Fermentation, a type of anaerobic respiration, an energy-releasing process that takes place in the absence of free (chemically uncombined) oxygen. Fermentation results in the breakdown of carbohydrates and other complex organic substances into simpler substances.

Certain organisms perform fermentation to obtain the energy they need to carry on their life processes. (Most organisms obtain the energy for these processes through aerobic respiration, in the presence of free oxygen.) Various microorganisms, including yeasts and certain molds and bacteria, obtain their energy through fermentation. Many of the fermentation processes result in products that are important in medicine, food preparation, and other fields.

The specific product resulting from fermentation is determined by the type of microorganism carrying on the process and the substance in which the fermentation occurs. For example, wine is the product of yeast fermentation in fruit juice, while beer is the product of yeast fermentation in grain. Antibiotics (drugs used to fight infectious diseases) are obtained from both bacterial and mold fermentation. Fermentation by various microorganisms is used to produce substances called enzymes, which are used in many medical and industrial processes to speed up chemical reactions. Vinegar and cheese are products of bacterial fermentation. Yeast fermentation is used to make leavened bread.