Although many new strategies have developed for cost-effective conversion of lig- nocellulosic biomass, there is still a need for improvements for making biorefineries economically viable. The amount and cost of hydrolytic enzymes required to efficiently degrade lignocellulosic biomass is a critical part of the economics of feasible bioethanol production processes. Although commercial enzyme producers have been involved in intensive research with many novel technologies to develop enzyme cocktail with better hydrolytic efficiency, information on a clear composition and amount of enzyme cocktail required for complete hydrolysis of lignocel- lulose is lacking. Moreover, Banerjee et al. (2010a) argue that there is a presence of many nonspecific enzymes which are not essential for lignocellulose degradation in commercial mixtures and removal of those enzymes could increase the specific activity and thus lower the enzyme cost. However, no direct cost comparisons have been performed to evaluate whether combinations of individual enzymes would be cheaper than commercial mixtures. More research focus is required to remove the techno-economic barriers of efficient enzymatic hydrolysis. Following are major challenges that need to be addressed for optimizing the sustainable and economic biofuel production:
- • The process of enzymatic hydrolysis of lignocellulosic biomass with expensive commercial enzymes needs replacement with the inexpensive crude enzymes produced on-site with higher catalytic efficiency by using cheap fractions from the biorefinery.
- • The selection between customized enzyme cocktails with individual enzymes or enzyme mono-components and commercial crude mixtures of enzymes.
- • The development of robust microbial strains that are tolerant to elevated process parameters and inhibitors present in biomass. These developed strains should be able to efficiently degrade all sugars available in the biomass into concentrated hydrolysate with high productivity.
- • The development of approaches for high throughput screening for enzyme systems with optimized compositions.
- • A strategy for the process integration (CBP) to reduce the complexity by reducing the number of steps involved in overall production process.
• The deployment of 3-R strategy: reduction, recycling, and reuse of any by-products and wastes generated in the process to reduce the energy and cost and save the environment.
Continuous efforts made by leading enzyme companies such as Novozymes and Genencor and new developments in biomass processing established by major players in the market such as DuPont (http://biosciences.dupont.com/) and POET-DSM (http://www.dsm.com/) indicated the significant developments in the area at industrial scale. Published literatures and patents from research institutions show the hard work in the creation of robust enzyme-producing microorganisms with thermotolerance, cellulosome production, protein engineering, consolidated bio-processing viable organism, use of biorefinery materials as carbon source for microorganisms’ growth, etc. Evaluation of current challenges and potential strategies to mitigate them provides us with a number of promising approaches to build an efficient and cost-effective bioconversion processing system.