Fermented foods are made when microorganisms like bacteria, yeast, or molds transform sugars and starches in food into acids, alcohol, or gases. The result is food that lasts longer, tastes more complex, and often contains live beneficial microbes. The list spans nearly every food category and every culinary tradition on the planet, from Korean kimchi to Ethiopian injera to Eastern European kefir.
How Fermentation Works
At its core, fermentation is an ancient form of food preservation. Bacteria or yeast feed on carbohydrates in food under low-oxygen conditions and produce lactic acid, acetic acid, or alcohol as byproducts. Lactic acid bacteria, particularly species in the Lactobacillus family, are the workhorses behind most fermented foods. The acid they generate drops the pH of the food, preventing harmful bacteria from growing while creating that characteristic tangy flavor.
Some fermentation relies on bacteria already present on the food itself. Cabbage naturally carries the microbes needed to become sauerkraut. Other ferments use a specific starter culture, like the grain-like clusters added to milk to make kefir or the SCOBY (a rubbery disc of bacteria and yeast) used to brew kombucha.
Fermented Vegetables
Vegetables are some of the simplest foods to ferment. Salt, time, and the bacteria already living on the produce do most of the work.
- Sauerkraut: Shredded cabbage fermented in salt. Unpasteurized versions sold in the refrigerated section contain live microbes. Shelf-stable jars have typically been heat-treated.
- Kimchi: A Korean staple made from cabbage, radishes, leeks, cucumbers, or sweet potatoes mixed with chili, garlic, ginger, and fish sauce, then fermented. It’s both a side dish and a cooking ingredient.
- Fermented pickles: Cucumbers brined in saltwater and left to ferment naturally. These are different from most supermarket pickles, which are simply soaked in vinegar and never actually fermented.
- Curtido: A Central American relish of fermented cabbage, onions, and carrots, similar to sauerkraut but seasoned with oregano and chili.
- Vegetable brine drinks: The liquid left over from vegetable ferments, sometimes consumed on its own as a probiotic-rich tonic.
Fermented Dairy
Dairy fermentation is driven by lactic acid bacteria converting lactose (milk sugar) into lactic acid. This is why many people who struggle with lactose find fermented dairy easier to digest.
- Yogurt: Milk fermented primarily by Lactobacillus and Streptococcus bacteria. The live cultures are what distinguish yogurt from thickened milk products.
- Kefir: A drinkable fermented milk with a thinner consistency than yogurt. It’s made with kefir grains, which contain a diverse mix of bacteria, acetic acid bacteria, yeasts, and even fungi. This makes kefir one of the most microbially complex fermented foods available. Non-dairy versions made from coconut water or nut milk also exist.
- Fermented cottage cheese: Cottage cheese made with live cultures rather than just acid. Check the label for “live active cultures.”
- Buttermilk: Traditional buttermilk is the liquid left after churning cultured cream into butter. Cultured buttermilk sold in stores is milk fermented with lactic acid bacteria.
- Aged cheese: Cheeses like cheddar, gouda, and parmesan undergo bacterial fermentation during aging. Microbial populations can reach up to 100 million organisms per gram during ripening.
- Koumiss: Fermented mare’s milk, traditional in Central Asian cultures, with a slightly alcoholic and fizzy quality.
Fermented Soy and Legumes
Soy-based ferments are central to East and Southeast Asian cooking. Fermentation transforms soybeans from a relatively hard-to-digest legume into highly flavorful, nutrient-dense foods.
- Tempeh: Whole soybeans bound together by a white mold (Rhizopus) into a firm, sliceable block. Originally Indonesian, it’s now widely available in Western supermarkets as a plant-based protein source.
- Miso: A thick paste made from soybeans fermented with salt and a mold called koji. Used as the base for miso soup and as a seasoning in sauces and marinades. Fermentation can range from weeks to years.
- Natto: Soybeans fermented with Bacillus subtilis, producing a sticky, stringy texture and a pungent flavor. Natto is one of the richest natural sources of vitamin K2 (specifically the MK-7 form), which plays a role in bone health and calcium metabolism. More than 90% of the vitamin K2 produced by the fermenting bacteria is in this highly bioavailable form.
- Soy sauce: Made by fermenting soybeans and wheat with mold, then aging the mixture in brine. Naturally brewed soy sauce takes months; cheaper versions use chemical hydrolysis and skip fermentation entirely.
