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 degreesC) for 15 min. The gelatinization rate was measured after treatment by using microscopic m easurements of the loss of birefringence of the granules. This method was p reviously calibrated by differential scanning calorimetry. Curves of isogel atinization were found to be quite similar to a pressure-temperature (P-T) diagram of unfolding proteins. Results were first analyzed by considering t he thermodynamic aspects related to the dT/dP curve shifts. On the basis of equations already shown for proteins, the P-T gelatinization diagram of wh eat starch would shaw different kinds of thermal contributions, suggesting endothermic, athermic, or exothermic melting reactions. Second, as a practi cal consequence, these previous P-T areas corresponded to specific gelatini zation conditions as confirmed by hydration evaluation measured by starch s welling index. Depending on the pressure-temperature conditions, gelatiniza tion would involve hydration. Lowering the pressure and temperature resulte d in a complete gelatinization with less hydration in comparison with a the rmal treatment at atmospheric pressure. A hydration model based on an energ etic approach was proposed.