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Characterization of oxidative changes in salted herring (Clupea harengus) during ripening.

Salted herring were prepared in barrels according to a traditional recipe. The biochemical changes in the fish and in the brine were monitored during a prolonged ripening period (12 months). The process was followed by measuring pH, protein, salt, dry matter, free fatty acids, and lipid content in the brine and in the fish according to standard protocols. The results showed that most of the biochemical changes occurred at an early stage in the ripening process. Lipid oxidation was followed in the fish muscle using spectroscopic determination for lipid hydroperoxide (PV) and by GC-MS for determination of secondary oxidation products. Protein oxidation was determined using spectrophotometric determination of protein carbonyl groups. To follow protein degradation (proteolysis) and protein oxidation SDS-PAGE and immunoblotting for protein carbonyl were performed on both brine and fish during the ripening period. Results revealed that no lipid oxidation occurred in fish muscle during ripening but a significant level of protein oxidation was detected. Finally, iron alpha-tocopherol, and 3-methylbutanal levels were also measured. Alpha-tocopherol levels decreased during ripening, further supporting that oxidative reactions took place. Peroxidase activity was demonstrated in the brine, suggesting that hemoglobin might be a crucial parameter, which might trigger protein oxidation. This indicates that protein oxidation might be important for the development of the characteristic organoleptic properties of salted herring.