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Application of constant current, low voltage electrical stimulation systems to pig carcasses and its effects on pork quality
Meat Science 65 (2003) 1309–1313 www.elsevier.com/locate/meatsci
Application of constant current, low voltage electrical stimulation systems to pig carcasses and its effects on pork quality H.A. Channon*, P.J. Walker, M.G. Kerr, S.R. Baud Department of Primary Industries, Victorian Institute of Animal Science, 600 Sneydes Road, Werribee, Victoria 3030, Australia Received 18 September 2002; received in revised form 20 January 2003; accepted 30 January 2003
Abstract This study examined the effectiveness of a constant current, low voltage electrical stimulation system on improving pork quality when applied to pigs at 2 min post-exsanguination. A total of 48 female DurocLarge White/Landrace pigs of 85–90 kg liveweight were randomly allocated immediately prior to slaughter to one of four constant current electrical stimulation treatments: control (no electrical stimulation), 50, 200 and 400 mA. Stimulation was applied to pig carcasses at 2 min post-exsanguination for 30 s. No differences (P> 0.05) in WB shear force values, muscle lightness or PSE incidence of pork M. longissimus lumborum (LL) was found due to electrical stimulation treatment. Muscle pH of the LL muscle was lower (P <0.001) in carcasses in the 200 and 400 mA treatments compared to those from carcasses in both the 50 mA and control treatment groups, when measured at the various time points from 40 min to 8 h post-slaughter. Although carcasses stimulated with 200 and 400 mA had higher percentage drip loss (P <0.05) and purge (P< 0.001), this was not found to impact WB shear force values, muscle lightness or PSE incidence. # 2003 Elsevier Ltd. All rights reserved. Keywords: Pork; Electrical stimulation; Meat quality
1. Introduction Bennett (1997) reported that Australian consumers consider pork to be tough and dry in comparison to other meats. The high incidence of pale, soft exudative (PSE) meat, inadequate intramuscular fat, cold shortening, inadequate ageing and overcooking of pork by consumers may be impacting pork tenderness. Processing practices (including aitchbone hanging, ageing of pork and the enhancement of pork quality by addition of brine solutions) can be used to varying degrees to reduce variability and improve consistency of eating quality attributes of pork. Channon, Reynolds, and Baud (2001) reported that both aitchbone hanging and ageing pork for 7 days post-slaughter can positively influence eating quality attributes of pork. However, there is considerable reluctance by the Australian pig processing sector to modify carcass hanging procedures from the achilles tendon to the aitchbone due to additional labour and chiller space required for aitchbonehung carcasses. Additionally, the practice of purchasing * Corresponding author. Tel.: +613-9742-0415; fax: +613-97420400. E-mail address: [email protected] (H.A. Channon). 0309-1740/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0309-1740(03)00051-2
aged pork by the Australian retail sector is not common, with the majority of pork purchased as carcasses rather than as boxed pork. Electrical stimulation of pig carcasses may be a commercially acceptable option available to the Australian pork industry to improve eating quality attributes of pork as it can be applied online without requiring additional labour support. Electrical stimulation is most effective in improving tenderness when carcass cooling is fast enough to induce cold shortening of muscle fibres post-slaughter. Electrical stimulation prevents cold shortening by causing a more rapid rate of post slaughter metabolism in muscle. This results in a lower muscle pH while muscle temperatures are still high which maximises proteolysis (Dutson & Pearson, 1985). However, care must be taken with electrical stimulation of pork carcasses as the rapid metabolism induced by electrical stimulation could result in an increased occurrence of PSE (Crenwelge, Terrell, Dutson, Smith, & Carpenter, 1984; Warriss et al., 1995). Electrical stimulation using constant voltage (variable current) electrical stimulation systems is not used in Australian abattoirs as a method of improving eating quality due to the risk of increasing the incidence of PSE. Previous studies that have evaluated the application of either high or low voltage electrical stimulation to
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pigs at 3–5 min post-slaughter (Bowker, Wynveen, Grant, & Gerrard, 1999; Rees, Trout, & Warner, 1999a; Rees, Trout, & Warner 1999b; Taylor & Martoccia, 1995) and at 20 min post-slaughter (Dransfield, Jones, & MacFie, 1991; Maribo, Ertberg, Andersson, Barton-Gade, & Moller, 1999; Taylor & Martoccia, 1995; Taylor, Nute, & Warkup, 1995a, Taylor, Perry, & Warkup, 1995b; Warriss et al., 1995) have been conducted using constant voltage systems. These studies have demonstrated that the effects of electrical stimulation (using both low and high voltage), on the incidence of PSE, rate of pH fall post-slaughter, drip loss and muscle lightness are inconsistent. A possible explanation for these inconsistent effects of conventional, constant voltage electrical stimulation systems on pork quality traits may be that the peak current delivered during stimulation can vary considerably due to varying resistance. Furthermore, the level of the peak current delivered during stimulation using constant voltage systems may also be directly related to the rate of glycolysis post slaughter and, if excessive, trigger the development of PSE. A low voltage electrical stimulation system, with modified frequency and waveform, that can deliver constant current has been developed in Australia and may provide the Australian pig industry with an opportunity to improve the tenderness of pork without inducing PSE. This system has been developed to be as effective as high voltage systems, but safer for abattoir workers and cheaper to install compared with conventional high voltage electrical stimulation systems. This study was conducted to determine the potential for improving pork quality by applying low voltage electrical stimulation to pig carcasses at 2 min postexsanguination using different levels of constant current.
