What Are Fermented Foods?



We all love fermented foods. From more traditional ones such as yogurt and cheese to the trendy ones for Western cuisines such as kefir, kombucha, and kimchi.

Traditionally, our ancestors have used fermentation to preserve raw food. These days, fermented foods are part of our diet. During the fermentation process, certain types of bacteria, yeast, and even molds (think about blue cheese) break down compounds in the raw food to produce smaller molecules that are nutritious, and change the way the food tastes.

Yogurt, for example, is made from milk by lactic-acid bacteria that use the sugars in the milk to grow and multiply. Thanks to their feeding habits, these tiny bacteria change the milk's texture, flavor, and nutrients [1].

More specifically, fermented foods have been defined as “foods made through desired microbial growth and enzymatic conversion of food components [2].”

Are fermented foods the same as probiotics?

To answer this question, let’s go back in time and understand how probiotic research started. More than 100 years ago, the Nobel laureate Elie Metchnikoff noticed that Bulgarian peasants who consumed yogurt tended to live above 100 years of age. That’s pretty impressive! 

Metchnikoff proposed that by eating yogurt regularly, we can help balance our gut health and live longer [3]. 

In modern times, the benefits of yogurt have been attributed to a particular bacterial species: Lactobacillus bulgaricus, one of the first isolated probiotic species.

Scientists quickly realized that we could consume more beneficial bacteria by isolating them from fermented foods, and later from the human gut. By isolating probiotic bacteria, scientists can study them and manufacture products with different properties [4]. These days, probiotic supplements contain well-defined and characterized live microorganisms, with known composition, concentrations, and documented health benefits [5]. These species include Lactobacillus, Bifidobacterium, Streptococcus, and yeasts [6].

These microorganisms are naturally found on the surface of raw foods used for fermentation, or are added to initiate the fermentation process. Under the appropriate conditions, they start growing and fermenting food [1], [6]. When you eat fermented foods, you eat these beneficial bacteria. 

What distinguishes fermented foods from probiotics is that we don’t always know what exact bacterial species or strains are present in fermented food, especially if the food is a natural ferment, instead of the result of known organisms that were deliberately added to start the fermentation process. We don’t always know their concentration and whether they have a beneficial health effect. We also don’t know if they can survive passage through the gut. Fermented foods may contain live bacteria, but they can’t be classified as probiotics since the microbes in them are uncharacterized [2].

However, there is a particular class of fermented food products known as probiotic fermented foods. These are fermented foods to which well-studied probiotic strains have been added [2]. Some examples are Activia and Yakult, which you can find in supermarkets.

Are fermented foods good for you?

One of the major benefits of fermented foods is their nutritional content. Fermentation makes some compounds become easily digestible [1], [6], [7]; and it can also increase the amount of beneficial bioactive substances such as vitamins and amino acid derivatives [1], [7]. 

Fermentation can help preserve food for a longer time by making it difficult for toxigenic bacteria to multiply in the food instead [7].

Many fermented foods also contain live microbes. These bacteria or yeasts may come in contact with your gut microbiome and inhibit pathogens or boost your immune system. Foods with live microbes are those that have not been heat-treated, such as: 

  • Yogurt
  • Cheese
  • Kefir
  • Miso
  • Kimchi
  • Sauerkraut
  • Kombucha

Even some beers have live bacteria following the fermentation process. 

Not all fermented foods have live bacteria. For example, sourdough is made with live yeast [1]. But the moment you bake the bread, the yeast dies. This doesn’t mean that the product is not fermented; it only means that the yeast was killed. 

Fermented foods without live microorganisms include:

  • Sourdough
  • Many sausages
  • Vinegar
  • Soy sauce
  • Most beers and wine
  • Chocolate 

Many foods on the market may look fermented but aren’t. These are shelf stable (no need to refrigerate) and their acidic taste doesn’t come from fermentation, but from adding an acid, like vinegar.

Foods that may look fermented but aren’t include:

  • Shelf-stable sauerkraut
  • Pickles
  • Other pickled vegetables, like beets 
  • Pickled eggs
  • Acidified cottage cheese

Bacteria in fermented foods don’t necessarily colonize your gut 

Even if you eat plenty of fermented foods, this doesn’t mean that the bacteria in them will colonize your gut.

The beneficial bacteria found in fermented foods are mostly transient species that will not become permanent residents in your gut [2], [6]. The same is true for most probiotic bacteria [4]. 

Lactic-acid bacteria found in fermented foods have been shown to survive the harsh stomach environment and safely reach the gut, but this isn’t always guaranteed. Many bacteria may not reach your intestine. Most of the time, we don't even know what type of bacteria live in your homemade kimchi or sauerkraut, so we can’t trace them in your gut microbiome.

But keep eating fermented foods.

They can support your digestive health in many different ways [1], [6], [7]. The benefits of fermented foods are far beyond the live microorganisms present in them. 

Even if the bacteria don’t colonize your gut, they can still provide health benefits while passing by. They may be able to modulate the gut microbiome [1], [8] and interact with host immune and epithelial cells . Fermented foods are also rich in nutrients that your gut microbiome can use.

In fact, some studies show that your gut microbiome can become more diverse upon eating yogurt, kimchi, and kombucha regularly [9]. This is also associated with improved immune response. Other studies show that regularly eating yogurt, kefir, kimchi, or sauerkraut may help with obesity, type 2 diabetes, and cardiometabolic disease [1], [7], [10]–[12].


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[7] N. Şanlier, B. B. Gökcen, and A. C. Sezgin, “Health benefits of fermented foods,” Crit. Rev. Food Sci. Nutr., vol. 59, no. 3, pp. 506–527, 2019, doi: 10.1080/10408398.2017.1383355.

[8] L. T. Stiemsma, R. E. Nakamura, J. G. Nguyen, and K. B. Michels, “Does Consumption of Fermented Foods Modify the Human Gut Microbiota?,” J. Nutr., vol. 150, no. 7, pp. 1680–1692, Jul. 2020, doi: 10.1093/jn/nxaa077.

[9] H. C. Wastyk et al., “Gut-microbiota-targeted diets modulate human immune status,” Cell, vol. 184, no. 16, pp. 4137-4153.e14, Aug. 2021, doi: 10.1016/j.cell.2021.06.019.

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[11] I. Sluijs et al., “The amount and type of dairy product intake and incident type 2 diabetes: results from the EPIC-InterAct Study,” Am. J. Clin. Nutr., vol. 96, no. 2, pp. 382–390, Aug. 2012, doi: 10.3945/ajcn.111.021907.

[12] Y. Lee, Y.-S. Cha, Y. Park, and M. Lee, “PPARγ2 C1431T Polymorphism Interacts with the Antiobesogenic Effects of Kochujang, a Korean Fermented, Soybean-Based Red Pepper Paste, in Overweight/Obese Subjects: A 12-Week, Double-Blind Randomized Clinical Trial,” J. Med. Food, vol. 20, no. 6, pp. 610–617, Jun. 2017, doi: 10.1089/jmf.2016.3911.