Impact of high voltage electric field thawing on the quality properties of frozen mutton

Introduction: Freezing and thawing are complex heat transfer processes and the chemical and physical changes that occur during these processes can affect the product quality (Li and Sun, 2002). During the thawing process, physicochemical changes and microorganism activities damage the food products (Bertola et al, 1994; Kalichevsky, Knorr, & Lillford, 1995). Several factors can affect the degree of quality loss, including the rate of thawing, temperature, and microorganisms (Jia et al, 2017). The rate and quality of the thawing process are the fundamental needs of the meat industry, and accordingly, many studies have investigated the thawing conditions to achieve the best quality of meat products (Jia et al, 2017). Modern thawing methods such as high pressure, microwave, ohmic, radiofrequency, ultrasonic, and high voltage electric field thawing can improve the thawing rate at the low temperature (Hsieh et al., 2010). In recent years, the use of high voltage electric fields (HVEF) has been considered due to low thermal damage, no chemical consumption, and being economical. In HVEF thawing, a strong electric field is used between point and plate electrodes in an environment of dielectric fluid to create a force, followed by a secondary motion in the fluid. The secondary flow of fluid name ionic wind, or corona wind, results from localized air ionization around the point electrode. By increasing the corona flow, the volume fluctuation and air velocity increase, which results in an increase in the heat transfer coefficient (Singh et al., 2012). In this case, the thin electrode is an electric discharge electrode and another electrode is a ground electrode. One of the advantages of using an electric field for thawing is the antimicrobial effect of this method. In addition, the electric field can affect oxygen molecules of air and generates free radicals of ozone molecules, which is a strong oxidizing agent with a high disinfection capacity that various studies reported the effect of ozone depletion on poultry meat (Yang and Chen, 1979; Jaksch et al., 2004). The present study was conducted to study the effect of HVEF thawing of mutton on the process rate and its qualitative and microbial properties. For this purpose, three different strengths of the electric fields were produced by varying the voltage at a constant electrode distance, and the HVEF thawing was compared with the conventional air thawing.
Materials and Methods: The fresh mutton meat was cut into cube pieces (4 × 4 × 3 cm3, 35 g) and after packing in polyethylene bags were frozen by a freezing tunnel at -30° C for 1 hour and stored at -18 ° C until analysis. The HVEF system was consists of a generator with adjustable voltages between -50 and 50 kV and a maximum output current of 5 mA (Ls50KV5mA, China), multiple needle and plate electrodes, and a wooden enclosure. The copper plate electrode was connected to the negative pole and the spot electrode connected to the positive pole of the high voltage power. The thawing process was performed at an electrode distance of 4.5 cm and 3 different voltages of 7.6, 9, and 10.3 kV at a temperature of 25 ˚C. The thawing process proceeded until the center of samples reached 0˚C and an optical fiber thermocouple (FOB 651A, Canada) was used to determine the end time of thawing. The evaporation, thawing, cooking and total losses were measured by weighting the frozen samples before and after thawing and cooking (Mousakhani- Ganjeh et al., 2015). The surface color, pH value and total microbial count of meat samples were measured after thawing.
Results and discussions: Thawing of frozen mutton was carried out at 3 voltages of 7.6, 9, and 10.35 kV, and at an electrode distance of 4.5 cm that is equal to the field strength of 1.7, 2.0, and 2.3 kV/cm. Thawing of the control sample was performed at the same condition without field application. Results showed that the thawing rate of the frozen samples significantly increased with increasing field strength so that the thawing time at the near-spark voltage in comparison with the control sample reduced 45%. Investigation of different losses of mutton samples thawed under different strengths of the electric field showed that the evaporation loss of the thawed samples under HVEF increased by arise the electric field strength, conversely, the thawing loss was reduced with increasing the electric field strength. The control sample had the lowest cooking loss and the thawed sample under the strength of 2.0 kV/cm had the highest cooking loss. As the results show, the pH value increased with increasing the strength of the electric field, but this increase was not statistically significant (p>0.05). Investigation of the color parameters of thawed samples showed a* index, as a particular index in the meat color, decreased and L* index increased with increasing the electric field strength. The results of microbial counts of thawed samples under different strengths of the electric field showed a significant decrease (P <0.05) in the total viable counts in thawed samples under HVEF compared to the control sample. Total microbial count of HVEF thawed samples under strengths of 1.7, 2.0, and 2.3 kV/cm decreased to 4.8, 4.69, and 4.5 log CFU/g, respectively.
Conclusion: This study investigated the HVEF thawing of frozen mutton in comparison with air thawing. The results showed that the HVEF thawing of frozen mutton improved the thawing rate and the evaporation, cooking and total loss of the conventional thawed sample was lower than the HVEF thawed samples while the thawing loss decreased by applying the electric field during the thawing process. In addition, HVEF thawing process reduced the total microbial count of thawed samples by the production of ozone and negative ions that this effect improved with increasing the electric field strength.