THE BIOCHEMICAL AND SENSORY PROPERTIES OF GILTHEAD SEA BREAM (SPARUS AURATA) FROZEN AT DIFFERENT CHARACTERISTIC FREEZING TIMES

ABSTRACT

Skinned, vacuum‐packed, postrigor gilthead sea bream fillets were frozen individually with characteristic freezing times of 2, 18, 74 and 640 min (times to cool the thermal center of the fillets from ?1 to ?7C). Immediately after freezing, the fillets were thawed, and their quality was evaluated with tests related to muscle integrity, myofibrillar protein denaturation and aggregation, lipid degradation and changes in tenderness. The muscle integrity indices (activities of α‐glucosidase, β‐N‐acetyl‐glucosaminidase and (β‐hydroxy‐acyl‐coenzyme‐A dehydrogenase and the amount and protein content of centrifugal tissue fluids) showed that the freezing process itself clearly affected the integrity of muscles. Freezing of fillets with characteristic freezing time of 74 min caused more damage to muscles and hydrolysis of lipids than the other freezing times. Adenosine triphosphatase (ATP) ase activities and Ca²⁺ sensitivity of actomyosin extracts suggested that the freezing process itself, but not the freezing times, caused structural damage to myofibrillar proteins. No difference in the levels of salt‐soluble proteins and sulfhydryl contents in actomyosin extracts was found between the fresh and frozen fillets. Sensory evaluations showed that the cooked frozen fillets were less tender than the cooked fresh ones.

PRACTICAL APPLICATION

In commercial seafood industries, seafoods are frozen at a range of freezing times (rates) that depends mostly on the type of seafood, the type of freezer and freezer's operating conditions. Based on the work reported in the literature, freezing of seafoods at different freezing times may furnish favorable conditions for alterations in muscle structure, muscle proteins and lipids, and textural properties in general. These changes are related to alterations in quality of frozen seafoods and may affect their market. Therefore, knowledge of optimal conditions with respect to freezing rates for freezing commercially important seafoods, as farmed gilthead seabream is, is relevant to the seafood industry. This information can be obtained by experimental studies on changes in the chemical, biochemical, physical and sensory properties of seafoods frozen at different freezing times.