Fermentation

History

Theory

Uses

Resources

In its broadest sense, fermentation refers to any biochemical process by which large organic molecules are broken down to simpler molecules as the result of the action of microorganisms. Virtually all organisms can perform fermentation, and some rely on it exclusively for their energy needs. The most familiar type of fermentation is the conversion of sugars and starches to alcohol by enzymes in yeast. To distinguish this reaction from other kinds of fermentation, the process is sometimes known as alcoholic or ethanolic fermentation.

History

The chemistry of fermentation was first investigated by French microbiologist Louis Pasteur (1822–1895) in 1860, who called the process la vie sans air, or life without air. Fermentation is indeed carried out without oxygen, and is therefore called an anaerobic process (an- meaning without; aero meaning air ).

Organisms that can use either air or not are called facultative aerobes, while those that must live without air are called obligate anaerobes. When oxygen is low, facultative aerobes, such as yeast, switch from cellular respiration, which requires oxygen, to fermentation.

Ethanolic fermentation was one of the first chemical reactions observed by humans. In nature, various types of food go bad as a result of bacterial action. Early in history, humans discovered that this kind of change could result in the formation of products that were enjoyable to consume. The spoilage (fermentation) of fruit juices, for example, resulted in the formation of primitive forms of wine.

The mechanism by which fermentation occurs was the subject of extensive debate in the early 1800s. It was a key issue among those arguing over the concept of vitalism, the notion that living organisms are in some way inherently different from non-living objects. One aspect in this debate centered on the role of so-called ferments in the conversion of sugars and starches to alcohol. Vitalists argued that ferments (what is now known as enzymes) are inextricably linked to a living cell. Destroy a cell and ferments can no longer cause fermentation, they said.

A crucial experiment on this issue was carried out in 1896 by German chemist Eduard Buchner (1860–1917). Buchner ground up a group of cells with sand until they were destroyed. He then extracted the liquid that remained and added it to a sugar solution. His assumption was that fermentation could no longer occur since the cells that had held the ferments were dead, so they no longer carried the life force needed to bring about fermentation. He was amazed to discover that the cell-free liquid did indeed cause fermentation. It was obvious that the ferments themselves, distinct from any living organism, could cause fermentation.

Theory

The chemical reaction that occurs in fermentation can be described quite easily. Starch is converted to simple sugars such as sucrose and glucose. Those sugars are then converted to alcohol (ethyl alcohol) and carbon dioxide. This description does not adequately convey the complexity of the fermentation process itself. During the 1930s, two German biochemists, Gustav Embden (1872–1933) and Otto Meyerhof (1884–1951), worked out the sequence of reactions by which glucose ferments. Glycolysis is sometimes called the Emden-Myerhof pathway.

In a sequence of twelve reactions, glucose is converted to ethyl alcohol and carbon dioxide. A number of enzymes are needed to carry out this sequence of reactions, the most important of which is zymase, found in yeast cells. These enzymes are sensitive to environmental conditions in which they live. When the concentration of alcohol reaches about 14%, they are inactivated. For this reason, no fermentation product (such as wine) can have an alcoholic concentration of more than about 14%.

Uses

The alcoholic beverages that can be produced by fermentation vary widely, depending primarily on two factors—the plant that is fermented and the enzymes used for fermentation. Human societies use, of course, the materials that are available to them. Thus, various peoples have used grapes, berries, corn, rice, wheat, honey, potatoes, barley, hops, cactus juice, cassava roots, and other plant materials for fermentation. The products of such reactions are various forms of beer, wine or distilled liquors, which may be given specific names depending on the source from which they come. In Japan, for example, rice wine is known as sake. Wine prepared from honey is known as mead. Beer is the fermentation product of barley, hops, and/or malt sugar.

Early in human history, people used naturally occurring yeast for fermentation. The products of such reactions depended on whatever enzymes might occur in “wild” yeast. Today, winemakers are able to select from a variety of specially cultured yeast that control the precise direction that fermentation will take.

