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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 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
The term "ferment" comes from the Latin word fervere, which means "to boil." Fermentation was described by late 14th century alchemists, but not in the modern sense. The chemical process of fermentation became a subject of scientific investigation about the year 1600.Hulton Deutsch/Contributor/Getty Images
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. However, Pasteur was unsuccessful in his attempts to extract the enzyme responsible for fermentation from yeast cells. In 1897, German chemist Eduard Buechner ground yeast, extracted fluid from them, and found the liquid could ferment a sugar solution. Buechner's experiment is considered the beginning of the science of biochemistry, earning him the 1907 Nobel Prize in chemistry.
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.
- 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
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)
Hydrogen and Methane Gas Production
The process of fermentation may yield hydrogen gas and methane gas.
Methanogenic archaea undergo a disproportionation reaction in which one electron is transferred from a carbonyl of a carboxylic acid group to a methyl group of acetic acid to yield methane and carbon dioxide gas.
Many types of fermentation yield hydrogen gas. The product may be used by the organism to regenerate NAD+ from NADH. Hydrogen gas may be used as a substrate by sulfate reducers and methanogens. Humans experience hydrogen gas production from intestinal bacteria, producing flatus.
- 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.
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