Development of lipid oxidation and flesh colour in frozen stored fillets of Norwegian spring-spawning herring ( Clupea harengus L.). Effects of treatment with ascorbic acid

Frozen stored fillets from Norwegian spring spawning herring ( Clupea harengus L.) become discoloured. Based on the hypothesis that lipid oxidation in the fillets caused discoloration, we designed an experiment where ascorbic acid was added to the water (2 g l −1) in RSW-tanks on board a fishing vessel and/or by spray (20 g l −1) after filleting but before freezing. Treatment with ascorbic acid on board and on the fillet line led to 8- and 17–24-fold increases, respectively, in the ascorbic acid concentration in the herring fillets. Ascorbic acid treatment on the fillet line, but not the treatment on board, protected the herring fillets against oxidation during freezing, an effect that could be detected until 9 weeks of storage at −30 °C. Between 9 and 14 weeks of storage there was a new burst of lipid oxidation where the treatments had no effect. There was a large variation in colour within each fillet and between fillets in the same group. During freezing, the amount of yellow colour increased substantially and the amount of red colour increased in the posterior end of the fillets, causing a visible change in appearance. Thereafter only minor changes occurred until week 10 but, in week 30, the fillets had become less red, causing a less fresh and more grey appearance. Compared to the within fillet and within group variations, there were only minor effects of ascorbic acid treatment on colour. It is suggested that shelf life of frozen herring fillets should be set at between 9 and 14 weeks at −30 °C. Treatment with ascorbic acid will not alleviate the discoloration of frozen herring fillets.