Combined steam-explosion toward vacuum and dilute-acid spraying of wheat straw. Impact of severity factor on enzymatic hydrolysis

This study deals with the development of an eco-friendly hydrothermal pretreatment of wheat straw. This pretreatment involves moderate temperatures without generation of liquid fractions allowing to reduce the energy input consumption and the environmental impact and enhance the enzymatic hydrolysis. For this purpose, the synergistic effect of impregnation of wheat straw by dilute sulfuric acid and DIC (in french: Détente instantannée Contrôlée) pretreatment on enzymatic hydrolysis has been investigated. DIC process involves subjecting the lignocellulosic biomass to saturated steam pressure (0.3–0.7 MPa), followed by a sudden decompression toward vacuum (5 kPa). The optimization of processing conditions was carried out using a full-factorial design, in respect to processing temperature (133–165 °C), residence time (5–40 min) and sulfuric acid concentration (0.70–2.20%). These processing conditions were converted into a single combined severity factor (CS), relating pH, temperature and residence time of pretreatment, ranged from −2.22 to 1.25. The efficiency of proposed pretreatment was measured through the enzymatic digestibility of lignocellulose and the responses parameters of experimental design were the monomeric glucose and xylose. The most influential factor on biomass bioconversion was sulfuric acid concentration followed by temperature and processing time. The optimum values were 2.2%, 165 °C and 40 min, for acid concentration, temperature and residence time, respectively. Strong correlations were observed between the enzymatic hydrolysis and conditions of pretreatment through the analysis of CS. The intense conditions have contributed to disrupt the biomass structure make it less dense leading to a higher specific surface area (ABET). Thus, increasing in accessibility of cellulose to enzymes has improved the rate and yields of bioconversion.