In its strictest sense, fermentation (formerly called zymnosis) is the energy-yielding anaerobic metabolic breakdown of a nutrient molecule, such as glucose, without net oxidation. Fermentation yields lactate, acetic acid, ethanol, or some other simple product.

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.

This process is often used to produce or preserve food. Fermentation typically refers to the fermentation of sugar to alcohol using yeast, but other fermentation processes include the making of yogurt. The science of fermentation is known as zymology.

Fermentation usually implies that the action of the microoganisms is desirable.

Brewing is the production of alcoholic beverages through fermentation. This is the method used in beer production, although the term can be used for other drinks such as sake, mead and wine. The term is also sometimes used to refer to any chemical mixing process.

Brewing has a very long history, and archeological evidence tells us that this technique was used in ancient Egypt. Descriptions of various beer recipes can be found in Sumerian writings, some of the oldest known writing of any sort.

The brewing industry is part of most western economies.


Brewing beer

The brewing process typically begins with malted barley, although other grains including wheat, rye, oats and sorghum are sometimes malted and used for brewing. A malted grain has been allowed to germinate to a limited extent, and is then kilned at controlled levels of moisture and temperature. Germination produces the increased levels of enzymes necessary for mashing and also begins the cellular degradation of the grain which permits extraction of its components during the mash and lauter. Malts that are more extensively germinated prior to kilning are also more degraded or "modified" and thus require less intensive mashing to create soluble extract. Grains kilned at higher moisture levels produce sweet tasting unfermentable caramel products and this flavor and caramel color is carried into the finished beer. Malts kilned at higher temperatures or for longer periods have a toasted or roasted character that strongly influences beer colour and flavour.

Coarsely ground malt, known as grist, is mixed with heated water in a vat called a "mash tun" for a process known as "mashing". The primary function of the mash is to apply numerous natural enzymes to the degradation of malt into simpler soluble components which are later extracted into the liquid fraction. It is also necessary to gelatinize starch to permit its degradation, and this requires achieving a gelatinization temperature which is dependant on the source of the starch. Greater than 70% of malt solids may be extracted in a typical mash.

Among the major groups of mash enzymes are amylases which degrade starch into sugars and dextrins, glucanases break down gummy beta-glucans to reduce viscosity, proteases which degrade grain proteins into peptides and amino acids crucial for yeast metabolism. Proteins are responsible for undesirable beer haze and also desirable beer foam or head, so controled degradation of protein is critical to beer quality. Numerous other enzyme reactions occur in the mash.

The brewmaster may exert limited control over the rate and extent of various enzyme activity in the mash by controlling the pH, the amount of water, but more often the time and temperature of mash "rests". Rest times and temperatures are chosen based the on the type of malt, its modification level, the inclusion of unmalted grains, and the desired character of the finished beer. An important example of enzyme activity control is the selection of the "saccharification" rest temperature. This rest relies on several amylase enzymes to convert starch into fermentable sugars and unfermentable dextrins, with a rest temperature in the range of 149 to 162 F (65 to 72 C). At the lower end of this temperature range beta-amylase rapidly converts starch to fermentable maltose sugar and produces a highly fermentable extract which results in more alcohol and less dextrins in the finished beer. At the higher end of this temperature range beta-amylase becomes unstable and inactive, and the beers produced have more dextrinous body and mouthfeel.

Traditionally British ale malts were very well modified requiring minimal mashing, while traditional Continental European malts particularly those for used for lagers in Germany and the Czech Republic were less modified requiring intensive mashing with numerous rests. Modern commercial barley malts are generally very well modified usually requiring only a single brief mash rest.

After the mash the resulting liquid is strained from the grains in a process known as "lautering". The liquid fraction is then known as sweet wort where the term "sweet" refers to the absence of bittering hops. The separation involves a slotted "false bottom" or other form of manifold which acts as a strainer. The lautering separation occurs in two phases. First the mash liquid is strained from the solids, then additional hot water is added to the solids and strained in a repeated "sparge" process.

The sweet wort is moved into a large tank known as a "copper" or kettle where it is boiled with hops and sometimes other ingredients such as herbs or sugars. The boiling process serves to terminate enzymatic processes, precipitate proteins, isomerize hop resins, concentrate and sterilize the wort. Hops add flavour, aroma and bitterness to the beer. At the end of the boil, the hopped wort settles to clarify it in a vessel called a "whirl-pool" and the clarified wort is then cooled.

The wort is then moved into a "fermentation vessel" where yeast is added or "pitched" with it. The yeast converts the sugars from the malt into alcohol, carbon dioxide and other components through a process called Glycolysis. After a week or so, the fresh (or "green") beer is run off into conditioning tanks. After conditioning for a week or longer, the beer is often filtered to remove yeast and particulates. The "bright beer" is then ready for serving or packaging.


The primary benefit of fermentation is the conversion, e.g. converting juice into wine, grains into beer, and carbohydrates into carbon dioxide to leaven bread.

According to Steinkraus (1995), traditionally 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, alcoholic, acetic acid, and alkaline fermentations"
  3. "Enrichment of food substrates biologically 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 benefits 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.

Fermentated foods, by region


Since fruits ferment naturally, fermentation precedes human history. However, humans began to take control of the fermentation process at some point. There is strong evidence that people were fermenting beverages in Babylon circa 5000 BCE, ancient Egypt circa 3000 BCE, pre-Hispanic Mexico circa 2000 BCE, and Sudan circa 1500 BCE. There is also evidence of leavened bread in ancient Egypt circa 1500 BCE and of milk fermentation in Babylon circa 3000 BCE. The Chinese were probably the first to develop vegetable fermentation.


Yeast produce ethanol and carbon dioxide gas. When the ferment has a high concentration of pectin minute quantities of methanol can be produced. Usually only one of the products is desired; in bread the alcohol is baked out, and in alcohol production the carbon dioxide is released into the atmosphere.

Products produced by fermentation, such as ethanol and lactic acid, are used as a substitute for oxygen. During cellular respiration, oxygen is the final electron acceptor. However, when no oxygen is available, ethanol and lactic acid is used in place of oxygen.

Depending on the type of fermentation, some products are harmful to people's health.

Bacteria generally produce acids. Vinegar (acetic acid) is the direct result of bacterial fermentation. In milk, the acid coagulates the casein, producing curds. In pickling, the acid preserves the food from pathogenic and putrefactive bacteria.

See also


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

da:Fermentering de:Fermentation es:Fermentacin fr:Fermentation id:Fermentasi it:Fermentazione lt:Fermentacija nl:Fermentatie ja:醗酵 pl:Fermentacja pt:Fermentao sq:Fermentimi su:Fermentasi sv:Fermentering zh:发酵


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