World Aquaculture 29 Feed composition and oxidative stability of rainbow trout during frozen storage Charlotte Jacobsen, Grethe Hyldig and Caroline P. Baron1 The rainbow trout (Oncorhynchus mykiss) farming industry has increased drastically in the last few decades reaching 600 million t in 2007, with Norway and Chile being the main producers (FAO 2010). Rainbow trout are fatty fish with a muscle fat content of approximately 10 to 15 percent and are, therefore, a good source of the long chain omega-3 fatty acids, which they obtain from their diet. Indeed, feed formulations for carnivorous fishes primarily contain fishmeal (about 30 percent) and fish oil (about 20 percent), which contains high amounts of omega-3 polyunsaturated fatty acids (PUFA), as essential ingredients. However, manipulation of the feed is seen as an interesting strategy for improving sustainability and optimizing utilization of marine resources by, for example, increasing feed digestibility, improving fish performance but also reducing the dependency of the sector on costly ingredients. Increasing pressure on fish oil producers for an enhanced product, in combination with unpredictable raw material abundance make fish oil a limited and costly feed ingredient. In contrast, vegetable oil is an abundant, sustainable resource and is readily available commercially. Several investigations have been performed to evaluate the replacement of fish oil by vegetable oil in several fish species including rainbow trout (Caballero et al. 2002, Schlechtriem et al. 2007). Most of the studies have looked at fish nutrition, feed digestibility and fish growth. Only a few investigations have been performed to determine the impact of the ingredients provided in the feed on the oxidative stability of the fish during subsequent frozen storage. However, it is often forgotten that the quality of fish muscle and its stability during storage can be linked to a large extent to pre-harvest parameters, such feeding regime, environmental factors, as well as the physiological status of the fish. In addition, processing conditions and storage conditions also affect the quality of the fish. Therefore, pre-harvest parameter impacts on fish quality need to be further investigated if premium frozen quality fish are to be found in our supermarkets. Oxidation in Fish during Storage Fatty fish species, such as salmon and trout, are good sources of long chain omega-3 PUFA, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), which have been recognized for their important nutritional and health properties. A high intake of omega-3 PUFA has been associated with the prevention of cardiovascular diseases and neurodegenerative diseases such as Alzheimers (Sahidi and Miraliakkbra 2004, 2005). However, omega-3 fatty acids are very susceptible to oxidative degradation, which means that they combine with oxygen via free radical reactions leading to the formation of primary oxidation products, including lipid hydroperoxide (peroxides). Peroxides do not have any taste but are easily decomposed into secondary oxidation products that are volatile and impart rancid odor and flavor (volatiles, Figure 1). Oxidative reactions in fish and fish products can have severe consequences and can affect not only the flavor and the odor but also the color by interacting with pigments, such as carotenoids, the texture by interacting with proteins and the nutritional value because of degradation of valuable compounds, such as fatty acids and essential amino acids. It can also to some extent affect the safety as a result of the potential formation of reactive and toxic aldehydes, such as 4-HNE and 4-HHE (4-hydroxy-2-nonenal and 4-hydroxy2-hexenal). In general oxidation of omega-3 fatty acids in fish leads to the development of unpleasant flavor, reduced nutritional value and, in general, low consumer appeal. A schematic representation of lipid oxidation and its consequences in fish is presented in Figure 1. Oxidation in fresh fish is selFig. 1. Lipid oxidation in fish and it consequences for fish quality. (With EPA and DHA long chain omega-3 fatty acids, eicosapentaenoic and docosahexaenoic acid).
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