Effects of freshness on thawing drip and ice crystal formation in frozen spotted mackerel Scomber australasicus

The effects of freezing and thawing on the meat of spotted mackerel Scomber australasicus were studied. The fish meat was kept in iced water for different time intervals before being frozen at -20 degrees C for one month. The ice crystals in muscle tissue of fish meat stored in iced water for a short time (pre-rigor mortis) before being frozen were small and were observed mainly in the intracellular spaces, while those of the fish kept for a long time (post-rigor mortis) before being frozen, e.g. for 4 days, were large and were observed mostly around the connective tissues. The amount of thawing drip of the latter was larger than that of the former. In this drip free amino acid, probably derived from decomposition of the protein, was detected. These findings suggest that the protein degrades along with the decline in freshness, and that the degraded tissue causes the formation of large ice crystals, resulting in a large amount of thawing drip. This phenomenon may be one of the dominant factors causing the deterioration in quality of frozen spotted mackerel.