Effect of ultrasound irradiation on ice crystal size distribution in frozen agar gel samples

Power ultrasound has been proven to be useful in promoting the nucleation of ice and affecting the crystal growth phenomenon during freezing. In the present research, the effect of ultrasound assisted freezing on the ice crystal size distribution of agar gel samples was studied. The samples were frozen in an ethylene glycol–water mixture (− 20 °C) in an ultrasonic bath system. To evaluate nucleation, ultrasound (25 kHz) was applied for 3 s at different temperatures during supercooling. Moreover, irradiation was carried out for different durations (90, 180 and 270 s), intensities (0.07, 0.25 and 0.42 W cm− 2) and duty cycles (30, 50 and 70%) in the phase change period. Results indicated that ultrasound irradiation at different temperatures was able to initiate nucleation in agar gel samples after a short delay. Microstructure evaluation of the samples revealed that the size distribution of ice crystals was affected by ultrasound induced nucleation temperature. Decreasing the nucleation temperature resulted in smaller ice crystals. Irradiation in the phase change stage significantly affected the microstructure as well. Smaller ice crystals were observed in comparison with the non-irradiated sample. However, no regular relationship between the evaluated parameters (intensity, duration and duty cycle) and the crystal sizes was detected. More investigations are needed to study the mechanism of freezing and the rheology of the final product as affected by ultrasound irradiation.Industrial relevanceThe results of this study suggested that ultrasound can be used as a potential tool to decrease the size of ice crystals in solid foods for the food industry.

► The ice crystal size distribution in ultrasound assisted frozen agar gel samples was studied.
► Size distribution of ice crystals was affected by ultrasound induced nucleation.
► Irradiation in the phase change stage significantly affected the microstructure.
► Smaller ice crystals were observed in comparison with the non-irradiated sample.
► No regular relationship between intensity or duration and crystal sizes was detected.