Effects Of Voids In Fruits And Vegetables On Ultrasound-Assisted Immersion Freezing

As one of the thriving emerging freezing technology,ultrasound-assisted immersion freezing?UF?has been widely concerned in the food industry recently,and a series of correlational studies have been carried out.Based on the existing researches,the auther concludes that the strengthening effects of ultrasound on immersion freezing is mainly influenced by three aspects of the solid foods,namely the complexity of the structure,the heterogeneity of the internal media and the existence of voids.All these factors will cause an attenuation of the ultrasound to some extent,and this is detrimental to UF.In this paper,we firstly selected radish tissue as solid food model to study the strengthening effects of ultrasound treatment conditions on freezing and select out the optimal parameters of ultrasound treatment.Then,radishes with different air content were obtained by vacuum degassing treatments,and they were treated by UF to investigate the effects of gas content.Finally,apple,radish and potato were used to represent the high,medium and low voids solid food models respectively,and the effects of voids were studied.In the comprehensive evaluation of the freezing,the freezing rate as well as the quality of frozen products were considered synthetically.And the quality of frozen products was analyzed from the macroscopic?hardness,drip loss?and microscopic?calcium content,total phenol content,microstructure?perspectives.The main research results are as follows:1.During the UF treatment of radish,the freezing rate of the sample increased with the increase of ultrasound power?when the ultrasound power is 180 W,the freezing rate increases by 3.33% vs when the ultrasound power is 540 W,the freezing rate increases by 11.44%?,but with the increase of ultrasound power,the quality of frozen products tended to enhance first and then decline.It can be seen that in the process of UF,the high freezing rate can not guarantee the high quality of frozen products.Therefore,it is insufficient to evaluate the freezing effect only from the freezing rate;2.Ultrasound treatment is applied to the sample in the pre-cooling,phase transition and tempering stage of the freezing process respectively,and it was found that the application of ultrasound only in the phase transition stage can significantly reduce the freezing time?p<0.05?.DSC results showed that in the phase transition stage,the ultrasound can promote the heat transfer and accelerate the release of latent heat.CLSM showed that ultrasound can promote nucleation and reduce the size of ice crystals,indicating the UF treatment can protect the integrity of cells.Therefore,the ultrasound treatment should be acted on the phase transition stage during the freezing process;3.For solid foods,the degassing treatment can effectively improve the ultrasound effect on immersion freezing.The samples were first degassed by C?D_{-0.02 MPa}^{5 min}?D_{-0.05 MPa}^{5 min} and D_{-0.09 MPa}^{5 min} treatments respectively and then treated by UF,the freezing rate of these samples were increased by 20.00%,27.00%,29.00% and 33.00% respectively.The analysis of samples' hardness,Ca²⁺ content and CLSM showed that after UF treatment the sample of D_{-0.09 MPa}^{5 min} remained intact structure.This may be due to the reduction of gas content leading to a reduction of ultrasound attenuation,and thus the effect of ultrasound is obvious;4.For plant tissue,there was a power function relationship between the ratio of the freezing rate strengthened by ultrasound and the voids in plant tissue,the function equation was y = 0.018_x^-1.057?R² = 0.994?.In addition,the changes of the voids and quality of frozen products show an opposite trend in UF treatment,therefore,UF technology was suitable for the food material which has high density and dense structure;5.For solid food materials,the difference of the Young's modulus,the gas content and the voids of the materials will lead to the difference of the acoustic impedance and the attenuation coefficient.The difference in acoustic properties of the materials may be the essential cause of the significant difference of the UF effect in solid foods.