Comparison of ultrasonic velocities in dispersive and nondispersive food materials

Ultrasonic techniques are increasingly being used to evaluate the properties of food materials. Interpretation of the structure and dynamics on the basis of measured ultrasonic parameters requires rigorous definition of ultrasonic parameters such as velocity, especially since many food materials can display considerable dispersive behavior (changes in velocity with frequency). Agar gel (2% w/v) and agar gel (2% w/v) with a regular array of bubbles (8% volume fraction) were chosen as nondispersive and dispersive materials, respectively. Frequency and time domain techniques were used to analyze velocities. Signal, phase, and group velocities were identical in the agar gel and were indistinguishable from those of water (1500 m s(-1)), indicating the predominant effect of the bulk modulus of the water they contain on the longitudinal modulus of the gel. In contrast, the inclusion of the bubbles in the agar gel led to strongly dispersive behavior, with group velocities varying by 1000 m s(-1) above and below the 1500 m s(-1) of the agar gel without bubbles, depending on frequency. The addition of bubbles also led to strong attenuation in the agar gel with a peak occurring at a frequency associated with a band gap arising from destructive interference of sound waves. The results show that care must be taken when comparing ultrasonic parameters derived from experiments on food materials performed at different frequencies or with different ultrasonic techniques.