30 December 2011 environment as well as physiology are also important in relation to fish quality. For example, the fatty acid profile of the feed as well as its antioxidant content are often reflected in the fish muscle and greatly affect the oxidative stability of the fish during subsequent storage. The potential to manipulate these two parameters in relation to oxidative stability is further discussed in relation to our experimental data and our findings. Feed Composition vs. Fish Muscle Composition: You are what you eat! “You are what you eat” is also true for fish. Indeed, the feeding regime of rainbow trout is directly reflected in its muscle composition. In a recent trial (Baron et al. 2009), rainbow trout were fed an experimental diet for 64 days with feed prepared with different oil types (fish oil versus rapeseed oil) and different carotenoid pigments (astaxanthin, canthaxanthin or no pigment). A total of six diets were obtained (Figure 2. The fatty acid profile of the feed was directly reflected in the muscle (Table 1). Feed prepared with fish oil contained a high proportion of saturated fatty acids (29 percent) and of omega-3 fatty acids (25 percent), while feed prepared with vegetable oil contained a lower proportion of saturated fatty acids and omega-3 fatty acids (8 and 10 percent). As a consequence, the muscle from the fish fed fish oil contained higher proportions of omega-3 fatty acids (28 percent) when compared to muscle from fish fed vegetable oil (15 percent). In addition, not only is the fatty acid profile of the feed reflected in the fish muscle but also the presence of minor compounds, such as tocopherols (vitamin E) present in the oil. Feed prepared with fish oil contained lower amounts of tocopherols than feed prepared with vegetable oil with 20 and 35 mg/kg for α-tocopherol and 4.5 and 70 mg/kg for γ-tocopherols (Table 1). The α-tocopherol content of the fish muscle ranged from 20 to 35 mg/kg fish muscle with significantly higher values for fish fed vegetable oil. It is noteworthy that fish muscle containing no carotenoid pigments contained a higher level of α-tocopherol. In contrast to α-tocopherol, γ-tocopherol content in the feed was not reflected in the fish muscle to the same extent. The different tocopherols seemed to be deposited differently in the muscle tissue and is has been claimed that this was because of the difference in the location of the tocopherols in the muscle cells (Ackman et al. 1998). Carotenoids such as astaxanthin and canthaxanthin are color additives used in salmon and trout feeds to enhance the orange color of the muscle tissue. They are costly ingredients and can represent up to 25 percent of the cost of the feed. The levels allowed in the feed in the USA are 80mg/ kg for both carotenoids (FDA 2010). In Europe the levels allowed in feed are 25 mg/kg canthaxanthin and 100 mg/ kg for astaxanthin or for the combination of the two. The initial pigment content in the feed preparation was set at 200 mg/kg, which is well above allowed levels and the carotenoid content in the fish muscle reached approximately 10 mg/kg for both pigments. The carotenoid content in the flesh was one twentieth of the carotenoid content present Fig. 2. Experimental design. Table 1. Percentage of saturated, monounsaturated (MUFA), polyunsaturated (PUFA) and omega-3 fatty acids and tocopherols (vitamin E) content in the feed and in the fish fed either fish oil or vegetable oil.a Fish Oil Vegetable Oil Feed Muscle Feed Muscle Σ Saturated (%) 29.3 22.1 8.6 14.6 Σ MUFA(%) 33.0 32.1 60.8 49.2 Σ PUFA (%) 29.8 35.6 29.2 28.3 Σ Omega-3 (%) 25.2 28.7 10.8 15.5 α-Tocopherol (mg/kg) 21.7 20.8 36.6 29.9 γ-Tocopherol(mg/kg) 4.2 0.8 69.5 4.5 aReprinted and adapted with permission from American Chemical Society 2009, (Baron et al. 2009). dom a problem as bacterial growth and spoilage will occur faster than will oxidative reactions, however in frozen fish, the reverse is true, making oxidative deterioration the most important quality related issue. In addition, the number of frozen fish products proposed to consumers is increasing drastically and prolonged storage at suboptimal temperature can lead to serious oxidative damage. Different strategies have been proposed to prevent oxidative deterioration in muscle during frozen storage; for example, using packaging material with a low oxygen barrier, adding antioxidants, or even reducing storage temperature and light exposure, but these approaches are related to the post-harvest processing and storage conditions (Erikson and Hung 1997). However, pre-harvest parameters related to fish nutrition, living conditions and
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