Fermented foods have been part of the human diet for thousands of years.
But what actually makes a food fermented?
Fermentation occurs when microorganisms such as bacteria or yeasts transform components naturally present in food.
In milk, microorganisms can convert lactose into lactic acid. This gives yogurt and kefir their characteristic acidity and helps change their texture.
Similar processes are involved in foods such as cheese, miso, sauerkraut, kimchi and sourdough bread.
Fermentation can do more than change taste and texture.
During the process, microorganisms can transform nutrients and produce organic acids, peptides and other biologically active compounds. These changes can influence the nutritional properties of the final food.
Fermented does not always mean probiotic
This is an important distinction.
Some fermented foods contain live microorganisms when we eat them. Certain yogurts and kefirs are common examples.
Others undergo additional processing that reduces or eliminates live microorganisms. Sourdough bread, for example, is baked after fermentation.
A food can therefore be fermented without being a probiotic food.
Fermented foods are not all the same
The term covers an exceptionally diverse group of foods.
Yogurt, kefir, cheese, fermented vegetables, miso and sourdough can differ greatly in protein, fiber, fat, salt and sugar content. They can also differ in the microorganisms involved and whether those microorganisms remain alive in the final product.
This means that fermentation alone does not determine whether a food is healthy.
Long-term research also suggests that different fermented foods may have different associations with health. In a large 2026 analysis of prospective studies, the most consistent associations with lower mortality were observed for fermented milk products. Evidence for many other fermented foods was more limited.
Understanding the food itself therefore matters as much as knowing that it has been fermented.
