Retrogradation of amylosucrase treated waxy corn starch by temperature cycling: structural and digestibility properties

Abstract

In this study, waxy corn starch was treated by amylosucrase (AS), followed by retrogradation under different temperature cycles or isothermal storage. Various retrogradation conditions were used to accelerate crystallization of amylopectin chains elongated by AS: temperature cycling with a low temperature of 4 °C for 1 day and a high temperature of 20, 40, or 60 °C for 1 day for 3 or 7 cycles, and isothermal storage at 4 °C for 6 days or 14 days. The properties of AS-treated starches were determined by high-performance anion-exchange chromatography and the iodine binding capacity (IBC). The properties of retrograded starches were determined by X-ray diffractometry, differential scanning calorimetry, 13C CP/MAS NMR spectroscopy, and in vitro digestibility. After the AS treatment, the proportion of short chains (DP ≤ 12 and DP 13-24) decreased, whereas that of long chains (DP 25-36 and DP ≥ 37) increased. Also, IBC was similar with that of normal corn starch. Following the retrogradation for 14 days, the X-ray diffraction patterns of all AS-treated starches showed a B-type polymorph. In the control starches, compared to starches stored only at 4 °C, the starches stored under the temperature cycles exhibited lower relative crystallinity and melting enthalpy due to the melting of unstable crystallites during storage at higher temperatures. Little difference in ordered structure content was observed. On the other hand, the AS-treated starches stored under the temperature cycles exhibited higher relative crystallinity, melting enthalpy, ordered structure content, and resistant starch content than the isothermal storage at 4 °C. Among all starches, the AS-treated starch retrograded under 4/60 °C cycled condition particularly showed the most striking changes in structural and digestibility properties. Conclusively, this study demonstrated that temperature cycling could induce reassociation of elongated chains by AS treatment and accelerate retrogradation. It also suggested that retrogradation by temperature cycling brought more changes in structural and digestibility properties of AS-treated starch than control starch.