Ultrasonic investigation of wheat starch retrogradation

A recently developed technique, based on ultrasonic wave propagation, was applied to study the effect of storage time on the retrogradation process of wheat starch extrudates during storage at room temperature. A specifically designed experimental set-up enabled to propagate in situ ultrasonic waves continuously for several days at constant water content and storage temperature (∼34% water and 25 °C). The velocity and the attenuation of the ultrasonic waves changed as a result of the recrystallisation process of amylopectin molecules of wheat starch samples during storage.The propagation of ultrasonic waves, acting as a dynamic mechanical deformation at high frequency (10 MHz), gave access to the complex longitudinal modulus that, compared to the complex Young modulus obtained from low frequency DMA, enabled a better insight of the changes in viscoelastic behaviour of starch extrudates during retrogradation. The results of the ultrasonic monitoring of starch retrogradation were correlated with those obtained from X-ray diffraction and differential scanning calorimetry.The present study demonstrated the potential of the ultrasonic technique in detecting the changes in physical properties of concentrated starch systems.