The impact of annealing, acetylation, and dual modification on the structure and physicochemical properties of waxy starches

Egodage, Rasanjali (2019) The impact of annealing, acetylation, and dual modification on the structure and physicochemical properties of waxy starches. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

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 morphology, molecular structure and properties of waxy corn (WC), waxy barley (WB), waxy rice (WR) and waxy potato (WP) 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 h, followed by acetylation (10%). Scanning electron microscopy showed that granular morphology did not change upon ANN, but granular surface became rough and slight fragmentations, and indentations were observed upon acetylation and dual modification. In the 1H-NMR spectra, the characteristic peaks for acetyl protons (2.01-2.06 ppm) were detected. The introduction of acetyl groups was confirmed by FTIR spectroscopy with new absorption bands appearing at around 1737, 1375, 1252 cm⁻¹. X-ray diffraction pattern remained unchanged upon modification. The relative crystallinity of annealed starches remained unchanged except for WC, while acetylation and dual modification decreased relative crystallinity. Gelatinization temperatures increased on ANN and dual modification but decreased on acetylation. The gelatinization enthalpy remained unchanged upon ANN but decreased upon acetylation and dual modification. Acid hydrolysis decreased in WP, but remained unchanged in other starches on ANN. In vitro digestibility did not significantly change upon annealing except for WP however, it significantly decreased upon acetylation. Acetylation decreased RDS and SDS contents but increased that of RS. The dual modification resulted in lower in vitro digestibility and increased the RS content to a similar or higher extent than that of 20% acetic anhydride treatment, except for WR. Retrogradation characteristics were affected by modifications. Annealing increased light transmittance in WC and WR, while it decreased in WP. However, the impact of ANN on retrogradation endotherms was only marginal. Acetylation decreased retrogradation as evidenced by increased light transmittance and lower retrogradation enthalpy. The increase in light transmittance of dual modified starches was higher than acetylated starches with 10% acetic anhydride and the decrease in retrogradation enthalpy was significantly lower (p<0.05) than that of 5% acetic anhydride treatment in WC and WP, and retrogradation endotherms were not observed in dual modified WR and WB. 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) indicating glycosidic and cross ring fragmentations of the precursor sodiated molecular ions. ESI-MS analysis of hydrolyzed acetylated and dual modified starches resulted in oligomers with various substitutions (1-3 acetyl groups). Furthermore, ESI-MS/MS analysis showed that when there were two acetyl groups, those were not located in the same glucose molecule, but were located in two different glucose molecules. The results of this study demonstrated 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.

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