Modification of the Polyphenolic Profile and Enhancement of Antioxidant Activity of Waste Orange Peel Extracts Using Alkali-Catalyzed Ethanol Organosolv Treatment

Orange processing generates large amounts of waste orange peels (WOPs), which are a valuable source of bioactive polyphenols. This study investigated the use of mild (urea) and strong (sodium hydroxide) alkaline catalysts to enhance polyphenol extraction via an ethanol-based organosolv process. First, the two catalysts were evaluated in terms of process performance, extraction kinetics, and treatment severity. Subsequently, response surface methodology was applied to optimize the conditions, and the obtained extracts were characterized for their polyphenolic profile and antioxidant activity. The sodium hydroxide (SoHy)-catalyzed treatment, using 70% ethanol as solvent, was the most effective, yielding 33.4 ± 1.7 mg of total polyphenols (as gallic acid equivalents) per gram of dry mass. For both catalysts tested, the yield followed a severity-dependent linear model. Liquid chromatography–mass spectrometry of extracts produced under optimized conditions showed that hesperidin was the predominant polyphenolic constituent, but the SoHy-catalyzed treatment resulted in the generation of three novel compounds, tentatively identified as ethyl esters of p-coumaric, ferulic and sinapic acids. Such an effect was not observed in the extracts produced with the urea (Ur)-catalyzed treatment. This compositional modification was reflected on both the antiradical activity and ferric-reducing power, which were found to be significantly enhanced in the extracts produced via the SoHy-catalyzed treatment. These findings highlight how treatment conditions can be tuned to modify the polyphenolic composition of WOP extracts and reinforce antioxidant activity. Such insights could support the development of WOP valorization strategies within integrated biorefineries.