Tuesday, September 16, 2008

Fermentation

Introduction:
More commonly, fermentation is the conversion of biological materials, by enzymes, into carbon dioxide and alcohol. This is particularly for the utilization of foodstuffs.

The Merriam-webster Dictionary defines fermentation as:
"an enzymatically controlled anaerobic breakdown of an energy-rich compound (as a carbohydrate to carbon dioxide and alcohol or to an organic acid) ; broadly : an enzymatically controlled transformation of an organic compound".

Fermentation is the process of creating energy through the oxidative process of organic compounds, such as carbohydrates, by the use of endogenous electron acceptor. Fermentation is mostly carried out in an anaerobic environment. But some yeast cells also cause fermentation in the presence of oxygen (more or less) as long as sugar is there for consumption.

Reaction:
In the fermentation process, if we take the example of glucose, following reaction takes place:

Glucose (Sugar)----->Ethanol (Alcohol) + CO2 + Energy (in the form of ATP)

The early stages of this reaction follows part of the glycolysis pathway but most of the pathways depends on the sugar involved or used. The later stages of the pathway depends on the product obtained.

Processes of fermentation:
Primary fermentation: This is first step in the fermentation process, which involves the conversion of sugar to alcohol and carbon dioxide. This usually starts when we add yeast (or bacteria) to the organic compound and it starts to multiply and starts feeding on the fermentable sugars. Many of the aroma compounds are also produced durign the primary fermentation process. In human beings also, cells use fermentation as the first step in breaking down sugar.


Secondary fermentation: Stage of fermentation occuring from several weeks to several months. It is also referred to as "Malolactaic fermentation". Here, bacteria converts the malic acid to lactic acid. (2) This lactic acid in human beings is further broken down by the process of respiration and results in carbon dioxide and water. (This lactic acid accumulates in our muscles and causes pain, if we exert ourselves.)

Types of Fermentation:
Photofermentation: This is the type of fermentation that takes place by photosynthetic bacteria in the presence of light and involves the same steps as in the anaerobic conversion.

Dark fermentation: This fermentation takes place in the absence of light and involves the same steps as those of anaerobic conversion.

Thermophilic fermentation: Fermentation takes place in heating environment.

Uses of Fermentation:
(Michael D. Flythe et al.)Mankind has used fermentation to preserve animal feed for thousands of years. (1) Fermentation takes place in the large intestine of almost animals but in carnivores and omnivores, it produces very few calories but in case of herbivores this is the major cause of energy production. (2) Yeasts produce alcohols from sugars and this is used to produce spirits and ethanol. CO2 produced during the fermentation process is used to cause bubbles in bread. Bacteria which are used in the lactic acid formation are used in cheese making process and in making buttermilk, sour cream and yogurt.
(Ghasem Najafpour) In world war I, Germany found that glycerol can be generated from alcoholic fermentation. They developed an industrial scale fermentation process with a yield capacity of 1000 tons of glycerol per month.

References:
(1) http://www.vivo.colostate.edu/hbooks/pathphys/digestion/largegut/ferment.html

(2) http://www.historyoftheuniverse.com/ferment.html

Ghasem Najafpour. Chapter 10 - Application of Fermentation Processes. Biochemical Engineering and Biotechnology, 2007, Pages 252-262.

Michael D. Flythe, James B. Russell. Fermentation acids inhibit amino acid deamination by Clostridium sporogenes MD1 via a mechanism involving a decline in intracellular glutamate rather than protonmotive force. Microbiology 152 (2006), 2619-2624

Further Reading:
Fermentation Microbiology and Biotechnology, Second Edition by E. M. T. El-Mansi, C. F. A. Bryce, Arnold L. Demain and A. R. Allman

Practical Fermentation Technology by Brian McNeil and Linda Harvey

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