Abstract:To gain a deeper understanding about the effect of air on ultrasonic attenuation, the freezing efficiency of the ultrasound-assisted immersion freezing method was determined on tissue samples with pores comprising different air contents. Radish tissue was used as a food model. The air content in the vegetable tissues was adjusted by vacuum degassing treatments to obtain tissue pores with 9.91%, 6.46%, 4.01%, or 0% of air. These samples were then treated by ultrasound-assisted immersion freezing at a frequency of 28 kHz and power of 0.33 W/cm2, and the results were compared with those from samples subjected to normal immersion freezing at the same temperature. Consequently, relative to the samples that were frozen normally, the freezing rates of the samples with 9.91%, 6.46%, 4.01%, and 0% of air were improved by 20.00%, 27.00%, 29.00%, and 33.00%, respectively, after ultrasound-assisted immersion freezing, and the phase transition times of these samples were reduced by 16.00%, 24.00%, 27.00%, and 31.00%, respectively. With decreasing air content in the radishes, the freezing rate of the ultrasound-assisted immersion freezing method was increased significantly (p<0.05), and the hardness, microstructure, and calcium content of the thawed samples were significantly enhanced (p<0.05) as well. The results suggested that the higher air content in the radish tissue pores was a major factor causing the decrease in the efficiency of ultrasound-assisted immersion freezing, and degassing treatment could effectively improve the efficiency of this freezing technique on air-containing substances.
