Enzymatic Hydrolysis Technologies for the Production of Biofuels

Abstract

One of the major challenges in second-generation biofuel production is economical conversion of lignocellulosic biomass to fermentable sugars. The most commercially feasible method for conversion of biomass to fermentable sugars has been the combination of thermochemical and enzymatic hydrolysis treatment of biomass. Nevertheless, even with the most efficient pretreatment method, the use of cellulolytic enzymes accounts for more than half of the sugar production cost. As a result of the high cost of commercial hydrolytic enzyme and the low digestibility of pretreated biomass with minimal enzyme use, sugar production costs are not economical for commercial production of some fermentation products. One of the primary reasons for low digestibility of biomass at minimal enzyme doses is the limited enzyme accessibility to cellulose due to the presence of lignin. Other reasons include (1) the change in reactivity of cellulose during hydrolysis that occurs when amphiphilic substance is depleted and (2) deactivation of enzyme by sugars, sugar degradation products, and both soluble and insoluble lignin. With current pretreatment technologies, commercially relevant methods must be developed that would improve the performance of enzymes and make the application of lower enzyme doses feasible. An attractive solution is to generate on-site enzymes to allow for the utilization of cheap abundant protein activity in-house. This chapter reviews the cellulolytic enzyme system, the mechanism of action, rate-limiting factors of enzymatic hydrolysis, on-site enzyme production, and recovery of enzyme activity by enzyme recycling.