Simulation and experiment of thawing process of pork under vacuum condition

Abstract: This study aims to explore the temperature distribution and thawing rate of frozen pork under vacuum conditions (vacuum degree of 100 Pa) during the thawing process, as well as the effect of vacuum thawing on pork quality. A total 20 kg of foreleg meat was collected from the same male white pig (about 6 months old) that had not yet been cooled were purchased from a local Metro supermarket. The purchased pork was first placed in an incubator with ice bags and transported back to the laboratory within 30 minutes, and then taken out and placed on a sterile operating table for simple cleaning to remove the surface fat and fascia tissue, and finally cut into pieces, each piece weighing about 1 kg with roughly the same shape and size (12 cm×12 cm×12 cm). The thermocouples were inserted into the center of meat blocks to measure temperature, and marked the sample number, then the meat blocks that put in zip-lock bags were placed in a -35 ℃ freezer for freezing. The freezing was stopped when the thermocouples measured the core temperature of meat blocks to reach -20 ℃, and then the samples were taken out for vacuum thawing and air thawing experiments. In the vacuum thawing experiment, it was performed after connecting the thermocouples of samples to the temperature sensors of the thawing box and setting the thawing pressure of 100 Pa, the thawing temperature of 15 ℃. In the air thawing, the same thawing temperature was set for the experiment. Both thawing process was set to start when the temperature of sample center was -18 ℃, and then automatically recorded the sample temperature per 5 minutes until the sample center temperature was 4 ℃ to terminate the experiment and stop recording. After the thawing was completed, some properties of pork were tested, including the water retention (thaw loss rate, cooking loss rate and pH value) and texture properties (hardness, elasticity, chewiness, adhesiveness, and resilience). The vacuum thawing was a complex physical process, including solid heat transfer and water vapor condensation. In the simulation, the solid heat transfer with meat as the research object and the condensation process with moist air as the research object were separately calculated. The heat transfer model was established according to the actual size of the frozen pork sample. The water vapor condensation model used a two-dimensional steady-state model to calculate the physical field. The thawing process in air thawing method was simplified to a pure heat transfer process. The finite element analysis software COMSOL Multiphysics was used to simulate the vacuum thawing process and the air thawing process, respectively, and the numerical simulation results were verified by the experimental results. The results showed that the numerical simulation time for vacuum thawing and air thawing were 8.90 and 27.30 h, respectively, whereas, the physical experimental results were 8.83 and 28.40 h, respectively. The errors between the simulation and experimental data were 0.79% and 4.03%, respectively, indicating in the acceptable range of errors. After testing the quality of thawed pork, it was found that the thawing loss rate and the cooking loss rate in the vacuum thawed pork were 1.45% and 5.36%, respectively, whereas, that in the air thawing in the control group were 5.12% and 19.90%, respectively, indicating the impact was remarkable (P<0.05). After vacuum thawing, the pH value of vacuum thawed pork maintained the best, about 6.49 (pH measured before thawing is 6.50), indicating significantly better than 5.96 of the air thawing group (P<0.05). The texture of vacuum thawed pork was also better than that of air thawing group (P<0.05). Compared to natural air thawing, the vacuum thawing has a faster thawing rate, while better maintain the quality of meat after thawing. The established vacuum thawing model has better reliability to represent the heat transfer and thawing process in the vacuum thawing. The finding can provide a sound reference for the in-depth research in the field of vacuum thawing.s of vacuum thawing, which provides a certain reference for in-depth research in the field of vacuum thawing.