Fermentation is a case of what's old is new again - an ancient food-processing method turned hot trend, with companies churning out products incorporating kefir, kimchi, sauerkraut and kombucha that are promoted as being probiotics.
But do these foods offer true probiotic benefits? The short answer is probably not. But that doesn't mean these foods are without benefit.
First, let's clear up what "probiotic" means. Scientists define probiotics as live microorganisms (microbes) which, when administered in adequate amounts, confer a health benefit.
Probiotics can contribute to digestive and immune system health and crowd out harmful microbes in the gut (large intestine). They also create some nutrients, including vitamin K and many of the B vitamins, and help our bodies absorb other nutrients.
However, most fermented foods do not meet these criteria, and would not give you the same benefit as a probiotic supplement might. Robert Hutkins, a food science professor and researcher at the University of Nebraska at Lincoln and author of "Microbiology and Technology of Fermented Foods," said that most microbes - which include fungi as well as bacteria - in fermented foods have not been characterized or defined, let alone tested in clinical trials to see if they offer probiotic health benefits. This is partly due to logistics - the specific microbes in fermented foods vary by manufacturer, batch and location.
"We don't know how many bacteria are helping us," said Cynthia Lair, a nutrition faculty member at Bastyr University near Seattle, and author of "Sourdough on the Rise: How to Confidently Make Whole Grain Sourdough Breads at Home." "We haven't even identified them all, so they certainly haven't all been studied."
And then there's the issue of how many live microbes might be needed to confer any benefit. Some fermented foods - including fresh kimchi, sauerkraut and sour pickles, as well as yogurt, kefir and kombucha - can contain 1 million to 1 trillion live microbes per gram. With other foods, the microbes are killed by baking, pasteurization or filtering.
These include tempeh, most soy sauces, beer and wine, as well as any shelf-stable fermented foods. Aged cheeses retain few live microbes, and it was long thought that baking killed all the microbes in fermented breads. That is not the case, as Lair discovered when she created a new sourdough starter from a piece of one of her home-baked loaves.
A review published in August in the journal Nutrients found that clinical evidence for the role of fermented foods in digestive health and disease is extremely limited, although laboratory evidence is promising. The health effects of kefir - a fermented dairy beverage - are the best studied, but the rest of the fermented food world is more of a question mark. So if they're not "probiotic," what do fermented foods do for you? Plenty, as it turns out.
Fermentation helps preserve food by suppressing microbes that might make us sick - something humans figured out early on. "Properly made cheese and sausages would have lasted for months or years, providing a stable source of protein, minerals, and vitamins for when food resources were scarce," Hutkins said.
"Even today, fermented foods serve as an excellent source of these nutrients." He points to cheese and yogurt, which are among the best sources of calcium in the human diet, as well as fermented vegetables such as kimchi and sauerkraut, which contain B vitamins and vitamin C.
Hutkins said that even when there are no live microbes left in the finished food, the dead microbes may still enhance the food's nutritional value. For example, microbes in sourdough bread deactivate phytic acid, an "anti-nutrient," increasing the bioavailability of minerals and protein.
Certain antioxidant phytochemicals - natural compounds in plant foods that have demonstrated health benefits for humans - found in grains, fruits, and vegetables may also become more bioavailable during fermentation.
And if you enjoy a glass of wine in the evening, you will appreciate that the alcohol produced during fermentation helps pull phytochemicals out of the grapes and into the wine.
Fermentation transforms some foods in ways that enhances their digestibility, Hutkins said. "For example, lactose-intolerant individuals can consume yogurt, even though it contains lactose, because the yogurt microbes provide the lactase enzyme needed to digest that lactose," he said. "Lactose-intolerant individuals can usually also consume aged cheese, because the lactose is removed during fermentation."
While it's unclear where any benefits in fermented foods might come from - the microbes, byproducts of fermentation or the nutritional content of the food itself - what is clear is that fermentation enhances flavor. There are thousands of food-microbe combinations, each creating unique tastes and textures.
Just think of what happens when microbes convert milk to tangy yogurt or kefir, or fresh olives - which are so bitter that they are inedible - into something delicious.