In chemistry and biology, fermentation is a biochemical process that obtains energy from carbohydrates without using oxygen. Many foods come from fermentation, plus the process has industrial applications. Here is the definition of fermentation, examples of fermented products, and a look at how fermentation works.
Fermentation is a metabolic process in organisms that converts carbohydrates into chemical energy, without requiring oxygen. In other words, it is an anaerobic process. In contrast, cellular respiration produces energy, but it is an aerobic process (requires oxygen). In addition to energy molecules (such as ATP), fermentation produces a variety of molecules, including ethanol, carbon dioxide, lactic acid, methanol, hydrogen, methane, butyric acid, acetone, and acetic acid. Examples of organisms that carry out fermentation include fungi (yeast), animals (humans, cattle), and bacteria (Clostridium).
The word fermentation comes from the Latin word fervere, which means “to boil.”
While organisms use fermentation mainly for energy, people apply the process for making many products. You may know that beer, wine, and cheese come from fermentation, but some other examples may surprise you.
- Sour food containing lactic acid, such as kimchi, sauerkraut, pickles, and pepperoni
- Leavened bread
- Industrial alcohol, as for biofuels
- Sewage treatment involves fermentation.
- Human muscles initially use aerobic respiration, but switch to fermentation and produce lactic acid as an anaerobic energy supply.
- Bacteria in the human digestive tract perform fermentation, producing hydrogen gas and sometimes methane as flatus (farts). Herbivores, like cattle, release more methane.
Biochemistry of Yeast Fermentation – A Closer Look
The classic fermentation example is yeast fermentation of sucrose (a sugar) into ethanol and carbon dioxide. Each sucrose molecule consists of a glucose subunit and a fructose subunit. For every mole of glucose, fermentation produces two moles of ethanol, two moles of carbon dioxide, and two moles of adenosine triphosphate or ATP. The overall chemical reaction is as follows:
C6H12O6 → 2 C2H5OH + 2 CO2
But, fermentation is a process and not a single chemical reaction. It occurs in multiple steps.
(1) In the first step, the enzyme invertase breaks the glycosidic linkage between the glucose and fructose residues of sucrose.
C12H22O11 + H2O + invertase → 2 C6H12O6
(2) Next, glycolysis occurs. This is where each glucose molecule breaks into two pyruvate molecules. Glycolysis takes several steps, but here is the overall chemical equation:
glucose + 2 ADP + 2 inorganic phosphate → 2 pyruvate + 2 ATP + 2 NAD + 2 water + 2 protons
C6H12O6 + 2 ADP + 2 Pi + 2 NAD+ → 2 CH3COCOO− + 2 ATP + 2 NADH + 2 H2O + 2 H+
(3) Finally, pyruvate reacts and forms ethanol and carbon dioxide. This occurs in two steps and regenerates the oxidized NAD+ for glycolysis:
CH3COCOO− + H+ → CH3CHO + CO2 (catalyzed by pyruvate decarboxylase)
CH3CHO + NADH + H+ → C2H5OH + NAD+ (catalyzed by alcohol dehydrogenase)
These reactions convert two moles each of NAD+ and ADP into two moles each of NADH, ATP, and water.
Fermentation is not as efficient at energy production as cellular respiration, so organisms capable of both processes typically utilize respiration when oxygen is available. However, the presence of oxygen does not necessarily prevent fermentation from occurring. For example, yeast prefer fermentation over cellular respiration as long there is a sufficient sugar supply.
People have been using fermentation since at least the Neolithic (7000 to 6600 BCE), primarily for fermenting beverages and making cheese. However, it wasn’t until the nineteenth century that scientists started understanding the process. In 1837, Theodor Schwann observed yeast budding using a microscope and found that boiling grape juice prevented fermentation until new yeast was added. But, many chemists still believed fermentation was a simple chemical reaction that could occur without a living organism. In the 1850s and 1860s, Louis Pasteur repeated Schwann’s experiments and demonstrated fermentation came from living cells. However, he could not extract the enzyme responsible for the process. In 1897, German chemist Eduard Buechner ground up yeast, extracted fluid, and discovered this fluid fermented a sugar solution. His experiment earned him the 1907 Nobel Prize in Chemistry.
Studying fermentation for practical applications is called zymurgy. The name comes from a Greek word that literally means “the workings of fermentation.” The science of studying fermentation is zymology. A person who practices fermentation is a zymurgist, while a scientist specializing in fermentation is a zymologist.
Interesting Fermentation Facts
- Yeast fermentation produces carbon dioxide gas bubbles that expand during cooking and make baked goods rise. But, yeast also produces alcohol (ethanol). Less than 2% of this alcohol remains after baking.
- Overgrowth of yeast in the gut can cause auto-intoxication. This is where yeast produces ethanol that gets in the blood stream and causes intoxication even when a person has not been drinking.
- Yeast converts sugar into ethanol, which is safe for human consumption. But, if high levels of pectin are present, one fermentation product is toxic methanol.
- Akhavan, Bobak; Luis Ostrosky-Zeichner; Thomas, Eric (2019). “Drunk Without Drinking: A Case of Auto-Brewery Syndrome.” ACG Case Reports Journal. 6(9): e00208. doi:10.14309/crj.0000000000000208
- Hui, Y. H. (2004). Handbook of Vegetable Preservation and Processing. New York: M. Dekker. ISBN 0-8247-4301-6.
- Klein, Donald W.; Lansing M.; Harley, John (2006). Microbiology (6th ed.). New York: McGraw-Hill. ISBN 978-0-07-255678-0.
- Purves, William K.; Sadava, David E.; Orians, Gordon H.; Heller, H. Craig (2003). Life, the Science of Biology (7th ed.). Sunderland, Mass.: Sinauer Associates. ISBN 978-0-7167-9856-9.
- Steinkraus, Keith (2018). Handbook of Indigenous Fermented Foods (2nd ed.). CRC Press. ISBN 9781351442510.
- Tortora, Gerard J.; Funke, Berdell R.; Case, Christine L. (2010). Microbiology: An Introduction (10th ed.). San Francisco, CA: Pearson Benjamin Cummings. ISBN 978-0-321-58202-7.