Michael E. Himmel and Stephen K. Picataggio

1.1 The modern lignocellulose biorefinery

Alternative and renewable fuels derived from lignocellulosic biomass offer the potential to reduce our dependence on imported oil, support national economic growth, and miti­gate global climate change (1, 2). However, breakthrough technologies are still needed to overcome barriers to developing cost-effective processes for converting biomass to fuels and chemicals. These needed breakthroughs include improved pretreatment processes that boost the yield of fermentable sugars while minimizing the formation and release of toxic byprod­ucts; low-cost cellulases that hydrolyze crystalline cellulose; and microbial biocatalysts that enable rapid and efficient fermentation of the mixed sugars in cellulosic hydrolysates (3).

We also understand that feedstock costs will be a major component of the commodity end-product cost of biomass-derived liquid fuel products, such as ethanol and butanol. Therefore, perhaps the highest near-term priority is boosting the yield of lignocellulose — derived sugars. Yield issues touch many other critical biorefinery operations, including particle size reduction, pretreatment and detoxification, solids/liquid separation, enzyme hydrolysis, and fermentation of sugars to products.

It is now apparent that new process scenarios are also important for ensuring the success of future energy biorefineries. For example, the consolidation of existing process schemes may deliver significant economic and technical advantages. The well-known direct micro­bial conversion process proposed that a single microorganism could produce the cellulase enzymes and ferment sugars released from biomass to ethanol in high concentrations. Such a strain does not exist today, but could be constructed with suitable acquisition of a new, deeper understanding of various critical metabolic and enzymatic processes occurring in selected bacteria and yeast. Novel microbes may also allow a staged process to optimize these steps separately.