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Fermentation

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Fermentation in progress
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Fermentation in progress

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

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.

Contents

History

Since fruits ferment naturally, fermentation precedes human history. Since prehistoric times, however, humans have been taking control of the fermentation process. The earliest evidence of winemaking dates from 5400 BCE, 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 [link]. 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.

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.

Typical examples of fermentation products are ethanol, 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 glucose (C6H12O6) into exactly two molecules of ethanol (C2H6O) and two molecules of carbon dioxide (CO2).

The burning sensation in muscles during hard exercise used to be attributed to the production of lactic acid during a shift to anaerobic glycolysis, 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:

  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. 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 [link]. 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.

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.

Chemical Equation

::C6H12O6 → 2C2H5OH + 2CO2 + 2 ATP (Energy Released:118 kJ mol−1)
Word Equation
::Sugar (glucose) → Alcohol + Carbon Dioxide + Energy (ATP)

Zymology

Zymology is the science term for fermentation. It deals with the biochemical processes involved in fermentation, with yeast selection and physiology, and with the practical issues of brewing. Zymology is occasionally known as zymurgy.

Fermentation

Fermentation can be simply defined, in this context, as the conversion of sugar molecules into ethanol and carbon dioxide by yeast.

C6H12O6 => 2CO2 + 2C2H5OH

History

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.

See also

References

External links

 

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