Electronarcosis as a pre-slaughter method for rainbow trout (Oncorhynchus mykiss) improves meat quality

Pre-slaughter and slaughter procedures in fish can induce acute stress disorder triggering a sequence of hormonal, biochemical and osmoregulatory responses. These responses can compromise the quality of the meat, do increase muscle activity, promoting the abrupt fall in pH after death and speeding deployment and resolution of rigor mortis. In Brazil, the slaughter of farmed fish is done by immersion in water and ice (Hypothermia), a method considered too stressful, and that decreases the shelf life of the fish. Rainbow trout (Oncorhynchus mykiss) is a species of cold water, and therefore the use of hypothermia may not be efficient. The present study evaluated the effects of eletronarcosis and hypothermia as pre-slaughter stunning methods on the quality of meat of rainbow trout stored in ice during 7 days. Four treatments were used to stunning fish: Hypothermia (water and ice on 1:1), E1 - Eletronarcosis 1 (1200 Hz for 3s), E2 - Eletronarcosis 2 (1500 Hz for 3s) and Anesthesia/control (Eugenol previously diluted in ethanol) in which the fish were immersed for 3 minutes. After stunning the fish were slaughtered by cutting from the gills and bled. Were evaluated rigor mortis, muscle pH index, dielectric properties, volatile nitrogenous bases (VNB) and thiobarbituric acid reactive substances (TBARS) at various times after the death. It took 67 minutes for the fish subjected to low temperature (hypothermia) to become apparently stunned and the complete rigor mortis (100%) was reached 3 hours after death. In other treatments, the deployment of rigor occurred 24 h after slaughter. The pH of the fish muscle stunned by eletronarcosis, independent of the frequency used, showed a gradual decline, reaching the minimum value observed (E1: 6.012; E2:5.934) at 72 h after death, while in other treatments the decline of muscle pH occurred abruptly reaching the smallest averages (Anesthesia: 6.265; Hypothermia: 6.304) only 6 h after slaughter. For trout stunned by hypothermia, the dielectric properties of the meat were higher (P <0.05) at times 0 and 3h after slaughter, and there were no differences in the other samplings. Immediately after slaughter VNB concentration was higher (P <0.05) in hypothermia stunned fish (14.487 mg N / 100 grams). For TBARS, there was no significant difference between the assay methods tested. We concluded that the eletronarcosis with a frequency of 1200 Hz is indicated to stunned trout in an effective and fast way, preserving the quality of the meat.