In its strictest sense fermentation 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 some 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.

Contents

1 Uses 1.1 Fermentated foods, by region 2 History 3 Mixed and Multiple Fermentations 4 Biology of Fermentation 4.1 Agents 4.2 Energy Sources 4.2.1 Sugars 4.2.2 Complex Carbohydrates 4.3 Environment 4.3.1 Temperature 4.3.2 Oxygen 4.3.3 Acidity 4.3.4 Available water 4.3.5 Alcohol 4.3.6 Other Chemicals 4.4 Products 5 References 6 External Links

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), 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 fermentation s"
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

• World wide: olives, vinegar, wine, yoghurt
• India: achar, gundruk , Indian pickles
• South East Asia: asinan, bai-ming, belacan, burong mangga, dalok, jeruk, fish sauce, kimchi, leppet-so, miang, nata de coco, nata de pina , naw-mai-dong, pak-siam-dong, paw-tsay, phak-dong, phonlami-dong, fish paste, sajur asin, sambal tempo-jak, santol, si-sek-chai, sunki, tang-chai, tempeh, tempoyak, vanilla
• East Asia: cha-ts’ai, dan moogi, dongchimi, hiroshimana, hot pepper sauce, jangagee, kachdoo kigactuki, kakduggi, kimchi, miso, mootsanji, nara senkei, natto, narazuke, nozawana, nukamiso-zuke, oigee, oiji, oiso baegi, omizuke, pow tsai, red in snow, sake, seokbakji, siozuke, soy sauce, szechwan cabbage, tai-tan tsoi, takana, takuan, totkal kimchi, tsa tzai, tsu, umeboshi, wasabi-zuke , yen tsai
• Central Asia: kumys (mare milk), shubat (camel milk)
• Africa: hibiscus seed, hot pepper sauce, lamoun makbouss, mauoloh, msir, mslalla, oilseed , ogili, ogiri
• Americas: pickles, sauerkraut, lupin seed, oilseed , vanilla, fermented walrus meat / fish / birds
• Middle East: kushuk, lamoun makbouss, mekhalel, torshi, tursu
• Europe: sauerkraut, kephir and other sour milk products, filmjölk, fermented Baltic Herring, alcohol

History

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.

Mixed and Multiple Fermentations

Sometimes multiple breeds or strains of the same yeast or bacteria may be fermenting together. Usually this is considered just a single fermentation. Sometimes two different strains may become dominant as conditions change; for example in the fermentation of sugar, one type of yeast may predominate at first, before a more alcohol-tolerant strains takes over as the alcohol concentration increases.

Many products require multiple types of fermenting agents to produce the final product. The most common example is vinegar; yeast converts sugars to alcohol, then aerobic bacteria convert the alcohol to acetic acid. In addition to fermentation by yeast, wine-making may also employ a type of fermentation known as malolactic fermentation.

There are undesirable multiple fermentations, too: Wine can sometimes become 'ropey' from lactic acid bacteria.

Biology of Fermentation

Agents

Yeast are an important type of fermenting agent, and used in the production of bread and alcohol, as well as most other fermentations involving carbohydrates. Although they are a type of fungus, yeast have some special properties that distinguish them. Other fungi, aside from mushrooms, don't usually produce anything edible or otherwise useful. Also, yeast can thrive on many types of nutrients under most conditions; e.g. they can function with and without oxygen, unlike most bacteria.

Bacteria are also used frequently in fermentation. Acetobacter and lactobacter are the two main useful types. Lactobacter are anaerobic and work on dairy products, and produce cheeses, yogurts, etc. They are naturally found in unpasteurized milk. Acetobacter are aerobic bacteria which produce acetic acid, or vinegar.

Energy Sources

See: Carbohydrates

Sugars

"Ordinary cane sugar, uncrystallizable fruit sugar, grape sugar and glucose, are the three most important varieties. Fruit sugar exists in all of the sub-acid fruits such as grapes, currants, apples, peaches, etc. When these fruits are dried, the fruit sugar changes to grape sugar forming the whitish grains which are seen on the outside of prunes, raisins, etc. Grape sugar is found to a limited extent in fruits associated with fruit sugar. Cane sugar is readily changed by the action of acids or ferments into fruit sugar, and the latter into grape sugar, but the process cannot be reversed. Grape sugar is the only fermentable variety, the others becoming changed into it before fermentation."

Complex Carbohydrates

"Under the influence of acids, or diastare, a principle existing in germinating grains, starch is changed first into gum (dextrine) and afterwards into grape sugar. Hence one of our most important sources of alcohol is to be found in the starch of barley, corn, wheat, potatoes, etc. Wood may be converted into grape sugar by the action of strong sulphuric acid which is afterwards neutralized. An attempt to produce alcohol in this way on a commercial scale was made in France, but was not successful."