2. Materials and methods A total of 48 female DurocLarge White/Landrace cross pigs of 85–90 kg liveweight were slaughtered following overnight lairage at a pilot abattoir. Low voltage electrical stimulation was applied to pig carcasses at 2 min post exsanguination using a rectal probe and a nose clip. Pigs were randomly allocated to one of four constant current electrical stimulation treatments immediately prior to slaughter (n=12 per electrical stimulation treatment): 1. Control: no stimulation. 2. Current 50 mA, 80V peak, frequency 14 Hz. Stimulation applied for 30 sec at 2 min postexsanguination. 3. Current 200 mA, 80V peak, frequency 14 Hz. Stimulation applied for 30 sec at 2 min postexsanguination.
4. Current 400 mA, 80V peak, frequency 14 Hz. Stimulation applied for 30 sec at 2 min postexsanguination. All pigs were stunned using 90% CO2 in a dip-lift stunner (Butina APS, Denmark) for a total of 108 s, shackled by the right leg after stunning, exsanguinated vertically and then electrically stimulated. Following stimulation, carcasses were placed in a dehairer at 62 C for 5 min, with any remaining hair removed using a knife and flame. Carcasses were then eviscerated before being chilled for 24 h set at 2–4 C. 2.1. Pork quality measurements Muscle pH and temperature of M. longissimus lumborum (LL) (caudal to the last rib) was measured at 40 min and 2, 3.5, 5, 6.5 and 8 h post-slaughter using a portable pH meter (Jenco pH meter, Model 6007) fitted with a polypropylene spear-type gel electrode (Ionode IS425, Brisbane, QLD) and a temperature adjusting probe. Ultimate pH and temperature was also measured in the LL muscle at 24 h post-slaughter. At 24 h post-slaughter, a section of the LL muscle was removed from the right side of each carcass. Meat colour was measured on a 4 cm thick chop at 24 h postslaughter following exposure of a cut surface to air for 10 min. A Byk Gardner Chromameter (Byk Gardner, Germany, Color Guide sphere; Cat. No. 6830) set on the L*, a*, b* system using D65 lighting and a 10 C standard observer and an 11mm aperture in the measuring head. Drip loss was determined on the LL muscle at 24 h post-slaughter using a modified method of Rasmussen and Andersson (1995). A sample of pork loin was cut to a 20-g (115 cm) strip, weighed and weight recorded. The sample was then wrapped in square netting (20 cm2), suspended in a sealed 120-ml plastic container and allowed to stand in a 4 C chiller for 24 h. The sample was then removed from the container, gently rolled in paper towelling and reweighed to determine percentage drip loss. Carcasses were classified as pale, soft and exudative (based on pHu < 5.6, L* value > 50, drip loss > 5%) as defined by Warner, Kauffman, and Greaser (1997), where pHu is defined as ultimate pH at 24 h postslaughter. LL muscle samples for Warner–Bratzler (WB) shear force assessment were aged at 2 C for 2 d post-slaughter. All samples were cut into 100 2 g blocks, cooked in plastic bags in a water bath at 80 C for 1 h (to a 70– 73 C internal temperature) and then cooled in cold running water for 30 min, as described by Bouton, Harris, and Shorthose (1971). Samples were then dried with paper towels to remove excess moisture and reweighed to determine cooking loss. Each sample was cut
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parallel to the muscle fibres into five replicates of 1 cm2 cross-section. Tenderness was measured using a Warner–Bratzler shear force blade fitted to an Instron Universal Testing Machine Model 4465, with crosshead speed of 300 mm/min and a 5 kN load cell. Purge was determined by weighing the LL sample destined for WB shear force measurement prior to vacuum packaging. Upon removal from the vacuum bag, each sample was dried with paper towels to remove excess moisture and re-weighed to determine purge. 2.2. Statistical analyses Pigs were slaughtered over two slaughter days (n=24 per day) with six blocks containing four pigs (one from each stimulation treatment) per slaughter day. A randomised block design was used to allocate pigs immediately prior to slaughter into electrical stimulation treatments. Data was analysed using Genstat 5 for Windows, Release 4.2 (Payne, Lane, & Genstat 5 Committee, 1987) by Analysis of Variance to determine predicted means and standard error of differences for objective pork attributes due to stimulation treatment. A comparison of the incidence of PSE pork between treatments was made using the chisquare goodness of fit test (Snedecor & Cochran, 1980).
