The development of a rigor mortis-like stiffening and the biochemical changes associated with it were investigated in tilapia (Oreochromis aureus/niloticus hybrid), a tropical freshwater species, and common carp (Cyprinus carpio), a temperate freshwater fish, during storage in ice (OT) and at ambient temperature (22°C). Onset of stiffening in carp occurred between 16 and 17 hr after death at both temperatures but full stiffness developed much later and was a longer duration at 0°C. In tilapia, onset occurred after 7 hr at 22°C and full stiffness was established after 19 hr. However, at O' C, tilapia experienced a cold shock reaction such that they stiffened within minutes of being placed in ice and were fully rigid within 8 hr. Resolution of stiffness in this species also occurred later at 0°C. The rate of ATP degradation was similar under both storage conditions in tilapia but more rapid at ambient temperature in carp. Although the rate of lactic acid accumulation was faster at the higher temperature in tilapia, it was not nearly so marked as for carp. Objective measurement of contractions in excised muscle fibres from trout (Salmo gairdnerii) and tilapia indicated that reducing the temperature delayed the occurrence of the contraction and reduced its intensity. It was concluded that cold shock stiffening and rigor mortis stiffening are different.
The effect on fillet yields and quality of the cold shock reaction of tropical fish when they are iced immediately following death was investigated. Tilapia (Oreochromis mossambicuslniloticus hybrid) were subjected to three handling treatments: filleting immediately following death; icing the whole fish immediately following death and filleting after 3 days; ageing the fish for 6 hr at ambient temperature prior to icing and then filleting after 3 days. All fillets were stored on ice until 9 days after death. Pre-rigor filleting resulted in the highest filleting yields with the least drip loss and gaping on storage but the shortest shelf life. This was accompanied by the highest post-mortem metabolic rate. The lowest yields and highest drip loss were obtained by icing the fish immediately at death followed by filleting after 3 days. A 6-hr delay before icing gave yields and drip loss which were between these two extremes and also the slowest post-mortem metabolism. The two post-rigor filleting procedures produced fillets with increased gaping but a longer shelf life compared with pre-rigor filleting. The implications of the cold shock reaction in terms of recommended codes of practice and handling operations in tropical fisheries are discussed.
A supply of the Bolivian fish sabalo, Prochilodw platensis, was stored at -15 " and -30 "C. Samples were taken at intervals for analysis and taste panel assessment. No significant change was observed in 20 months at either temperature in the pH or in the percentage of protein soluble in 5% sodium chloride. The fish remained acceptable to the taste panel throughout. Thus the storage life of the species is shown to be in excess of 20 months at both -15 " and -30 "C.
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