Fermentation (food)

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Fermentation refers most commonly to the conversion of sugar to alcohol using yeast. However, a more general definition would be the chemical conversion of carbohydrates into alcohols, acids or gas.

The process is used to produce wine, beer and leavened bread, but fermentation is also employed in preservation to create lactic acid in sour foods such as pickles, kimchi and yogurt. The science of fermentation is known as zymology.


In its strictest sense, fermentation (formerly called zymosis) is the anaerobic metabolic breakdown of a nutrient molecule, such as glucose, without net oxidation. Fermentation does not release all the available energy in a molecule; it merely allows glycolysis (a process that yields two ATP per glucose) to continue by replenishing reduced coenzymes. Depending on which organism it is taking place in, fermentation may yield lactate, acetic acid, ethanol, or other reduced metabolites. Yeast produces ethanol and CO2; human muscle (under anaerobic conditions) produces lactic acid.

Fermentation is also used much more broadly to refer to the bulk growth of microorganisms on a growth medium. No distinction is made between aerobic and anaerobic metabolism when the word is used in this sense. When the conversion with additives fed to the organisms, as in producing variants of penicillin such as ampicillin and amoxicillin, the products are called semi-synthetic.


See also: History of beer and Brewing

Fruit fermentation by wild yeasts is natural; many fruits have a visible powdery surface (called "bloom") on the skin, therefore fermentation precedes human history. Since prehistoric times, however, humans have been taking control of the fermentation process. The results of fermentation are more predictable when known yeast cultures are used. Often all wild yeasts are killed prior to the introduction of cutured yeasts.

The earliest evidence of winemaking dates from 5400 BC, in Iran near western Azerbaijan Province, south of where the city of Orumieh is today. 7000 year old jars of wine have been excavated in the Zagros Mountains, which are now on display at the University of Pennsylvania [1]. There is strong evidence that people were fermenting beverages in Babylon circa 5000 BC , ancient Egypt circa 3000 BC , pre-Hispanic Mexico circa 2000 BC , and Sudan circa 1500 BC.

Yeast (called "leaven" in ancient times) is also used to produce carbon dioxide bubbles in bread dough; this is then baked to make a lighter bread product (alcohol is evaporated during baking). There is evidence of leavened bread in ancient Egypt circa 1500 BC.

Milk fermentation is thought to have begun in Babylon circa 3000 BCTemplate:Citation needed. The Chinese were probably the first to develop vegetable fermentation.

French chemist Louis Pasteur was the first zymologist, when in 1857 he connected yeast to fermentation. Pasteur originally defined fermentation as respiration without air.

Pasteur performed careful research and concluded, "I am of the opinion that alcoholic fermentation never occurs without simultaneous organization, development and multiplication of cells.... If asked, in what consists the chemical act whereby the sugar is decomposed ... I am completely ignorant of it.".

The German Eduard Buchner, winner of the 1907 Nobel Prize in chemistry, later determined that fermentation was actually caused by a yeast secretion that he termed zymase.

The research efforts undertaken by the Danish Carlsberg scientists greatly accelerated the gain of knowledge about yeast and brewing. The Carlsberg scientists are generally acknowledged with jump-starting the entire field of molecular biology.


The primary benefit of fermentation is the conversion of sugars and other carbohydrates, e.g., converting juice into wine, grains into beer, carbohydrates into carbon dioxide to leaven bread, and sugars in vegetables into preservative organic acids.

According to Steinkraus (1995), food fermentation serves five main purposes:

  1. Enrichment of the diet through development of a diversity of flavors, aromas, and textures in food substrates
  2. Preservation of substantial amounts of food through lactic acid, alcohol, acetic acid, and alkaline fermentations
  3. Biological enrichment of food substrates with protein, essential amino acids, essential fatty acids, and vitamins
  4. Detoxification during food-fermentation processing
  5. A decrease in cooking times and fuel requirements

Fermentation has some uses exclusive to foods. Fermentation can produce important nutrients or eliminate antinutrients. Food can be preserved by fermentation, since fermentation uses up food energy and can make conditions unsuitable for undesirable microorganisms. For example, in pickling the acid produced by the dominant bacteria inhibit the growth of all other microorganisms. Depending on the type of fermentation, some products (e.g., fusel alcohol) can be harmful to people's health.

In alchemy, fermentation is often the same as putrefaction, meaning to allow the substance to naturally rot or decompose.

Fermented foods, by region

Fermentation vs. putrefaction/rancidification

There are a number of animal-based foods from different parts of the world that are described as being "fermented." However, the term is erroneous when applied to such foods because fermentation properly means the decomposition of carbohydrates, and since animal tissues are composed of proteins and lipids, and contain at most only traces of carbohydrates, the operative processes in the transformation undergone by these foods are actually putrefaction and rancidification.

The difference is more than technical since the end products of these processes are quite different from those of fermentation, and also because putrefied/rancidified foods are often dangerous for human consumption. For instance, Alaska, despite its small population, witnesses more cases of botulism than any other U.S. state [2]. This is caused by the traditional Eskimo practice of allowing animal products such as whole fish, fish heads, walrus, sea lion and whale flippers, beaver tails, seal oil, birds, etc., to decompose for an extended period of time before being consumed raw. The risk is exacerbated when a plastic container is used for this purpose instead of the old-fashioned method, a grass-lined hole, as the botulinum bacteria thrive in the anaerobic conditions created by the former method.

Other putrefied/rancidified foods include fish sauce from Southeast Asia, Icelandic hákarl, fermented Baltic herring and certain speciality sausages from Sweden, and Limburger cheese. Most putrefied/rancified foods are considered to have an exceptionally foul odor, but if the process of decomposition is allowed to reach completion, the smell is greatly diminished, as with some varieties of fish sauce.

Fermentation usually implies that the action of the microorganisms is desirable. Occasionally wines are enhanced through the process of cofermentation. When fermentation stops prior to complete conversion of sugar to alcohol, a stuck fermentation is said to have occurred.


  • Steinkraus, K. H., Ed. (1995). Handbook of Indigenous Fermented Foods. New York, Marcel Dekker, Inc.
  • The 1811 Household Cyclopedia