Investigating the influence of CaCl2 induced surface gelatinization of red adzuki bean starch on its citric acid esterification modification: Structure–property related mechanism

[Display omitted] •CaCl2 can induced surface gelatinization on red adzuki bean starch shell.•Citric acid esterification intensified starch granule surface fragmentation and amylose leaching.•Synergistic modifications induced a sharp reduction in molecular weight and viscosity of starch.•Esterification inhibited the digestive enzyme binding site thereby increasing resistant starch content.•Surface gelatinization promoted the modification of starch esterification. This study prepared starch with different degrees of gelatinization (10 % and 30 %) by CaCl2, and further combined with citric acid at different concentrations (10 %, 20 %, and 30 %) for esterification. The results indicated that CaCl2 induced surface gelatinization and the esterification promoted its fragmentation. This synergistic modification induced depolymerization of starch chains and a decrease in molecular weight, although starch's Maltese cross, growth rings, and crystal type remained unchanged. Moreover, the synergistic modification induces a short-range ordered structure loss and the enthalpy decrease of starch, which induce starch pasting properties decrease and solubility increase. Furthermore, the introduction of ester groups by esterification increases the resistant starch content by limiting the binding sites for digestive enzymes. In conclusion, surface gelatinization can increase active reaction sites by disrupting the starch shell structure, and serves as a potential pretreatment to enhance the deep application of esterified starch in food and pharmaceutical fields.