To rationalize the substitution priority of octenyl succinic group along amylopectin chain: An analysis from the change of lamellar structure

Under controlled supply of octenyl succinic anhydride (OSA), the priority of OSA substitution along amylopectin chain was revealed to follow backbone branch-branching point-chain pattern in our previous study, but whether or how this priority pattern was related to the distribution of amylopectin chains within undispersed starch granule was unclear. To reveal the reason for this substitution priority of OSA, changes in the lamellar structure associated with the branching point and chain structure of amylopectin, were studied. Firstly, X-ray diffraction (XRD) test suggested that initial branching-point substitutions at low substitution degree (DS, ≤0.7%) could decrease the crystallinity of starch. A 0.08 nm increase in the thickness of intermediate region and a 9.5% reduction in the lamellar compactness (Dm) were revealed by small angle X-ray scattering, indicating that initial structural changes were resulted from the substitution of backbone branch closely connected to crystal region. When DS reached 1.5%, the loosened lamellar structure triggered branch-chain substitutions, leading to a 6% decrease in β-limit value, despite largely available branching points at this point. Then, the chain substitution resulted in increased thickness of crystalline lamellae dc and further reduced Dm, which in turn promoted more chain substitutions at high DS. Changes in the lamellar structure, crystallinity, granular morphology, particle size and the substituent distribution of starch well rationalized the ordered substitution of OSA at molecular level. [Display omitted] •Lamella structural change well explained molecular level OSA substitution priority.•Initial branching-point substitutions could loosen crystalline lamella structure.•Loosened crystalline lamella structure triggered branch-chain substitutions.•OSA substitution position mutually affected with starch lamellar structure.