ULTRAMICROSCOPICAL STRUCTURES AND LIQUID LOSS IN HEATED COD (GADUS-MORHUA L) AND SALMON (SALMO-SALAR) MUSCLE

This study was performed in order to assess the effect of heating in p re- and post-rigor muscle of fed cod, wild cod and farmed salmon harve sted at different times of the year. The structural changes in muscle samples pre-heated from 5 to 60 degrees C were qualitatively evaluated using both light and transmission electron microscopy techniques. The microstructural changes are discussed in relation to the liquid loss measured by a low-speed centrifugation test. The heat-induced structur al changes varied between the fish tested, reflecting different degree s of post mortem degradation prior to heating, the muscle-pa and speci es-specific structural properties. The fed fish, both cod and salmon, underwent the most severe structural degradation. This reflected both the low muscle pH and the more severe post mortem degradation observed in these fish prior to heating, compared with the wild cod. Heating c aused extensive shrinkage of the myofibrils and hence, widened intermy ofibrillar and extracellular spaces in both the fed cod and the salmon muscle. In the sample of wild cod muscle, the extracellular spaces we re narrow and the myofibrils were closely packed. The difference in he at-induced liquid loss of the fed compared with the wild cod muscle co incides with their different structural features, as observed both by LM and TEM. The better liquid-holding properties of the salmon muscle than the cod muscle are attributed to the species-specific ultrastruct ural features as observed with TEM. In addition to the denser appearan ce of the salmon myofibres, it is suggested that both fat droplets and aggregated sarcoplasmic proteins filling the intermyofibrillar and ex tracellular spaces are important in preventing release of liquid upon heating.