Valorisation of Agri-Food Residues: Comparative Analysis of Phenolic Composition, Antioxidant Activity and Extraction Methods

Phenolic compounds are recognized for their antioxidant properties and potential application in sustainable functional materials. In this study, the phenolic composition and antioxidant activity of four agro-industrial residues—rice husk, spent hops, and oregano and thyme stems—were evaluated to examine the influence of the extraction method and plant matrix on bioactive compound recovery. Extracts obtained by ultrasound-assisted extraction (UAE) and conventional water bath (WB) extraction were characterized using spectrophotometric assays for total phenolic content (TPC) and antioxidant activity (DPPH and ABTS), as well as HPLC analysis for phenolic profiling. Oregano and thyme stem extracts exhibited the highest phenolic content and antioxidant activity, whereas rice husk extracts showed the lowest concentrations of detectable phenolics. Hydroxycinnamic acids, particularly p-coumaric and ferulic acids, predominated across the studied matrices. Multivariate analysis indicated that matrix composition had a greater influence on phenolic profile and antioxidant behaviour than extraction method, with UAE providing comparable results to those obtained by WB extraction while requiring a substantially shorter extraction time. Pearson correlation analysis further suggested that antioxidant activity results from the combined interaction of multiple phenolic compounds rather than isolated constituents. Overall, these residues show potential as sustainable sources of phenolic-rich extracts for functional material applications.