Extraction, characterization and antioxidant activity of free and bound polyphenols from Undaria pinnatifida

Free polyphenols (FP) and bound polyphenols (BP) were extracted from Undaria pinnatifida, with processes optimized by response surface methodology. BP accounted for approximately 65% of total polyphenols. UHPLC-QTOF-MS/MS tentatively identified 28 phenolic compounds in BP and 3 in FP, with characteristic brown algal polyphenols, such as eckol-type phlorotannin derivatives and highly oxidized phlorotannin dimers, exclusively detected in BP. They showed stronger DPPH radical scavenging capacity than FP, even surpassing tea polyphenols at 3.5 mg/mL. These findings demonstrate that alkaline extraction is essential for accessing polyphenolic resources otherwise inaccessible through ethanol extraction, and BP possess potential as natural antioxidants. This study may provide a reference for the comprehensive utilization of UP polyphenolic resources.