Investigation of protein denaturation and pigment fading in farmed steelhead (Onchorhychus mykiss) fillets during frozen storage

The present experiment was conducted to measure the extent and nature of quality loss in farmed steelhead fillets during frozen storage, to determine the possible causes of steelhead pigment fading as a result of frozen storage and to suggest some methods for improving storage conditions in order to obtain good-quality frozen steelhead. This two-part study confirms the importance of frozen temperature and appropriate storage conditions on the extension of product shelf life. The fading phenomenon of steelhead fillets during 10 months of frozen storage at -20C was subsequently studied in detail over a 6-week period for fillets stored at both -5 and -30C. Associated protein denaturation, tough/dry texture and rancidity were also examined. Temperature had a stronger effect on the development of rancidity in steelhead fillets than the duration of the storage period for fillets stored for up to 6 weeks. Highest levels of oxidation products were observed in the fillets stored at -5C even in the short-term storage experiment while extensive lipid oxidation, toughening, expressible fluid loss on thawed fillets and apparent pigment fading took place during long-term storage at -20C. The results from sensory evaluation of texture, color and flavor agreed well with the chemical assessment of rancidity. Flavor scores from the fillets stored at -5C revealed a slightly oily/rancid taste as compared to the samples at -30C after 6 weeks of storage. The expressible fluid results indicated that the binding of water to protein decreased significantly in fillets stored at -5C as compared to the matching fillets at -30C and during long-term storage at -20C. The increases in expressible fluid correlated positively with fading (L*) and negatively with redness (a*). The quantity of extractable protein nitrogen (EPN) decreased drastically over the 6-week period in both temperature groups (-5 and -30C), but during this relatively short experiment, the EPN levels between temperature treatments were not significantly different. In both experiments, fillet redness faded dramatically as a result of frozen storage. Abusive cold storage at -5C resulted in far more fading than at -30C even after 6 weeks. Fading was measured both subjectively and objectively using reflectance colorimetry, and an increase in whiteness value was concomitant with a decrease in redness. It is suggested that whiteness may be used as an accurate subjective parameter for color fading in frozen storage. Redness values were significantly higher in the fillets at -30C. The study showed that pigment fading was not a result of a decrease in carotenoid concentration but may be related to protein denaturation, which causes muscle tissue appearance to change from translucent to opaque, thereby giving the illusion of pigment fading.