Degradation of pullulans of narrow molecular weight distribution—the role of aldehydes in the oxidation of polysaccharides

Degradation of pullulan samples of narrow molecular weight distribution was studied in O2 and N2 atmosphere at 180 °C and in air at 80 °C, 65% RH. At higher temperatures, size exclusion chromatographic analyses provided evidence for non-random degradation and formation of cross-links, a phenomenon depending on the concentration of carbonyl groups. Initial degradation rates in oxidative and inert environments were comparable, while at longer reaction times extensive cross-linking was typical of degradation in oxygen. °C, in a moderately alkaline environment, the degradation remains random, as demonstrated by the mono-modal distributions of molecular weights. Furthermore, there was no evidence of cross-linking. Correlations between the concentration of aldehyde end-groups in the pullulan samples, rates of degradation and the content of peroxides after a pre-oxidation treatment, as determined by the use of chemiluminometry, were demonstrated. This leads to the conclusion that aldehyde groups, via the peroxide intermediates, have the decisive influence on the rates of oxidative degradation of polysaccharides.