What Is Fermented Food? A Beginner's Guide

Fermented food is food or drink transformed by live microorganisms — mostly bacteria and yeasts. These microbes feed on natural sugars and starches and convert them into acids, gases or alcohol. That process changes the flavour, texture and keeping quality of the food, and in many cases adds live bacteria. Sauerkraut, kimchi, miso, tempeh, yoghurt, kefir and kombucha are all fermented.

What does fermentation actually do?

Fermentation is one of the oldest ways of preserving food, used for thousands of years before refrigeration existed. The principle is simple. You create conditions — usually salt, water and a lack of oxygen — where helpful microbes thrive and spoilage microbes cannot.

In a jar of cabbage and salt, naturally present lactic acid bacteria get to work. They consume the sugars in the cabbage and release lactic acid. That acid is what gives sauerkraut and kimchi their sour tang, and it lowers the pH enough to stop the bacteria that cause rot. The same family of bacteria sours yoghurt and kefir. In miso, tempeh and soy sauce, a mould called koji (Aspergillus oryzae) and other microbes break down soya beans and grains. In kombucha, a culture of yeasts and bacteria ferments sweetened tea.

So fermentation is not decay. It is a controlled transformation carried out by microbes we want, which is why fermented foods can keep for weeks or months.

What are the most common fermented foods?

You have almost certainly eaten fermented food already. The best-known examples include:

• Sauerkraut — finely cut cabbage fermented in its own brine.
• Kimchi — the Korean staple of fermented cabbage and vegetables, usually spiced.
• Miso — a savoury Japanese paste made from fermented soya beans and grains.
• Tempeh — a firm cake of fermented whole soya beans, originally from Indonesia.
• Yoghurt and kefir — milk (or water, in the case of water kefir) fermented by bacteria and yeasts.
• Kombucha — lightly fizzy fermented tea.
• Tamari and soy sauce — fermented soya bean seasonings.

Bread, cheese, wine, beer, chocolate and coffee are fermented too, which shows how much of everyday eating depends on microbes.

Are fermented foods good for you? What the science says

This is where it pays to stick to the evidence rather than the hype. The clearest human trial to date came from the Stanford School of Medicine and was published in the journal Cell in July 2021.

Researchers put 36 healthy adults on one of two diets for 10 weeks: one high in fermented foods (such as yoghurt, kefir, fermented vegetables, vegetable brine drinks and kombucha), the other high in fibre. The fermented-food group showed two measurable changes:

• An increase in the diversity of their gut bacteria — generally regarded as a marker of a healthy gut — with bigger servings producing a stronger effect.
• A drop in 19 inflammatory proteins in the blood, alongside less activation of certain immune cells.

The high-fibre group, interestingly, did not show the same rise in microbial diversity over the same period.

A diet rich in fermented foods increased the diversity of gut microbes and lowered molecular signs of inflammation, according to the Stanford trial.

It is worth being honest about the limits. This was a small, short study in healthy adults, so it is early evidence rather than the final word, and fermented foods are not a treatment for any condition. But it is real, peer-reviewed human data pointing in a consistent direction, and you can read the Stanford Medicine summary of the study for the detail.

Do all fermented foods contain live bacteria?

No — and this catches a lot of people out. Live cultures only survive in foods that have not been heated after fermentation. Pasteurisation, baking and cooking all kill the microbes. A loaf of sourdough and a bowl of cooked miso soup are still fermented and still taste of it, but they no longer contain living bacteria.

If your aim is to eat live cultures, check the label for the words raw, unpasteurised or live cultures, and keep the product in the fridge. Tinned or shelf-stable "sauerkraut" sold at room temperature has usually been pasteurised. Both versions have a place; just know which one you are buying.

Fermented or pickled — what's the difference?

The two are easy to confuse because both produce sour, tangy food. The difference is the source of the acid. In fermentation, living microbes make the acid themselves. In quick pickling, you pour vinegar over the food, adding acid from the outside with no microbes involved. A naturally fermented sauerkraut is alive with bacteria; a jar of malt-vinegar pickles is not, unless the label specifically says it was fermented.

Where to start

If you are new to fermented food, begin with one item you already like the sound of and add a spoonful or small serving to meals you eat anyway. Stir miso into a broth, add a forkful of sauerkraut or kimchi to a salad, sandwich or grain bowl, or swap minced meat for tempeh in a stir-fry. Introduce it gradually — a small daily serving is more sustainable than a large one-off — and let your taste and digestion adjust.

Frequently asked questions

What is fermented food in simple terms?

It is food or drink changed by living microbes — bacteria and yeasts — that turn its natural sugars into acids, gases or alcohol. That is what makes sauerkraut sour, miso savoury and kombucha fizzy.

Are fermented foods good for your gut?

A 2021 Stanford trial published in Cell found that a diet rich in fermented foods increased gut microbial diversity and lowered inflammatory markers over 10 weeks. It is early evidence from a small study rather than proof, but it is a credible, measurable effect.

How much fermented food should I eat?

There is no official UK guideline. A small daily serving, such as a couple of forkfuls of sauerkraut or kimchi or a spoon of miso, is a sensible and sustainable place to start. Build up gradually.

Do fermented foods need to be refrigerated?

Raw and unpasteurised products do, because the cultures are alive and active. Pasteurised, shelf-stable versions can be stored at room temperature until opened. Always follow the storage instructions on the label.

Can everyone eat fermented foods?

Most healthy adults can. If you are pregnant, immunocompromised, or managing a condition such as histamine intolerance, check with a doctor or registered dietitian first, particularly for raw, unpasteurised products.

Source: Wastyk, H. C., et al. “Gut-microbiota-targeted diets modulate human immune status.” Cell, 12 July 2021. Plain-English summary via Stanford Medicine. This article is general information, not medical advice.