Antioxidant activities of modified spent coffee extracts in different lipid media

Natural phenolic antioxidants are considered promising alternatives to synthetic antioxidants; however, their application in lipid-based systems is often limited by their low lipophilicity and poor compatibility with non-polar media. Although several chemical modification strategies have been proposed to overcome this limitation, a systematic comparison of their effects on antioxidant performance across different lipid matrices remains lacking. This study investigated the influence of molecular hybridization, methylation, and decarboxylation on the structure, lipophilicity, and antioxidant activity of phenolic extracts obtained from spent coffee grounds. The modified extracts were characterized by total phenolic content (TPC), DPPH radical scavenging activity, UHPLC phenolic profiling, FTIR spectroscopy, and octanol/water partition coefficient (logP), followed by evaluation in flaxseed oil, oil-in-water emulsion, butter, and linoleic acid model systems using peroxide value, UV-specific absorbance (K 232 and K 270 ), rancimat induction time, DPPH, and β-carotene bleaching assays. Structural modification increased the extract’s lipophilicity by reducing hydroxyl groups and enriching hydrophobic moieties, while chlorogenic acid derivatives remained the dominant phenolics despite undergoing partial transformation. Antioxidant performance depended strongly on the lipid matrix. Molecularly hybridized extracts exhibited the greatest oxidative stability in emulsions and butter, whereas the effectiveness of modified extracts in highly unsaturated lipid systems was more variable; in some cases, the modified extracts exhibited pro-oxidant behavior. These findings demonstrate that improving lipophilicity alone is insufficient to predict antioxidant performance, which is governed by the compatibility between the phenolic structure and the lipid matrix. The study provides a systematic framework for designing matrix-specific natural antioxidants from spent coffee grounds through targeted chemical modification.