Changes in Freshness during Frozen Storage of Farmed Coho Salmon: Effect of Replacement of Synthetic Antioxidants by Natural Ones in Fish Feeds

The addition of antioxidants to the diets for salmonids is crucial to prevent or delay deterioration of fish muscle. Most of the antioxidants used in these diets are synthetic, and their safety is more and more being questioned. The aim of this research was to compare the effects of substituting synthetic antioxidants with natural ones on selected biochemical freshness indicators of frozen coho salmon Oncorhynchus kisutch muscle. Three different feeds were tested: a conventional diet with the synthetic antioxidants butylated hydroxytoluene and ethoxyquin (diet 1) was supplied to farmed coho salmon in parallel with two different diets that included natural antioxidants (a tocopherol isomer-rich mixture [diet 2] and a tocopherol isomer-rosemary extract mixture [diet 3]). Five individuals of each group were analyzed every 3 months during frozen storage. The biochemical indicators measured were total volatile basic nitrogen (TVB-N), dimethylamine content, formaldehyde content, and tissue pH. Levels of TVB-N were significantly different between the three diets; TVB-N content was the lower for salmon fed diet 3 than for those fed diet 1, a result that was attributed to the antimicrobial action of the rosemary extracts. Formaldehyde content was similar among the salmon fed diets with different antioxidants, and pH values were always lowered to maximum limits allowed by international legislation. These results support the use of natural antioxidants in the diet of Chilean coho salmon for export.