Studies On Freezing And Thawing Of Red Radish And Their Related Mechanisms By Low Frequency Ultrasound

Low frequency ultrasound(LFU) assisted freezing is a novel and promising food freezing technology, which combines ultrasonic technology and immersion freezing technology together. Heretofore, the mechanism of the ultrasound assisted freezing on the solid materials is still unclear. In addition, the coolant easily and uncontrollably penetrates into the materials during ultrasound assisted freezing process. This leads to the pollution of frozen materials and change of the original flavor. Furthermore, freezing is one of the most effective preservation methods and widely used in the fruits and vegetables processing. However, big ice crystals form during freezing process due to the high moisture content in fruits and vegetables, resulting in the quality attributes decline. Therefore, regarding the red radish as material, this research focuses on study on the mechanism of ultrasound assisted freezing of red radish, preventing the penetration of the coolant into the materials and improving the quality attributes.The distribution of ultrasonic intensities of the freezer was studied to make bedding for subsequent research. Results showed that the distributions under different liquid(30% CaCl2) levels, at different vertical locations and power levels of the freezer had obvious differences. Therefore, it is crucial to choose the suitable location for ultrasound assisted freezing. After the experimental data analysis, the suitable conditions are as follows: the liquid level should be 100 mm, the materials should be placed in the middle of freezer’s bottom and the optimum vertical distance of 5 mm to 30 mm from the freezer’s bottom should be maintained.The effects of LFU on the nucleation temperature and quality attributes of frozen red radishes were investigated. Results showed that LFU can induce nucleation occurrence of red radishes. Statistical analysis showed that there was a linear relationship between nucleation temperature(y) and ultrasound application onset temperature(x); the regression equation was y=0.7x-1.2(R2=0.96). Additionally, LFU can significantly improve freezing rate, reduce the damage of cell structures, decrease drip loss, promote texture property and improve retention of nutrients of the red radishes. Therefore, the quality of frozen red radishes was improved.The wrapped treatment was adopted to prevent the coolant uptake during ultrasound assisted freezing process. Due to the decrease of the heat transfer coefficient after wrapped treatment, the freezing rate of wrapped red radishes reduced during ultrasound assisted freezing. In addition, the drip loss of frozen sample was increased, while the firmness decreased. However, the wrapped treatment can prevent the coolant penetrating into the red radishes during ultrasound freezing process. Therefore, the problem of coolant uptake was solved and the total quality of frozen red radishes improved.Due to high moisture content of red radishes, the LFU assisted osmotic dehydration was applied to improve the quality of frozen products. The results showed that LFU can significantly improve the mass and heat transfer efficiency during osmotic dehydration process; this was because the “Sponge effect” of LFU generated micro-channels inside the cellular tissues of red radishes according tomicrostructure analysis. After the subsequent freezing process, the frozen product with osmotic dehydrated treatment possessed higher quality. In addition, with equal water content, the characteristic freezing time of LFU assisted osmotic dehydrated sample was shorter than that of osmotic dehydrated sample; this was probably because the LFU assisted osmotic dehydrated sample had lower freezable water content from the analysis of DSC.The effect of LFU assisted thawing on the thawing rate under aerial and water mediums was investigated. The results showed that LFU can increase the thawing rate of frozen red radishes under both mediums. In addition, the effect of different thawing methods on the thawing rates, quality attributes and microstructure of red radishes were also studied. The results showed that microwave thawing had the highest thawing rate, but maximum amount of drip loss and the most severely damaged microstructure, while refrigerator thawing had the lowest thawing rate, but minimum amount of drip loss and the least damage to microstructure. The LFU assisted thawing significantly increased thawing rate and retained the color and Lascorbic acid of red radishes. This illustrates that LFU assisted thawing is a promising technology in the future food industry.Because the intercellular space of red radish contained a certain amount of air, it was not suitable to use red radish as the material to study the mechanism of LFU assisted freezing. The solid model system, which simulated the main component of red radish, was comprised of 3% gelatin, 4% sucrose and 93% distilled water. Then the carbon dioxide absorption regularity of solid model system under different pressurized carbon dioxide pressures was studied. The effects of different carbon dioxide content and LFU conditions on the nucleation temperatures and delays of solid model system were also investigated. In addition, the effects of pressurized carbon dioxide and LFU treatment on the freezing time and ice crystals size and distributions were also studied. Results showed that the absorption rate was increased with increasing carbon dioxide treated pressure and faster absoption was observed at the initial stage. The treatment of pressurized carbon dioxide, LFU treatment or the combined treatments can significantly promote the occurrence of nucleation. In addition, the combined treatments had synergy effect on the occurrence of nucleation. Cavitation effect was the main but not the only mechanism for LFU inducing the nucleation of solid model system. From the results, it can be found that the solid model system can well reflect the nucleation of the red radishes. In addition, pressurized carbon dioxide and LFU treatment can improve freezing rate and decrease freezing time. Compared to untreated sample, the ice crystal size of samples with pressurized carbon dioxide treatment, LFU treatment and the combined treatment decreased 25%, 29% and 53%, respectively.