Towards Increasing the Productivity of Lignocellulosic Bioethanol: Rational Strategies Fueled by Modeling

Abstract

Bioethanol is not only currently the most widely used biofuel, but also potentially the most promising alternative to fossil fuels. The majority of bioethanol in today’s use is made from sucrose-containing (e.g., sugarcane, sugar beet, and sweet sorghum) or starch-based feedstocks (e.g., corn, wheat, rice, barley, and potatoes). The excessive production of such crop-based (first generation) bioethanol, however, imposes an adverse effect on global food supply. A sustainable alternative feedstock which can be used for non-crop (second generation) bioethanol is lignocellulosic biomass such as rice straw (Binod et al., 2010), wheat straw (Talebnia et al., 2010), corn stover (Kadam & McMillan, 2003), switchgrass (Keshwani & Cheng, 2009), sugarcane bagasse (Cardona et al., 2010), and various other agriculture and forest residues. Lignocellulose primarily consists of cellulose, hemicellulose and lignin. Cellulose is a homopolymer of glucose, while hemicellulose is a heteropolymer of pentoses (i.e., xylose and arabinose) and hexoses (i.e., glucose, mannose, and galactose) sugars. Lignin is a rich source of aromatic carbon compounds but extremely recalcitrant. Lignocellulose is decomposed via pretreatment and hydrolysis into a spectrum of sugars in which glucose and xylose are the first and second most dominant. These cellulosic sugars are finally converted to bioethanol by fermentation. The lignocellulosic bioethanol has not yet been produced on a commercial scale due to lack of cost-effectiveness. For ensuring its economical viability, comprehensive efforts are required to reduce cost (and maximize the profit) throughout the entire process from biomass to bioethanol. In the current discussion, we limit ourselves to the fermentation step only and examine various issues with increasing bioethanol productivity. Cost-benefit analysis of the fermentation process shows that the processing cost is more dominant (two-thirds of the total cost) than the feed cost (Lange, 2007; Wingren et al., 2003). It is thus important to improve the processing efficiency, not just the sugar conversion alone. In this regard, increasing the productivity should be a preferred target over increasing the yield, not only in the reactor optimization, but also in strain improvement. The yeast Saccharomyces cerevisiae has typically been used for the production of crop-based bioethanol. This wild-type strain is, however, not suitable for converting cellulosic sugars as it can efficiently ferment glucose but hardly xylose. Considerable effort has been made to