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The effect of chilling, electrical stimulation and conditioning on pork eating quality
Meal Science 39 (1995) 339-347 0 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0309-l 740/951%07.00 ELSEVIER
0309-1740(94)E0001
I-T
A. A. Taylor, G. R. Nute Department of Food Animal Science, School of Veterinary Science, University of Bristol, Langford, Bristol, UK, BS18 7DY &
Meat & Livestock Commission, PO Box 44, Winterhill House, Snowdon Drive, Milton Keynes, UK, MK6 1AX
(Received 10 January 1994; revised version received IO March 1994; accepted 22 March 1994)
ABSTRACT Tlze eflect of three dt&rent post-sluughtertreutmentsand subsequent conditioning times on the eating quality of pork was studied, using u totul of 72 pigs (NJ-90 kg live wt). The treatments were: (A) holding in air at >IO”C for 3 h, followed by chilling in air at 1°C; (B) chilling in air at 1°C; (C) high voltage electrical stimulation(ES) ut 20 min post-sluughter, followed by Treatment B. The qualityattributes were measured in M. longissimus thoracis et lumborum (LTL) and in M. semimembranosus CW. There was little dtflerence in cooling rate between the three treatments; the major eflect on quality cume from the use of ES in Treatment C. ES reduced pH at 45 min by approximatelyO-3units, and achieved pH values at 3 h post-slaughter of 5.64 (LTL) and 5.87 (Sm) but did not produce PSE meat. Drip losses were generally low, but were slightly higher with Treatment C. By all three instrumental texture parameters, LTL from Treatment C was significantlymore tender than from A and B at 4, 7 and 12 days post-slaughter, suggesting that either some cold-toughening with A and B was overcome by ES in treatment C or that ES had some other beneJicia1 action. Conditioning at 1°C improved the tenderness of LTL from 4 to 7 days and further to 12 days. Taste panelling of loin chops und
ICP
34u
A. A. Taylor, G. R. Nute, C. C. Warkup Sm roasts confirmed that Treatment C gave significantly more tender meat than A and B, and that ageing from 4 to 7 &ys and further to 12 days significantly improved tenderness. High voltage electrical stimulation at 20 min post-slaughter followed by cooling in air at 1°C (Treatment C) produced loin muscle which was more tender at 4 days than at 12 days with the other treatments.
INTRODUCTION Despite the faster rate of post-mortem glycolysis in pig muscle compared with beef or lamb, cold-toughening in pork has been reported by several workers (Marsh et al., 1972; Gigiel & James, 1984; Dransfield & Lockyer, 1985; BartonGade et al., 1987; Moller & Vestergaard, 1988). In most cases, the toughening has been associated with rapid chilling procedures. Several studies have demonstrated that electrical stimulation (ES) can improve the tenderness of pork, presumably by creating conditions where cold-toughening can not occur, but, on the other hand, some researchers have found no tenderising effect from ES, or even harmful effects such as increased drip loss. Taylor & Tantikov ( 1989, 1992) and Dransfield et al. (1991) have however shown a clear improvement in pork tenderness when high voltage ES was applied to carcasses at 20 min post-slaughter. ES had a tenderising effect not only with carcasses which were rapidly chilled, but also to some extent with carcasses which had been conventionally chilled at 1°C and should therefore have been less prone to cold-toughening. The present study was designed to examine further the effects of ES in pigs, and particularly to determine whether tenderness can be improved in carcasses subjected to slow, conventional chilling at 1°C. The level of tenderness has also been compared with that obtained with an even slower process where chilling was delayed by 3 h.
EXPERIMENTAL
The experimental design required six groups of 12 pigs, each group consisting of six entire males and six gilts. The pigs were selected to be of bacon weight (80-90 kg live weight) oi Large White-Landrace breeding, low incidence of the halothane gene and with Pz backfat thickness measuring 8-12 mm.
he three post-slaughter treatments examined were: (A) Delay at 10°C until 3 h post-slaughter, before cooling at 1°C until 24 h post-slaughter. ) Cooling at l°C from 45 min until 24 h post-slaughter. (C) High valtage ES and cooling at 1°C from 45 min until 24 h post-slaughter.
