Industrial application of electrical stimulation and maturation. Its effect on the quality of poultry meat

For economic and hygienic reasons, slaughter and cutting processes in poultry processing plants must be as quick as possible. If the meat is removed from the carcass before rigor mortis resolution, muscle fibers contract and shorten, resulting in hardening of the meat. Rapid subsequent freezing to which fillets are subject may even increase these adverse effects, due to the risk of shortening by cold temperatures (Honikel et al., 1986). Some researchers consider maturity prior to the cut as the most important way to ensure tenderness in breast meat without diminishing other characteristics of quality (Fletcher, 2002). However, the proposed minimum maturing times differ. Stewart et al. (1984) and Lyon et al. (1985) have suggested a minimum time of 4 hours, while Thielke et al. (2005) proponed a minimum of 6 hours. Young et al. (1999) found differences in tenderness from 2 hours of maturation on, Northcutt et al. (2001) from 4 hours on, and Lyon & Lyon (2000) from 6 hours on. Another procedure that has been proposed is the application of electrical stimulation (ES). However, this technology is still in discussion, since results are inconclusive. Various authors have discussed the effect of the ES on the functional properties of bird meat, suggesting that results vary according to conditions of work (variables of stimulation, stage in which it takes place and slaughter conditions). For the above reasons, the need arises to have data related to the quality of poultry meat that are obtained according to the production characteristics of each region.
The objective of this study has been to assess the incidence of electrical stimulation and the effect of various maturation times on the quality of the meat of chicken breast fillets.

Birds (360 males, 48 days old and 2.8 ± 0.2 kg of live weight) were slaughtered in industrial conditions. Stunning (72 V, 8 s), bleeding (2 min), scalding (55° C, 2 min) was performed; half of the birds were electrically stimulated (45 V, 50 mA) immediately after plucking (with ES) and the other half were not (No ES). Then, in both cases, evisceration, washing and cooling in water (1 ° C) were performed. Subsequently, (1h y 15 min post mortem) carcasses were subject to maturation: 0, 2, 4, 6, 8 or 24 hours (at 3 ± 1ºC) and automatically cut to obtain the fillets, which individually deep-frozen (-30 ± 2ºC). Immediately after cutting, pH in the pectoralis major muscle was determined (using a puncture electrode, Oakton, Singapore). After freezing-defreezing, tenderness was determined through Warner-Bratzler (WB) maximum cut-force, for which fillets were individually placed in air-tight plastic bags in a bath of water at 100 ° C until an internal temperature of 71 ° C, controlled by a multiple temperature recorder (Yokogawa, mod.). DX106-1-2, China) was reached. Four cylinders of 1.30 cm in diameter were extracted for each sample, the maximum cutting force (kgf) was determined using a texture analyzer (Stable Micro Systems TXT, United Kingdom) with a Warner-Bratzler cell (AMSA, 1995). Losses through cooking were calculated as the difference between the weight of each fillet before and after cooking, expressed as a percentage of the weight of the initial sample. The results were analyzed using Statgraphics Centurion XV (StatPoint Tech, Inc., Warrenton, VA, U.S.A.) and the ANOVAs and Duncan tests were performed. In all cases, the significance level used was 0.05.

The pH showed no significant differences as a result of ES. Regarding the influence of maturation time, the longer maturation time lasted, the lower pH became, the first significant decrease in both cases (Without ES and With ES) occurred between 0 and 2 hours of maturation (Table 1).

Table 1. Average values (± Standard deviation) of pH and Warner - Bratzler (WB) tenderness in matured fillets (Mad) in different periods, with and without electrical stimulation.

Alvarado & Sams (2000) found significantly lower pH values 1.25 h post mortem, compared to the controls without ES (450 mA). Castañeda et al. (2005) reported that electrical stimulation (450 mA, 450 V) accelerated the decrease in pH when the breast meat was boned 2 h post mortem. In this work, the absence of a in pH by effect of ES could be due to the fact that the electrical characteristic that produces the stimulation of muscle fiber is the amperage (Sams, 1999) and, in our case, we used much lower amperages (of approx. 50 mA) than those used in the experiences cited above. Lyon et al. (1989), studying different ES voltages, found that the pH was significantly lower in birds stimulated at 200 V and 350 V, compared to those stimulated at 50 V and to controls without stimulation. Furthermore, it should be noted that experiences differ in various conditions of processing, such as the facility in which electrical stimulation is applied, the time of application and the pulsing used, manual or mechanized processing, type and conditions of cooling, among others.

Considering the effect of ES on WB tenderness (Table 1), differences (with and without ES) are manifested in the fillets subject to maturation for 2, 4 and 6 h. In reference to the changes brought about by the effect of different maturation times, fillet without ES improved significantly in tenderness from 6 h. on of maturation, while products with ES show a substantial improvement already after 2 h. This would indicate an acceleration of the tendering process as a result of ES. According to Devine et al. (2001) if birds are cut within 65 min post mortem, a decrease in tenderness may occur, and it is precisely in this situation where electrical stimulation (230 V) can result in a significant improvement of the quality of the product, with a minimum of handling. Dickens et al. (2002) found that electrically stimulated fillets (200 V) that were cut after 2 h cooling, require an estimate of 50 percent less shear force for cutting than non-stimulated fillets, although these differences were not observed in fillets that were cut after 24 hours cooling. Other researchers reported an improvement in the tenderness of fillets electrically stimulated within 1.25 h post mortem (Alvarado & Sams, 2000); 2 h post mortem (Craig et al., 1999) or after 3 h cooling (Young et al., 2005). According to Sams (1999), low-intensity electrical stimulation systems (currents up to 200 mA per bird), such as the one applied in this study, induce contractions and accelerate the development of rigor mortis, but the resulting meat tenderness does not increase enough as to be able to eliminate completely maturation prior to cutting.

Losses through cooking were generally lower in the fillets with ES (Table 2), compared with fillets without ES. Similar results were obtained by Dickens et al. (2002), working with electrically stimulated and boned fillets after 2 hours of cooling, compared to  the same product without stimulation, recording losses of 22.6% (with ES) vs. 24.4% (without ES). These percentages are slightly higher than those observed in the present work for the same maturation times. Other authors report that implementation of ES reduced losses through cooking in fillets that were cut 1.25 h post mortem (24.6 per cent with ES, 29.8% without ES), while they found no differences 0.25 to 24 h post mortem (Alvarado & Sams, 2000). On the other hand, Lyon et al. (2002) noted an increase in the losses by cooking in stimulated fillets (22.8%), compared to non-stimulated fillets (22.0%), the same as Young et al. (1999). Kahraman et al. (2011) found no differences in this parameter as a result of ES working with high voltages (500 V). As stated above, the differences found could be attributed to the different work conditions.

Table 2: Average values (± standard deviation) of losses through cooking in matured fillets (Mad) during different periods, with and without electrical stimulation.

The results of this study suggest that the application of electrical stimulation accelerates the tenderness process of fillets matured for short periods (between 2 and 6 h). On top of this, ES reduces losses through cooking and does not alter the pH in the working conditions tested.
Maturation time improves tenderness of fillets stimulated from 2 h on, while tenderness in non-stimulated fillets occurs from 6 hours on. In both cases the best result is confirmed in matured meat, 24 hours.
Finally, and although the mechanisms of action are not fully known, it could be argued that electrical stimulation contributes positively to the tenderness-increasing process, proving to be effective in fillets after short maturation periods.

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