The term ‘fermentation’ is derived from the Latin verb fervere, to boil, thus describing the appearance of the action of yeast on extracts of fruit or malted grain. The appearance of boiling is due to the production of carbon dioxide bubbles caused by the anaerobic catabolism of the sugars present in the extract. However, fermentation has different meanings according to biochemists and to industrial microbiologists. Biochemically, it relates to the generation of energy by the catabolism of organic compounds, whereas its meaning in industrial microbiology tends to be much broader.

In alcoholic fermentation, the substrates that are mainly sugars are fermented, with ethanol as the main product. It is widely distributed among microorganisms. Even plants switch to this pathway for a short period under anaerobic conditions. However, the yeast cell, especially the species of Saccharomyces is the main alcohol producer. Some bacteria, particularly Z. mobilis, which only utilize hex — oses, can also produce ethanol from glucose [150]. In other bacteria, the alcohol is not a predominant end product. Certain yeasts including S. cerevisiae can also ferment pentose sugar, xylose to ethanol though the yield is lower compared to the fermentation of hexoses. The production of alcohol by the action of yeast on malt or fruit extracts has been carried out on a large scale for many years and was the first ‘industrial’ process for the production of a microbial metabolite. Thus, industrial microbiologists have extended the term fermentation to describe any process for the production of a product by the mass culture of a microorganism. It may be noted that the fermentation equipment makes upto 10-25% of the total fixed capital cost of an ethanol plant depending upon its design.