2-42: The effect of temperature and hemicellulose removal on enzymatic digestion of cellulose for pretreatment of switchgrass with dilute acid followed by hot water

Various factors are considered to contribute to biomass recalcitrance, with an example being the intricate hemicellulose -lignin network that surrounds cellulose microfibrils and restricts access by enzymes. Dilute acid pretreatment has advantages for selectively removing hemicelluloses with high yields and increasing cellulose digestibility in the resulting solids but faces costly materials of construction and other challenges.  It remains unclear as to the relative importance of removing or altering lignin versus hemicellulose in improving enzymatic hydrolysis of biomass solids following pretreatment.  In this study, switchgrass was pretreated in two stages. A high throughput pretreatment and co-hydrolysis system was first applied to screen different combinations of acid loadings and temperatures in each stage. Then, conditions of 1% w/w acid loading over a range of 80 to 120^oC in Stage one followed by reaction of the washed solids at 160 and 180^oC with just water in Stage two were selected to further explore how enzymatic digestion of cellulose changed with hemicellulose removal for different histories of temperatures and acid concentrations.  Enzymatic hydrolysis of solids from each stage was compared in terms of reactivity, digestibility, release of oligomers, and relationships with hemicellulose removal.  It was found that glucose yields from enzymatic hydrolysis of cellulose in the pretreated solids differed significantly with the conditions applied for similar hemicellulose removal.  Furthermore, pretreatment at higher temperatures in the second stage significantly increased cellulose digestibility even though no obvious change in composition was apparent.  The effect of different pretreatment temperatures and their impact on recalcitrance was evaluated.