Effects of slaughtering methods on stress and post mortem changes in rainbow trout

The study evaluated the effects on stress and freshness evolution in rainbow trout (Oncorhynchus mykiss) slaughtered by percussion or immersion in cold saline water (-4/-8 °C; 5‰ NaCl) during 5 minutes. A total of 100 rainbow trout were sampled for measuring stress indicators (cortisol, malondialdheyde-MDA) in plasma and tissues (30 fish; 15 per treatment) and for evaluating freshness (pH, rigor mortis index, fillet and skin colour and muscular texture) at 3, 48, 168 and 264 h of storage (70 fish; 35 per treatment). When cold immersion was used compared to percussion, trout showed lower cortisol in skin mucus (4.33 vs10.4 ng/ml; P<0.001), muscle (14.8 vs 31.4 ng/g; P<0.01) and fin (34.9 vs 70.6 ng/g; P<0.01), and higher muscle MDA (20.0 vs 10.5 nmol; P<0.001). Plasma cortisol (184 ng/ml, on average) and MDA (13.7 nmol) were not affected. As for freshness evolution, the cold immersion sped up the onset of rigor mortis compared with percussion, but no differences were found between slaughtering methods in fillet pH (6.46), colour (L* 47.3; a* −0.89; b* 10.1), TVB-N (17.6 mg/100 g of fillet), fatty acids composition (% total FA) (EPA: 0.73%; DHA: 2.66%; PUFA n-3: 7.67%) or texture profile analysis. Some minor changes were observed according to storage time, that is TVB-N showed the highest value at 264 h of storage (15.8, 13.8, 14.2 vs 26.5 mg/100 g of fillet); pH changed from 6.57 at 3 h of storage to 6.39 and 6.37 at 48 and 168 h to raise at 6.50 at 264 h (P<0.001). In conclusion, based on cortisol changes, cold immersion was likely to be a less stressful slaughtering method compared to percussion. As for freshness, some differences were found on rigor mortis onset between the two methods, which however did not produce relevant effects on fish quality traits.