Chilling, electrical stimulation and conditioning on pork quality
341
The ES conditions were: 700 V peak, 12.5 Hz, for 90 s, applied at 20 min post-slaughter to whole carcasses. For all treatments the air speeds during chilling were ~0.5 m/s. ES was applied between hook electrodes inserted in the hind legs near the Achilles tendon and in the severed neck muscles. At each slaughter time pigs were allocated at random so that two boars and two gilts were subjected to each of the three treatments. Backfat thickness
This was measured in the P2 position on the hot carcass, using an intrascope. Intramuscular fat
The percentage of fat in a portion of A4.Zongissimus thoracis et fumborum (LTL) caudal to the last rib was estimated by extracting with diethyl ether in a Soxhlet apparatus. Cooling rate
Carcasses were split before chilling and temperatures recorded during the 24 h period by inserting thermocouples into LTL at the last rib and the deepest part of the hind leg of one carcass in each treatment at each slaughter time.
The pH was measured in each carcass in LTL at the 10/l lth rib and in the it4. semimembrunosus (Sm), at 45 min, 3 h and 24 h post-slaughter. For each measurement, 1 g muscle was removed, homogenised in 10 ml iodoacetate solution and the pH measured with a Radiometer p meter and combined electrode. c Colour
The colour of a section of LTL caudal to the last rib was assessed at 24 h postslaughter, using CIELAB L* (lightness), a* and b* colour coordinates, measured on a Minolta Chroma Meter. rip loss
A section of LTL from each carcass adjacent to that used for colour measurement was used for recording drip loss, by suspending a 25 mm thick slice of the muscle inside a plastic bag, so that the drip accumulating over 3 days could be weighed. Inst~~eniaI
tex
A 30 cm section of LTL was removed from one side of each carcass at 24 h and divided to give three portions (one forward from the last rib and two rear of the last rib) which were individually vacuum packed and aged at l°C for assessment at 4, 7 and 12 days post-slaughter. The Sm was removed and treated similarly to provide a sample at 4 days. The muscle samples were cooked in a water bath at
342
A. A. Taylor, 6. R. Nute, C. C Warkup
80°C to a centre temperature of 78°C and cooled overnight. Then six blocks (10 x 10 X 20 mm) were cut from each muscle sample, with the longest side
parallel to the fibre direction, and sheared using Volodkevitch-type jaws on a Stevens CR Analyser, three measurements were recorded during shearing: yield force (kg), compression force (kg) and total work done (J X lO-2). Taste panelling From the opposite side of each carcass, three 10 cm long sections of LTL were removed for instrumental texture measurement at 24 h post-slaughter, vacuum packed and aged at 1°C until 4, 7 and 12 days after slaughter. After ageing, the vacuum packed samples were blast frozen and stored at -20°C until required for assessment, at which time they were thawed and cut into 25 mm thick steaks. These were cooked on a pre-heated griddle, turning every 3 min until the centre temperature was 80°C. The steaks were cut into 2 X 3 cm blocks and submitted, hot, to 10 panellists. At each session, assessors received samples of each of the three treatments and two ageing times which they evaluated on an eight-point scale ranging from ‘extremely tough’ (1) to ‘extremely tender’ (8). From the same side of each carcass, the Sm was removed at 24 h post-slaughter, vacuum packed and aged at 1°C until 4 days post-slaughter, The muscle samples were roasted to an internal temperature of 8O”C, and panellists given I cm thick slices to evaluate. Analysis of data The data from boars and gilts were subjected to analysis of variance to examine the effects of sex and post-slaughter treatments and possible interactions. There was no effect of sex, and therefore the data from boars and gilts were combined. Treatment differences were tested for significance at the 5% level, based on the standard error of the difference between means obtained from the analysis of variance.
