Ultrasonic monitoring of sol–gel transition of natural hydrocolloids

The sol–gel transition of iota carrageenan and pectin is followed by measuring the evolution of ultrasonic velocity of a compressional wave at a frequency of 500 kHz. Measurements are performed as a function of temperature, from about 90 to 20 °C. Results are compared to those obtained in rheology in oscillatory conditions. It is shown that the ultrasonic velocity is sensitive to the sol–gel transition when going from a viscous to a “solid-like” state with a large elastic component. However, the method fails when the transition occurs gradually to give a weak elastic gel.