Conversion of simple hexose sugars, such as glucose and mannose, in fermentation into ethanol can take place anaerobically as follows:

C6H12O6 (Hexoses) Micr°°rganisms> 2C2H5OH (ethanol) + 2CO2

If the entire sugar is converted into ethanol according to the above reac­tion, the yield of ethanol will be 0.51 g/g of the consumed sugars, mean­ing that from 1.0 g of glucose, 0.51 g of ethanol can be produced. This is the theoretical yield of ethanol from hexoses. However, the ethanol yield obtained in fermentation does not usually exceed 90-95% of the theoretical yield, since part of the carbon source in sugars is converted to biomass of the microorganisms and other by-products such as glycerol and acetic acid [9, 31].

A similar reaction for anaerobic conversion of pentoses, such as xylose to ethanol, might be considered. Xylose is generally converted first to xylulose by a one-step reaction catalyzed by xylose isomerase (XI) in many bacteria, or by a two-step reaction through xylitol in yeasts and fungi. It can then be converted to ethanol anaerobically through a pentose phosphate pathway (PPP) and glycolysis. The general reaction can be written as

3C5H10O5 (Pentoses)———- ii——— * 5C2H5OH (Ethanol) + 5CO2

In this case, we can expect a theoretical ethanol yield of 0.51 g/g from xylose, as we had from glucose. However, the redox imbalance and slow rate of ATP formation are two major factors that make anaerobic ethanol production from xylose very difficult [32, 33]. A few anaerobic ethanol — producing strains have been developed from xylose in research labora­tories, but no strain is so far available for industrial-scale processes. Attempts have been made to overcome this problem of xylose assimila­tion by cometabolization or working with microaerobic conditions, where oxygen is available at low concentrations. A number of microorganisms can produce ethanol aerobically from xylose, where the practical yield of ethanol from xylose and other pentoses is usually lower than its the­oretical yield. The challenges in ethanol production from xylose have been reviewed by van Maris et al. [34].