An introduction to the special section on application of leading pretreatments to switchgrass by the Biomass Refining Consortium for Applied Fundamentals and Innovation (CAFI).

Pretreatment is among the most costly steps in the overall biological conversion of cellulosic biomass to fuels and chemicals. Pretreatment also has a profound effect on all other operations from choice and preparation of feedstock all the way to final product recovery and use of process residues. Yet, because cellulosic biomass is naturally resistant to breakdown to release the sugars contained in its cellulose and hemicellulose components, pretreatment is vital to preparing cellulosic biomass for biological conversion to sugars with high yields, and the economics become even worse if it is left out of the process. Thus, this conundrum led to the statement that ‘‘the only operation more expensive than pretreatment is no pretreatment.’’ Over the years, numerous biological, chemical, and physical pretreatment approaches have been tried to prepare a wide range of plants for enzymatic hydrolysis followed by fermentation of the sugars released. However, the variety of biomass substrates, enzymes, enzyme loadings and formulations, and reaction conditions applied coupled with significant differences in definitions and approaches to reporting data made it very difficult to compare results among pretreatments. Nonetheless, it became evident that thermochemical approaches in particular, did an effective job of making biomass susceptible to enzyme attack with high yields of sugars. Furthermore, some pretreatments that appeared to be successful in achieving high yields from the coupled operations of pretreatment and enzymatic hydrolysis employed dilute acid while other pretreatments that applied dilute alkali appeared to be equally successful. Such a wide pH range resulted in removal of different portions of biomass, confounding simple assignment of a fundamental mechanism that could account for pretreatment performance. For example, low pH pretreatment with dilute sulfuric acid removed most of the hemicellulose from biomass, while pretreatment at high pH with lime removed much more lignin and left a large portion of hemicellulose in the pretreated solids. Thus, a better understanding of how pretreatment impacted enzymatic hydrolysis was also needed to tie together the different pretreatment technologies and support definition of pretreatments that could perform better while reducing costs. Against this background, a group of leaders in development of pretreatment technology came together in late 1999 and early 2000 with the goal of applying competing pretreatment technologies to a common source of feedstocks and sharing the same enzymes. Furthermore, the team sought to apply identical methods to analyze compositions of all biomass streams. In addition, proto-