Background. During ice storage of fish a series of biochemical changes occur at a relatively rapid pace. The changes involve both nitrogen compounds and the lipid fraction. Lipid damage, through hydrolysis and oxidation reactions, can lead to important losses in nutritive value and quality during chilled storage. Oxidised lipids are a source of free radical compounds and highly reactive peroxides. The aim of this study was to assess the quality of two economically valuable species of Baltic fish, especially the quality of lipids, during storage in ice. Materials and methods. The analysis involved two Baltic fish: cod, Gadus morhua Linnaeus, 1758—a lean fish (about 1% of fat in flesh) and flounder, Platichthys flesus  (Linnaeus, 1758)—an average-fat fish (about 4% of fat in flesh). Descriptive testing was used for quality determination of fish and the chemical changes of nitrogen compounds (TVB-N)(Total volatile bases nitrogen) and lipids (PV—peroxide value and AsV—anisidine value) were analysed. Changes of FA (fatty acids) were based on the qualitative interpretation of chromatograms. Results. In cod, peroxide value reached after 10 days a value of 45.47 mg O* ∙ 100 g–1 of lipids. In flounder peroxide value reached the maximum of 26.98 mg O* ∙ 100 g–1 after 12 days. The initial anisidine values did not differ significantly and remained at a similar level during storage. It was found that the most significant changes involved loss of EPA. After fifteen days flounder lost approximately 6.81 mg of EPA ∙ g–1 of fat (about 10% of initial amount), whereas cod about 9 mg EPA ∙ g–1 of fat (about 26.5% of initial amount). The observed loss of DHA was much lower, in particular in flounder. Conclusion. Different dynamics of oxidative changes were observed. In cod primary oxidation products increased linearly but in flounder such dynamic growth occurred after 12 days of storage still being almost twice as low as that of cod. No significant differences were observed in the secondary oxidation products between the fish. The level of PUFA and especially the level of EPA and DHA decreased. The loss of PUFA were higher in the lipid fraction of cod than flounder and were respectively about 2 pp and about 0.4 pp for each day of storage. Moreover, the dynamics of loss of EPA was greater than DHA.

fish quality, shelf life of fish, lipid quality, EPA and DHA changes