Many fermented foods are associated with live bacteria and other microorganisms.
But what role do these microorganisms actually play?
Microorganisms are essential to fermentation itself. Bacteria and yeasts transform components of the original food and, in doing so, change its taste, texture and biological properties.
Some of these microorganisms remain alive in the food when we eat it. Others do not.
Understanding this difference helps explain why fermented foods can affect the body in different ways.
Which fermented foods contain live microorganisms?
Natural yogurt is produced using bacterial cultures and can contain live microorganisms when consumed.
Kefir is different. It is traditionally produced using a more diverse microbial community that includes both bacteria and yeasts.
Greek yogurt is generally strained or concentrated yogurt. Whether live cultures remain depends on how the product is made and processed.
Skyr is a cultured dairy product traditionally closer to a fresh cheese, although it is commonly eaten like yogurt.
Cultured buttermilk, or maślanka, is produced through bacterial fermentation and can also contain live cultures.
Even within the same type of food, the microorganisms present can therefore differ between products.
What can live microorganisms do?
When consumed, microorganisms from fermented foods enter an already complex ecosystem in the digestive tract.
Some can survive passage through parts of the gastrointestinal tract and temporarily interact with the existing gut microbiome.
They do not necessarily become permanent residents of the intestine.
Their presence may nevertheless influence microbial activity and interactions within the gut environment.
Importantly, a fermented food containing live microorganisms is not automatically probiotic. Probiotics are specific live microorganisms that have demonstrated a health benefit when consumed in adequate amounts.
What if the microorganisms are no longer alive?
This does not mean that fermentation has had no biological effect on the food.
During fermentation, microorganisms can produce organic acids, peptides and other bioactive compounds. These compounds may remain in the final product even when the microorganisms themselves are no longer present.
This happens in many familiar foods.
Sourdough bread is fermented and then baked. Cocoa beans are fermented before being dried and roasted. Some fermented foods are pasteurized — heated to reduce potentially harmful microorganisms and improve safety or shelf life. This can also reduce or eliminate live cultures.
The final food can therefore retain changes created during fermentation without containing the microorganisms that originally produced them.
Live microorganisms are important, but they are only one component of the biology of fermented foods.
