Almost any food containing sugars, starches, or proteins can ferment. Fermentation is the process where microbes, mainly bacteria and yeast, break down organic compounds in the absence of oxygen (or with limited oxygen) to produce acids, alcohol, or gases. The range of things that ferment spans from milk and grains to fruits, vegetables, meat, and even fiber sitting in your gut right now.
Sugars and Starches: The Primary Fuel
Fermentation starts with carbohydrates. Simple sugars like glucose, fructose, and sucrose are the easiest fuel for microbes to consume. This is why fruit juice left at room temperature will start to bubble on its own: wild yeasts on the fruit skin begin converting those sugars into alcohol and carbon dioxide. In wine production, yeast converts grape sugars into ethanol, typically reaching 11 to 13% alcohol by volume.
Starches ferment too, but they need an extra step. Enzymes first break the starch chains into simpler sugars, which microbes then consume. This is what happens when grains are mashed for beer brewing. Even resistant starch, the kind found in legumes, unripe bananas, and cooked-then-cooled potatoes, ferments readily once it reaches the bacteria in your large intestine.
Lactose, the sugar in milk, is another major fermentation substrate. Bacteria convert it into lactic acid, which is how yogurt, kefir, and cheese get their tangy flavor and thick texture. Milk from cows, goats, sheep, and even buffalo and camels serves as the base for fermented dairy products across the world.
Dietary Fiber Ferments in Your Gut
Your colon is essentially a fermentation vessel. Bacteria living there break down dietary fibers that your own digestive enzymes can’t touch. The major end products are short-chain fatty acids, specifically acetate, propionate, and butyrate. These compounds fuel the cells lining your colon, influence inflammation, and play a role in metabolism and immune function.
Not all fibers ferment equally. Highly fermentable types include beta-glucans from oats and barley, pectins from apples and berries, inulin from onions and chicory root, and fructooligosaccharides from asparagus and Jerusalem artichokes. These act as food for beneficial gut bacteria, which is why they’re often called prebiotics. Less fermentable fibers, like cellulose from wheat bran, pass through mostly intact and add bulk to stool instead.
Proteins and Amino Acids
Carbohydrates aren’t the only things that ferment. When microbes run low on available sugars, they switch to digesting proteins. Yeast and bacteria produce enzymes that break proteins into amino acids, then convert those amino acids into a group of compounds called biogenic amines, along with carbon dioxide and ammonia. This is the process behind strongly flavored fermented foods like fish sauce, soy sauce, and aged cheese. It’s also why these foods can develop intense, savory, or pungent aromas that purely sugar-based fermentations don’t produce.
Three Core Types of Fermentation
Lactic Acid Fermentation
Lactic acid bacteria consume sugars and produce lactic acid as their main output. This drops the pH of the food, creating an acidic environment that preserves it and gives it a sour taste. Sauerkraut is a classic example: bacteria on raw cabbage produce lactic and acetic acids along with carbon dioxide, quickly lowering the pH enough to inhibit spoilage organisms. The same basic process creates kimchi, pickles, yogurt, sourdough bread, miso, salami, and dozens of traditional foods from every continent. In Nigerian ogi, a fermented cereal porridge, lactic acid concentrations reach about 0.65%, while acetic acid hits around 0.11%.
Alcoholic Fermentation
Yeast, particularly strains of brewer’s yeast, converts sugars into ethanol and carbon dioxide under low-oxygen conditions. This is the foundation of beer, wine, cider, mead, and spirits. During cocoa and coffee processing, yeasts in the fruit pulp consume sucrose, glucose, and fructose to produce ethanol and CO2, which contribute to flavor development. In spontaneous fermentations, wild non-brewer’s yeasts dominate early on, but they die off once ethanol concentrations reach about 4 to 5%, allowing hardier strains to finish the job.
Acetic Acid Fermentation
Vinegar is the product of a two-stage process. First, yeast converts sugars to alcohol. Then a different group of bacteria oxidizes that ethanol into acetic acid. Unlike the other two types, this step requires oxygen. The bacteria perform best at around 30°C with moderate aeration. Too little oxygen slows the process; too much causes ethanol and acetic acid to evaporate, reducing yield. Under optimized conditions, the conversion takes roughly 25 to 29 hours.
What Controls Whether Fermentation Succeeds
Temperature and acidity are the two biggest levers. Lactic acid bacteria generally thrive between 35°C and 50°C, with higher temperatures and a pH around 5.5 favoring lactic acid production. Cooler temperatures slow fermentation, which is why sauerkraut made in a cool cellar takes weeks while a warm ferment can finish in days. Salt concentration also matters in vegetable ferments: it suppresses unwanted bacteria early on while allowing salt-tolerant lactic acid bacteria to establish themselves.
The critical safety threshold is a pH of 4.6. Below this level, the bacterium responsible for botulism cannot grow. Most properly fermented foods drop well below this number. Sauerkraut, kimchi, and vinegar-based pickles typically reach a pH between 3.0 and 4.0, which is why fermented vegetables have been a safe preservation method for thousands of years.
Fermented Foods Around the World
The substrate determines the product. Milk ferments into airag (mare’s milk, Mongolia), amasi (cow’s milk, South Africa), dadih (buffalo milk, Indonesia), and kefir (goat, sheep, or cow’s milk, Caucasus region). Rice ferments into ang-kak (red rice, China) and dosa batter (rice and black gram, India). Tef flour becomes injera, the spongy Ethiopian flatbread. Soybeans become miso, tempeh, and soy sauce. Cabbage becomes sauerkraut in Germany and kimchi in Korea. Grapes become wine, barley becomes beer, and apples become cider.
What ties all of these together is the same basic principle: microbes consuming organic compounds and transforming them into something with a longer shelf life, a different flavor, and often greater nutritional availability. If a food contains sugars, starches, or proteins, and microbes can access it, fermentation can happen.