Introduction

The term “fermentation” was first used by Pasteur to define respiration in the absence of free molecular oxygen. Fermentation can be broadly defined as respiration that occurs in the dark (no photosynthesis) and does not involve the use of free molecular oxygen, nitrate ions, or nitrite ions as the final electron acceptors of degraded organic compounds. Therefore, respiration may occur through several fermentative pathways including sulfate reduction, mixed acid production, and methane production. Fermentation is a form of anaerobic respiration. The bacteria that perform fermentation are facultative anaerobes and anaerobes. Fermentation involves the transformation of organic compounds to various inorganic and organic products. During fermentation a portion of an organic compound may be oxidized while another portion is reduced. It is from this oxidation-reduction of organic compounds that fermenting bacteria obtain their energy and produce numerous simplistic and soluble organic compounds. Fermentative bacteria are capable of performing a variety of oxidation-reduction reactions involving organic compounds, carbon dioxide, carbon monoxide (CO), molecular hydrogen, and sulfur compounds. Fermentative bacteria include facultative anaerobes, aerotolerant anaerobes, and strict anaerobes. Some fermentative bacteria such as the clostridia and Escherichia coli produce a large variety of products, whereas other fermentative bacteria such as Acetobacterium produce a very small number of products. As environmental or operational conditions change, for example, pH and temperature, the bacteria that are active and inactive also change. These changes in activity are responsible for changes in the types and quantities of compounds that are produced through fermentation.

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The Microbiology of Anaerobic Digesters, by Michael H. Gerardi 693-8 Copyright © 2003 by John Wiley & Sons, Inc