Comparison of different thawing methods quality of mullet fillets (Liza aurata)

Aquatic products are becoming a vital dietary component in humans’ daily life, due to their high protein and low fat, especially rich in polyunsaturated fatty acids (omega-3 and omega-6). Golden grey mullet, Liza aurata, have a commercial value for fisheries, especially in internal waters. This species usually lives inshore, lagoons and estuaries, and rarely move into freshwater. Additionally, they are widely distributed in the Mediterranean Sea and the Black Sea, Atlantic coasts from the Azores and Madeira northward to the British Isles, and the southern coasts of Norway and Sweden. Many technologies have been used in the food industry to extend the shelf-life of fish for ease of transportation, such as freezing which is the most effective method to ensure food quality and safety. Consequently, the frozen fish should be thawed before further processing. Generally speaking, the quality of frozen food is highly associated with the thawing process. Improper thawing methods have caused unacceptable changes of the chemical and physical properties of frozen fish such as drip loss, enzymatic reaction, lipid and protein oxidation, and inhomogeneity. There are different thawing methods in foods reported, such as air thawing, water thawing, refrigerator thawing, microwave thawing, ultrasonic thawing, high-voltage electric field thawing, high-pressure thawing, and so on. However, each thawing method has its advantages and disadvantages. Therefore, it is necessary to explore a better thawing method that is beneficial for maintaining the quality of frozen fish. The purpose of this study was to evaluate the effects of different thawing methods on the chemical, microbial, sensory, and physical quality of goldfish fillets after thawing.

Twenty live L. aurata weighted 400± 30 g were purchased and were kept in powdered ice in a ratio of 1 to 1 in medium-sized tanks and immediately transferred to the laboratory of the National Center for Aquatic Processing Research. After arrival, the butterfly method was used to wash fishes, then their skin and internal organs were removed. Two back fillets removed from the back. Each fillet was individually packed in a polythene bag to prevent moisture loss and frozen in storage at-35°C and then maintained at -18 °C for 60 days. The fillets were randomly divided into three groups and thawed by different treatments as follows: thawing in the microwave (8°C), ambient temperature )21°C) and refrigerator temperature (4°C). The duration of thawing in each of the above conditions lasted 1 hour, 10 minutes, and 8h, respectively. The experiments were performed at the National Center for Aquatic Processing Research in Bandar Anzali (affiliated to the Iranian Fisheries Research Institute) and at the Food Industry Laboratory of the Azad University of North Tehran Branch in October and November 2016. To determine the approximate amount of protein, fat, moisture, and ash sampling was performed in zero phases (before freezing) and final stage (after 2-months storage) after thawing. For measuring some content of mullet fillets (such as TVB-N and TBA, pH, etc.), color assessment, and weight loss, sampling was done three times during 2-months storage (zero phase, after one month, and after two months). Moisture, ash and protein contents, TVB-N, and pH were measured. Also, fat content and TBA were determined. Moreover, the color assessment was done using a colorimeter (NR60CP Precision Colorimeter, 3nh, China). All experiments were performed in triplicates. The normality of obtained data was checked by Kolmogorov – Smirnov method. All data were presented as mean values ± standard error (SE), which were compared using one-way analysis of variance (ANOVA) following Duncan’s multiple range test. A P-value of less than 0.05 was considered significant. This study was done in the summer of 2018.

The results indicated that the amount of thiobarbituric acid (TBAs) and volatile nitrogen (TVB-N) and pH were significantly different between treatments. The highest value of the first two parameters and the lowest pH was found in microwave thawing (p<0.05). Also, a significant increase in thiobarbituric acid levels was observed in all three thawing methods. This may indicate increasing the fat oxidation, which may be due to the release of oxidative enzymes and peroxidants from disintegrated cells due to freezing. Regarding the moisture content, crude protein, fat content, and ash content, the results showed that thawing in ambient temperature and refrigerator is better than that thawing in the microwave. It must be due to the formation of drip in the process of thawing and the solubility of ash and protein in drip. Also, it should be related to the high energy of microwaves which causes the removal of intermittent water from fish’s bodies. also, color indices showed that the thawing fillets at ambient temperature had higher quality than at refrigerator and microwave temperature treatment, except for L*, which is the highest in refrigerator temperature.

Based on the results of this research, it can be noted that the freezing of the aquatic environment at refrigerator temperature will maintain the nutritional value of the food after dehumidification, the most suitable method for the thawing of the fillet.