The heat stability of the proteins present in cod, siganus and tilapia fish muscle were compared using differential scanning calorimetry and by measuring the solubility of the proteins in weak and strong salt solutions following heat treatment at selected temperatures from 25" to 100°C for 1 hr. It was found that the stability of both the collagens of the connective tissue and the myosins of the myofibrillar proteins varied between species, the more stable proteins being those from the fish found in the waters of higher ambient temperatures. The stability of the sarcoplasmic proteins also exhibited some species dependence but the actins present in the three species were all of similar thermal stability. The thermograms obtained by scanning calorimetry indicated that during frozen storage some changes in the nature of the myosin molecule occurred. It is suggested that scanning calorimetry may be a rapid and simple means of following the so-called cold store denaturation of frozen fish but more work is required to confirm this.

The thermal stability of fish muscle proteins varies between species with considerable implications for storage and processing properties. Myosins were isolated jiom cod and snapper, cold-and tropical-water fish respectively, and their thermal melting characteristics were compared using differential scanning calorimetry over a range of pH and ionic strength conditions. At pH 6 and ionic strength 0.06 M , cod myosin exhibited a thermal melting transition 10 K lower than snapper myosin, reflecting a comparable differential in transitions attributed to myosin in intact muscle jiom the two species at 315 and 325 K , respectively. However, with increasing p H and ionic strength, conditions favourahle to disaggregation and dissolution of myosin in vitro, the snapper myosin transition decreased to that exhibited by cod myosin, which remained essentially unaflected by the conditions. I t is suggested that differences in the aggregation characteristics of fish myosins may explain in part the different thermal melting properties in vitro.

Samples of cod, whiting, herring, lemon sole and skate muscle were frozen and stored at -8°C for up to 20 months. Samples of cod muscle were also stored at -30°C.During storage at -8°C the decrease in the solubility of the muscle proteins in 5 % NaCl (denaturation) was most rapid in whiting followed in order by skate, cod, herring and lemon sole. Little change was detected in the NaCl solubility of cod muscle protein stored at -30°C. An increase in the pH was found to occur in all the species examined. The relationship between these changes and the toughening that occurs in fish muscle during frozen storage is considered. No nutritionally significant decrease in the lysine availability or the in vitro digestibility of the muscle proteins was found to occur in any of the species examined. In whiting, lemon sole and skate very small, statistically significant, increases in the available lysine content were detected after 7 months of storage at -8°C and this may indicate that unfolding of the protein chains had occurred.

The thermal stabilities of the proteins of a range of fish muscles of different habitat temperatures were determined by differential scanning calorimetry before and after frozen storage at -20°C.In whole muscle a clear relationship was seen between habitat temperature and the thermal denaturation of myosin, which persisted when isolated myosins were analysed under conditions close to physiological pH and ionic strength. The ionic environment of the myosin molecules in the whole tissue will, however, not be exactly the same as in the myosin solution.N o significant correlations were seen between habitat temperature, ultimate pH and other analytical parameters.After frozen storage, the myosin transitions in red snapper, a warm water species, was markedly changed in whole muscle but not in isolated myosin, suggesting the post mortem development of an interaction with other muscle proteins. In contrast, in cod, a cold water species, changes in myosin transitions were very similar, both in whole muscle and isolated myosin.The implications of species differences in the 'melting' of myosin domains and changes in textural quality during frozen storage are discussed briefly.

Indonesian mackerel (Rastrelliger kanagurta) were sun dried or salted and sun dried and stored at 30°C for up to 20 weeks in air. During processing and storage polyunsaturated fatty acids oxidized to form thiobarbituric acid (TBA) reactive substances and fluorescent compounds. TBA values increased with increasing salt content (range 2.3-20.7%). Positive, highly significant correlations were found between the odour of the stored products and both fluorescent pigment production and the degree of browning. Amino acid analysis indicated loss of lysine, glutamic acid, serine, histidine, arginine and the sulphur containing amino acids, but these losses were far too small to account for the massive decreases found in net protein utilization (NPU) of the stored products, measured by rat-feeding trials. The latter were highly correlated with the fluorescent pigment content and degree of browning in products of intermediate (-14%) salt content.