The impact of Annealing, Acetylation and Dual modification on the structure and properties of waxy starches

Physical and chemical modification techniques are often used to modify native starch to enhance its positive attributes which are required in industrial processes. The present study focused on modification of waxy starches using annealing (ANN), acetylation, and dual modification (ANN-acetylation) and to evaluate their impact on the molecular structure and properties of waxy corn, waxy barley, waxy rice and waxy potato starches. In addition, the uses of methylation and mass spectrometry analysis helped to unravel the structures of native and modified starches. Acetylated starches were prepared using different levels of acetic anhydride (5, 10, and 20%) and dual modification was achieved by ANN for 72 hours, followed by acetylation (10%). Scanning electron microscopy showed that granular morphology didn’t change upon ANN, but granular surface become rough upon acetylation and dual modification. In the 1H-NMR spectra, the characteristic peaks for acetyl proton (2.01-2.08 ppm) were detected. The introduction of acetyl groups were confirmed by FTIR spectroscopy. Relative crystallinity and gelatinization enthalpy remained unchanged upon ANN but decreased upon acetylation and dual modification. Gelatinization temperatures increased on ANN and dual modification but decreased on acetylation. In vitro digestibility did not significantly change upon annealing, however, decreased upon acetylation. Acetylation decreased RDS and SDS contents but increased that of RS. Dual modification resulted lower in vitro digestibility and higher RS content. ESI-MS analysis of permethylated native starches resulted in a series of sodiated molecular ions and low energy collision induced dissociation tandem mass spectrometry analysis (CID-MS/MS) indicated glycosidic and cross ring fragmentations of the precursor sodiated molecular ions. ESI-MS analysis of hydrolysed acetylated starches resulted oligomers with various substitutions. The results of this study showed that dual modification has a promising effect on improving functionality in waxy starches, and that tandem mass spectrometry can be used to study the structure of chemically modified starches.