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Influence of lipid oxidation and protein solubility on bioelectrical properties of fish during frozen storage

In our earlier studies, the potential of the bioelectrical measurements for differentiation the frozen fish fillets with low lipid content was evaluated. The results showed that the frozen fillets with low lipid content may be differentiated by bioelectrical measurements and that these measurements possibly reflect changes on proteins during frozen storage. Unlike the frozen fish with low lipid content where shelf life is primarily limited by the loss of protein functionality, the shelf life of fish with higher lipid content is most often limited by the oxidation of fatty acids. The occurrence of lipid oxidation in frozen fish may lead to the changes in flavor, appearance and also, protein functionality. With the aim of assessing the relationship between changes on proteins and lipids with bioelectrical measurements of frozen fish with higher lipid content, frozen Atlantic chub mackerels with lipid content between 7 and 10%, were purchased from a retailer and examined after 1, 2, 3 and 5 months of storage. The HP 4294A Precise LCR meter was used to measure impedance magnitude (|Z|) and phase (φ) at 200 frequencies from 100 Hz to 100 MHz. The protein solubility, fatty acid content and the oxidation products of fatty acids were also determined. Impedance measurements (phase) showed the best results for detecting the differences among the fillets at frequencies between 10 and 30 MHz. The correlation between phase and protein solubility was moderate. Protein solubility was strongly negatively correlated with lipid content and with some products of fatty acids oxidation (octanal and heptanal), which confirmed the interaction between lipid oxidation and changes in proteins. Overall, the results indicated that lipid oxidation resulted in loss of protein solubility but the correlation between protein solubility and impedance measurements is not as strong as when measured on the fish with lower fat content.

fish, lipid oxidation, bioelectrical impedance

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