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ABSTRACT. Electrical stunning is considered one of less stressful fish stunning method. However, for its efficiency, it is important to determine the best way of application of the electrical variables. The aim of this study was to evaluate the effect of different voltages (100, 150, and 200 V) of alternating current (AC) or direct current (DC), frequency of 50 Hz, for 180 s exposure, concerning the aspects of behavior and meat quality of pacu (Piaractus mesopotamicus). Fish subjected to AC remained unconscious longer, evaluated by behavior analysis, when compared to DC. The pacu subjected to 100 AC, 200 AC and 100 DC showed less bleeding in the fillets. The lightness (L*) of the fillets were higher in 150 AC, 150 DC and 100 DC, followed by 200 and 100 AC. The redness (a*) in pacu fillets were moderate in treatments 200 and 150 AC and less intense in treatments 100 DC and 100 AC. The yellowness (b*) was higher in fish fillets treatment 150 DC and lower in 100 AC. Therefore, the 200 V alternating current (AC) used in water stunning caused apparent unconsciousness of up to 4 min. after exposure and less bleeding in fillets, thus it is a suitable stunning method to be recommended for the pacu processing industry.

Pacu (Piaractus mesopotamicus) is one of the most important fish farmed species in Brazil due to its hardy, fast growing, omnivorous feeding behavior, and good consumers acceptance. However, studies on the effect of different slaughtering procedures on quality of frozen pacu fillets have not been carried out. Therefore, this study aimed to evaluate the effect of electrical stunning, CO 2 narcosis, or water/ice immersion on the quality (lipid oxidation -TBARS, protein oxidation -carbonyl and thiols, protein denaturation, and instrumental color -L*, a*, and b*) of frozen (-18 °C) pacu fillets stored for up to 6 months. Electrical stunning became the fillets darker and red, besides leading to higher protein oxidation (carbonyl and thiols), and protein denaturation. CO 2 narcosis resulting in more pale flesh, with higher protein oxidation. The slaughtering by water/ice immersion was the best alternative for obtaining high quality frozen pacu fillets with lower oxidation and protein denaturation. A quality loss was observed for all pacu fillets stored at -18 °C over the months, as evidenced by the increase in lipid and protein oxidation, color changes, and higher protein denaturation. Additionaly, a maximum storage period of 3 to 4 months is recommended for frozen pacu fillets.