Effect of dilute acid pretreatment conditions and washing on the production of inhibitors and on recovery of sugars during wheat straw enzymatic hydrolysis

Pretreatment is an essential process to break down recalcitrant biomass and dilute acid hydrolysis is one of the most efficient and cost effective pretreatment technologies available today. However there are potential disadvantages in using dilute acid as a pretreatment, such as the production of degradation products, which inhibits the ensuing processing chain and limits its adoption. In this work, wheat straw was pretreated under varying dilute acid conditions; the resulting degradation products were determined and the quality of sugar stream generated via enzymatic saccharification was monitored. The dilute acid pretreatment conditions were: temperatures of 140 and 160 °C, sulfuric acid concentrations of 5, 10 and 20 dm3 m−3 and reaction times of 10, 20, 30, 45 and 60 min. Pretreated wheat straw was washed with six dilutions of water and hydrolyzed with commercial cellulase enzymes for 24–48 h. Optimal conditions for pretreating wheat straw were determined as: 140 °C, 10 dm3 m−3 sulfuric acid concentration and a 30 min reaction time. At these conditions, the glucose yield from wheat straw was maximized at 89% of the theoretical maximum, while the concentrations of formic acid, furfural, acetic acid and 5-hydroxymethylfurfural were 32.37 ± 4.91, 12.08 ± 1.69, 7.98 ± 1.02 and 1.14 ± 0.22 g kg−1, respectively. Increases in pretreatment severity led to increases in inhibitor generation, as well as a 27% reduction in monosaccharide yield. Rinsing with deionized water was effective in removing inhibitors, such as 86% of furfural. The formation of inhibitors was thus observed to depend on dilute acid pretreatment conditions.