| Influences of starch molecular structure on hypochlorite-catalyzed oxidation of starches were investigated in this study. Potato, rice, and four types of corn starches varying in amlyose content (0, 25, 50, and 70%) were oxidized with NaOCl at different NaOCl concentrations (0.8, 2, and 5%). The Carbonyl and carboxyl contents and the extent of structure depolymerization of oxidized starches increased with increasing NaOCl concentration. Oxidation initially took place in the amorphous lamellae and then advanced to the crystalline lamellae depending on the amount of oxidant and starch organization. Oxidized potato starch had the highest carbonyl and carboxyl contents while oxidized 70% amylose corn starch (AMC) had the lowest. The carboxyl and carbonyl contents of oxidized corn starches decreased with increasing amylose content. Carboxyl groups were found to be more concentrated on amylopectin than on amylose, particularly in 50% and 70% AMCs. Amylose was more susceptible to oxidative depolymerization than amylopectin. Among oxidized corn starches, degradation of amylopectin long chains (DP > 24) was more pronounced in waxy corn starch (WC) and common corns starch (CC) than in AMCs. Oxidation decreased gelatinization temperature and enthalpy with WC showing the most decrease and 70% AMC showing the least. Retrogradation of amylopectin slightly increased after oxidation with increasing oxidation level. The peak viscosities of oxidized WC and CC were higher than those of their native counterparts at 0.8% NaOCl, but this increase was not observed in AMCs. By using a chemical surface gelatinization method, more carboxyl groups were found at the periphery than at the core of starch granules, and this difference in carboxyl distribution was more pronounced in the oxidized 70% AMC. For CC, most amylose depolymerization from oxidation occurred at the surface. Oxidized starches swelled to a greater extent than their unmodified counterparts at all levels of surface removal. Efficiency of hypochlorite oxidation on starches in terms of the carbonyl/carboxyl contents, the extent of structure depolymerization, and the rate and site of the reaction were strongly affected by granule organization. Starch origins and variation in the proportion of amylose and amylopectin indirectly influenced the efficiency of oxidation and consequently the physicochemical properties of the resultant oxidized starches. |
