Shahnaz Fattahi; Nafiseh Zamindar
Abstract
Introduction: Fish is the most extremely perishable food products (Roiha et al, 2018) and its quality may be affected by many elements (García et al, 2015). The techniques used for freezing and thawing processes play an essential function in maintaining the quality of the frozen foods. If freezing ...
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Introduction: Fish is the most extremely perishable food products (Roiha et al, 2018) and its quality may be affected by many elements (García et al, 2015). The techniques used for freezing and thawing processes play an essential function in maintaining the quality of the frozen foods. If freezing and thawing processes are carried out accurately, the quality of frozen fish will be near to fresh quality after thawing (Duygu and Ümit, 2015). There are various methods for thawing of frozen food products like still air thawing, air blast, refrigerator thawing, cold and warm water thawing, warm salt-water thawing, static water thawing and a number of innovative approaches such as high voltage electrostatic field (HVEF) thawing, ohmic, microwave, pressure-assisted, acoustic, radio frequency, infra-red and high hydrostatic pressure thawing (Mousakhani-Ganjeh et al, 2015; Mousakhani-Ganjeh et al, 2016). During Ohmic heating (OH), electrical energy is altered to thermal energy within a conductor by applying an alternative current across the food product (Darvishi et al, 2013). The energy is approximately completely dispersed within the heated food product (Shim et al, 2010). A large number of applications exists for ohmic treatments including heating, cooking, thawing, blanching, evaporation, dehydration, pasteurization, fermentation and extraction (Athmaselvi et al, 2017; Boldaji and Borghei, 2015). Materials and methods: Tuna fish (Thunnus albacares) obtained from Esfahan Pegah Co., Isfahan, Iran and transferred to the laboratory. The white fresh fish muscles were divided into cubes (3 × 3 × 3 cm3) and instantly frozen at -30°C after vacuum packaging in polyethylene bags. The frozen samples kept at -18°C until the day of experiments. The experimental apparatus of ohmic heating comprised of a power supply, an isolating variable transformer, power analyzer and a microprocessor board. The ohmic cell was made of PTFE cylinder with two displacement stainless steel electrodes. Temperature was measured with a K-type thermocouple. The electrical conductivity (σ, mS/cm) of the brine solutions was determined before thawing at 25°C. The electrical conductivity of fish minced paste was also measured at 4°C. In the present study, the tuna cubes (27 cm3) were thawed under constant ohmic power intensity (50 V) at three different brine concentrations (0.3, 0.4 and 0.5 NaCl, %w/v) during three time intervals (0, 24 and 48 hours). The parameters associated with the quality of tuna such as thawing time, thawing rate, thawing, cooking and total losses, centrifugal loss, lipid oxidation, texture and color were investigated during OH thawing. Changes in the thawing, cooking and total losses, WHC, texture, L*, a*, b* and color changes were studied as kinetic models. Results and Discussion: Results showed that, thawing under immersion ohmic thawing significantly decreased the thawing time of frozen tuna fish cubes. Suitable kinetic models with the highest regression coefficient described dependent parameters. In addition, kinetic models were validated by R2 and RMSE. Statistical analysis showed that the interaction effects of two parameters (concentration * time) on thawing loss were significant (p0.05). The lowest thawing loss was observed at 0.3 and 0.5 %w/v NaCl immediately after thawing. The lowest cooking loss was seen at highest brine concentration and the lowest total loss was observed at time zero after thawing (p0.01). The WHC of meat product is one of the most important factors affecting economical value and meat quality due to the weight change during thawing. In this study, WHC increased by increasing the brine concentration. In addition, oxidation of lipids did not occur, and TBA index did not differ significantly. This was probably due to the high speed of immersion ohmic thawing. The highest shear force was related to the treated sample with the lowest brine concentration. The highest a* was indicated immediately after thawing. Immersion ohmic thawing saved time and led to reducing the losses parameters. In addition, WHC was maintained at the highest possible level. Fish and fishery products are very perishable materials and no oxidation was observed during immersion ohmic thawing regarding the presence of NaCl,.
