Dual modification manipulates rice starch characteristics following debranching and propionate esterification

The effect of dual modifications of pullulanase debranch and propionylation on rice starch property was investigated in this study. The occurrence of the propionylation in starch molecules was confirmed due to the formation of a new peak at 1750 cm −1 in FTIR spectrum and the characteristic peaks around 1.0 (methyl) and 2.2 (methylene) ppm in NMR compared with the native starch. Interestingly, because of the higher tendency of molecular retrogradation following the debranching, the current results revealed that there was no positive correlation between the amylose content (AC) and the debranching time. Debranching treatment increased the degree of substitute (DS) of the propionyl starch, and the AC was positively correlated with DS (R = 0.7753*), indicating that AC is an important element affecting propionyl reaction due to the exposure of more available hydroxyl groups during debranching. Meanwhile, debranching led to an increased crystallinity level and the crystallinity pattern changed from A-type to B-type, which is associated with the changes in their corresponding hydrolysis resistance. The NMR data also proved that the propionylation involves the replacements of the glucose in the starch structure (C-2, C-3 and C-6) with acyl groups of the desired complexes. Furthermore, the sequence of the reactions that occur in the process of propionylation and their mechanisms are proposed in the current study.