Crystal structure changes of native and retrograded starches modified by high hydrostatic pressure: Physical dual modification

The effect of high hydrostatic pressure (HHP) on the structural changes of starches with different state of organization was investigated. Starch in native state (granular - NS) and in retrograded state (granular structure not present - RetS) were adjusted to 40% (w/v) of moisture and then treated at pressures of 300 and 600 MPa for 5 min at 10 °C. The structural changes of NS and RetS were significantly and differently affected by the HHP treatment. When the pressure increased, orthorhombic unit cell of NS showed an increase in crystal parameter of c-axis and a decrease of a- and b-axes. Crystallinity, crystallite size, short-range ordered structure (DO), organization of the double helix structures (DD), and enthalpy change (ΔH) gradually decreased. After treatment at 300 MPa, the granules get aggregated while keeping their granular structure, promoting a higher viscosity. At 600 MPa, leached material, aggregated granules, destroyed, and with collapsed “ellipsoid-shape” structure were observed, which contributes to decreasing the viscosity. In RetS, the dual physical modification achieved after pressurizing the hydrothermally treated starch, resulted in a significant increase in the crystal parameters (a, b, and c) of hexagonal unit cell. Furthermore, the crystallinity, crystallite size, DD, DO, ΔH, and viscosity were improved, while a gradual decrease of the amounts of cavities was observed. Crystal structure changes in RetS showed that the double helices of unit cell were stabilized by dual modification, while the unit cell was affected after pressurization of NS. This investigation is the first evidence of the application of HHP on RetS.