Seasonal development of nutrient composition, lipid oxidation and colour of fillets from Norwegian spring-spawning herring (Clupea harengus L.)

The Norwegian spring-spawning herring (Clupea harengus L.) forms the basis of extensive fisheries off the Norwegian coast from September to January each year. Herring is filleted, frozen and exported, mainly to the East European markets. Fillets from herring caught late in the season tend to be discoloured and it was hypothesised that this was due to lipid oxidation. Herring fillets from fish caught throughout the season were sampled at a local filleting plant. The fillets had been frozen and stored at −30 °C for varying periods. Dry matter and lipid content, fatty acid composition of lipids, vitamin C and E, lipid oxidation and the colour of the fillets were monitored. Lipid and dry matter content of the fillets decreased during the season from 175 to 75 g kg−1 and 364 to 260 g kg−1, respectively. Vitamin C content decreased during the season and varied between 3.3 and 1.0 mg kg−1, while vitamin E varied between 21 and 7 mg kg−1. The change in fatty acid composition was characterised by a decrease in polyunsaturated and saturated fatty acids and an increase in monounsaturated fatty acids during the season. Thiobarbituric acid reactive substances (TBARS) varied between 43 and 122 nmol g−1 and peroxide value (PV) between 2.6 and 6.8 μmol g−1. The fillets became darker, redder and yellower toward the end of the season, especially after spawning migration had commenced. There was a negative correlation between vitamin E content and both TBARS and PV, indicating that vitamin E protected against lipid oxidation or that vitamin E was consumed during lipid oxidation. Very little connection was found between lipid oxidation and colour development. It was concluded that the colour deterioration in the fillets was not due to lipid oxidation, but may have been caused by the change in proximate composition, less lipid leading to less white and more transparent fillets when the dark muscle and blood stains became more visible.