Environment

Control of the fermentation process is achieved by controlling the conditions in which the medium is stored. All organisms have a particular range of conditions in which they are able to thrive; outside this range their action is surpressed. The best example of this are honey and molasses, which are full of simple sugars, yet are stable for decades: the low water availability retards any microbial growth. In general, conditions are controlled so that only the desired fermentation is able to take place.

Microbes: environments for optimal growth

	Typically Desirable			Typically Undesirable
	Yeast	Lactobacter	Acetobacter	Fungi	Putrefactive Bacteria	Pathogenic Bacteria
Temperature, °C	10–35	37–50	25–35	10–35	21–38	20–45
Oxygen	Any	Anaerobic	Aerobic	Any	Anaerobic	Usually Aerobic
Acidity (pH)	3.7–4.6	3.1–4.5	3–8	2–8.5	6–8.2	4.6–9.3
Available Water	>0.8	>0.9	>0.9	>0.7	>0.9	>0.9
Alcohol by Volume	<15%	<13%	<7%	<15%	intolerant	intolerant

Selected fermentation media

	Honey	Milk	Fresh apple	Apple juice	Fruit wine	Dried apricot	Cucumber	Cucumber Pickles
Acidity (pH)	3.7–4.2	6.4–6.8	3.3–3.9	3.4–4.0	3.3–3.5	3.3–3.5	5.1–5.8	2.2–4.6
Available Water	0.5–0.7	0.97	0.97	0.97	0.97	0.50–0.75	0.97	0.93
Alcohol by Volume	0%	0%	0%	0%	5%	0%	0%	0%

Temperature

Temperature affects all microbes and enzymes greatly. High temperatures destroy microbes, and low temperatures suspend them, although the temperatures that these happen at depend on the particular microbe.

Oxygen

Microorganisms have differing relationships to air; some require it, others require its absence. In the production of alcohol, often a fermentation lock is used to prevent oxygen from entering the ferment, since this prevents the growth of aerobic bacteria. Conversely, in the production of vinegar oxygen is bubbled through the ferment to encourage aerobic acid-producing bacteria.

Acidity

In general, yeasts prefer acidic conditions, while bacteria prefer neutral conditions. Fruits are naturally acidic, so the action of yeasts and fungi is encouraged.

Available water

There are two types of water in a medium: bound and available. Bound water, for example ice, is not available to microbes. The presence of some available water is necessary for fermentation to occur.

Water availability is measured by water activity. Pure water is defined to have a water activity of 1. Fungi usually require at least 0.7, yeast at least 0.8, and bacteria 0.9. Dried foods, molasses and honey cannot support fermentation since their water activity is 0.65 or lower. When honey is used in ferments, it must be mixed with water first.

Alcohol

Yeast cannot survive in high alcohol concentrations. This is why distillation is necessary — no natural fermentation can produce greater than 20% alcohol by volume. Only a few types of yeast can survive up to even 20%, many can only tolerate 5%–10%.

Other Chemicals

There are specific chemicals that have a great effect on fermentation.

Nitrogen is necessary for the growth of microorganisms; it is frequently added to ferments that lack it, such as those made from honey. Without added nitrogen, it can take years to produce mead from a honey ferment.

Sulfites are commonly used in wine production. When grapes are first harvested, they are covered with natural yeasts. If the winemaker wants a more controlled fermentation, they will kill these yeasts with sulphites and use their own yeast culture. Also, sulphites are used to prevent a fermentation from continuing. For example, the winemaker may want to make a wine with more sugar left in it, or prevent fermentation after bottling. Uncontrolled fermentation after bottling is undesirable, because if too much carbon dioxide is developed, the bottle may explode.

Products

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.

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.

References

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

External Links

• fao.org: Fermented fruits and vegetables: a global perspective http://www.fao.org/docrep/x0560E/x0560E00.htm
• fao.org: Fermented cereals: a global perspective http://www.fao.org/docrep/x2184E/x2184E00.htm
• fao.org: Fermented fish in Africa http://www.fao.org/DOCREP/T0685E/T0685E00.HTM
• Conditions for Pathogenic bacterial growth http://seafood.ucdavis.edu/Pubs/pathogen.htm
• Approximate pH of Foods and Food Products http://www.cfsan.fda.gov/%7Ecomm/lacf-phs.html
• What is fermentation? http://www.tempeh.info/fermentation/fermentation.html
• Water Activity http://www.foodtechsource.com/rcenter/tech_data/td_water.htm