Ultrasound-Assisted Acid-Catalyzed Isopropanol Organosolv Treatment of Grape Stems Induces Alterations in the Polyphenolic Composition and Antioxidant Activity

Grape stems (GSs), a key by-product of winemaking, are a rich source of bioactive polyphenols and have potential applications in functional foods, antioxidant additives, and cosmetics. In this study, the production of polyphenol-rich extracts from GS using an ultrasound-assisted, acid-catalyzed organosolv process was explored, using isopropanol as solvent. Sulfuric acid served as an effective catalyst for breaking down lignocellulosic material, and the process conditions were optimized using response surface methodology to maximize efficiency. Following an initial single-factor analysis, sulfuric acid and isopropanol concentrations were selected as the main variables. The optimal conditions—54% isopropanol and 1% sulfuric acid—yielded the highest polyphenol content (50.9 ± 4.4 mg caffeic acid equivalents per gram of dry mass). Further analysis using liquid chromatography–mass spectrometry revealed that acid catalysis significantly altered the polyphenolic composition of the extracts. These changes enhanced antioxidant potency, as expressed by the radical scavenging activity but not by the reducing power, while the exact nature of the newly formed compounds was unclear. Overall, the results provide useful insights into modifying the polyphenolic profile of grape stems to obtain extracts with improved antioxidant capacity, supporting more sustainable and value-added uses of winemaking by-products within biorefinery systems.