Comparison of methods of differentiating between fresh and frozen–thawed fish or fillets

Abstract

AbstractFreezing is an efficient way of storing fish. Objectively though, it is very hard to determine whether a fish has been previously frozen. Following an appraisal of various methods, we selected a physical determination (torrymeter), a physiological examination (eye lens) and three enzymatic assays (α‐glucosidase, β‐N‐acetylglucosaminidase and β‐hydroxyacyl‐CoA‐dehydrogenase) and applied them to three species: plaice (Pleuronectes platessa), whiting (Merlangus merlangus) and mackerel (Scomber scombrus). We also compared the results obtained following slow and rapid freezing and investigated how spoilage affects the torrymeter measurements and α‐glucosidase assay values. For whole fish the physical method using the torrymeter is a reliable indicator. For fish fillets we recommend the enzymatic method using the α‐glucosidase assay, which should be accompanied by measurement of the freshness to avoid confusing a frozen–thawed fish and a fish in an advanced stage of spoilage. The values noted for fresh and thawed whiting and plaice indicated cut‐off values of 0.15 for whiting and 0.5 for plaice, above which it can be asserted that the sample had been frozen.© 2002 Society of Chemical Industry