Simultaneous Optimization of Alkaline and Acid–Alkaline Pretreatments Applied to Rice Straw to Produce Glucose Correlated with Chemical and Morphological Effects

Rice straw is a relevant and potential feedstock for bioethanol production due to its abundance and availability around the globe. In this study, a fractional factorial design (FFD 2) was applied to simultaneously evaluate the influence of only alkaline and acid-alkaline pretreatment conditions in glucose release, considering the glucose release after 12 and 24 h of enzymatic hydrolysis as responses and predicting alternatives for the fractionation of rice straw components. Hydrolysis yields (HY) higher than 90% were achieved using low enzyme loads (8 FPU/g substrate) after only 24 h of hydrolysis under optimized pretreatment conditions. Simultaneous DOE optimization showed that the acid step is optional to achieve higher HY but can contribute to a more holistic use of the hemicellulose fraction. Also, it significantly increased the hydrolysis efficiency compared to untreated rice straw (HY = 18%). Enzymatic hydrolysis with a different type of enzymatic cocktail in the optimized conditions using higher solid contents resulted in high cellulose conversion (up to 85%), showing the robustness of the DOE optimization and its applicability as a starting point for optimizations using other types of enzymes. Chemical and morphological analyses were also carried out to understand the effect of the treatments, aiming to achieve pretreatment and hydrolysis processes more effective for lignocellulosic biorefineries. Graphical Abstract: (Figure presented.)