Acoustic Methods for Characterizing Physical Properties of Food Products

We found that the change with time of milk curd strength during the process of lactic acid fermentation and the change with time of freezing rate during the freezing process of food material can be monitored by measuring ultrasonic velocity. While the ultrasonic velocities of fruits and vegetables depend on not the “hardness” but the gas volume fractions of their tissues, the propagation velocities of impact vibration have strong correlations with their elastic properties. We have developed a measurement system for volume/density of food material by using phenomena of Helmholtz acoustic resonance. We found that the Helmholtz resonant frequency for food material having a pore structure is lower than that of non-porous food material having the same volume. Based on the theoretical consideration on the relations between the physical properties of porous food and the reduction of resonant frequency, we have also developed a measurement system for evaluating beer foam characteristics and bread texture. In another study we have found that the frequency of crack vibrations generated during chewing deep-fried foods has a close correlation with the results of sensory evaluation of their crispness.