Structural Analysis of Condensed Tannins in Hydrothermal Extracts from Wasted Bark by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

Abstract

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) was applied to a structural analysis of condensed tannins extracted from wasted bark through a hydrothermal process under different temperatures. The mass spectra of hydrothermal extracts obtained at 100°C and 150°C showed a series of peaks for the tannin oligomers up to 8-mers, consisting mainly of the combination of three types of flavan-3-ol units. This result suggests that hydrothermal extraction under these temperatures leads to a rapid recovery of the tannin components while keeping their oligomer structure. On the other hand, the ion peak of the tannin oligomer was almost missing from the mass spectrum of extracts obtained at 200°C, suggesting that degradation and/or polymerization of the tannins occurred significantly through the extraction procedure under this temperature. Furthermore, the number-averaged molar masses of the tannin components in each extract sample were calculated on the basis of their corresponding mass spectra, and interpreted in terms of the antioxidant activity. This interpretation revealed that 1) the antioxidant activities for the tannin components increased with an increase in their number-averaged molar masses, and 2) the tannin components in hydrothermal extracts obtained under lower temperatures showed relatively higher antioxidant activities than α-tocopherol.