Behaviors of starches evaluated at high heating temperatures using a new model of Rapid Visco Analyzer - RVA 4800

Rapid Visco Analyzer (RVA) is a reliable viscometer that is commonly utilized to measure the pasting properties of starch. The conventional model of RVA can only determine starch pasting properties at heating temperatures up to 95 degrees C, which thus cannot evaluate the performance of starch related to high-temperature processing, such as retorting, jet-cooking and extrusion. In this study, RVA 4800 - the latest model of the viscometer having high-temperature capability - was deployed to determine the pasting properties of representative waxy, normal and high-amylose starches of 1.8-69.7% amylose contents at heating temperatures of 95-140 degrees C. As the temperature increased from 95 to 140 degrees C, the pasting temperatures and peak viscosities of most waxy and normal starches remained unchanged, but their holding strengths and final viscosities decreased, which could be attributed to thixotropic breakdown and thermal degradation of starch molecules. Consequently, the paste adhesiveness or gel hardness of most waxy and normal starches was reduced at higher temperatures. By contrast, heating at temperatures above 120 degrees C completely gelatinized wrinkled pea and high-amylose maize starches, allowing the starch granules to swell for viscosity development and subsequent gel formation. Matrix structures of the freeze-dried starch pastes and gels were observed under scanning electron microscope to elucidate how the changes at granular and molecular levels at the tested cooking temperatures influenced the pasting and gelling properties of different starches. This research offered new insights into the relationships between the thermal properties, pasting properties and gelling ability of various starches at heating temperatures of 95-140 degrees C by using RVA 4800.