Ethyl alcohol is not the only useful product of fermentation. The carbon dioxide generated during fermentation is also an important component of many baked goods. When the batter for bread is mixed, for example, a small amount of sugar and yeast is added. During the rising period, sugar is fermented by enzymes in the yeast, with the formation of carbon dioxide gas. The carbon dioxide gives the batter bulkiness and texture that would be lacking without the fermentation process.

Fermentation has a number of commercial applications beyond those described thus far. Many occur in the food preparation and processing industry. A variety of bacteria are used in the production of olives, cucumber pickles, and sauerkraut from the raw olives, cucumbers, and cabbage, respectively.

KEY TERMS

Vitalism —The concept that compounds found within living organisms are somehow inherently differ from those found in non-living objects.

Wastewater —Water that carries away the waste products of personal, municipal, and industrial operations.

The selection of exactly the right bacteria and the right conditions (for example, acidity and salt concentration) is an art in producing food products with exactly the desired flavors. An interesting line of research in the food sciences is aimed at the production of edible food products by the fermentation of petroleum.

In some cases, antibiotics and other drugs can be prepared by fermentation if no other commercially efficient method is available. For example, the important drug cortisone can be prepared by the fermentation of a plant steroid known as diosgenin. The enzymes used in the reaction are provided by the mold Rhizopus nigricans.

One of the most successful commercial applications of fermentation has been the production of ethyl alcohol for use in gasohol. Gasohol is a mixture of about 90% gasoline and 10% alcohol. The alcohol needed for this product can be obtained from the fermentation of agricultural and municipal wastes. The use of gasohol provides a promising method for using renewable resources (plant material) to extend the availability of a nonrenewable resource (gasoline).

Another application of the fermentation process is in the treatment of wastewater. In the activated sludge process, aerobic bacteria are used to ferment organic material in wastewater. Solid wastes are converted to carbon dioxide, water, and mineral salts.

See also Ethanol ; Enzyme.

Resources

BOOKS

Bamforth, Charles W. Food, Fermentation, and Microorganisms. Oxford, UK, and Ames, Iowa: Blackwell Science, 2005.

Bloch, Daniel R., ed. Organic Chemistry Demystified. New York: McGraw-Hill, 2006.

Loudon, G. Mark. Organic Chemistry. Oxford: Oxford University Press, 2002.

Moog, Richard Samuel. Chemistry: A Guide Inquiry. New York: Wiley, 2005.

Tro, Nivaldo J. Introductory Chemistry. Upper Saddle River, NJ: Pearson Education, 2006.

David E. Newton

FERMENTATION

FERMENTATION. Fermentation is one of the oldest known food preservation techniques. Along with drying and salting, fermentation was a key method of extending the life of foods, allowing them to be available, and eaten safely, in times of scarcity or seasonal nonavailability. These methods helped allow the transition from hunting and gathering to organized food cultivation and storage, which took place some ten to fifteen thousand years ago in the Middle East.

Fermentation involves the action of desirable microorganisms, or their enzymes, on food ingredients to make biochemical changes, which cause significant modification to the food. Often lactic-acid bacteria convert the carbohydrate energy source of food, such as lactose in milk, to lactic acid; examples are yogurt and cheeses from milk, and pickles from fruits and vegetables. Alternatively, yeasts, often of the Saccharomyces species, may convert the glucose to ethanol and carbon dioxide in leavened breads, or the sugars in grain or fruit beverages to beers and wines. Molds also can be active in certain fermentations, such as Stilton cheese and soy sauce. It is estimated that about one-third of all the food we consume is fermented. World estimates for beer consumption are about 22 million gallons, and a total of 15 million tons of some one thousand varieties of cheese are eaten annually.

