Ra. Reid et al., STRUCTURAL AND CHEMICAL-CHANGES IN THE MUSCLE OF CHUM SALMON (ONCORHYNCHUS-KETA) DURING SPAWNING MIGRATION, Food research international, 26(1), 1993, pp. 1-9
The white muscle of migrating chum salmon (Oncorhynchus keta) becomes
increasingly soft and watery as the fish matures and progresses toward
s its spawning grounds. Flesh colour, measured by HunterLab parameters
a and a/L, was a better indicator of maturity grade than muscle fat,
protein or moisture. In the transition from grade 1, silver-bright, to
grade 4, spawning, chum muscle protein decreased from 17.9% to 15.8%,
fat decreased from 4.9% to 1.0% and moisture increased from 74.2% to
79.5%. Compositional changes and concurrent textural degradation of th
e flesh may be attributed to the physical stress of migration, starvat
ion, and hormonal changes during the development of the reproductive s
ystem. The structural basis for loss of firm texture was investigated
with light and transmission electron microscopy (TEM) by comparing the
immature (silver-bright) and mature (spawning) salmon. Light microgra
phs revealed an apparent loss in myofibrillar material within muscle c
ells as the salmon matured. With TEM, local or focal areas of degenera
tion were visible in both the spawning and silver-bright fish, but cha
nges were more severe in the mature fish. Given a series of micrograph
s of muscle, judges could distinguish between the two groups using a s
coring system based on defined criteria of myofibril 'splitting', 'sme
aring', 'thinning', and 'inter-myofibril spaces'.