Rainbow trout (Salmo gairdneri) were stunned by electrocution, exposure to elevated concentrations of carbon dioxide and by a blow to the head, and subsequently bled. The fish were stored ungutted in ice for up to 15 days, and the changes in the textural properties of the flesh of the fish were measured by a sensory panel and with a texturometer. Parallel changes in the concentrations of spoilage-related biochemical constituents, in water-holding capacity and in bacterial counts were also determined. Slaughter by electrocution and by carbon dioxide narcotization led to a greater initial production of lactic acid and a slightly reduced pH, compared with slaughter by a blow to the head. No significant differences were found in the values of the other indices of quality, either immediately after death or during post-mortem storage. ?Correspondent.
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K . Azam, I. M. Mackie and J. SmithStation (TRS), Aberdeen. They were held in aerated tanks for 7 days at TRS before slaughter.
Methods of slaughterElectrical stunning of the fish was carried out in an apparatus constructed at TRS consisting of a holding tank (737 x 559x381 mm) with an electrode at each end spanning the full width and depth of the tank. A portable generator provided the power to the electrodes. For AC stunning (sinusoidal wave form) 225 V at 50 Hz, and for DC stunning (full wave rectification of AC sinusoidal wave form) 175 V were applied for 10 s. A circuit breaker microswitch ensured that no power could be supplied to the electrodes until the perspex safety cover was placed over the tank. After the fish were stunned, they were removed from the tank and one set of gill arches in each was cut for bleeding. The fish were then transferred to fresh water for completion of bleeding. Carbon dioxide immobilization was accomplished by immersing the fish in water treated with carbon dioxide. The water was prepared by bubbling a regulated flow of carbon dioxide gas from a cylinder through a perforated pipe anchored to the bottom of the tank. The fish became narcotized after 2-5 min and turned belly upwards; they were then taken out and bled in fresh, oxygenated water as above,The concentrations of dissolved carbon dioxide and oxygen in the water were measured with a carbon dioxide test kit (Camlab, Model CA-23 HW01936-01) and with a digital oxygen meter (PTI-401) respectively. The concentrations of C02 and O2 before flushing with COz were 2.5 and 12.4 mg 1-' respectively, and after flushing, 450 and 4.5 mg 1-I.A blow to the head was delivered with a wooden club, and the fish were bled as above.
Storing and sampling of fishThe fish were stored in ice at 0-4°C for up to 15 days. Five fish from each treatment were withdrawn after 6 h, and 1,5,10 and 15 days of storage. They were filleted and 3 g of fish tissue excised immediately from the same part of each fillet (ten fillets for each treatment), giving a combined sample of 30 g for microbiological analyses. The fillets were then sealed in polythene bags under vacuum, and held at -30°C until they were thawed for an...
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