Radio Frequency Thawing Process Of Frozen Pork

This paper mainly studied radio frequency(RF) thawing process of frozen pork to reduce thawing time, improve thawing efficiency and assure quality of thawed pork. Firstly, dielectric properties of pork were investigated. Then effects of electrode distance, thickness of frozen pork and intermittent time on RF thawing were explored. Thirdly, temperature change in pork during RF thawing process was simulated via software. Finally, three different thawing methods(RF thawing, air thawing and microwave(MW) thawing) were compared.A vector network analyzer was used to determine dielectric constant and dielectric loss factor of pork from-18 to 20℃ at 27 MHz, 915 MHz and 2450 MHz. Penetration depths of MW and RF in pork under different temperature were calculated. Results showed that dielectric constant, dielectric loss factor of pork and penetration depth of electromagnetic wave at 27 MHz in pork were the highest, and those at 2450 MHz were the lowest. At the three frequencies, dielectric constants of pork increased slowly with the increase of temperature from-18 to-5℃, then increased rapidly from-5 to-1℃, finally stayed stable from-1 to 20℃. From-18 to 20℃, at 27 MHz, dielectric loss factor of pork increased with the increase of temperature and the increase rate was the highest between-5 and-1℃, higher between-1 and 20℃, the lowest between-18 and-5℃; at 915 MHz and 2450 MHz, dielectric loss factors of pork increased slightly with the increase of temperature. Penetration depth of 27 MHz decreased significantly with the increase of temperature, fast during-18~-3℃, slow during-3~20℃, while penetration depths of 915 MHz and 2450 MHz decreased slightly with the increase of temperature.Effects of process conditions on thawing were investigated using a RF machine(Rated power, 6k W). Proper RF thawing condition was acquired by measuring quality of thawed pork. Results showed that thawing loss of juice decreased with the increase of electrode distance, and thawing loss of distance 14 cm was significantly higher than that of other distances. Colony forming units(CFU) of distance 14 cm was the highest and CFU of other distances had no significant difference. Pork quality thawed at various electrode distances had no difference, except that the pork quality at distance 14 cm was lower than others. The thawing loss of pork thickness ranged from 6 to 10 cm, had slight difference, and the thawing loss increased significantly when the thickness was higher than 10 cm. With the increase of thickness, CFU of thawed pork decreased. Partly overheating emerged when thickness was 12 cm or higher. Thawing loss increased with the increase of interval time. Overheating of pork can be avoided when interval time was 3 min and 5 min. Suitable RF thawing condition was as follows: Electrode distance was between 15 and 24 cm, thickness of frozen pork was less than 10 cm. When pork thickness was more than 10 cm, interval thawing was adopted, and interval time was 3 min.An electro-quasi-static and heat transfer coupled model was established using finite element method to simulate the process of RF thawing. Temperature changes during RF thawing of frozen pork were investigated using theoretical calculation and experimental determination. The simulation results of effects of electrode distance and meat thickness on thawing were investigated. Results showed a good consistency between the model predictions and experimental measurements. Thawing rate is fast(-18~-5℃), then slow(-5~-1℃), and again fast(>-1℃).The thawing duration increased with the increase of electrode distance and meat thickness. The longer the distance between electrodes and the smaller the thickness of frozen pork, the more even was the temperature distribution.Three thawing methods(RF thawing, MW thawing and air thawing) were compared by thawed pork quality, temperature distribution evenness and quality of pork ball made of the three kinds of thawed pork. Results showed that RF thawing took the least time(0.63h), about one sixteenth of air thawing(9.5h), and a bit less than MW. Quality of pork thawed by RF was the best, and quality of pork thawed by air was the worst. Temperature difference in pork thawed by RF, air and MW was 8℃, 8℃ and 36 ℃ respectively, and temperature distribution was much more even in pork thawed by RF and air. During frozen storage of a month, sensory quality, texture properties and CFU of the three kinds of pork balls had no significant difference(p<0.05). With the extension of storage time, significant difference showed up. After four months of frozen storage, the meat balls made of pork thawed by RF and MW had better sensory quality, texture properties and lower CFU than air thawing.