with the difference in redox potential of the substrate and the end product providing energy for ATP synthesis (Figure 1). In most fermentations, the same substrate is used as both reductant and oxidant, whereas in some amino acid fermenting organisms, one amino acid is oxidized and another is reduced (Stickland reaction). The oxidation reaction is coupled to substrate level phosphorylation whereas the reduction reaction is usually not. The fermentation end products are excreted. The nature of these products is different in various species, and the various fermentation pathways are named after their main products (Gottschalk and Peinemann, 1992) (Figure 2).