Table 1 Means and standard error of the difference between means (S.E.D.) for pigs in each electrical stimulation treatment for muscle pH and temperature measured from 40 min to 24 h post-slaughter in the M. longissimus thoracis et lumborum (LTL) muscle Variate
Electrical stimulation treatment Control
Muscle pH 40 min 6.69 2h 6.53 3.5 h 6.31 5h 6.12 6.5 h 5.96 8h 5.87 24 h 5.43 Temperature ( C) 40 min 36.2 2h 24.4 3.5 h 16.7 5h 12.2 6.5 h 9.6 8h 7.9 24 h 7.6
6.66 6.44 6.26 6.12 6.02 5.95 5.46 36.3 23.1 14.9 10.1 8.5 6.7 7.7
200 mA
P value
0.062 0.110 0.062 0.090 0.082 0.072 0.027
<0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.458
400 mA
6.14 5.80 5.59 5.50 5.45 5.45 5.42
6.13 5.70 5.53 5.47 5.45 5.42 5.42
37.0 25.5 16.4 12.0 9.2 7.3 7.5
36.7 24.5 15.8 11.2 8.7 6.9 7.4
0.51 0.86 0.58 0.45 0.40 0.32 0.14
0.361 0.077 0.025 0.023 0.030 0.005 0.230
Table 2 Means and standard error of the difference between means (S.E.D.) for pigs in each electrical stimulation treatment for drip loss (%), purge (%), muscle lightness (L*), Warner–Bratzler (WB) shear force (kg), sarcomere length (nm), cooking loss (%) and PSE incidence (%) in the M. longissimus thoracis et lumborum muscle Variate
Electrical Stimulation treatment
S.E.D. P value
Control 50 mA 200 mA 400 mA
3. Results Muscle pH, measured from 40 min to 8 h postslaughter was lower at each time interval (P < 0.001) in LL muscles from carcasses in the 200 and 400 mA electrical stimulation treatments compared with those from the 50 mA and control (no stimulation) treatments (Table 1). No significant difference (P > 0.05) in muscle pH between stimulation treatments was found at 24 h post-slaughter. Although muscle temperature decline post-slaughter was influenced (P < 0.05) by stimulation treatment from 3.5 to 8 h post-slaughter, these treatment differences were not consistent with increasing current levels applied during stimulation. Drip and purge losses were higher (P < 0.001) from LL muscle obtained from carcasses in both the 200 and 400 mA treatments compared with those from the 50 mA and control treatments (Table 2). No differences in drip or purge losses occurred (P> 0.05) between carcasses stimulated with 50 mA and those in the control treatment. No differences (P > 0.05) in either muscle lightness (CIE L*) or the incidence of PSE occurred due to stimulation treatment. No significant differences in WB shear force values (P > 0.05) occurred due to electrical stimulation treatment. No effect of stimulation treatment (P > 0.05) occurred for cooking loss.
50 mA
S.E.D.
Drip loss (%) 3.35 Purge (%) 2.72 Muscle lightness (L*) 49.1 WB shear force (kg) 3.74 Sarcomere length (nm) 1.97 Cooking loss (%) 34.18 PSE incidencea (%) a
0
3.90 2.48 47.3 3.77 2.00 34.57 0
5.00 4.74 48.3 3.56 1.98 34.31 8
5.18 5.38 49.2 3.56 1.98 35.06 16
0.671 0.614 1.06 0.251 0.07 0.545
0.028 <0.001 0.278 0.747 0.986 0.398 n.s.
Chi-square analysis.