RESULTS
Hot carcass weight, P2 backfat thickness and intramuscular fat expressed as a percentage of wet weight, are shown in Table 1. The only treatment difference wzs in intramuscular fat where the mean value for carcasses in Treatment A was slightly higher than in B and C (P = 0.025). This was largely because two of the pigs in this group had fat levels above 2.0%. Cooling rate The delayed chilling of Treatment A reduced the temperature of deep LTL to 10°C by 6.2 h after slaughter, and the deep leg to 10°C by 9.4 h. With Treatments B and C where there was no delay before chilling, the temperature in LTL fell to 10°C in 5.6 h and in deep leg in 9.0 h. Despite the delay with Treatment A, there was only a slight difference in cooling rate in the deep muscle.
Chilling, electrical stimulation and conditioning on pork quality
343
TABLE I Carcass Characteristics. Mean Values of 24 Pigs per Treatment Treatment A Hot carcass weight (kg) Backfat thickness (P2) (mm) Intramuscular fat (%)
71.93 9.62 1*14a
B
c
s.e.d.
Significance
72.55 9.57 0.87b
72.38 9.58 0.83b
1.230 0.572 0.123
NS NS co.05
Means with different superscripts are significantly different (P < O-05).
pH changes changes in pH in LTL and Sm are shown in Table 2. The The post-slaughter effect of ES in Treatment C is clearly shown in the pH measured at 45 min (after ES) and at 3 h, the values being significantly (P < 0.001) lower than in the unstimulated carcasses. There was no significant difference with treatment in ultimate pH, which was approximately 5.5 for both muscles.
Colour The mean of the CIELAB colour coefficients L* (lightness), a* (green-red) and b* (blue-yellow) are shown in Table 3, along with the calculated values for Saturation and Hue. Only in the case of lightness was there any significant difference with Treatment C, where ES had been used, producing slightly lighter meat. There was no evidence of PSE meat.
pH at 45 min, 3 h and 24 h Post-slaughter in M. longissimus thoracis et lumborum (LTL) and h4. semimembranosus (Sm) Muscles. Mean Values of 24 pigs per Treatment Treatment
LTL 45 min 3h 24 h Sm 45 min 3h 24 h
s. e.d.
Significance
A
B
6.63” 6a25a 5.51
6.50” 6.20” 5.51
6.22b 564b 5.46
0.070 0,087 0,025
6~65~ 6.43” 5.53
6.53b 6.25b 5.53
6.26’ 5.87’ 5.51
0.063 0.079 0,018
co.00 1 <0*001 NS
Means with different superscripts are significantly different (P < 0.05).
344
A. A. Taylor. G. R. Nufe, C. C. Warkup
TABLE 3 Colour (L*, a*, b*), Saturation and Hue at 24 h Post-slaughter in LTL and Drip Loss (% by wt) over 72 h at 1°C from LTL and Sm. Mean Value!: of 24 pigs per Treatment Treatment
Colour, in LTL L* a* b+ Saturation Hue Drip loss LTL Sm
A
B
c
s.e.d.
Sign$cance
52.58” 6.05 4.80 7-74 38.3 1
52-68a 6.24 4.74 7.85 37.61
54.65b 6.21 5.15 8.08 39.68
0.657 O-387 o-305 0.463 l-256
3.24 1.89
3.63 2.25
4.18 2.23
O-387 O-228
NS NS
Means with different superscripts are significantly different (P < 0.05).
TABLE 4 Instrumental Texture (Yf, Cf, W) of LTL at 4, 7 and 12-days Post-slaughter, and Sm at 4 days post-slaughter. Mean Values of 24 pigs per Treatment Treatment A
Yield force (Yf) (kg) LTL 4 days 7 days 12 days Sm 4 days CO;~~ force (Cf) (kg)
B
C
s.e.d.
