4.1.3 General scope

In ethanol fermentation saccharine materials like glucose, fructose, and sucrose are metabolized by yeast strains through glycolysis pathway (Embden-Meyerhof Pathway) to produce ethanol and carbon dioxide in anaerobic condition (Eq. (5.2.1)). In this reaction two molecules of ATP are generated from one molecule of glucose and are used as energy for growth of yeast cells. Mankind has long known and utilized ethanol fermentation for brewery of alcohol drinks, manufacture of fermented food, bakery and so on for several thousands years. In the medieval period, people came to know how to make distilled liquor. Ethanol became available in the field of various chemical industries, beverage and food industry, medical use, as well as fuel, since great progress was achieved with technologies of fermentation and distillation in 19-20th century. Large amount of fuel ethanol has been produced from corn in the USA and from sugar cane in Brazil for the purpose to alternate fossil fuel and to prevent global warming, particularly after two times of oil crisis in 1970’s. Extensive research and development are undergoing in the world on technologies for ethanol production from various cellulosic materials which are available in large amount and do not compete with food utilization.

C6H12O6 ^ 2C2H5OH + 2CO2 (5.2.1)

100 g 51.14 g 48.86 g

Ethanol fermentation is a biological reaction at room temperatures and under atmospheric pressure. Saccharomyces cerevisiae is the yeast which is widely used for industrial and fuel ethanol production, and has excellent ethanol fermenting ability and ethanol tolerance. Yeast strains produce 51.14 g of ethanol from 100 g of glucose according to Eq. (5.2.1). In this reaction nearly 50% weight of glucose is lost as carbon dioxide, but about 91% of energy contained in glucose (2.872 MJ/mol) is retained in ethanol. Therefore, ethanol fermentation is an excellent biological process to convert biomass to liquid fuel ethanol. Yeast cells were first isolated from beer as pure cultures in 1883 in Denmark and a great deal of work were carried out on their metabolic pathway of ethanol fermentation. S. cerevisiae can ferment many sugars including glucose, fructose, galactose, mannose, sucrose, maltose, except pentoses like xylose and arabinose. Pichia stipitis and Pachysolen tannophilus are known as yeasts capable of fermenting pentoses, but they are not so tolerant to ethanol as S. cerevisiae is. Strain improvement research to construct strains of S. cerevisiae having pentose fermentability is undergoing in many laboratories.

Next to S. cerevisiae, Zymomomas mobilis is excellent bacterium to ferment a limited range of sugars of glucose, fructose and sucrose to ethanol. Fermentation yield and fermentation rate of Z. mobilis are supposed to be better than those of yeast S. cerevisiae, but Z. mobilis is not so tolerant to ethanol as S. cerevisiae is. Zymobacterpalmae, isolated in 1980’s in Japan, has ethanol fermenting ability similar to that of Z. mobilisand its genome base sequence has been determined recently. Strain improvement of Z. mobilis and Z. palmae regarding pentose and mannose fermentation has been successfully made in Japan. Pentoses are contained at relatively high concentration in hard woods and herbaceous plants, and mannose is a characteristic component of soft woods.

DNA recombinant strains of Escherichia coli and Corynebacterium glutamicum having ethanol fermenting ability have been constructed through biotechnology. Other ethanol fermenting bacteria like hetero-lactic acid bacteria (Lactobacillus), cellulose degrading Clostridium bateria, and anaerobic thermophilic bacteria Themoanaerobacterhave been known so far, but they can produce ethanol at relatively low concentration and with byproducts like organic acids. Therefore, these bacteria are considered difficult for the industrial use at least for the present.