Fermented Beverages and Foods

Fermentation is often the key to the safe, enjoyable consumption of perishable food materials, as it changes their composition, flavor, and texture. For example, milk is a nutritious but highly perishable beverage. Originally, in the Middle East, milk carried in animal-skin containers, often on horseback, would sour naturally, to produce acidic fermented milk. The combined action of the two lactic-acid bacteria, Streptococcus lactis, producing lactic acid, and Lactobacillus bulgaricus, producing lactic acid and acetaldehyde, a major contributor to flavor, are involved in yogurt production. The Tartars of Central Asia used the milk of horses, donkeys, or camels to produce a fizzy, gray acidic and alcoholic drink, kumiss, in which yeasts were active.

In acid conditions, the milk protein, casein, denatures and is precipitated to form a curd, producing cottage and soft cheese. By stirring and pressing, whey is removed and a more solid curd is produced, which by ripening or maturation produces semi-hard or hard cheeses. Surface-active bacteria of Brevibacterium linens are active in producing the aroma of Limburger type cheeses, while the blue molds of the genus Penicillium give Stilton and Gorgonzola cheeses their character.

The use of Saccharomyces yeasts has allowed the production of a range of fermented beverages, enabling safe consumption of liquid when fresh water supplies are not available. Lagers, the light golden, gassy beverage made by "bottom" yeast fermentation of cereal extracts, were first made in the regions of Germany and Czechoslovakia, but are now produced and consumed throughout the world. In Africa, a thick, sour alcoholic beverage is made from sorghum or millet, or sometimes maize or banana. These sorghum beers are important sources of nutrients, particularly B vitamins, to people on marginal diets in these regions. The Romans planted extensive vineyards in North Africa to harvest and ferment their grapes into wine, thereby producing a fermented beverage that could be readily stored, transported, and consumed when and where required.

Distillation of these alcoholic beverages, such as whiskey from beers, brandies from grape wines, or arrack from palm or rice wine, further extend our range of drinks and play important cultural roles in festivities.

Fermentation Vessels and Starter Cultures

Art meets science in the production of fermented foods. Traditional practices are passed down through generations of producers, often small in scale, and consumption patterns often have great cultural importance. In Scandinavia, traditionally the brides and mothers jealously guard their own supplies of sourdough starters, so that they can always make the desired bread for their partners and families. In West Africa, a homeowner keeps a supply of dawadawa, a dried fermented African locust bean paste (Parkia species); it is used to give everyday soups and stews the desired "meaty" flavor, while also providing important nutrients, such as riboflavin, the B vitamin that protects against blindness, which is endemic to the region due to nutritional deficiency.

In Korea, few meals are complete without kimchi, a pickled fermented cabbage, which may also contain fish and other components. The practice of every home having their own kimchi jars, often on their verandahs, originated as a way of preserving vegetables through the cold winter season, providing year-round vitamin C. Kimchi together with kochujang, the fermented red pepper paste, give Korean preparations a unique and characteristic attractive color and flavor.

Where food fermentation occurred naturally as conditions favored particular organisms, an important art arose to encourage the desired fermentation organisms, while preventing undesirable microorganisms from developing, for successful fermented food production.

Food storage often took place in earthenware vessels, whose semipermeable inner walls were difficult to clean completely. This allowed a biofilm of desirable microorganisms to remain, to initiate a successful fermentation of the next batch of food. Because of their significance, the vessels themselves were artistically designed and treasured. Interesting examples can be seen in museum collections, such as the Nezu Museum in Tokyo, Japan, and a museum dedicated to kimchi in Seoul, South Korea.

In Europe, the fermented meat producers, while using ceramic or metallic vats with smoother, more easily cleaned surfaces, developed the technique of "backslopping" to introduce a small quantity of the fermenting liquor from the previous batch of meat to initiate successful fermentation.

