An X-ray study of hydrothermally treated potato starch

Potato starch was annealed at temperatures from 44 to 51 °C (66% moisture) and heat moisture treated (HMT) at 90–130 °C (at 17–26% moisture). The structural effects of the various hydrothermal treatments were examined with differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). Annealing and HMT both increased the DSC gelatinisation temperatures. The former narrowed the gelatinisation temperature range, while the latter broadened it. Gelatinisation enthalpies and WAXD crystallinities were unaffected by annealing treatments, while they decreased by HMT at 90–120 °C. Furthermore, HMT triggered a gradual transition from B- to A-type crystallinity when treatment temperatures increased. In contrast to B-type crystallinity, A-type crystallinity is reduced by post-HMT evacuation of the accumulated steam. The latter clearly prevented growth or perfection of A-type nuclei/crystallites formed during HMT at elevated temperatures. Compared to native starch, annealed samples showed a more intense 9 nm scattering maximum. This suggested a more efficient packing of crystallites in dense lamellae. HMT, on the other hand, resulted in the development of a diffuse SAXS background, which became more prominent for samples treated at higher temperatures, and eventually replaced the 9 nm scattering maximum. The stacked lamellae, present in native and annealed starches, are thus clearly disrupted in the HMT process.