Fermentation: bread, cheese, yogurt and more

Fermentation is the process by which microorganisms such as yeasts and bacteria break down sugars and other compounds, producing acids, gases or alcohol. People used fermentation for thousands of years before anyone knew microbes existed.

Evidence of fermented drinks goes back more than 9,000 years, with residues found on pottery at Jiahu in China. In the 1850s and 1860s Louis Pasteur showed that fermentation is carried out by living microorganisms, not a purely chemical process.

• Preservation: acids and alcohol produced by microbes inhibit spoilage organisms.
• Flavour: fermentation creates sourness, depth and complex aromas.
• Digestibility: microbes can break down compounds that are hard for people to digest, such as much of the lactose in yogurt.
• Nutrition: some fermentations increase certain vitamins or make nutrients easier to absorb.
• Safety: before refrigeration, fermentation was one of the most reliable ways to store food through winter.

Yeasts are single-celled fungi. When they ferment sugars without much oxygen, they produce alcohol and carbon dioxide. In wine, yeast ferments the sugars in grape juice; in beer, it ferments sugars released from malted grains.

In bread, the carbon dioxide inflates the dough, and most of the alcohol evaporates during baking. Sourdough uses a starter, a mix of flour and water kept alive with wild yeasts and lactic acid bacteria, which gives the bread its tangy flavour and helps it keep longer.

Lactic acid bacteria convert sugars into lactic acid, which lowers the pH and gives a pleasantly sour taste. They are responsible for yogurt, where bacteria ferment milk sugar and cause milk proteins to thicken, and for many cheeses, where fermentation is one step alongside curdling, pressing and ageing.

The same bacteria make sauerkraut from cabbage in Central Europe and kimchi from cabbage, radish, garlic and chilli in Korea. Salt is key: it draws water out of vegetables and discourages harmful microbes while lactic acid bacteria thrive.

Some fermentations use moulds. In Japan and China, a mould called koji, Aspergillus oryzae, breaks down starches and proteins in soybeans and grains, leading to soy sauce, miso and sake. Blue cheeses such as Roquefort get their veins and sharp flavour from Penicillium moulds.

Vinegar is made in two stages: yeast turns sugars into alcohol, then acetic acid bacteria turn the alcohol into acetic acid. Coffee beans and cocoa beans are also fermented after harvest, a step that is important for developing their characteristic flavours.

Some fermented foods, such as live yogurt and kefir, contain living microbes when eaten. Researchers are actively studying how fermented foods affect the community of microbes in the gut, and a 2021 Stanford study found that a diet high in fermented foods increased gut microbial diversity in participants.

The science is still developing, and claims made for some products go beyond the evidence. Home fermentation should also be done carefully, following reliable recipes, since improper conditions can allow harmful bacteria to grow.