In many cases, dried grains or balls of the derived fermenting microorganisms on cereal or other substrates would be used to start fermentation. Baker's yeast may be used in this work. Kefir grains are used in North Africa, the Middle East, and Russia for production of kefir, laban, or leben fermented milks. Ragi is used in Indonesia and throughout East and Southeast Asia as inoculum for lao-chao and other fermented foods.

Cultural Diversity

The production, consumption, and enjoyment of different fermented foods reflects the diversity of cultures and cuisines that make up our varied world. In Chinese and Japanese cuisines, shoyu, or soy sauce, is added almost universally to dishes, while the Indian vegetarian diet depends on fermented cereals and legumes, often in combinations, as in dosas and vadas. The art and science of fermenting meat to a wide range of salamis are vital to the enjoyment of Eastern and Central Europeans, while Italian food market stall holders proudly display their mold-covered fermented sausages and traditional cheeses.

As people migrate, they normally carry their traditional fermented food practices with them. The range of fermented cheeses and meats in Latin America reflects the European origins of these populations, and the wineries of Chile were originally established by French families. Consumers of imported wine, chocolate, coffee, or tea are all beneficiaries of the internationalism and significance of fermented foods.

See also Beer; Bread ; Cheese ; Meat ; Microorganisms ; Preserving ; Spirits ; Wine.

BIBLIOGRAPHY

Campbell-Platt, Geoffrey. Fermented Foods of the World: A Dictionary and Guide. London: Butterworth, 1987

Steinkraus, Keith, ed. Handbook of Indigenous Fermented Foods. 2nd ed. NewYork: Marcel Dekker, 1995.

Wood, Brian J. B., ed. Microbiology of Fermented Foods. 2nd ed. London: Blackie, 1998.

Geoffrey Campbell-Platt

Fermentation

Fermentation is a chemical process that breaks down carbohydrates and other organic materials and produces energy without using oxygen. This process is carried out by microorganisms such as bacteria, molds, and fungi. Alcohol fermentation is a well-known type of fermentation where sugar is broken down into alcohol and carbon dioxide.

LOUIS PASTEUR

One of the most extraordinary scientists in history, French chemist and microbiologist (a person specializing in the study of microorganisms) Louis Pasteur (1822–1895) is considered the founder of microbiology. He also contributed to our understanding of fermentation (a chemical process that breaks down carbohydrates and other organic materials and produces energy without using oxygen), developed the germ theory of disease, improved immunization, and proved that heating kills microorganisms (an organism of microscopic size). This process of using heat was named pasteurization after the famed scientist.

Louis Pasteur was born in Dole, France, and his family moved to Arbois when he was very young. He attended school there and appeared to be a mediocre student in every way. Still, he stayed in school despite near poverty, and after attending a lecture in chemistry and being inspired by it, he decided to study this new and fascinating subject. Studying chemistry, he suddenly became an excellent student, and by the age of twenty-six earned his Ph.D. and made a major discovery concerning crystals for which he won a national award.

By 1854, the "mediocre" student had become dean of the Faculty of Sciences at the University of Lille, and was asked by the French wine industry to help them with their spoilage problem. Very often, wine and beer spoiled, or went sour, as they aged, ruining tons of good beverage without anyone knowing why or what to do. Pasteur took up the problem and discovered almost immediately with his microscope that the yeast (various single-celled fungi capable of fermenting carbohydrates) in sour wine had an elongated shape, while the yeast in good wine was spherical, or round. When the good (round) yeast ferments, it produces alcohol. When the bad (elongated) yeast ferments, it produces lactic acid (a syrupy liquid). He suggested heating the wine or beer gently at about 120°F (48.9°C) after it had been properly made. Pasteur stated that the heat would kill any yeast left, especially the bad yeast, and if the wine were properly corked, it would not go sour. Heating wine seemed barbaric to the French, but they tried the experiment and it worked. Ever since, a gentle heating that kills unwanted microorganisms has been called pasteurization. Besides beer and wine, milk also is now pasteurized.

