Microbial fermentation

Fermentation is a specific type of heterotrophic metabolism that uses organic carbon instead of oxygen as a terminal electron acceptor. This means that these organisms do not use an electron transport chain to oxidize NADH to NAD⁺ and therefore must have an alternative method of using this reducing power and maintaining a supply of NAD⁺ for the proper functioning of normal metabolic pathways (e.g. glycolysis). As oxygen is not required, fermentative organisms are anaerobic.

Many organisms can use fermentation under anaerobic conditions and aerobic respiration when oxygen is present. These organisms are facultative anaerobes. To avoid the overproduction of NADH, obligately fermentative organisms usually do not have a complete citric acid cycle. Instead of using an ATP synthase as in respiration, ATP in fermentative organisms is produced by substrate-level phosphorylation where a phosphate group is transferred from a high-energy organic compound to ADP to form ATP. As a result of the need to produce high energy phosphate-containing organic compounds (generally in the form of CoA -esters) fermentative organisms use NADH and other cofactors to produce many different reduced metabolic by-products, often including hydrogen gas (H₂). These reduced organic compounds are generally small organic acids and alcohols derived from pyruvate, the end product of glycolysis. Examples include ethanol, acetate, lactate, and butyrate. Fermentative organisms are very important industrially and are used to make many different types of food products. The different metabolic end products produced by each specific bacterial species are responsible for the different tastes and properties of each food.

The two main types of fermentation are alcoholic fermentation and lactic acid fermentation (Fig.2). The two main types of fermentation are:

1) Alcoholic fermentation

2) Lactic acid fermentation

  Fig. 2. Lactic acid and ethanolic fermentations

Both types have the same reactants: Pyruvic acid and NADH, both of which are products of glycolysis.

In alcoholic fermentation, the major products are alcohol and carbon dioxide. In lactic acid fermentation, the major product is lactic acid.

For both types of fermentation, there is a side product: NAD+ which is recycled back to glycolysis so that small amounts of ATP can continue to be produced in the absence of oxygen.

The chemical equations below summarize the fermentation of sucrose , whose chemical formula is
C₁₂H₂₂O₁₁. One mole of sucrose is converted into four moles of ethanol and four moles of carbon dioxide :

C₁₂H₂₂O₁₁+H₂O + invertase →2C₆H₁₂O₆

C₆H₁₂O₆+Zymase → 2C₂H₅OH + 2CO₂

The process of lactic acid fermentation using glucose is summarized below. In homolactic fermentation, one molecule of glucose is converted to two molecules of lactic acid:^[3]

C₆H₁₂O₆ → 2CH₃CHOHCOOH

In heterolactic fermentation, the reaction proceeds as follows, with one molecule of glucose converted to one molecule of lactic acid, one molecule of ethanol, and one molecule of carbon dioxide:

C₆H₁₂O₆→ CH₃CHOHCOOH + C₂H₅OH + CO₂

 

REFERENCES:

Text Books:

1. Jeffery C. Pommerville. Alcamo's Fundamentals of Microbiology (Tenth Edition). Jones and Bartlett Student edition.

2. Gerard J. Tortora, Berdell R. Funke, Christine L. Case. Pearson - Microbiology: An Introduction. Benjamin Cummings.

Reference Books:

1. Lansing M. Prescott, John P. Harley and Donald A. Klein. Microbiology. Mc Graw Hill companies.