Effects of hot water extraction (HWE) of Douglas fir as a pre-process for the sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL)

Abstract

Abstract The process named “sulfite pretreatment to overcome recalcitrance of lignocellulose” (SPORL) is designed for the digestion of softwoods within the framework of the biorefinery concept. “Hot water extraction (HWE)” is an environmentally benign and low-cost pretreatment, which only needs water as a reagent. In the present study, HWE has been investigated as a pre-process prior to SPORL with Douglas fir as feedstock. The SPORL parameters, namely, temperature and treatment time, were in the range 135–155°C and 40–120 min, respectively, while the sulfuric acid concentration was 0.2–0.4% (v/v). The aim was to maximize the enzymatic digestibility of the treated wood. The severity of SPORL at different pretreatment conditions was characterized by the combined severity factor (CSF). The HWE pre-process led to a two-fold increase in specific surface area of the substrate. More hemicellulosic-derived simple sugars were dissolved in the spent liquor (SL) as the CSF increased from 1.23 to 1.82. A maximum enzymatic digestibility of 64.3% was attained when SPORL was conducted at 155°C for 120 min with a sulfuric acid concentration of 0.4% (v/v). A considerably high enzymatic digestibility (~55–60%) is still achievable by incorporating HWE prior to SPORL, even if the SPORL severity is reduced, namely to a lower temperature (145°C), a shorter time (80 min), and a lower acid volume (0.3% v/v).