Evaluation of products obtained from oxidative degradation of (1-3) (1-4)-β-d-glucan

Cereal β-glucan is a water-soluble cell wall polysaccharide, which has positive health effects on humans. Oxidative Degradation of β-glucan may occur during food processing, leading to the loss in physiological functionality of β-glucan. Oxidative degradation can result in cleavages of polysaccharide chain, the formation of oxidised functional groups (e.g. carbonyls) along the chain or the release of carboxylic acids (e.g. formic acid). In the case of β-glucan, chain scission and the formation of oxidised functional groups due to hydroxyl-radical induced oxidation has been shown, but the identification of released carboxylic acids has not been done. The aim was therefore to study the oxidation pathway of β-glucan, by analysing its degradation products. The focus was the release of carboxylic acids, especially formic acid. The change in molecular structure of β-glucan after the release of formic acid was also analysed. Barley β-glucan water solutions were oxidised with H2O2 and ascorbic acid at different concentrations (5, 10, 40, 70 mM), in the presence of 1 mM FeSO4·7H2O. Samples were collected on 1, 2 and 4 days and formic acid was analysed using formic acid assay kit. To evaluate the structure of oxidised β-glucan, part of the samples underwent reduction to convert any carbonyl groups back to hydroxyl groups. The oligosaccharide composition and monosaccharide composition of samples were then analysed. Results showed that formic acid was formed in H2O2 treated β-glucan and its content was positively correlated with H2O2 concentration in the presence of Fe2+. Formic acid was also formed in ascorbic acid treated β-glucan but an obvious increase in formic acid content at increased ascorbic acid concentration was not observed. Formic acid accumulated in β-glucan solution over time. Monosaccharide composition showed that samples were mainly composed of glucose. In H2O2 treated β-glucan, however, an additional component was observed which was identified to be arabinose. Arabinose was reduced by reducing agent, indicating that arabinose was formed at the reducing end of oxidised β-glucan. The content of arabinose increased with increasing H2O2 concentration, which was concomitant with a decreasing glucose content. Arabinose content decreased from oxidation day 1 to day 4. Oxidative degradation of β-glucan is proposed to proceed progressively, with random chain scission and degradation of the reducing ends. Formic acid was released due to oxidation and arabinose was formed at the reducing end. As oxidation proceeded, we suggest that the reducing end unit was degraded stepwise to release formic acid. Formic acid is demonstrated to be the oxidation product of β-glucan for the first time. The released formic acid was well related to the degree of oxidation induced by H2O2 and Fe2+. Therefore, formic acid can be used as an indicator for the oxidation of β-glucan induced by H2O2 and Fe2+.