Saving the French wine industry made Pasteur a hero, so it is not surprising that its silk industry asked him to do the same thing for them. He did by showing them how to get rid of a killer parasite (an often harmful organism that lives on or in a different organism) that was killing the silk worms. This led Pasteur to work with communicable (contagious) diseases. He had long felt that disease was something that was caused by unseeable organisms and then was spread person-to-person. By now, Pasteur had considerable experience using his microscope to identify different kinds of microorganisms such as bacteria and fungi. So when he decided to work on what is now called the germ theory of disease, he was following one of his favorite sayings, "Chance favors the prepared mind." Pasteur then developed techniques for culturing (growing) and examining several disease-causing bacteria. He identified both Staphylococcus and Streptococcus, which cause serious, sometimes fatal infections, and also cultured the bacteria that cause cholera. It was in working with these infectious bacteria that Pasteur realized that weakening them allows them to be used as a vaccine. From this discovery, he developed a vaccine for the disease anthrax, as well as one for rabies, a deadly disease contracted from the bite of an infected, rabid animal. If any individual had achieved one or two of these accomplishments, he or she would be considered among the pioneers in the life sciences. Yet Pasteur did all this and more. His germ theory of disease is considered by many scientists as the single most important medical discovery of all time because it not only showed doctors how to fight and prevent disease, but it supplied the all-important correct theory that would guide future research. Pasteur is truly one of the giants of biology.

Fermented products have been used by people for thousands of years, primarily to make the alcohol in beer and wine and to make bread dough rise. Although they did not understand what made it happen, the ancient Egyptians knew that if they allowed bread dough to stand for several hours, it became lighter and better tasting than if baked immediately. What they did not know was that the dough was lightened by the carbon dioxide gas produced by the fermentation of sugar. This happened not because the Egyptians knew enough to add yeast (a single-celled fungus) to the dough, but because leaving the dough uncovered allowed microscopic organisms like yeast and bacteria to float in on the breeze and break down the dough's sugars into alcohol and carbon dioxide. The carbon dioxide gas then became trapped in the dough and made it rise, while the alcohol would evaporate during baking. The Egyptians also discovered that by allowing certain grains like barley to begin to spoil, they could obtain a drink with a pleasing side effect (alcohol). The same effect could be achieved by allowing grapes to spoil since grapes contain yeast that grow naturally on their skins.

Throughout history, the process of fermentation was shrouded in mystery and superstition. During the seventeenth century, the English chemist Robert Boyle (1627–1691) correctly predicted that an understanding of the fermentation process would lead to the discovery of the causes of other phenomena like disease. Boyle's prediction came true when the French chemist, Louis Pasteur (1822–1895), proved that yeast caused fermentation in beer and wine. After this discovery, Pasteur turned his research toward the spread of diseases caused by other microorganisms.

Pasteur's work saved France's wine industry, which could not understand why its burgundy wine was spoiling. Pasteur discovered that wine normally contained yeast cells that produced alcohol. However, he also realized that wine containing bacteria and other microorganisms produced lactic acid when they fermented, and thus spoiled the wine. Pasteur showed that fermentation caused by living organisms is too small to be seen without a microscope, and that the end product of the fermentation process depends on both what is being fermented and what microbes are the catalyst (something that starts a chemical reaction). Pasteur taught France's wine industry how to kill unwanted bacteria by the gentle heating of the wine at about 120°F (48.9°C). This process is called pasteurization after the great scientist.

Today, fermentation is well understood and can be controlled. Fermentation is a large part of today's food industry, with some form of fermentation taking place in the production of many food products like yogurt, buttermilk, cheese, soy sauce, cured meats, pickled vegetables, and chocolate, as well as in alcoholic beverages and bread. In some cases, antibiotics and other medications can be produced by fermentation, as can ethyl alcohol that is added to gasoline to produce gasohol. Fermentation is also critical to today's disposal of solid waste by converting it to carbon dioxide, water, and mineral salts.

