Integrated Valorization of Rice Husk by Pressurized Liquid Extraction: Phenolic-Rich Extract Recovery, Reduced 5-HMF...

Rice husk, a by-product of rice processing, is a natural source of phenolic compounds and cellulose. Although pressurized liquid extraction (PLE) is an efficient technique for recovering antioxidant compounds, its application to rice husk using water-ethanol mixtures and its effect on the chemical and structural properties of the resulting post-extraction lignocellulosic residues remain unexplored. Thus, this study evaluated water-ethanol mixtures (0-60%) at 90-150 °C under PLE conditions. The process was assessed in terms of recovery of antioxidant compounds, co-extraction of reducing sugars, and formation of 5-hydroxymethylfurfural (5-HMF), along with structural and chemical characterization of the post-extraction residue. The highest total phenolic content (3.23 mg GAE/g dry weight) and antioxidant capacity (88.05 μmol TE/g dry weight) were obtained with 60% ethanol at 150 °C, significantly outperforming water-only extraction. The concentrations of gallic acid, caffeic acid, and vanillin in the extracts increased under these conditions. Ethanol played a dual role, acting as an antisolvent that reduced sugar co-extraction while simultaneously suppressing 5-HMF formation. SEM images of the post-extraction residue revealed cell wall disruption at higher temperatures and ethanol concentrations, consistent with the release of phenolic compounds. XRD and FTIR confirmed that the native cellulose I structure and characteristic functional groups were preserved across all extraction conditions. This approach supports the integrated valorization of rice husk by simultaneously producing phenolic-rich extracts and a structurally preserved cellulosic co-product.