Proximate composition and quality evaluation of Thunnus tonggol and Thunnus albacares landed in Chabahar coast during storage in two states of ice and freezing

Considering that the tuna fishes are used as raw material for canning therefore it is necessary to identify the suitable method of catching and keeping them in order to maintain thequality of the raw material. Usually, changes in fish after catching until consumption or processing may be due to reasons such as chemical and enzymatic reactions, oxidative spoilage and bacterial growth, which cause the production and increase of some chemical compounds such as total volatile nitrogen, peroxide number, thiobarbituric acid, change in pH results in a decrease in the quality and shelf life of the fish (Rezaei and Hosseini, 2008), so the proper method of fishing and the correct method of storage and transportation after catching can play an important role in controlling these changes and creating the desired quality in fish and its products include canned fish. The principle components of this fishes muscles are water, fat, protein, minerals and vitamin compounds. Biochemical methods based on production of trimethylamine (TMA), total volatile basic nitrogen (TVB-N) and lipid oxidation have also commonly used to evaluate fish quality (Jinadasa et al., 2015). Thunnus tonggol and Thunnus albacares belongs to the family Scombridae and they are pelagic large size marine fish (Jinadasa et al., 2015).. Thunnus tonggol and Thunnus albacares fisheries are of crucial importance to Iran due to their economic importance of fresh, frozen and canning trade. The aim of this study is therefore to determine the nutritional value and quality of the Thunnus tonggol and Thunnus albacares that landed in Chabahr coast using somechemical indices, to established the limit of acceptability and determine if the type of catching and storage as effects on the rate of deterioration.

The samples studied in the present research are Thunnus tonggol and Thunnus albacares fishes with average length and weight of 69.50±2.62 cm and 4123±69.45 gr and 75.00±11.115 cm and 7399±103.44 gr respectively, which were caught by local fishermen using fishing boats using two methods of fishing with hooks and gillnets, and until they reached the shore of Briss in Chabahar, they were in ice in some boats and in others they were kept frozen. After landing the fishes on the beach for qualitative evaluation, the studied samples were divided into 8 groups based on the type of fishing method (hook and gillnet) and the method of storage after catching (keeping in ice and freezing) on the float. The standard method of AOAC (2005) was used to measure the proximate compositions include moisture, crude protein, crude fat, and crude ash of the samples. The determination of the pH value was performed according to Chen et al. (2019). The determination of the TMA and TVB-N value was performed according to Li et al. (2019). Peroxide value was measured according to the iodometric method provided by Egan et al. (1979) through fish oil extraction using methanol-chloroform and anisidine index was performed according to the AOCS (1998) method with a standard number of 90-18 Cd based on the reaction between the anisidine reagent diluted in Glacial acetic acid with aldehydes in the fat sample.

In the present study, the results of the proximate composition in the muscles of Thunnus tonggol and Thunnus albacares fish species showed that the moisture content was above 70-75% in all groups of fish, except for the Thunnus tonggol fish caught with the hook method and kept in freezing condition. Usually, the largest percentage of biochemical compounds in the fish body is made up of water, which is less than 80% in medium-fat fish such as tuna (Amiri et al., 2015). Tuna fish have high protein (22.26-22.2 gr per 100 gr of muscle) and low fat content (0.2-2.70 gr per 100 gr of muscle) and high moisture content in fish with low fat content (Rani et al., 2016), that this was also observed in the present study. The amount of moisture and protein in Thunnus tonggol fish was slightly higher than Thunnus albacares fish. The difference in the proximate composition of fish may be due to feeding or fishing ground and season (Ali et al., 2020). In terms of fat content, Thunnus tonggol fish had more fat than Thunnus albacares fish, but the percentage of fat in both species was less than 5%. Usually, the amount of fat in tuna fish is less than 5% and they are classified as lean fish (Bahurmiz, 2019). The amount of ash was not much different in both types of Thunnus tonggol and Thunnus albacares fish, this issue shows the same nutritional value of Thunnus tonggol and Thunnus albacares fish in terms of minerals, because ash is a indices for measuring the mineral content of different foods, including fish. The amount of moisture, fat and protein in Thunnus tonggol fish caught with gillnet and kept in frozen state has decreased significantly compared to other fish groups (p<0.05), but there was no difference in the amount of ash between different groups of Thunnus tonggol fish (p>0.05). The reduction of protein in food products stored at freezing temperature can be caused by protein denaturation as a result of the increase of ionic bonds in the intracellular tissue, followed by the migration of water to the extracellular tissue.The reduction of fat during the storage period of fish may be due to the activity of enzymes involved in the hydrolytic spoilage of fat and its conversion into free fatty acids (Kusuma et al., 2017). In Thunnus albacares fish, except for the moisture, which was significantly reduced in the group of fish caught with gillnet and kept in frozen state compared to other groups (p<0.05), the amount of other proximate compounds did not differ significantly between different groups of fish (p>0.05) and the highest amount of these compounds was related to the group of Thunnus albacares fishes caught with hooks and kept in frozen condition. The measurement of chemical quality indicators showed that the pH level in Thunnus tonggol fish caught with gillnet and kept in ice increased significantly (p<0.05) and in Thunnus albacares fish caught with hooks and kept in freezing condition, decreased significantly (p<0.05). The decrease in pH indicates the phenomenon of glycolysis and as a result the production of lactic acid and its subsequent increase to a value higher than 7 also indicates the loss of quality due to the production of volatile compounds and the start of autolytic activity (Done et al., 2018). The peroxide value, thiobarbituric acid and TMA in the groups caught by the hook method and kept in the frozen state was significantly lower than the other groups, but the anisidine index did not show significant difference in all the fish samples (p>0.05). The amount of TVB-N in Thunnus tonggol fish had no significant difference and in Thunnus albacares fish it was 13.21 ± 1.53 mg/100g in the method of catching with gillnet and kept in ice, it was significantly higher than other groups (p<0.05). The amount of these compounds was lower than the acceptale limit (Jinadasa et al., 2015), which indicates the proper quality of the fish

Overall, the results of this study showed that fishes caught with hooks and kept in frozen condition have better nutritional value and quality than the other groups.