Fermentation

Fermentation typically refers to the conversion or changing of sugar to alcohol using yeast. The process is often used to produce wine and beer, but fermentation is also employed in preservation to create lactic acid in sour foods such as pickled cucumbers, 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.

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.

Reaction

The reaction differs according to the sugar being used in the process of anaerobic respiration, below, the sugar will be glucose (C6H12O6) the simplest sugar.

Symbol Equation

C6H12O6 --> 2C2H5OH + 2CO2 + Energy Released (118kJ mol-1)

Word Equation

Sugar (glucose) --> Alcohol + Carbon Dioxide + Energy

History

Since fruits ferment naturally, fermentation precedes human history. Since prehistoric times, however, humans have been taking control of the fermentation process. 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. There is also evidence of leavened bread in ancient Egypt circa 1500 BC and of milk fermentation in Babylon circa 3000 BC. The Chinese were probably the first to develop vegetable fermentation.

Biochemistry

Fermentation is a process that is important in anaerobic conditions when there is no oxidative phosphorylation to maintain the production of ATP (Adenosine triphosphate) by glycolysis. During fermentation pyruvate is metabolised to various different compounds. Homolactic fermentation is the production of lactic acid from pyruvate; alcoholic fermentation is the conversion of pyruvate into ethanol and carbon dioxide; and heterolactic fermentation is the production of lactic acid as well as other acids and alcohols.

Textbook examples of fermentation products are ethanol (drinkable alcohol), lactic acid, and hydrogen. However, more exotic compounds can be produced by fermentation, such as butyric acid and acetone.

Although the final step of fermentation (conversion of pyruvate to fermentation end-products) does not produce energy, it is critical for an anaerobic cell since it regenerates nicotinamide adenine dinucleotide (NAD+), which is required for glycolysis. This is important for normal cellular function, as glycolysis is the only source of ATP in anaerobic conditions.

Fermentation products contain chemical energy (they are not fully oxidized) but are considered waste products since they cannot be metabolised further without the use of oxygen (or other more highly-oxidized electron acceptors). A consequence is that the production of ATP by fermentation is less efficient than oxidative phosphorylation, where pyruvate is fully oxidised to carbon dioxide. Fermentation produces two ATP molecules per molecule of glucose compared to approximately 36 by aerobic respiration. Even in vertebrates, however, it is used as an effective means of energy production during short, intense periods of exertion, where the transport of oxygen to the muscles is insufficient to maintain aerobic metabolism. While fermentation is helpful during short, intense periods of exertion, it is not sustained over extended periods in complex aerobic organisms. In humans, for example, lactic acid fermentation provides energy for a period ranging from 30 seconds to 2 minutes. The speed at which ATP is produced is about 100 times that of oxidative phosphorylation. The pH in the cytoplasm quickly drops when lactic acid accumulates in the muscle, eventually inhibiting enzymes involved in glycolysis.

Products

Products produced by fermentation are actually waste products produced during the reduction of pyruvate to regenerate NAD+ in the absence of oxygen.

When yeast ferments, it breaks down the sugar(C6H12O6) into exactly two molecules of ethanol (C2H6O) and two molecules of carbon dioxide (CO2).

- Ethanol fermentation (done by yeast and some types of bacteria) breaks the pyruvate down into ethanol and carbon dioxide. It is important in bread-making, brewing, and wine-making. 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.
- Lactic acid fermentation breaks down the pyruvate into lactic acid. It occurs in the muscles of animals when they need energy faster than the blood can supply oxygen. It also occurs in some bacteria and some fungi. It is this type of bacteria that convert lactose into lactic acid in yogurt, giving it its sour taste.

The burning sensation in muscles during hard exercise used to be attributed to the production of lactic acid during a shift to anaerobic glycolosis, as oxygen is converted to carbon dioxide by aerobic glycolysis faster than the body can replenish it; but muscle soreness and stiffness after hard exercise is actually due to microtrauma of the muscle fibres. The body falls back on this less-efficient but faster method of producing ATP under low-oxygen conditions. This is thought to have been the primary means of energy production in earlier organisms before oxygen was at high concentration in the atmosphere and thus would represent a more ancient form of energy production in cells. The liver later gets rid of this excess lactate by transforming it back into an important glycolysis intermediate called pyruvate. Aerobic glycolysis is a method employed by muscle cells for the production of lower-intensity energy over a longer period of time.

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

Uses

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), food fermentation serves five main purposes:

- Enrichment of the diet through development of a diversity of flavors, aromas, and textures in food substrates
- Preservation of substantial amounts of food through lactic acid, alcoholic, acetic acid, and alkaline fermentations
- Biological enrichment of food substrates with protein, essential amino acids, essential fatty acids, and vitamins
- Detoxification during food-fermentation processing
- 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. 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

- Worldwide: alcohol, wine, vinegar, olives, yogurt
- Asia - East and Southeast Asia: asinan, bai-ming, belacan, burong mangga, dalok, doenjang, jeruk, fish sauce, kimchi, leppet-so, miang, nata de coco, nata de pina, natto, naw-mai-dong, pak-siam-dong, paw-tsaynob in snow, prahok, sake, seokbakji, soy sauce, stinky tofu, szechwan cabbage, tai-tan tsoi, takuan, tempeh, totkal kimchi, yen tsai, zha cai
- Central Asia: kumis (mare milk), kefir, shubat (camel milk)
- India: achar, gundruk, mixed pickle, idli
- Africa: garri, hibiscus seed, hot pepper sauce, injera, lamoun makbouss, mauoloh, msir, mslalla, oilseed, ogili, ogiri
- Americas: cheese, pickling (pickled vegetables), sauerkraut, lupin seed, oilseed, chocolate, vanilla, fermented fish, fish heads, walrus, seal oil, birds (in Inuit cooking)
- Middle East: kushuk, lamoun makbouss, mekhalel, torshi, tursu, boza
- Europe: cheese, sauerkraut, soured milk products such as quark, kefir and filmjolk, fermented Baltic herring, sausages

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