4. Discussion In this study, electrical stimulation of pig carcasses at 2 min post-exsanguination with 200 and 400 mA constant current for 30 s resulted in both higher drip and purge losses, following vacuum packaging, from the LL muscle. This may have resulted from the apparent faster rate of pH decline from 40 min to 8 h post-slaughter of LL muscles from carcasses stimulated with 200 and 400 mA current compared with those from carcasses in the 50 mA and control treatments. Taylor and Tantikov (1992), in a study using high voltage electrical stimulation, also found that stimulation within 5 min postslaughter increased drip loss from the LD muscle. Overall, the level of toughness across all experimental treatments was extremely low (i.e. the pork from this production system was already tender), with only 2% of
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all pork LL muscles having WB shear force values greater than 5 kg [a threshold level above which consumers may consider pork to be tough (Hofmeyr, 1998)]. This was a surprising and unexpected result considering that the pork had been aged for only 2 d post-slaughter. Channon et al. (2001) found that 41% of pork LL muscles had WB shear force values greater than 5 kg following ageing for 2 days post-slaughter. It is noteworthy that electrical stimulation treatment did not impact WB shear force values of pork loin, particularly those obtained from carcasses in the 200 and 400 mA treatments, despite the higher drip loss levels observed. This suggests that the faster rate of pH decline post-slaughter in pork LL muscle does not necessarily result in increased toughness, at least when objectively assessed. Although Unruh, Kastner, Kropf, Dikeman, and Hunt (1986) stated that if the rate of muscle pH decline in beef is too fast at high muscle temperatures, a slight increase in toughness might result due to rigor onset at high temperatures, this was not observed in this study. Previous studies (Dransfield et al., 1991; Taylor & Tantikov, 1992; Warriss et al., 1995) have found that when pig carcasses are electrically stimulated using high voltage and chilled conventionally, a high level of protein denaturation can result. It is not known if this occurred in this study as the solubility of sarcoplasmic and myofibrillar proteins was not determined. It is noteworthy that, although, electrical stimulation with high constant current levels (200 and 400 mA) resulted in high drip loss percentages, neither muscle lightness nor cooking loss were significantly influenced by stimulation treatment in this study. Taylor et al. (1995b) also found that the application of high voltage electrical stimulation at 20 min postslaughter to pig carcasses did not influence meat colour, even though drip loss increased. However, these results do suggest that the application of either 200 or 400 mA current for 30 s at 2 min post-exsanguination was not ideal due to the high drip and purge losses found from the LL muscle. In comparison, the application of 50 mA current did not result in pork with quality attributes dissimilar to pork from unstimulated carcasses. It is suggested that any further work conducted with this new, constant current, low voltage electrical stimulation system be conducted using a current level between 50 and 200 mA applied for 30 s at 2 min post-exsanguination in order to minimise drip loss whilst enhancing pork tenderness.
Acknowledgements The authors are appreciative of the funding provided by Australian Pork Limited to undertake this work. The technical assistance provided by Paul Meredith, Paul Weston, Bob Nightingale and Matt McDonagh and statistical advice provided by Kym Butler was appreciated and is gratefully acknowledged.
References Bennett, J. (1997). Eating Quality Assurance for Pig Meat. Final Report 1266 to the Pig Research and Development Corporation, Canberra, ACT, Australia. Bouton, P. E., Harris, P. V., & Shorthose, W. R. (1971). Effect of ultimate pH upon the water holding capacity and the tenderness of mutton. Journal of Food Science, 36, 435. Bowker, B. C., Wynveen, E. J., Grant, A. L., & Gerrard, D. E. (1999). Effects of electrical stimulation on early postmortem muscle pH and temperature declines in pigs from different genetic lines and halothane genotypes. Meat Science, 53, 125–133. Channon, H.A., Reynolds, J. and Baud, S.R. (2001). Identifying pathways to ensure acceptable eating quality of pork. Final Report to the Pig Research and Development Corporation, DV 136/1385, Canberra, Australia. Crenwelge, D. D., Terrell, R. N., Dutson, T. R., Smith, G. C., & Carpenter, Z. L. (1984). Effects of time post mortem of electrical stimulation and post-mortem chilling method on pork quality and palatability traits. Journal of Food Science, 49, 294– 297. Dransfield, E., Ledwith, M. J., & Taylor, A. A. (1991). Effect of electrical stimulation, hip suspension and ageing on