Distribution of Octenyl Succinate Groups in Octenyl Succinic Anhydride Modified Waxy Maize Starch

The location of octenyl succinate groups within octenyl succinic anhydride (OSA)-modified waxy maize (WM) starch granules was studied in order to better understand the relationship between the structure and physical properties of OSA starches. OSA starches of D.S. 0.03—0.11 were prepared by reaction between starch, OSA and NaOH in aqueous suspension; the native granular structure of starch was retained after reaction. Backscattered electron imaging of osmium-stained, sectioned OSA starch granules showed a uniform distribution of OSA groups over the cross-section of the granules. Anion-exchange chromatography of OSA starches solubilized in water showed that most of the amylopectin molecules contain some negative charge, suggesting that most of the starch granule is accessible to and reacts with OSA. However, after partial debranching with pullulanase, more of the resulting chains were neutral than would be expected on statistical grounds, suggesting heterogeneity in OSA substitution at the branch level. X-ray photoelectron spectroscopy suggested that the concentration of OSAgroups on the immediate surface of the OSA starch granules was about 3—4 times that of the bulk.