Bread, cheese, yoghurt, beer, wine, soy sauce, pickles, chocolate, coffee. These foods seem to have nothing in common — until you realise that every one of them is the work of microbes. They are all products of fermentation, one of the oldest and most widespread techniques humans have ever used to make food.

What fermentation is

Fermentation is the transformation of food by microorganisms — chiefly bacteria, yeasts and moulds. These tiny living things feed on the sugars and starches in food and, as they do, produce new compounds: acids, alcohol, gases and a wide range of flavour molecules.

The food that comes out the other side is changed. It may be tangier, more savoury, fizzier, more digestible or longer-lasting than it was before. In essence, fermentation is controlled microbial activity that we have learned to steer toward results we want, rather than letting food simply rot.

How it works

Every fermentation follows the same basic logic. You provide microbes with food, the right conditions and time, and they do the rest.

  • The microbes may come from the food itself, from the surrounding environment, or be deliberately added as a "starter culture" (as with yoghurt or sourdough).
  • Their fuel is the sugars and starches present in the ingredients.
  • Their output depends on the microbe. Yeasts often produce alcohol and carbon dioxide. Certain bacteria produce lactic acid, which gives a sour tang. Others produce the savoury, complex flavours of aged and cured foods.

A couple of classic patterns show up again and again:

  • Lactic acid fermentation, where bacteria turn sugars into acid. This is behind yoghurt, sauerkraut, kimchi and traditional pickles.
  • Alcoholic fermentation, where yeasts turn sugars into alcohol and gas. This is behind beer, wine and the rise of bread dough.

Fermentation is less about adding something to food than about inviting the right microbes to a controlled feast and benefiting from what they leave behind.

Why people ferment food

Across history and cultures, fermentation has stuck around for two big reasons.

Flavour. Fermentation creates tastes that are difficult or impossible to achieve otherwise. The tang of yoghurt, the funk of aged cheese, the savoury depth of soy sauce and miso, the complexity of wine — all are built by microbes breaking ingredients down and rearranging them into something richer.

Preservation. Long before refrigeration, fermentation was a way to keep food from spoiling. The acids and alcohol that microbes produce create conditions that are hostile to the organisms that cause rot. A cabbage turned into sauerkraut, or milk turned into cheese, lasts far longer than the raw ingredient. This is why so many traditional preserved foods are fermented.

A useful way to think about it: fermentation lets helpful microbes get to the food first, crowding out and outcompeting the harmful ones.

Fermented foods around the world

Once you start looking, fermentation is nearly universal. A small sample:

  • Dairy: yoghurt, kefir, and the world's many cheeses.
  • Grains: bread (especially sourdough), beer, and many traditional porridges.
  • Vegetables: sauerkraut, kimchi, and naturally fermented pickles.
  • Soybeans: soy sauce, miso, tempeh and natto.
  • Fruit: wine, cider and vinegar.
  • Others: the fermentation steps that develop the flavour of coffee, cocoa and tea.

Almost every food culture independently discovered fermentation, which is a sign of just how useful it is.

Live cultures versus cooked foods

A common point of confusion is whether fermented foods contain living microbes. The answer is: it depends.

Some fermented foods can be eaten with their cultures still alive — fresh yoghurt and unpasteurised sauerkraut, for instance. Others are cooked, baked or pasteurised after fermenting, which removes the live microbes while keeping the flavours and other changes they created. Bread is fermented, but baking kills the yeast. Most shelf-stable jarred products are pasteurised for safety and storage. So a food can be genuinely fermented and still contain no live organisms by the time you eat it.

The bottom line

Fermentation is the quiet partnership between people and microbes that gave us a huge share of the world's most beloved foods. By letting bacteria, yeasts and moulds transform sugars and starches, we get new flavours and longer-lasting food at the same time. It is ancient, global and still going strong in kitchens and factories everywhere — proof that some of the best food science is also some of the oldest.