Structural and ultrastructural changes on muscle tissue of sea bass, Dicentrarchus labrax L., after cooking and freezing

Fish flesh undergoes structural changes during postmortem storage and processing, which could significantly influence flesh quality. This study is aimed to characterize the structural changes of the muscle tissue of the sea bass in the fresh raw state (3 h postmortem), and after different treatments (cooking and thawing). Ten reared sea bass (approximate body length 30-40 cm) were used and muscle tissue processed in fresh raw state (FR), after cooking for 5 or 10 min (FC5 or FC10), after thawing (ThR), and after thawing/cooking (ThC5, ThC10). Light and electron microscopy techniques were used to describe and quantify muscle changes after the different processing methods. Additionally, morphometry was used to estimate muscle fibre size and percentage of interstitial material. In FR samples typical early postmortem muscle tissue changes were observed: fibre to fibre detachment, detachment of myofibrils to endomysium, increase of the intermyofibrillar spaces, together with swelling of some organelle, mainly mitochondria and sarcoplasmic reticulum, and the appearance of abundant vesicles within the muscle fibres. Cooking in boiling water produced massive protein coagulation and shrinkage of muscle fibres with subsequent water loss. Thus, the connective tissue (collagen), sarcolemma and myofibrils lost their typical ultrastructural features. The interfibrillar spaces showed abundant amorphous material, which was slightly higher in specimens with smaller fibres and in muscle samples cooked for 10 min. Myofibrils were packed or distorted and abundant electron dense granular aggregates appeared at interstitial and subsarcolemmal spaces. In ThR muscle samples the formation of ice crystals during the freezing process produced abundant clear spaces occupied by liquids at the interstitial spaces and inside the muscle fibres. The reciprocal arrangement of thick versus thin contractile filaments was altered in transversal sections, most sarcolemmas were broken and the intermyofibrillar spaces significantly increased. Cooking of thawed muscle samples caused massive protein coagulation and disintegration of myofibrils. The most significant feature in these samples was the appearance of intrafibrillar cavities (holes) within the muscle fibres, which were occupied by liquids, amorphous material and granular aggregates. Also, endomysium was often replaced by a dense "chain like" line of small granular aggregates. Correlation between the structural changes, as described in the present study, and textural and organoleptic characteristics would contribute to define the optimal conditions of postmortem processing of sea bass flesh. (C) 2005 Elsevier B.V. All rights reserved.