quality of chilled pig meat. Meat Science, 29, 129–139. Dutson, T. R., & Pearson, A. M. (1985). Post mortem conditioning of meat. In A. M. Pearson, & T. R. Dutson (Eds.), Advances in meat research 1. Electrical Stimulation (pp. 45–72). Connecticut, USA: AVI Publishing Co Inc. Hofmeyr, C. D. (1998). Pork quality measured in Melbourne retail outlets. Milne’s Pork Journal, March 20(2), 24–25. Maribo, H., Ertbjerg, P., Andersson, M., Barton-Gade, P., & Moller, A. J. (1999). Electrical stimulation of pigs—effect on pH fall, meat quality and cathepsin B+L activity. Meat Science, 52, 179– 187. Payne, R. W., Lane, R. W., & Genstat 5 Committee. (1987). Genstat 5 Reference Manual. Oxford: Oxford Science Publishers. Rasmussen, A. J., & Andersson, M. (1996). New methods for determination of drip loss in pork muscles. In Meat for the Consumer, 42nd International Congress of Meat Science and Technology (pp. 286–287). Matforsk, Lillehammer, Norway. Rees, M. P., Trout, G. R., & Warner, R. D. (1999a). Effect of stunning method, electrical stimulation and pelvic suspension on the rate of ageing in pork. 45th International Congress of Meat Science and Technology (pp. 469–470), Tokyo, Japan, P-56. Rees, M. P., Trout, G. R., & Warner, R. D. (1999b). Effect of stunning method and electrical stimulation on the rate of ageing in pork. In P. D. Cranwell (Ed.), Manipulating Pig Production VII’ (pp. 188). Werribee, Victoria, Australia: Australasian Pig Science Association. Snedecor, G. W., & Cochran, W. G. (1980). Tests of hypotheses. In Statistical methods (pp. 75–78) (7th ed.). Iowa State University Press: Ames, IA. Taylor, A. A., & Tantikov, M. Z. (1992). Effect of different electrical stimulation and chilling treatments on pork quality. Meat Science, 31, 381–395. Taylor, A. A., & Martoccia, L. (1995). The effect of low voltage and high voltage electrical stimulation on pork quality. Meat Science, 39, 319–326. Taylor, A. A., Nute, G. R., & Warkup, C. C. (1995a). The effect of chilling, electrical stimulation and conditioning on pork eating quality. Meat Science, 39, 339–347. Taylor, A. A., Perry, A. M., & Warkup, C. C. (1995b). Improving pork quality by electrical stimulation or pelvic suspension of carcasses. Meat Science, 39, 327–337. Unruh, J. A., Kastner, C. L., Kropf, D. H., Dikeman, M. E., & Hunt, M. C. (1986). Effect of low voltage electrical stimulation during
H.A. Channon et al. / Meat Science 65 (2003) 1309–1313 exsanguination on meat quality and display colour stability. Meat Science, 18, 282–293. Warner, R. D., Kauffman, R. G., & Greaser, M. L. (1997). Muscle protein changes post-mortem in relation to pork quality traits. Meat Science, 45, 339–352.
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Warriss, P. D., Brown, S. N., Nute, G. R., Knowles, T. G., Edwards, J. E., Perry, A. M., & Johnson, S. P. (1995). Potential interactions between the effects of post-slaughter stress and post-mortem electrical stimulation of the carcasses on meat quality. Meat Science, 41, 55–68.
Application of constant current, low voltage electrical stimulation systems to pig carcasses and its effects on pork quality H.A. Channon*, P.J. Walker, M.G. Kerr, S.R. Baud Department of Primary Industries, Victorian Institute of Animal Science, 600 Sneydes Road, Werribee, Victoria 3030, Australia Received 18 September 2002; received in revised form 20 January 2003; accepted 30 January 2003
Abstract This study examined the effectiveness of a constant current, low voltage electrical stimulation system on improving pork quality when applied to pigs at 2 min post-exsanguination. A total of 48 female DurocLarge White/Landrace pigs of 85–90 kg liveweight were randomly allocated immediately prior to slaughter to one of four constant current electrical stimulation treatments: control (no electrical stimulation), 50, 200 and 400 mA. Stimulation was applied to pig carcasses at 2 min post-exsanguination for 30 s. No differences (P> 0.05) in WB shear force values, muscle lightness or PSE incidence of pork M. longissimus lumborum (LL) was found due to electrical stimulation treatment. Muscle pH of the LL muscle was lower (P <0.001) in carcasses in the 200 and 400 mA treatments compared to those from carcasses in both the 50 mA and control treatment groups, when measured at the various time points from 40 min to 8 h post-slaughter. Although carcasses stimulated with 200 and 400 mA had higher percentage drip loss (P <0.05) and purge (P< 0.001), this was not found to impact WB shear force values, muscle lightness or PSE incidence. # 2003 Elsevier Ltd. All rights reserved. Keywords: Pork; Electrical stimulation; Meat quality