- Doenjang: A Korean fermented soybean paste, earthier and chunkier than miso.
Fermented Grains and Breads
Grain-based ferments appear in cultures worldwide, often as staple foods rather than side dishes.
- Sourdough bread: Made with a starter of wild yeast and lactic acid bacteria instead of commercial yeast. The fermentation process breaks down some of the compounds in wheat that cause digestive discomfort, including certain FODMAPs.
- Injera: The spongy, slightly sour flatbread that’s the national staple of Ethiopia. Traditionally made from tef flour fermented with wild Lactobacillus and yeast, it doubles as both plate and utensil.
- Idli: A steamed cake from South India and Sri Lanka made from fermented rice and black gram (a type of lentil). The batter ferments overnight, creating a light, airy texture.
- Dosa: A thin, crispy crepe made from the same fermented rice-and-lentil batter as idli, but cooked on a flat griddle.
- Dhokla: A steamed, fermented cake from western India made from chickpea flour.
Fermented Beverages
Kombucha and kefir are the two fermented drinks with the strongest evidence behind them, and they’re the ones most gastroenterologists point to when recommending probiotic-rich beverages.
Kombucha is brewed by fermenting sweetened tea with a SCOBY. The result is a fizzy, slightly tart drink with a small amount of residual sugar and trace alcohol (typically under 0.5%). Water kefir is a dairy-free alternative made by fermenting sugar water with water kefir grains. Beet kvass, a traditional Eastern European drink, is made by fermenting beets in saltwater.
Boza, popular in Turkey and the Balkans, is a thick, mildly sweet beverage made from fermented grains like millet or wheat. It has a porridge-like consistency and a low alcohol content.
What Fermented Foods Do for Your Gut
A 10-week clinical trial at Stanford University assigned 36 healthy adults to either a high-fiber diet or a diet rich in fermented foods like yogurt, kefir, kimchi, fermented vegetables, and kombucha. The fermented food group showed an increase in overall gut microbial diversity, with stronger effects from larger servings. Four types of immune cells showed reduced activation, and levels of 19 inflammatory proteins in the blood decreased. One of those proteins, interleukin 6, is linked to rheumatoid arthritis, type 2 diabetes, and chronic stress.
Many fermented foods can contain up to 100 million live microorganisms per gram when eaten without further cooking or processing. These microbes can interact with the existing gut microbiome or temporarily establish themselves there. The key word is “live”: cooking, baking, or pasteurizing fermented foods kills most of the bacteria, so sourdough bread and shelf-stable sauerkraut won’t deliver the same probiotic benefit as raw sauerkraut or a cup of kefir.
How to Tell if a Product Is Truly Fermented
Not everything labeled “fermented” actually contains live cultures, and not everything that tastes sour was fermented. Here’s what to look for when shopping:
Check where it’s stored. Most fermented foods with live microbes are in the refrigerated section. Shelf-stable versions have usually been pasteurized, which kills the fermenting organisms while preserving the flavor. Look for labels that say “fermented,” “cultured,” “live active cultures,” “raw,” or “wild.” If the ingredient list includes vinegar, the product may have been acidified for shelf stability rather than naturally fermented. Many supermarket pickles fall into this category.
Dairy ferments like yogurt and kefir typically list the specific bacterial species on the label. Vegetable ferments like sauerkraut and kimchi often don’t, because their microbial communities are more complex and harder to standardize. Some large-scale commercial products are pasteurized after fermentation and then have probiotic strains added back in. These aren’t the same as naturally fermented foods, but they can still provide some benefit.
How Much to Eat
There are no official dietary guidelines for fermented food intake. Stanford’s nutrition research group recommends starting with one serving per day and gradually increasing to at least two servings daily, or more as your digestive system adjusts. A serving is roughly the size of what you’d normally eat of that food: a cup of yogurt or kefir, a few tablespoons of sauerkraut or kimchi, a bowl of miso soup.
Starting slowly matters. Fermented foods introduce new microbes and their metabolic byproducts to your gut all at once, which can cause gas and bloating if you jump from zero to several servings a day. Most people adjust within a week or two. Mixing different types of fermented foods, like pairing yogurt at breakfast with kimchi at dinner, exposes your gut to a wider range of microbial species than eating the same fermented food repeatedly.