Integrated Valorization of Rice Husk by Pressurized Liquid Extraction: Phenolic-Rich Extract Recovery, Reduced 5-HMF Formation, and Preservation of a Cellulosic Co-Product

Abstract

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.