Metal-catalyzed lipid oxidation and changes of proteins in fish

Abstract

The addition of metals to pure fats and model systems has given us a picture of the part they play in lipid oxidation. But in complex substrates such as fish flesh the same metal ions catalyze reactions in other components of the muscle as well. A more complete picture of deterioration is obtained when we also examine the effect of metals on the other components of the muscle.

Although lean fish muscle contains 0.5% to 1% unsaturated lipids, during frozen storage it rarely goes rancid, as indicated by thiobarbituric acid (TBA) values or rancid odors. The lipids do oxidize, but instead of forming carbonyls and other compounds associated with rancidity, they become bound up in lipid-protein complexes, which accounts for the toughened texture of overstored or poorly stored frozen fish. It has been generally accepted that the formation of these lipid-protein polymers is brought about by a reaction between the proteins and oxidizing fatty acids. For fish of the gadoid or cod family, this is further complicated by the enzymic production of formaldehyde, which forms in the muscle during cold storage. The direct addition of formaldehyde to give concentrations of 0.001% to 0.05% caused marked reductions in the extractable protein of cod muscle. Because formaldehyde and dimethylamine (DMA) are produced in the stored muscle by the same reaction, the accumulation of the DMA can be used as a measure of this process which leads to protein insolubility. Removal of the dark lateral muscle from the fillets before freezing reduced the production of DMA and formaldehyde during storage and resulted in less loss of extractable protein.