Effects of Gamma Irradiation on Inhibition of Urease Activity and Fishy Smell in Mackerel (Scomber japonicus) during Refrigerated Storage

Abstract

IntroductionRecently, there has been an increasing demand for fish, as an excellent nutrient source rich in minerals such as selenium, and nitrogen compounds, including taurine and betaine, and vitamins [1]. However, fishy smell, induced by the deterioration of freshness after capture, is a major limiting factor in the consumption of fish, including the direct consumption and development of processed foods. Deterioration of fish is largely caused by autologous enzymes, microorganisms, and oxidation, while the substances that cause fishy smell due to freshness deterioration include trimethylamine (TMA), formaldehyde, hydrogen sulfide, methyl mercaptan, ammonia, and volatile basic nitrogen (VBN) [2][3][4].Elasmobranchs, such as rays and sharks, contain large amounts of urea in their muscles, and urease secreted by microorganisms decomposes urea to produce ammonia. In the case of fish, urease in muscles makes the pH alkaline after death and volatility increases, resulting in a strong ammonia-like smell [5,6]. Vibrio parahaemolyticus is a basophilic bacterium distributed in seawater and various seafood including fish and shellfish. V. parahaemolyticus is greatly affected by water temperature, salt concentration, pH, and levels of available organic matter. The species also produces various metabolites [7]. While several species of bacteria are reported to produce urease, about 8% of V. parahaemolyticus strains isolated from the marine environment were found to be urease-positive [8]. Therefore, to control the substances causing the fishy smell of fish and shellfish, it is important to inhibit the activity of enzymes that play important roles in the generation of the odor. Methods to suppress the fishy smell of fish mainly involve antioxidant action by adding various natural substances such as citron, coffee gourd, onion, ginger, Zanthoxylum piperitum, persimmon leaves, Agastachis Herba, rosemary, and sage extract [9-14]. However, so far, there has been a lack of research regarding the inhibition of the enzymes that generate fishy smells.Mackerel (Scomber japonicus) contains large amounts of n-3 fatty acids, such as eicosapentaenoic acid, 20:5n-3 (EPA), docosahexaenoic acid, 22:6n-3 (DHA), polyunsaturated fatty acids (PUFA), and peptide and proteinIn this study, gamma-irradiated mackerel (Scomber japonicus) meat was stored in a refrigerator for 20 days to examine the physicochemical changes related to fishy smell. The effect of gamma irradiation on the inhibition of the activity of crude urease extracted from Vibrio parahaemolyticus was also evaluated. Increased levels of trimethylamine (TMA) and volatile basic nitrogen (VBN) content, which are the main components causing fishy smell, were significantly reduced by day 20 of storage after gamma irradiation, indicating that freshness was maintained during storage. The ammonia nitrogen contents of 3, 7, 10, and 20 kGy gamma-irradiated groups were significantly decreased by 6.5, 15.2, 17.4, and 23.9%, respectively, compared to non-irradiated groups on day 20 of storage. In...