The identification of degradation products and degradation pathway of malvidin-3-glucoside and malvidin-3,5-diglucoside under microwave treatment

Understanding the degradation of anthocyanins (Acys) during microwave treatment is required for the application of this novel technology to fruit and vegetable processing. The pH 3.5 buffer solutions containing 50 mg/l malvidin-3-glucoside (Mv-3-glu) and malvidin-3,5-diglucoside (Mv-3,5-diglu), respectively, were treated with microwaves at 700 W. In comparison, the samples were heated at 98 ± 2 °C in a water bath. In total, nine degradation products were detected (using LC–UV/Vis–MS/MS) for each Acys after microwave treatment. Anthocyanone A, hydroxycoumarins and dihydroxy phenylacetaldehyde were characterised as the main degradation products of both Acys under microwave conditions, which were not detected in the thermal treatment. In addition, syringic acid, 2,4,6-trihydroxybenzaldehyde, coumarin glucosides and 2,6-dimethoxyphenol were found simultaneously under the microwave and thermal treatments. During the microwave treatment, hydrogen peroxide was formed and resulted in the Baeyer–Villiger-type oxidation of the Acys, which was the main degradation pathway. At the same time, the conventional thermal degradation pathway of Acys also occurred.