What Is Fermentation?

Definition, History, and Examples of Fermentation

Beer brewing
Adding yeast for beer to start fermentation. William Reavell / Getty Images

Fermentation is a process used to produce wine, beer, yogurt and other products. Here's a look at the chemical process that occurs during fermentation.

Fermentation Definition

Fermentation is a metabolic process in which an organism converts a carbohydrate, such as starch or a sugar, into an alcohol or an acid. For example, yeast performs fermentation to obtain energy by converting sugar into alcohol.

Bacteria perform fermentation, converting carbohydrates into lactic acid. The study of fermentation is called zymology.

History of Fermentation

Fermentation is a natural process. People applied fermentation to make products such as wine, mead, cheese, and beer long before the biochemical process was understood. In the 1850s and 1860s, Louis Pasteur became the first zymurgist or scientist to study fermentation when he demonstrated fermentation was caused by living cells.

Examples of Products Formed by Fermentation

Most people are aware of food and beverages that are fermentation products, but may not realize many important industrial products results from fermentation.

  • Beer
  • Wine
  • Yogurt
  • Cheese
  • Certain sour foods containing lactic acid, including sauerkraut, kimchi, and pepperoni
  • Bread leavening by yeast
  • Sewage treatment
  • Some industrial alcohol production, such as for biofuels
  • Hydrogen gas

Ethanol Fermentation

Yeast and certain bacteria perform ethanol fermentation where pyruvate (from glucose metabolism) is broken into ethanol and carbon dioxide.

The net chemical equation for the production of ethanol from glucose is:

C6H12O6 (glucose) → 2 C2H5OH (ethanol) + 2 CO2 (carbon dioxide)

Ethanol fermentation has used the production of beer, wine, and bread. It's worth noting that fermentation in the presence of high levels of pectin results in the production of small amounts of methanol, which is toxic when consumed.

Lactic Acid Fermentation

The pyruvate molecules from glucose metabolism (glycolysis) may be fermented into lactic acid. Lactic acid fermentation is used to convert lactose into lactic acid in yogurt production. It also occurs in animal muscles when the tissue requires energy at a faster rate than oxygen can be supplied. The next equation for lactic acid production from glucose is:

C6H12O6 (glucose) → 2 CH3CHOHCOOH (lactic acid)

The production of lactic acid from lactose and water may be summarized as:

C12H22O11 (lactose) + H2O (water) → 4 CH3CHOHCOOH (lactic acid)

Fermentation Facts

  • Fermentation is an anaerobic process, meaning it does not require oxygen in order to occur. However, even when oxygen is abundant, yeast cells prefer fermentation to aerobic respiration, provided a sufficient supply of sugar is available.
  • Fermentation occurs in the digestive system of humans and other animals.
  • In a rare medical condition called gut fermentation syndrome or auto-brewery syndrome, fermentation in the human digestive tract leads to intoxication by ethanol production.
  • Fermentation occurs in human muscle cells. Muscles can expend ATP faster than oxygen can be supplied. In this situation, ATP is produced by glycolysis, which does not use oxygen.
  • Although fermentation is a common pathway, it is not the only method used by organisms to obtain energy anaerobically. Some systems use sulfate as the final electron acceptor in the electron transport chain.