Type V collagen became solubilized in softened sardine muscle after 1 day of chilled storage with the concomitant weakening of pericellular connective tissue induced by disintegration of thin collagen fibrils. However, no significant changes were observed in the structure of interstitial connective tissue or biochemical properties of type I collagen. Z disk in myofibrils showed structural changes, but no significant loss of longitudinal continuity of myofibrils was observed even at the deteriorated Z disk from the muscle destroyed by compression test. On the other hand, tiger puffer muscle did not show significant softening during the storage, with no significant changes in structure of connective tissues and biochemical properties of collagens. These facts suggest that degradation of type V collagen causes disintegration of the thin collagen fibrils in pericellular connective tissue, weakening pericellular connective tissue and resulting in postharvest softening. Keywords: Collagen; postharvest storage; fish; connective tissue; muscle; collagen V

Bleeding caused the delay of muscle softening in yellowtail, horse mackerel, and striped jack, which are pelagic fish. Conversely, bleeding had no influence on the muscle firmness of red sea bream, flatfish, and rudder-fish, which are demersal fish. Transmission electron microscopy showed delay of degradation of pericellular collagen fibrils in bled yellowtail and horse mackerel. Striped jack showed slower weakening of the pericellular connective tissue in a compression test. However, the demersal fish had no structural difference due to bleeding. These results indicate that removal of blood could delay collagen fibril degradation and muscle softening of pelagic fish.

The three-dimensional structure of a collagen fibrillar network was examined by using a cell-macer ation/SEM method combined with measuring the breaking strength of rainbow trout, yellowtail, and tiger puffer muscles. Rainbow trout and yellowtail muscles were tenderized during storage, but tiger puffer muscle showed no tenderization even after 72 h storage. According to histological observations, the thickness of pericellular connective tissue in rainbow trout and yellowtail muscles became thinner, and the density of their collagenous fibrils decreased during 24 h storage. On the other hand, no struc tural change was observed during 72 h storage in tiger puffer muscle. These results showed that the struc tural change in the collagen fibrillar network corresponded to the post-mortem tenderization. This result agreed well with previous light microscope and transmission electron microscope observations.

A considerable amount of squid mantle collagen, 45% to 70% of total collagen, was not solubilized even after 30 min of heating in boiling water with its fibrous structure left intact. Pyridinoline, one of the major intermolecular crosslinks in matured collagen, was more predominantly included in the insoluble collagen than in the soluble one (p , 0.05). These results suggest that pyridinoline is closely related to the heat solubility of squid collagen.