1. Introduction Bennett (1997) reported that Australian consumers consider pork to be tough and dry in comparison to other meats. The high incidence of pale, soft exudative (PSE) meat, inadequate intramuscular fat, cold shortening, inadequate ageing and overcooking of pork by consumers may be impacting pork tenderness. Processing practices (including aitchbone hanging, ageing of pork and the enhancement of pork quality by addition of brine solutions) can be used to varying degrees to reduce variability and improve consistency of eating quality attributes of pork. Channon, Reynolds, and Baud (2001) reported that both aitchbone hanging and ageing pork for 7 days post-slaughter can positively influence eating quality attributes of pork. However, there is considerable reluctance by the Australian pig processing sector to modify carcass hanging procedures from the achilles tendon to the aitchbone due to additional labour and chiller space required for aitchbonehung carcasses. Additionally, the practice of purchasing * Corresponding author. Tel.: +613-9742-0415; fax: +613-97420400. E-mail address: [email protected] (H.A. Channon). 0309-1740/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0309-1740(03)00051-2
aged pork by the Australian retail sector is not common, with the majority of pork purchased as carcasses rather than as boxed pork. Electrical stimulation of pig carcasses may be a commercially acceptable option available to the Australian pork industry to improve eating quality attributes of pork as it can be applied online without requiring additional labour support. Electrical stimulation is most effective in improving tenderness when carcass cooling is fast enough to induce cold shortening of muscle fibres post-slaughter. Electrical stimulation prevents cold shortening by causing a more rapid rate of post slaughter metabolism in muscle. This results in a lower muscle pH while muscle temperatures are still high which maximises proteolysis (Dutson & Pearson, 1985). However, care must be taken with electrical stimulation of pork carcasses as the rapid metabolism induced by electrical stimulation could result in an increased occurrence of PSE (Crenwelge, Terrell, Dutson, Smith, & Carpenter, 1984; Warriss et al., 1995). Electrical stimulation using constant voltage (variable current) electrical stimulation systems is not used in Australian abattoirs as a method of improving eating quality due to the risk of increasing the incidence of PSE. Previous studies that have evaluated the application of either high or low voltage electrical stimulation to
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pigs at 3–5 min post-slaughter (Bowker, Wynveen, Grant, & Gerrard, 1999; Rees, Trout, & Warner, 1999a; Rees, Trout, & Warner 1999b; Taylor & Martoccia, 1995) and at 20 min post-slaughter (Dransfield, Jones, & MacFie, 1991; Maribo, Ertberg, Andersson, Barton-Gade, & Moller, 1999; Taylor & Martoccia, 1995; Taylor, Nute, & Warkup, 1995a, Taylor, Perry, & Warkup, 1995b; Warriss et al., 1995) have been conducted using constant voltage systems. These studies have demonstrated that the effects of electrical stimulation (using both low and high voltage), on the incidence of PSE, rate of pH fall post-slaughter, drip loss and muscle lightness are inconsistent. A possible explanation for these inconsistent effects of conventional, constant voltage electrical stimulation systems on pork quality traits may be that the peak current delivered during stimulation can vary considerably due to varying resistance. Furthermore, the level of the peak current delivered during stimulation using constant voltage systems may also be directly related to the rate of glycolysis post slaughter and, if excessive, trigger the development of PSE. A low voltage electrical stimulation system, with modified frequency and waveform, that can deliver constant current has been developed in Australia and may provide the Australian pig industry with an opportunity to improve the tenderness of pork without inducing PSE. This system has been developed to be as effective as high voltage systems, but safer for abattoir workers and cheaper to install compared with conventional high voltage electrical stimulation systems. This study was conducted to determine the potential for improving pork quality by applying low voltage electrical stimulation to pig carcasses at 2 min postexsanguination using different levels of constant current.
2. Materials and methods A total of 48 female DurocLarge White/Landrace cross pigs of 85–90 kg liveweight were slaughtered following overnight lairage at a pilot abattoir. Low voltage electrical stimulation was applied to pig carcasses at 2 min post exsanguination using a rectal probe and a nose clip. Pigs were randomly allocated to one of four constant current electrical stimulation treatments immediately prior to slaughter (n=12 per electrical stimulation treatment): 1. Control: no stimulation. 2. Current 50 mA, 80V peak, frequency 14 Hz. Stimulation applied for 30 sec at 2 min postexsanguination. 3. Current 200 mA, 80V peak, frequency 14 Hz. Stimulation applied for 30 sec at 2 min postexsanguination.
