Yeast eats sugar and produces alcohol and carbon dioxide. That is fermentation, and humans have used it for thousands of years to make beer, wine, and bread. The word comes from the Latin fervere, meaning to boil, because the bubbles made it look like the liquid was boiling. It is a metabolic process that converts sugars into acids, gases, or alcohol in the absence of oxygen.

In biology, fermentation is how cells generate energy without oxygen. Glycolysis breaks down glucose into pyruvate, and fermentation regenerates the electron carriers needed to keep glycolysis running. It produces less energy than aerobic respiration, but it is fast. Muscle cells use fermentation during intense exercise, producing lactic acid. Yeast uses it to produce ethanol.

In industry, fermentation is used to produce a wide range of products. Antibiotics like penicillin are made by fermenting mold. Citric acid is made by fermenting Aspergillus niger. Enzymes, vitamins, and amino acids are produced by fermentation. Biofuels like ethanol are produced by fermenting corn or sugarcane. The bioreactor is the modern fermentation vessel.

Fermentation is not just a biological process. It is an industrial workhorse. The same chemistry that makes bread rise makes insulin, antibiotics, and fuel.