Abstract: This study was conducted in order to explore the changes in muscle quality of sea bass during cold storage. The physiological and biochemical changes of fish meat during storage at 4 ℃ were evaluated in terms of its pH, total bacterial count, total volatile basic nitrogen (TVB-N) content, K value and texture. Morphological changes of muscle were evaluated by Masson staining and immuno-histochemical staining. The enzymatic activity of matrix metalloproteinases (MMPs) was determined by hydrolyzing fluorescent peptide substrates. The results showed that the pH of sea bass muscle decreased firstly and then increased during cold storage. The total bacterial count, TVB-N content and K value increased with storage time, and exceeded the threshold on the 8th day. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed that there was little change in the protein composition in sea bass muscle after 6 days of storage, while the myosin heavy chain was significantly degraded on the 8th day. Masson staining and immuno-histochemical staining of type I collagen showed that the endomysium was gradually degraded during storage. On the 12th day, an evident gap between type I collagen and muscle fibers was observed. The activity of MMPs increased significantly with storage time while the hardness of fish muscle decreased gradually. Hence, the softening of fish muscle during cold storage is closely related to collagen decomposition, and type I collagen degradation by MMPs may be the major reason for muscle softening after fish death.

Key words: sea bass; matrix metalloproteinase; texture; Masson staining; immuno-histochemical