Multi-Cycle steam explosion combined with subcritical water Hydrolysis: An integrated process for maximizing sugar yields from rice husk

This work investigated the valorization of rice husk (RH) through an integrated steam explosion (SE) and subcritical water hydrolysis (SWH) process, aiming to maximize the production of fermentable sugars while minimizing the formation of undesirable inhibitory molecules. The biomass was subjected to successive explosion cycles (1–3 cycles, 180 °C, 5 min each) followed by SWH (230 °C or 260 °C, S/F-80), with kinetic monitoring of the release of sugars, organic acids, and inhibitors. The results demonstrated that three cycles of SE combined with SWH at 230 °C represented the optimal condition, reaching 40.68 g of fermentable sugars/100 g RH – a 12x increase compared to the untreated biomass. This condition also drastically reduced the formation of inhibitors (HMF and furfural <0.6 g/100 g RH) and organic acids (Y_OA = 5.83 g/100 g RH), with maximum sugar production occurring between 6–10 min of hydrolysis (5.4 g FS/100 g min). In contrast, SWH at 260 °C led to higher thermal degradation, especially after multiple explosions, reducing yields. The optimal hydrolysis time was established at 20 min, after which sugar production rates approached zero. Sequential pretreatment with 3 cycles of SE followed by SWH at 230 °C for 20 min is the most efficient strategy to convert rice husk into fermentable sugars, offering operational and environmental advantages compared to conventional methods. These findings open perspectives for application in biorefineries, highlighting the potential of RH as a sustainable feedstock.