After the pre-treatment and the degradation stage that releases the sugar units it is possible to convert the carbohydrates to ethanol by the technique of fermentation. Baker’s yeast (Saccharomyces) is most commonly used and is able to convert glucose to ethanol under both aerobic and anaerobic conditions. During the fermentation process, two moles of carbon dioxide and two moles of ethanol are produced from one mole of a sugar unit. In order to achieve optimal fermentation there are several characteristics of the fermenting microorganisms that should be considered, e. g. temperature range, pH range (3.5-5.0 for yeast, 6.5-7.0 for bacteria), alcohol tolerance, growth rate, genetic stability, inhibitor tolerance, yield, etc. (Bai, Anderson, and Moo-Young, 2008). The fermentation process can occur either in separate batches or as a continuous process which is often more preferable economically (Sanchez and Cardona, 2008).

Distillation and purification

During the fermentation process it is important to separate the produced ethanol from the original liquid since several microorganisms are not able to survive the high concentration of ethanol (more than 15-20%). The remaining liquid contains ethanol and water (about 80%) and other soluble compounds and ethanol can be separated by distillation or supercritical fluid technology (Schacht, Zetzl, and Brunner, 2008). Unfortunately the distillation requires a lot of energy to obtain 95.6% ethanol (azeotrope mixture of ethanol and water). In the next step the ethanol is further purified (99%) by adding a drying agent to the solution or by molecular sieve adsorption. But the 99% ig (industrial grade) ethanol is hygroscopic and may absorb water again from the surrounding air during storage. The purification (i. e. dehydration) steps are necessary since the ethanol/gasoline blend will separate in the presence of water and is difficult to remix (Szulczyk, McCarl, and Cornforth, 2010).