SignjJicance
7.86” 7.29” 6.77”
8.00” 7.41” 6.92”
8-34
7.92
7.57
0.377
7.06” 6.68” 6.18”
5,60b 4~81~ 4.61’
O-532 0.485 0.478
744
7.28
0.388
NS
43-04” 41.39” 40.74”
36.41b 350lb 34*79b
2-i i7 1a993 1.976
43.63
43.06
2-l 17
NS
4 days 7.18” 7 days 6.54a 12 days 6.06” Sm 4 days 8.10 Work done (W) (J x lo-“) LTL 4 days 42-59” 7 days 41.74” 12 days 38.99” Sm 4 days 44.80
O-466 0,418 0.422
Means with different superscripts are significantly different (P < 0.05).
co.0 1
NS
co-0 1 co*00 1
Chilling,electricalstimulationund conditioningon pork quality
345
Drip loss The mean values for drip loss over 3 days from LTL and Sm are shown in Table 3. There was no significant treatment difference for either muscle although LTL samples in Treatment C tended to lose slightly more drip than those from the other two treatments. In this experiment, the drip losses were generally low.
Table 4 shows the mean values for the three texture parameters measured in LTL at 4, 7 and 12 days after slaughter and in Sm at 4 days. For LTL, Treatment C gave significantly more tender meat, expressed by all three parameters, than A and B. There was no significant difference between Treatments A and B. These results were true for all ageing times. The yield force (Yf) was significantly lower in Treatment C by about 1.5 kg per sample after 4 days (P = ONll), 7 days (P < 0.001) and 12 days (P = 0.002). The compression force (Cf) was significantly lower in Treatment C by more than 1.5 kg after 4 days (P = 0.006), 7 days (P c 0.001) and 12 days (P = O-01). Superimposed on the effect of treatment, an improvement in tenderness with ageing was observed in all parameters after 7 and 12 days. However, as measured by all three texture parameters, ageing up to 12 days after Treatments A and B did not achieve the degree of tenderness achieved in 4 days with Treatment C. The Sm muscle was measured only at 4 days post-slaughter and, although there was no significant overall treatment effect on texture parameters, samples from Treatment C tended to be less tough than the others in terms of yie!d, compression and work done. Taste panelling Panel scores for the tenderness of griddled loin slices and roast Sm joints are shown in Table 5. The mean values show quite clearly that Treatment C produced samples of LTL which at 4 days and 7 days were significantly more tender
TABLES Taste Panel Assessment of Tenderness of Griddled Loin Slices at 4, 7 and 12 days Postslaughter, and of Roast Sm at 4 days Post-slaughter. Eight Point Rating Scales from ‘Extremely Tough’ (1) to ‘Extremely Tender’(8). Mean Values of 24 Pigs per Treatment Treatment
LTL 4 days 7 days 12 days Sm 4 days
A
B
c
s. e. d.
Significance
3.64” 4-W 4.26
3~83~ 4*00” 4-32
4-28b 4-59b 4.65
0.203 o-192 O-214
co*01 CO.0i NS
4@Ia
4-2gb
4.w
0.119
Means with different superscripts are significantly different (P < O-05).
