Effects of a carbon monoxide stunning method on rigor mortis development, fillet quality and the oxidative stability of tench (Tinca tinca)

The growing economic interest in tench has led to the need for further information on the best slaughter methodologies for this species in order to respect animal welfare and preserve fillet quality. Sixty farmed tench (Tinca tinca) were randomly divided into three groups of killing: carbon monoxide (CO), electrical (ES) and percussive (PS) stunning. The behaviour of the fish, the onset of rigor mortis and the gill cortisol concentration were determined as stress indexes, while after rigor resolution, the quality of the fillets were followed over a period of 10 days of refrigerated storage (+2.5 °C) by determining physical and chemical properties of the fillets. The observations indicated that the fish did not seem to perceive CO negatively, and normal swimming activity was recorded. No external or internal damage was recorded for any of the killed fish. The evolution of the rigor mortis index indicated that CO reached full rigor at about 15 h post mortem, that is, an intermediate value was reached between ES (9 h) and PS (19 h). The fish stunned with CO showed significantly lower gill cortisol level than the ES and PS ones, that is, 0.408, 0.453 and 0.455 ng/mg protein, respectively. As far as the quality parameters are concerned, the slaughter method significantly affected pH, redness (a*) and yellowness (b*). No differences in pH were recorded for the killing procedures at the end of rigor resolution, whereas CO derived flesh presented the lowest pH from the second day till the end of the storage, when values of 6.44, 6.51, and 6.60 pH were found for CO, ES, and PS, respectively. The CO flesh presented the most stable redness index over the whole trial and the lowest b* value from the beginning. The drip loss values were unaffected by the killing method, but increased significantly with the storage time. Neither the slaughter method nor the storage negatively impacted on the fatty acid composition, which resulted to be equally rich in PUFA n3 and n6 (around 40 g/100 g total fatty acids). A numerical difference in the secondary lipid oxidation products emerged for the different killing groups at the end of the refrigerated storage. The CO flesh was the least oxidized, with the samples showing 1.33 mg MDA-equivalents/100 g muscle against 1.60 and 1.55 found for ES and PS, respectively.