Comparison of thermal and non-thermal extraction methods on free and bound phenolics in pomegranate peel

Pomegranate peel is a valuable byproduct for phenolic extraction. However, comprehensive analysis on novel extraction methods is lacking. In this study, free and bound phenolics, extracted by thermal (heating) and non-thermal methods (conventional solvent, high pressure, and ultrasound), were systematically compared by UHPLC-QTOF-MS and UPLC-QQQ-MS. The total content of free phenolics, approximately 10 times to that of bound phenolics, showed no significant difference across different extraction methods. The highest yield of bound phenolics was obtained by heating and high-pressure methods and the composition was closely dependent on the degree of alkaline hydrolysis. Many of bound phenolics obtained by thermal extraction were small molecules. Meanwhile, a greater proportion of larger compounds were obtained by high-pressure extraction, indicating that high pressure was gentler than thermal extraction. Genistin, nicotiflorin and myricetin-3-galactoside were first reported in pomegranate peel. This study provides a solid foundation for purposeful extraction of phenolics from pomegranate peels for industry.