The effect of reduction on the properties of the regioselectively oxidized starch granules prepared by bromide-free oxidation system

Abstract

Development of intact oxidized starch granules by regioselective oxidation technology is of interest and provides a new research direction for oxidized starch. In this study, new sodium tetrahydridoborate (NaBH4)-treated oxidized starch (OS-BH4) granules were prepared by a one-pot method, where native corn starch (NS) granules were oxidized by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)/sodium hypochlorite (NaClO) system followed by reduction with NaBH4. Oxidized starch (OS) granules without NaBH4 reduction were also prepared to investigate the effect of C6 aldehyde groups remained after TEMPO-mediated oxidation on properties of the granules. When degrees of oxidation were controlled to be not higher than 12%, both the OS and OS-BH4 granules had similar morphology to the NS granules with envelopes. Compared to the OS granules, except for lower pasting temperatures and dextrose equivalents, the OS-BH4 granules had higher molecular weights, degrees of polymerization (DP), peak viscosities, final viscosities, and swelling power. Difference of the properties was considered related to (1) repulsive forces formed between the C6 carboxylate groups, (2) C6 aldehyde groups with lower hydrophilicity than the C6 hydroxyl groups, and (3) some hemiacetal linkages formed between the C6 aldehyde groups and the hydroxyl groups. Furthermore, pregelatinized OS-BH4 granules were preliminarily prepared, which showed good swelling behavior with intact granular morphology in alkaline environment.