Stunning/slaughtering by cold shock in saline water: Effects on fish stress, post-mortem changes, and product quality in rainbow trout

This study evaluated the effects of two stunning/slaughtering methods, percussion vs. immersion in cold saline (-6 C, 5% NaCl) water, on stress status and freshness evolution in rainbow trout (Oncorhynchus mykiss). Stress indicators (cortisol, malondialdehyde-MDA) were measured in plasma, muscle, fin and skin mucus of 30 fish (15 per treatment), while freshness evolution and quality (pH, rigor mortis index, fillet and skin colour, and muscle texture) were assessed in 70 fish (whole fish and fillets; 35 per treatment) at 3, 48, 168, and 264 h of storage. Compared with percussion, cold immersion resulted in lower cortisol levels in skin mucus (-58%; P < 0.001), muscle (-53%; P < 0.01), and fin (-51%; P < 0.01), and higher muscle MDA (+90%; P < 0.001), while cortisol (184 ng/ml, on average) and MDA (13.7 nmol/mg protein) of plasma were not affected. Skin colour was darker in fish subjected to cold immersion compared with percussion. As for fillet, no differences were found in pH (6.46), L* (47.3), a* (-0.89), b* (10.1), TVB-N (17.6 mg/100 g of fillet), fatty acid profile, or texture, where only chewiness was higher (+28% and + 32% for whole fish and fillet, respectively; P < 0.05) when cold immersion was used compared with percussion. Over time, TVB-N showed the highest value at 264 h of storage (26.5 mg/ 100 g of fillet), and pH decreased from 6.57 at 3 h of storage to 6.39 at 48 h and 6.37 at 168 h, before increasing to 6.50 at 264 h (P < 0.001). Immersion in cold saline water was proved to be a viable alternative to percussion for stunning/slaughtering rainbow trout, being an ease technique, cost-effective, and with reduced labour, while producing a similar stress response amplitude and no relevant changes on product quality.