What Is Fermentation? Definition and Examples

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.

Key Takeaways: Fermentation

• Fermentation is an anaerobic (oxygen-free) energy-generating process.
• It converts carbohydrates into energy, alcohol, acids, or gases.
• Common fermentation products include ethanol, lactic acid, and carbon dioxide.
• It occurs in yeast, bacteria, and animal cells (e.g., muscle cells).
• Humans use fermentation to make bread, alcohol, yogurt, and biofuels.

Fermentation Definition

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.”

Fermentation Examples

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.

• Beer
• Wine
• Mead
• Liquor
• Cheese
• Yogurt
• 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.

Types of Fermentation

Fermentation takes several forms, depending on the organism and end products. While all fermentation is anaerobic and involves carbohydrate breakdown, each type has distinct biochemistry, products, and uses. Here are the main types of fermentation found in nature and industry:

Alcoholic Fermentation

Alcoholic fermentation converts sugars into ethanol and carbon dioxide. It is the most well-known type, especially in baking and alcohol production.

• Organisms: Saccharomyces cerevisiae (brewer’s and baker’s yeast), some bacteria
• Main products: Ethanol (C₂H₅OH) and CO₂
• Applications: Beer, wine, liquor, leavened bread, biofuels

Lactic Acid Fermentation

Lactic acid fermentation produces lactic acid instead of ethanol. It occurs in certain bacteria and in animal muscle cells under low oxygen.

• Organisms: Lactobacillus, Streptococcus, human muscle cells
• Main product: Lactic acid (C₃H₆O₃)
• Applications: Yogurt, sour cream, kimchi, sauerkraut, muscle fatigue response

There are two types of lactic acid fermentation:

• Homolactic (produces only lactic acid)
• Heterolactic (produces lactic acid, ethanol or acetic acid, and CO₂)

Acetic Acid Fermentation

Acetic acid fermentation oxidizes ethanol into acetic acid using aerobic bacteria.

• Organisms: Acetobacter, Gluconobacter
• Main product: Acetic acid (CH₃COOH)
• Applications: Vinegar production, kombucha brewing

Butyric Acid Fermentation

Butyric acid fermentation produces butyric acid and gases. It occurs in certain obligate anaerobic bacteria, especially in the gut and in spoiled food.

• Organisms: Clostridium butyricum, Clostridium acetobutylicum
• Main products: Butyric acid (C₄H₈O₂), CO₂, and hydrogen gas
• Applications: Cheese flavoring, industrial solvents (acetone, butanol)

Mixed Acid Fermentation

Mixed acid fermentation yields a complex mixture of acids and gases, typically seen in enteric bacteria.

• Organisms: Escherichia coli, other Enterobacteriaceae
• Main products: Lactic acid, acetic acid, ethanol, CO₂, H₂, and succinic acid
• Applications: Anaerobic waste digestion, microbial testing

Propionic Acid Fermentation

Propionic acid fermentation produces propionic acid and CO₂, giving Swiss cheese its distinctive holes and flavor.

• Organisms: Propionibacterium freudenreichii
• Main products: Propionic acid (C₃H₆O₂), acetic acid, CO₂
• Applications: Swiss-type cheeses, preservatives

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 P_i + 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.

History

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.

Related Terms

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.

Fermentation vs Cellular Respiration

Fermentation and cellular respiration are both metabolic pathways for converting glucose into energy, but they differ in oxygen use, energy yield, and byproducts. This section compares these processes side by side.

Summary

• Fermentation is faster but less efficient than cellular respiration.
• Some organisms switch between both, depending on oxygen availability (facultative anaerobes).
• In fermentation, the goal is to regenerate NAD⁺ so glycolysis can continue producing ATP.

References

• 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.