4. Current 400 mA, 80V peak, frequency 14 Hz. Stimulation applied for 30 sec at 2 min postexsanguination. All pigs were stunned using 90% CO2 in a dip-lift stunner (Butina APS, Denmark) for a total of 108 s, shackled by the right leg after stunning, exsanguinated vertically and then electrically stimulated. Following stimulation, carcasses were placed in a dehairer at 62 C for 5 min, with any remaining hair removed using a knife and flame. Carcasses were then eviscerated before being chilled for 24 h set at 2–4 C. 2.1. Pork quality measurements Muscle pH and temperature of M. longissimus lumborum (LL) (caudal to the last rib) was measured at 40 min and 2, 3.5, 5, 6.5 and 8 h post-slaughter using a portable pH meter (Jenco pH meter, Model 6007) fitted with a polypropylene spear-type gel electrode (Ionode IS425, Brisbane, QLD) and a temperature adjusting probe. Ultimate pH and temperature was also measured in the LL muscle at 24 h post-slaughter. At 24 h post-slaughter, a section of the LL muscle was removed from the right side of each carcass. Meat colour was measured on a 4 cm thick chop at 24 h postslaughter following exposure of a cut surface to air for 10 min. A Byk Gardner Chromameter (Byk Gardner, Germany, Color Guide sphere; Cat. No. 6830) set on the L*, a*, b* system using D65 lighting and a 10 C standard observer and an 11mm aperture in the measuring head. Drip loss was determined on the LL muscle at 24 h post-slaughter using a modified method of Rasmussen and Andersson (1995). A sample of pork loin was cut to a 20-g (115 cm) strip, weighed and weight recorded. The sample was then wrapped in square netting (20 cm2), suspended in a sealed 120-ml plastic container and allowed to stand in a 4 C chiller for 24 h. The sample was then removed from the container, gently rolled in paper towelling and reweighed to determine percentage drip loss. Carcasses were classified as pale, soft and exudative (based on pHu < 5.6, L* value > 50, drip loss > 5%) as defined by Warner, Kauffman, and Greaser (1997), where pHu is defined as ultimate pH at 24 h postslaughter. LL muscle samples for Warner–Bratzler (WB) shear force assessment were aged at 2 C for 2 d post-slaughter. All samples were cut into 100 2 g blocks, cooked in plastic bags in a water bath at 80 C for 1 h (to a 70– 73 C internal temperature) and then cooled in cold running water for 30 min, as described by Bouton, Harris, and Shorthose (1971). Samples were then dried with paper towels to remove excess moisture and reweighed to determine cooking loss. Each sample was cut
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parallel to the muscle fibres into five replicates of 1 cm2 cross-section. Tenderness was measured using a Warner–Bratzler shear force blade fitted to an Instron Universal Testing Machine Model 4465, with crosshead speed of 300 mm/min and a 5 kN load cell. Purge was determined by weighing the LL sample destined for WB shear force measurement prior to vacuum packaging. Upon removal from the vacuum bag, each sample was dried with paper towels to remove excess moisture and re-weighed to determine purge. 2.2. Statistical analyses Pigs were slaughtered over two slaughter days (n=24 per day) with six blocks containing four pigs (one from each stimulation treatment) per slaughter day. A randomised block design was used to allocate pigs immediately prior to slaughter into electrical stimulation treatments. Data was analysed using Genstat 5 for Windows, Release 4.2 (Payne, Lane, & Genstat 5 Committee, 1987) by Analysis of Variance to determine predicted means and standard error of differences for objective pork attributes due to stimulation treatment. A comparison of the incidence of PSE pork between treatments was made using the chisquare goodness of fit test (Snedecor & Cochran, 1980).
Table 1 Means and standard error of the difference between means (S.E.D.) for pigs in each electrical stimulation treatment for muscle pH and temperature measured from 40 min to 24 h post-slaughter in the M. longissimus thoracis et lumborum (LTL) muscle Variate
Electrical stimulation treatment Control
Muscle pH 40 min 6.69 2h 6.53 3.5 h 6.31 5h 6.12 6.5 h 5.96 8h 5.87 24 h 5.43 Temperature ( C) 40 min 36.2 2h 24.4 3.5 h 16.7 5h 12.2 6.5 h 9.6 8h 7.9 24 h 7.6
6.66 6.44 6.26 6.12 6.02 5.95 5.46 36.3 23.1 14.9 10.1 8.5 6.7 7.7
200 mA
P value
0.062 0.110 0.062 0.090 0.082 0.072 0.027
<0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.458
400 mA
6.14 5.80 5.59 5.50 5.45 5.45 5.42
6.13 5.70 5.53 5.47 5.45 5.42 5.42
37.0 25.5 16.4 12.0 9.2 7.3 7.5
36.7 24.5 15.8 11.2 8.7 6.9 7.4
0.51 0.86 0.58 0.45 0.40 0.32 0.14
0.361 0.077 0.025 0.023 0.030 0.005 0.230
Table 2 Means and standard error of the difference between means (S.E.D.) for pigs in each electrical stimulation treatment for drip loss (%), purge (%), muscle lightness (L*), Warner–Bratzler (WB) shear force (kg), sarcomere length (nm), cooking loss (%) and PSE incidence (%) in the M. longissimus thoracis et lumborum muscle Variate
Electrical Stimulation treatment
S.E.D. P value
Control 50 mA 200 mA 400 mA
3. Results Muscle pH, measured from 40 min to 8 h postslaughter was lower at each time interval (P < 0.001) in LL muscles from carcasses in the 200 and 400 mA electrical stimulation treatments compared with those from the 50 mA and control (no stimulation) treatments (Table 1). No significant difference (P > 0.05) in muscle pH between stimulation treatments was found at 24 h post-slaughter. Although muscle temperature decline post-slaughter was influenced (P < 0.05) by stimulation treatment from 3.5 to 8 h post-slaughter, these treatment differences were not consistent with increasing current levels applied during stimulation. Drip and purge losses were higher (P < 0.001) from LL muscle obtained from carcasses in both the 200 and 400 mA treatments compared with those from the 50 mA and control treatments (Table 2). No differences in drip or purge losses occurred (P> 0.05) between carcasses stimulated with 50 mA and those in the control treatment. No differences (P > 0.05) in either muscle lightness (CIE L*) or the incidence of PSE occurred due to stimulation treatment. No significant differences in WB shear force values (P > 0.05) occurred due to electrical stimulation treatment. No effect of stimulation treatment (P > 0.05) occurred for cooking loss.