[See alsoBacteria; Carbohydrates; Carbon Dioxide; Fungi ]

Fermentation

In its broadest sense, fermentation refers to any process by which large organic molecules are broken down to simpler molecules as the result of the action of microorganisms (organisms so small they can be seen only with the aid of a microscope). The most familiar type of fermentation is the process by which sugars and starches are converted to alcohol by enzymes in yeasts. (Enzymes are chemicals that act as catalysts, which spark reactions.) To distinguish this reaction from other kinds of fermentation, the process is sometimes known as alcoholic or ethanolic fermentation.

History

Ethanolic fermentation was one of the first chemical reactions observed by humans. In nature, various types of food "go bad" as a result of bacterial action. Early in history, humans discovered that this kind of change could result in the formation of products that were actually enjoyable to consume. The "spoilage" (fermentation) of fruit juices, for example, resulted in the formation of primitive forms of wine.

The mechanism by which fermentation occurs was the subject of extensive debate in the early 1800s. It was a key issue among those arguing over the concept of vitalism, the notion that living organisms are in some way essentially different from nonliving objects. One aspect in this debate centered on the role of so-called "ferments" in the conversion of sugars and starches to alcohol. Vitalists argued that ferments (what we now know as enzymes) are linked to a living cell. Destroy a cell, they said, and ferments can no longer cause fermentation.

A crucial experiment on this issue was carried out in 1896 by the German chemist Eduard Buchner (1860–1917). Buchner ground up a group of cells with sand until they were totally destroyed. He then extracted the liquid that remained and added it to a sugar solution. His assumption was that fermentation could no longer occur since the cells that had held the ferments were dead. Thus, they no longer carried the "life-force" needed to bring about fermentation. He was amazed to discover that the cell-free liquid did indeed cause fermentation. It was obvious that the ferments themselves, distinct from any living organism, could cause fermentation.

Words to Know

Enzyme: An organic compound that speeds up the rate of chemical reactions in living organisms.

Ferment: An early term used to describe the substances we now know as enzymes.

Gasohol: A synthetic fuel consisting of a mixture of about 90 percent gasoline and 10 percent alcohol.

Vitalism: The concept that compounds found within living organisms are somehow essentially different from those found in nonliving objects.

Wastewater: Water that carries away the waste products of personal, municipal, and industrial operations.

Wild yeast: A naturally occurring yeast.

Theory

The chemical reaction that occurs in fermentation can be described quite easily. Starch is converted to simple sugars such as sucrose and glucose. Those sugars are then converted to alcohol (ethyl alcohol) and carbon dioxide:

This description does not really provide an idea as to how complex the fermentation process really is. During the 1930s, two German biochemists, Gustav Embden (1874–1933) and Otto Meyerhof (1884–1951), worked out the sequence of reactions by which glucose ferments. Embden and Meyerhof found that it required a sequence of 12 reactions in order to accomplish the "simple" change from glucose to ethyl alcohol and carbon dioxide. A number of enzymes are needed to carry out this sequence of reactions, the most important of which is zymase, found in yeast cells. These enzymes are sensitive to environmental conditions in which they live. When the concentration of alcohol in a liquid reaches about 14 percent, they are inactivated. For this reason, no fermentation product (such as wine) can have an alcoholic concentration of more than about 14 percent.

Uses

The alcoholic beverages that can be produced by fermentation vary widely, depending primarily on two factors, the plant that is fermented and the enzymes used for fermentation. Human societies use, of course, the materials that are available to them. Thus, various peoples have used grapes, berries, corn, rice, wheat, honey, potatoes, barley, hops, cactus juice, cassava roots, and other plant materials for fermentation. The products of such reactions are various forms of beer, wine, or distilled liquors, which may be given specific names depending on the source from which they come. In Japan, for example, rice wine is known as sake. Wine prepared from honey is known as mead. Beer is the fermentation product of barley, hops, and/or malt sugar.

