Document Type : Full Research Paper
Authors
Department of Fisheries, Faculty of Marine Natural Resources, Khorramshahr University of Marine Science and Technology.
Abstract
Introduction: The Spangled emperor (Lethrinus nebulosus) is one of the valued fish species in Persian Gulf, which due to its high nutritional quality and excellent sensory properties, is preferred by the customers in the south of Iran. Because this species is consumed domestically, it is very important to extend its shelf life, which is normally quite limited when kept refrigerated. Vacuum packaging (VP) is one of the methods of the natural preservation in order to delay the degradation and maintain the quality of the products longer. VP is widely used as a supplement to ice or refrigeration to decrease the supply of oxygen to the aerobic bacteria in the flesh to extend the shelf life of product. Chitosan have been used in seafood products to inhibit the growth of bacteria in stored fish in refrigerator and retarded the oxidation of unsaturated fatty acids in fish muscle before vacuum packaging. Chitosan, a linear polysaccharide of randomly distributed β-(1-4)-linked D-glucosamine and N-acetyl-D-glucosamine, is a biocompatible polysaccharide obtained from deacetylation of chitin. Edible coating is a thin layer of edible material formed as a coating on a food, while an edible film is a preformed thin layer which once formed can be placed on or between food components. In food industry, chitosan coatings have been used successfully because of some advantages such as edibility, biodegradability, aesthetic appearance and barrier properties, being nontoxic and non-polluting, as well as carrier of foods additives (i.e.: antioxidants, antimicrobials). Therefore, these coatings can retain quality of raw, frozen and processed foods including fish items by preventing bacterial growth and delaying lipid oxidation.
Materials and methods: Lethrinus nebulosus with an average weight of 500 g was caught with gill net in the Persian Gulf, Khorramshahr, Iran in July 2016. Fish samples were placed in crushed ice with a fish/ ice ratio 1:3 (w/w) and transported to the fish processing laboratory with 2-3 h after catch. They were washed with tap water and two fillets were obtained from each fish after removing the head and gutted. Chitosan solution was prepared with 1% (w/v) chitosan (Sigma Chemical Co., medium molecular weight, viscosity 200-800 cP) in 1% v/v acetic acid. To achieve complete dispersion of chitosan, the solution was stirred at room temperature to dissolve completely. Glycerol was added at 0.75 ml/g concentration as a plasticizer and stirred for 10 min. All films were obtained by casting 100 ml film forming solution on a nonstick surface (16*27cm), dried at ambient temperature (20˚C) until a firm surface but still with adhesive properties was obtained. After evaporation the films were peeled off from the plates. Fillet samples were randomly assigned into four treatment lots consisting of: one control lot (un-coated), second lots packaged with VP, third lots wrapped with films prior to VP, fourth lots immersed for 30 s in chitosan solution. Then the fish fillets were removed and allowed to drain for 2 h at ambient temperature (20˚C) in order to form the edible coating. All samples were stored at 4±1˚C for 12 days. Chemical, microbiological and sensorial analyses were performed at 3-day intervals to determine the overall quality of fish.
Results and discussion: Combination of vacuum packaging and chitosan treated samples effectively retarded the TVB-N and TMA values and inhibited the growth of total viable count and psychrophilic count bacteria during refrigerated storage. Therefore, to extend the shelf life and delay the deterioration of fresh Spangled emperor fillets during refrigerated storage, chitosan coating prior to vacuum packaging is more appropriate. These coatings and films also showed antioxidant effects, since TBA and FFA values was lower than control samples at the end of the storage. There was no significant difference between coating and film in reducing TBA of fillets and bacterial contamination. Therefore, chitosan coating and film prior to vacuum packaging provides a type of active packaging that can be utilized as a safe preservative for fish under refrigerated storage.
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