50 mA
S.E.D.
Drip loss (%) 3.35 Purge (%) 2.72 Muscle lightness (L*) 49.1 WB shear force (kg) 3.74 Sarcomere length (nm) 1.97 Cooking loss (%) 34.18 PSE incidencea (%) a
0
3.90 2.48 47.3 3.77 2.00 34.57 0
5.00 4.74 48.3 3.56 1.98 34.31 8
5.18 5.38 49.2 3.56 1.98 35.06 16
0.671 0.614 1.06 0.251 0.07 0.545
0.028 <0.001 0.278 0.747 0.986 0.398 n.s.
Chi-square analysis.
4. Discussion In this study, electrical stimulation of pig carcasses at 2 min post-exsanguination with 200 and 400 mA constant current for 30 s resulted in both higher drip and purge losses, following vacuum packaging, from the LL muscle. This may have resulted from the apparent faster rate of pH decline from 40 min to 8 h post-slaughter of LL muscles from carcasses stimulated with 200 and 400 mA current compared with those from carcasses in the 50 mA and control treatments. Taylor and Tantikov (1992), in a study using high voltage electrical stimulation, also found that stimulation within 5 min postslaughter increased drip loss from the LD muscle. Overall, the level of toughness across all experimental treatments was extremely low (i.e. the pork from this production system was already tender), with only 2% of
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all pork LL muscles having WB shear force values greater than 5 kg [a threshold level above which consumers may consider pork to be tough (Hofmeyr, 1998)]. This was a surprising and unexpected result considering that the pork had been aged for only 2 d post-slaughter. Channon et al. (2001) found that 41% of pork LL muscles had WB shear force values greater than 5 kg following ageing for 2 days post-slaughter. It is noteworthy that electrical stimulation treatment did not impact WB shear force values of pork loin, particularly those obtained from carcasses in the 200 and 400 mA treatments, despite the higher drip loss levels observed. This suggests that the faster rate of pH decline post-slaughter in pork LL muscle does not necessarily result in increased toughness, at least when objectively assessed. Although Unruh, Kastner, Kropf, Dikeman, and Hunt (1986) stated that if the rate of muscle pH decline in beef is too fast at high muscle temperatures, a slight increase in toughness might result due to rigor onset at high temperatures, this was not observed in this study. Previous studies (Dransfield et al., 1991; Taylor & Tantikov, 1992; Warriss et al., 1995) have found that when pig carcasses are electrically stimulated using high voltage and chilled conventionally, a high level of protein denaturation can result. It is not known if this occurred in this study as the solubility of sarcoplasmic and myofibrillar proteins was not determined. It is noteworthy that, although, electrical stimulation with high constant current levels (200 and 400 mA) resulted in high drip loss percentages, neither muscle lightness nor cooking loss were significantly influenced by stimulation treatment in this study. Taylor et al. (1995b) also found that the application of high voltage electrical stimulation at 20 min postslaughter to pig carcasses did not influence meat colour, even though drip loss increased. However, these results do suggest that the application of either 200 or 400 mA current for 30 s at 2 min post-exsanguination was not ideal due to the high drip and purge losses found from the LL muscle. In comparison, the application of 50 mA current did not result in pork with quality attributes dissimilar to pork from unstimulated carcasses. It is suggested that any further work conducted with this new, constant current, low voltage electrical stimulation system be conducted using a current level between 50 and 200 mA applied for 30 s at 2 min post-exsanguination in order to minimise drip loss whilst enhancing pork tenderness.
Acknowledgements The authors are appreciative of the funding provided by Australian Pork Limited to undertake this work. The technical assistance provided by Paul Meredith, Paul Weston, Bob Nightingale and Matt McDonagh and statistical advice provided by Kym Butler was appreciated and is gratefully acknowledged.
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