Enzymatic Hydrolysis of Cellulose and Hemicellulose
In the production of cellulosic ethanol via cellulolysis process it is essential to hydrolyze the lignocellulosic biomass to fermentable sugars before the biochemical conversion to ethanol. This hydrolysis process is also known as saccharification, and during this step major components in the biomass, cellulose and hemicellulose are depolymerized to glucose and xylose respectively. In addition to this, other minor sugars are also produced in the hydrolysis step. If lignin is not removed in the pretreatment step, lignin remaining in the pretreated biomass is unaffected in the saccharification step. The lignin fraction usually precipitates during the saccharification and can be separated from the sugar solution by filtration or centrifugation. There are three basic routes to produce ethanol through the aqueous phase biomass hydrolysis or cellulolysis process as shown in Figure 6.1, and these routes are discussed as separate sections in Part 2 of this book. The hydrolysis step is a critical step in the cel — lulosic ethanol process and is one of the major hurdles in the realization of economical and large-scale cellulosic ethanol production.
In the first route, pretreated biomass is subjected to enzymatic hydrolysis to give fermentable sugars, which are then fermented to produce ethanol; the enzymatic hydrolysis step will be discussed in this chapter. This path is known as separate hydrolysis and fermentation (SHF). The second route is where the pretreated biomass is subjected to a cocktail of enzymes that can affect hydrolysis and fermentation in one pot; this method is commonly known as simultaneous saccharification and fermentation (SSF). The SSF technique will be discussed in Chapter 8. The third route is the direct acid hydrolysis of untreated biomass, which after neutralization, the sugars are subjected to fermentation to give ethanol; this direct hydrolysis step will be discussed in Chapter 7.