After pretreatment, hydrolysis converts the carbohydrate polymers into monomeric sugars. Although a variety of process configurations have been studied for conversion of cellulosic biomass into ethanol, enzymatic hydrolysis of cellulose provides opportunities to improve the technology so that biomass ethanol is competitive with other liquid fuels(Wyman, 1999). Novozymes (www. novozymes. com) and Genencor (www. genencor. com) are two companies leading research & development for advanced cellulosic ethanol enzymes. In early 2010, Novozymes said its new Cellic® CTec2 enzymes enable the biofuel industry to produce cellulosic ethanol at a price below US$ 2.00 per gallon for the initial commercial — scale plants that are scheduled to be in operation in 2011. This cost is on par with gasoline and conventional ethanol at current US market prices. According to Novozymes, the new enzyme can be used on different types of feedstock including corn cobs and stalks, wheat straw, sugarcane bagasse, and woodchips. The enzyme is designed to break down cellulose in biomass into sugars that can be fermented into ethanol. Genencor, a division of Danisco also introduced its enzyme Accellerase®, which is designed to do the same thing.

The selection of the enzymes needs to match the pretreatment technologies and the feedstock used, as well as the process. For example, if a dilute acid pretreatment is used, most of the hemicellulose is degraded, so hemicellulases is unnecessary. However, if an alkaline or hot-water pretreatment is used, the hemicellulose still needs to be hydrolyzed and hemicellulases will be needed.

The cellulose portion of the biomass is another difficulty. In order to efficiently break it down, a mixture of several enzymes with different activities is required. This mixture includes three basic types of enzymes.

1. Endoglucanases break bonds between adjacent sugar molecules in a cellulose chain, fragmenting the chain into shorter lengths. Endoglucanases act randomly along the cellulose chain, although they prefer amorphous regions where the chains are less crystalline.

2. Cellobiohydrolases attack cellulose chains from the ends of the chain. This exo — or processive action releases mainly cellobiose (glucose dimer). Because endoglucanases create new ends for cellobiohydrolases to act upon, the two classes of enzymes interact synergistically.

3. P-glucosidases break down short glucose chains, such as cellobiose, to release glucose. P-glucosidases are important as they act on cellobiose, which inhibits the action of the other cellulases as it builds up the hydrolysis reactor.