What is fermentation?

Fermentation is a metabolic process in which microorganisms (or sometimes muscle cells) break down sugars and other carbohydrates for energy without using oxygen, producing byproducts like gases, alcohol, or acids along the way.

The basic idea

Cells normally get energy by burning sugar with oxygen. When oxygen is scarce or absent, some organisms can still extract a smaller amount of energy from sugar through glycolysis. To keep that process running, they need to regenerate a molecule called NAD⁺, and fermentation is the step that does this by converting the sugar’s leftover products into something else. Those “something elses” are what we notice and use.

Main types

• Alcoholic fermentation: Yeast converts sugar into ethanol and carbon dioxide. This is how beer, wine, and cider get their alcohol, and, as in bread, how dough gets its lift.
• Lactic acid fermentation: Bacteria (like Lactobacillus) convert sugar into lactic acid. This gives yogurt, sauerkraut, kimchi, sourdough, and pickles their tang, and the acid also helps preserve the food. Your muscles do this too during intense exercise when oxygen runs short, which contributes to that burning feeling.
• Acetic acid fermentation: Bacteria turn ethanol into acetic acid, producing vinegar. It technically requires oxygen, so some people classify it separately.

Why people use it

• Preservation: Acids and alcohol make food inhospitable to spoilage microbes.
• Flavor and texture: Fermentation creates complex tastes in cheese, soy sauce, miso, tempeh, coffee, chocolate, and cured tea.
• Nutrition: It can make some nutrients more available and, in foods like yogurt and kimchi, adds live microbes.
• Industry: Fermentation is also used to produce biofuels, antibiotics, enzymes, and many medicines.

A note on terminology

In everyday food talk, “fermentation” covers any microbe-driven transformation of food. In strict biochemistry, it means the oxygen-free process described above. Industrial microbiology uses the word even more broadly for any large-scale microbial culture, including ones that use oxygen.