فهرست مطالب بابک کرمی

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.

This study is designed to investigate the effects of slow and quick freezing on fatty acids and microbial load of Oreochromis niloticus. Fillets were prepared in groups of slow and quick freezed and were preserved in -18ᵒ centigrade for six months. Changes in Fatty acids amounts and microbial load were monitored every month. Our results indicate statistically significant changes in fatty acid amounts during cold-room preservation in all tested samples. Fresh fillets of O.niloticus contain 24.84 percent of sutured fatty acids (SFAs), 36.14 percent of mono unsutured fatty acids (MUFAs) and 38.62 percent of poly un-sutured fatty acids (PUFAs). However, in the last month of sampling, SFA and MUFA levels significantly increased; whereas PUFA level decreased (p<0.05). In slow and quick freezed O.niloticus, SFA increased to 28.90% and 27.38%, MUFA reached 39.55% and 38.21%, and PUFA decreased to 30.56% and 35.22%, respetively. Changes in fatty acid levels were significantly less severe in fillets with quick freezing method (p<0.05). The results of total count indicate that quick freezed samples had lower numbers of microbial load in comparison with slow ones (p<0.05).

The aim of this study was to investigate the effects of slow and quick freezing on proximate compositions, drip loss, sensory properties changes, and microstructure of Red Tilapia (Oreochromis niloticus × Tilapia mosambicus) fillets. For this reason, skinned and deboned tilapia fillets were frozen by slow and quick frozen methods. The samples were packed and storedat -18 °C for six months. Proximate composition, drip loss, and sensory evaluation of the samples were determined every month. Microstructure of the samples was studied using Scanning Electron Microscopy (SEM) every two months. Results indicated that fresh tilapia fillets had 1.68, 20.26, 1.38, 78.06 percentage of fat, protein, ash and moisture content, respectively. The proximate compositions were changed during the storageperiod. Quick frozen samples had significantly (P<0.05) lower changes than the slow frozen samples. The amount of moisture, ash, protein and fat for samples with slow freezing were 73.56, 2.76, 17.56 and 0.73 percentage and for samples with quick freezing were 76.31, 1.89, 18.01 and 1.18 percentage, respectively. The percentage of the drip in the slow frozen samples was significantly higher than quick frozen samples (P<0.05). The amount of drip was 11.4 and 6.1 percentage for slow and quick freezing samples after six month. SEM micrographs also showed that the changes in the microstructure of the samples were different in the slow and frozen samples. Slow freezing methods caused higher damage in the microstructure of the sample than quick freezing methods. Sensory evaluation of the samples indicated that there was a better acceptability in the quick frozen samples than slow frozen sample (P<0.05).