Structural modification of raisin pectin fractions by electron beam irradiation

The effect of electron beam irradiation (EBI) on raisin pectin properties remains unclear regarding structural differences across pectin fractions. This study investigated the effects of EBI on the structural changes of three pectin fractions including WSP fraction, CSP fraction and NSP fraction in raisins as well as their correlations with polyphenol release and variations in antioxidant activity. EBI induced differential structural modifications of pectin fractions by disrupting hydrogen and glycosidic bonds, and altering molecular weight, crystallinity, monosaccharide composition and molecular conformation. EBI raised the molecular weight and HG content of WSP, reduced its RG-I ratio, Rg and Rc, leading to molecular aggregation and conformational contraction. Increasing irradiation doses induced de-esterification and molecular weight reduction in CSP, which exhibited maximum linearity and minimal branching at 4 kGy. NSP had the highest structural stability with intact HG and branched RG-I regions. The 4 kGy treatment maximized the release of (+)-catechin, (-)-Epigallocatechin, proanthocyanidins, protocatechuic acid, caffeic acid, quercetin, achieving the optimal antioxidant capacity. EBI degraded the RG-I and HG domains of WSP and the HG region of NSP, facilitating polyphenol release and enhancing the overall antioxidant activity of raisins. This work provides theoretical support for the processing of dried fruits via EBI technology.