Antioxidative properties of bayberry tannins with different mean degrees of polymerization: Controlled degradation based on hydroxyl radicals.

The antioxidant properties of condensed tannins (CTs) are closely related to the mean degree of polymerization (mDP), and CTs with low mDP show stronger antioxidant effects. Therefore, obtaining CTs with a low mDP are very meaningful in improving their antioxidant properties and utilization. In this study, hydroxyl radicals generated by the decomposition of hydrogen peroxide under UV irradiation were used to degrade bayberry tannins in a clean and controllable manner. Taking the formaldehyde reactivity as an index to control the mDP of the degradation product, the changes in antioxidant properties of bayberry tannins with different mDP were studied by the method of 1,1-diphenyl-2-trinitrophenylhydrazine (DPPH), and 2,2-azido-di(3-ethyl-benzothiazole-6-sulfonic acid)diammonium salt (ABTS). Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), gel permeation chromatography (GPC), carbon nuclear magnetic resonance (13C NMR), and reversed-phase HPLC-ESI-MS were used to characterize the mDP, molecular weight (Mw), and chemical structure of the degradation products of bayberry tannins in different degradation stages. Results showed that hydroxyl radicals could cause significant degradation of bayberry tannins, and the controllable degradation of bayberry tannins could be achieved with the formaldehyde reactivity as an index. At the degradation times of 0, 2, 4, 6, and 8 h, the mDP (Mw) of the degradation products were as follows: 5.22 (2457), 4.36 (1895), 3.36 (1534), 2.87 (1153), and 1.78 (813), respectively. The antioxidant activity of the degraded product increased with the decrease in the mDP, and the degraded products had the largest formaldehyde reactivity and the best oxidation resistance when degraded for 6 h. This study provided a new method to achieve clean and controllable degradation of tannins and supported those tannins with low mDP could provide higher antioxidant activity.