Recalcitrance: Fundamentals and Progress

Overcoming lignocellulosic biomass recalcitrance followed by enzymatic hydrolysis of structural polymeric carbohydrates (i.e., cost-efficient liberation of fermentable sugars) is perhaps the most challenging technical and economic barrier to success of biorefineries [1–3]. Lignocellulosic biomass is a natural composite having three main biopolymers (cellulose, hemicellulose, and lignin) intertwined chemically and physically. The central role of biomass pretreatment has been indicated by the fact that 12 oral presentations and 76 posters were given on the subject at this symposium. Pretreatment is among the most costly steps in biochemical conversion of biomass [4, 5], accounting for up to 40 % of the total processing cost [6]. Also, it affects the costs of other operations including size reduction prior to pretreatment and enzymatic hydrolysis and fermentation after pretreatment. Pretreatment can also strongly influence downstream costs involving detoxification if inhibitors were generated, enzymatic hydrolysis rate and enzyme loading, mixing power, product concentration, product purification, power generation, waste treatment demands, and other process variables [4]. Although a number of pretreatment methods have been proposed and investigated on th