Early in human history, people used naturally occurring yeasts for fermentation. The products of such reactions depended on whatever enzymes might occur in those "wild" yeasts. Today, wine-makers are able to select from a variety of specially cultured (grown) yeasts that control the precise direction that fermentation will take.

Ethyl alcohol is not the only useful product of fermentation. The carbon dioxide generated during fermentation is also an important component of many baked goods. When the batter for bread is mixed, for example, a small amount of sugar and yeast are added. During the rising period, sugar is fermented by enzymes in the yeast, with the formation of carbon dioxide gas. The carbon dioxide gives the batter bulkiness and texture that would be lacking without the fermentation process.

Fermentation has a number of commercial applications beyond those described thus far. Many occur in the food preparation and processing industry. A variety of bacteria are used in the production of olives, cucumber pickles, and sauerkraut from raw olives, cucumbers, and cabbage, respectively. The selection of exactly the right bacteria and the right conditions (for example, acidity and salt concentration) is an art in producing food products with exactly the desired flavors. An interesting line of research in the food sciences is aimed at the production of edible food products by the fermentation of petroleum.

In some cases, antibiotics and other drugs can be prepared by fermentation if no other commercially efficient method is available. For example, the important drug cortisone can be prepared by the fermentation of a plant steroid known as diosgenin. The enzymes used in the reaction are provided by the mold Rhizopus nigricans.

One of the most successful commercial applications of fermentation has been the production of ethyl alcohol for use in gasohol. Gasohol is a mixture of about 90 percent gasoline and 10 percent alcohol. The alcohol needed for this product can be obtained from the fermentation of agricultural and municipal wastes. The use of gasohol provides a promising method for using renewable resources (plant material) to extend the availability of a nonrenewable resource (gasoline).

Another application of the fermentation process is in the treatment of wastewater. In the activated sludge process, aerobic bacteria (bacteria that can live without oxygen) are used to ferment organic material in wastewater. Solid wastes are converted to carbon dioxide, water, and mineral salts.

[See also Alcohol; Bacteria; Brewing; Carbon dioxide; Enzyme; Yeast ]

Fermentation

In its broadest sense, fermentation refers to any process by which large organic molecules are broken down to simpler molecules as the result of the action of microorganisms . The most familiar type of fermentation is the conversion of sugars and starches to alcohol by enzymes in yeast . To distinguish this reaction from other kinds of fermentation, the process is sometimes known as alcoholic or ethanolic fermentation.

History

Ethanolic fermentation was one of the first chemical reactions observed by humans. In nature, various types of food "go bad" as a result of bacterial action. Early in history, humans discovered that this kind of change could result in the formation of products that were enjoyable to consume. The "spoilage" (fermentation) of fruit juices, for example, resulted in the formation of primitive forms of wine.

The mechanism by which fermentation occurs was the subject of extensive debate in the early 1800s. It was a key issue among those arguing over the concept of vitalism, the notion that living organisms are in some way inherently different from non-living objects. One aspect in this debate centered on the role of so-called "ferments" in the conversion of sugars and starches to alcohol. Vitalists argued that ferments (what we now know as enzymes) are inextricably linked to a living cell . Destroy a cell and ferments can no longer cause fermentation, they said.

A crucial experiment on this issue was carried out in 1896 by the German chemist Eduard Buchner. Buchner ground up a group of cells with sand until they were totally destroyed. He then extracted the liquid that remained and added it to a sugar solution . His assumption was that fermentation could no longer occur since the cells that had held the ferments were dead, so they no longer carried the "life-force" needed to bring about fermentation. He was amazed to discover that the cell-free liquid did indeed cause fermentation. It was obvious that the ferments themselves, distinct from any living organism , could cause fermentation.

