Unlocking the Potential of Lignocellulosic Biomass: Microwave and Hydrothermal Pretreatment to Improve the Production of High Value-Added Biorefinery Compounds

This study is focused on the performance of a hydrothermal reactor (HTR) and microwave-assisted (MW) pretreatments of sugar beet pulp (SBP), orange peel (OP), brewer spent grain (BSG), and rice husk (RH) to evaluate the extraction of high-value biorefinery compounds. The influence of temperature, duration of treatment, and energy consumption on hydrolysis efficiency was evaluated by quantifying total reducing sugars (TRS), proteins (PR), polyphenols (TP), and volatile fatty acids (VFA). MW pretreatment at 180 °C for 30 min yielded 18% TRS and 24% PR from OP, respectively. In contrast, HTR at 200 °C, for 60 min, achieved higher yields of 32% TRS and 22% PR for OP. BSG showed higher responsiveness under HTR, reaching 25% TRS and 20% PR at 220 °C after 120 min. The highest VFA production was 16 g H-Ac/L (BSG, HTR) and 3.2 g H-Ac/L (SBP, MW) after 120 and 5 min at 220 °C, respectively. From the point of view of energy consumption, MW pretreatment consumed significantly less energy (40.1 kJ/g) than HTR (70.85 kJ/g) under equivalent conditions (120 min at 220 °C). In addition, the MW pretreatment proved to be more energy-efficient for simpler substrates (SBP, OP), whereas HTR was optimal for complex biomasses (BSG, RH). Therefore, tailored pretreatment strategies based on substrate type are crucial to optimize energy consumption and maximize bioproduct recovery.