% of the weight of the vegetables). Maangchi salts the halved cabbages and lets them sit in a large bowl, turning them every 30 minutes, for two hours. Then, she rinses the cabbage halves thoroughly, tears them into quarters and sets them aside to drain.

For the seasoning paste, she makes a porridge of water, glutinous rice flour and sugar, to which she adds gochugaru red pepper flakes, minced garlic, ginger and onion, sliced spring onion, fish sauce (she says the Vietnamese brand Three Crab is best) and Korean salted shrimp (press the shrimp to get the juice, then finely chop before adding them). Next, julienne some daikon or Korean mu radish and some carrot, chop up some Korean herbs, such as buchu or minari (optional), and mix it all into the paste.

Spiciness is not compulsory in kimchi: the traditional baek, or white kimchi, is entirely without heat. Park and Choi also make a vegan version: instead of shrimp, they use yondu (a Korean vegan seasoning sauce) or cheonjang (light soy sauce) and add in sesame oil to the vegetable mix.

When the leaves are drained, cover each one thoroughly with your chosen paste, then roll up the cabbage quarters, packing them tightly in a large, wide container (earthenware, glass or BPA-free plastic), gently pressing down to get rid of air pockets. Let it sit out for a few days, watching it until bubbles form: “Every day, you need to check out your kimchi baby,” Maangchi says. It is ready when it is nicely soured. Store it in the fridge.

Kraut

Kraut has become a shorthand, in English, for all manner of variations on sauerkraut – AKA the German staple of fermented white cabbage. In Germany, as the food writer Meike Peters says, traditional fermented white cabbage is the simplest and the best. It is almost always served cooked, alongside mashed potato and a fatty sausage. “Adding character to it would feel weird to me,” says Peters. “You don’t need to fuss.”

The more playful krauts found all over the internet, then, feel closer to eastern European and Asian lactofermented cabbage preserves. The basic method, though, is the same.

Shred the cabbage, massage it, then scatter with good-quality salt and massage it again until it releases its juices. Morimoto suggests the amount of salt should be 2% of the weight of the chopped veg. Zak just goes by eye, generally adding about a tablespoon for a whole cabbage.

To red or white cabbage, Zak adds grated carrot, a bay leaf and sometimes a clove of garlic. Mandolined apple is often used for sweetness, but I have seen blueberries used, too; caraway, juniper and allspice berries or mustard seeds are good aromatics. Other good vegetables to include are kohlrabi, celeriac, red pepper and red onion.

Place the shredded veg with its salted juices in a large jar, pushing it down to ensure the brine fully covers the veg. Zak adds a little water, if necessary. As the lactofermentation process is anaerobic (without oxygen), the veg has to be submerged. You can cover it with a cabbage leaf, a clean piece of muslin or a small fermentation weight. Zak sometimes opts for a clean pebble.

Cover the jar, without sealing it, and leave it out at room temperature, until it is as sour as you like it. This can take a few days or a couple of weeks. The longer you leave a ferment, the more potent it gets, so keep tasting.

The veg will change colour – red cabbage can go blue, celeriac can darken – but as long as the taste is right, it is not a problem. It’s OK if the liquid goes slimy; it may be because it wasn’t salted enough, or it got too warm. Just add some lemon juice and put it in the fridge – the cultures should balance out again. Once the flavour is where you want it, seal the jar and refrigerate it, where it will keep for a few weeks.

Zak likes the cabbage very, very sour, she says, and eats it straight from the fridge (when her daughters have a cold, she gives them a spoonful of the brine). She also piles it on cheese on toast, with a layer of mayo and mustard spread beneath it. Peters makes a sauerkraut tartine with hummus in place of cheese, the kraut cold and not too sour. Crunchy fermented cabbage is an excellent addition to a salad or, indeed, the base of one: the Ukrainian way, according to the food writer Olia Hercules, is to serve the sour cabbage with a scattering of thinly sliced red onion, drizzled with unrefined sunflower oil.

From here, it is a small step into the wider world of lactofermentation. Tomatoes, wet garlic, watermelon rind, celery, syrups, sauces, passatas; the possibilities and the promise are boundless.

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• This article was amended on 28 February 2024. Songsoo Kim continues to be based in London, contrary to what an earlier version said.