Pressure−Temperature Phase Transition Diagram for Wheat Starch

Wheat starch suspensions in water (5% dry matter) were subjected to various pressures (0.1−600 MPa) and temperatures (−20 to 96 °C) for 15 min. The gelatinization rate was measured after treatment by using microscopic measurements of the loss of birefringence of the granules. This method was previously calibrated by differential scanning calorimetry. Curves of isogelatinization were found to be quite similar to a pressure−temperature (P−T) diagram of unfolding proteins. Results were first analyzed by considering the thermodynamic aspects related to the dT/dP curve shifts. On the basis of equations already shown for proteins, the P−T gelatinization diagram of wheat starch would show different kinds of thermal contributions, suggesting endothermic, athermic, or exothermic melting reactions. Second, as a practical consequence, these previous P−T areas corresponded to specific gelatinization conditions as confirmed by hydration evaluation measured by starch swelling index. Depending on the pressure−temperature conditions, gelatinization would involve hydration. Lowering the pressure and temperature resulted in a complete gelatinization with less hydration in comparison with a thermal treatment at atmospheric pressure. A hydration model based on an energetic approach was proposed. Keywords: Starch gelatinization; high hydrostatic pressure; phase diagram; hydration