Theory

The chemical reaction that occurs in fermentation can be described quite easily. Starch is converted to simple sugars such as sucrose and glucose. Those sugars are then converted to alcohol (ethyl alcohol) and carbon dioxide . This description does not adequately convey the complexity of the fermentation process itself. During the 1930s, two German biochemists, G. Embden and O. Meyerhof, worked out the sequence of reactions by which glucose ferments.

In a sequence of twelve reactions, glucose is converted to ethyl alcohol and carbon dioxide. A number of enzymes are needed to carry out this sequence of reactions, the most important of which is zymase, found in yeast cells. These enzymes are sensitive to environmental conditions in which they live. When the concentration of alcohol reaches about 14%, they are inactivated. For this reason, no fermentation product (such as wine) can have an alcoholic concentration of more than about 14%.

Uses

The alcoholic beverages that can be produced by fermentation vary widely, depending primarily on two factors—the plant that is fermented and the enzymes used for fermentation. Human societies use, of course, the materials that are available to them. Thus, various peoples have used grapes , berries, corn, rice , wheat , honey, potatoes, barley , hops, cactus juice, cassava roots, and other plant materials for fermentation. The products of such reactions are various forms of beer, wine or distilled liquors, which may be given specific names depending on the source from which they come. In Japan, for example, rice wine is known as sake. Wine prepared from honey is known as mead. Beer is the fermentation product of barley, hops, and/or malt sugar.

Early in human history, people used naturally occurring yeast for fermentation. The products of such reactions depended on whatever enzymes might occur in "wild" yeast. Today, wine-makers are able to select from a variety of specially cultured yeast that control the precise direction that fermentation will take.

Ethyl alcohol is not the only useful product of fermentation. The carbon dioxide generated during fermentation is also an important component of many baked goods. When the batter for bread is mixed, for example, a small amount of sugar and yeast is added. During the rising period, sugar is fermented by enzymes in the yeast, with the formation of carbon dioxide gas. The carbon dioxide gives the batter bulkiness and texture that would be lacking without the fermentation process.

Fermentation has a number of commercial applications beyond those described thus far. Many occur in the food preparation and processing industry. A variety of bacteria are used in the production of olives, cucumber pickles, and sauerkraut from the raw olives, cucumbers, and cabbage, respectively. The selection of exactly the right bacteria and the right conditions (for example, acidity and salt concentration) is an art in producing food products with exactly the desired flavors. An interesting line of research in the food sciences is aimed at the production of edible food products by the fermentation of petroleum .

In some cases, antibiotics and other drugs can be prepared by fermentation if no other commercially efficient method is available. For example, the important drug cortisone can be prepared by the fermentation of a plant steroid known as diosgenin. The enzymes used in the reaction are provided by the mold Rhizopus nigricans.

One of the most successful commercial applications of fermentation has been the production of ethyl alcohol for use in gasohol. Gasohol is a mixture of about 90% gasoline and 10% alcohol. The alcohol needed for this product can be obtained from the fermentation of agricultural and municipal wastes. The use of gasohol provides a promising method for using renewable resources (plant material) to extend the availability of a nonrenewable resource (gasoline).

Another application of the fermentation process is in the treatment of wastewater. In the activated sludge process, aerobic bacteria are used to ferment organic material in wastewater. Solid wastes are converted to carbon dioxide, water , and mineral salts.

See also Ethanol; Enzyme.

Resources

books

Baum, Stuart J., and Charles W. J. Scaife. Chemistry: A Life Science Approach. New York: Macmillan Publishing Company, Inc., 1975, Chapter 28.

Brady, James E., and John R. Holum. Fundamentals of Chemistry. 2nd edition. New York: John Wiley & Sons, 1984, p. 828A.

Loudon, G. Mark. Organic Chemistry. Oxford: Oxford University Press, 2002.

David E. Newton

KEY TERMS

Vitalism

—The concept that compounds found within living organisms are somehow inherently differ from those found in non-living objects.

Wastewater

—Water that carries away the waste products of personal, municipal, and industrial operations.