346
A. A. Taylor, G. R. Nute, C. C. Warkup
(P < O-01) than the other treatments. By 12 days the treatment differences were no longer significant. At 4 days, Treatment C produced Sm which was significantly more tender (P c 0.001) than the other treatments, and Treatment A gave Sm which was significantly tougher than the other treatments. The effect of ageing is seen in the LTL samples, where the tenderness, with all treatments, improved with ageing from 4 days to 7 days and 12 days. DISCUSSION Carcass characteristics, with the exception of intramuscular fat, were similar across all three treatments. The slightly higher intramuscular fat levels of carcasses in Treatment A might have been expected to enhance tenderness to a slight extent, but in fact Treatment A samples were still among the toughest. The 3 h delay before chilling in Treatment A had only a small effect on the cooling rate of LTL or the deeper Sm compared to carcasses where chilling commenced 45 min after slaughter. Even so, the deepest part of LTL had cooled to 10°C in little more than 6 h. In this case therefore, as with the earlier cooling in the other treatments, a certain amount of cold-toughening might be expected in some of the muscle. The similarity in cooling rates across treatments meant that the greatest effect on quality parameters was likely to come from the electrical stimulation in Treatment C. This was clearly demonstrated by the rapid pH fall in Treatment C, where values were approximately O-3 units lower at 45 min after slaughter, and approaching the ultimate pH by 3 h. The rapid pH fall was the likely cause of the slightly lighter colour of LTL with stimulation. This increased lightness was however not sufficiently pronounced to be visible to the eye. Although the general level of drip loss was low across all treatments, the slightly higher loss in Treatment C was probably due to ES. This tendency towards higher drip losses with ES was noted in earlier studies (Taylor & Tantikov, 1989, 1992) where it was also shown that it, could be avoided by faster chilling. The clearest effect of treatment was on meat tenderness measured both instrumentally and by taste panel. These results are consistent with those in the earlier studies and demonstrated the tenderising effect of ES. The improvement in tenderness in Treatment C by all instrumental parameters was considerable, compared with the other two treatments which did not incorporate ES. Treatment C with ES produced LTL which was more tender 4 days after slaughter than meat produced by the other two (non-ES) treatments after 12 days ageing. The improvement in tenderness with Treatment C was maintained throughout ageing, so that by 12 days, the yield and compression forces were still about l-5 kg lower than in Treatments A and B. The Sm was less affected by treatment, possibly because there was less likelihood of cold shortening. Nevertheless, Treatment C samples were the most tender by all three texture parameters, and slowly cooled samples from Treatment A the toughest. The instrumental texture results were strongly supported by taste panel assessment, with significantly higher ratings for Treatment C samples, not only for LTL but also for Sm where instrumental measurements were less discriminating between treatments. The magnitude of the improved tenderness judged by taste panels at 4 days was equal to, or greater than, the O-5 scale points improvement
Chilling. electrical stimulation and conditioning on pork quality
347
in panel rating of tenderness attributed by Warkup eCal. (1990) to rapid growth caused by ad iibitum feeding. The overall improvement in tenderness of LTL with ageing was also clearly demonstrated, so that by 12 days the ES effect was no longer significant. These results suggest t.5 at, cvcn *under the conventional chilling conditions used in this study, either some cold-toughening occurs in pig carcasses which can be effectively avoided by ES or the ES has some other beneficial effect. Delaying the onset of chilling by 3 h gave no improvement in tenderness, but all samples became more tender with ageing up to 12 days. The greatest improvement in tenderness however, came from applying high voltage ES at 20 min after slaughter. ACKNOWLEDGEMENTS The authors wish to thank Miss A. M. Perry, Mr S. P. Johnson and Mrs A. Baker for their expert technical assistance in the experimental work, and Mr G. A. Fursey and Mrs J. Bayntun for their help and advice in analysing the data. This research was part of a programme funded by the Meat and Livestock Commission.
REFERENCES Barton-Gade, P., Bejerholm, C. & Borup. V. (1987). Proc. 33rd ht. Cong. Meat Sci. and Technology, Helsinki, Finland, p_ 18I. Dransfield, E. & Lockyer, D. K. (1985). Meat Sci., 13, 19. Dransfield, E., Ledwith, M, J. & Taylor, A. A. (199 1). Mear Sci., 29, 129. Gigiel, A. J. & James, S. J. ( 1984). Meat Sci., 11, I. Marsh, B. B., Cassens, R. G., Kaufman, R. G. & Briskey, E. J. (1972). J. Food SC+., 37, 179.
Moller, A. J. & Vestergaard, T. (1988). Proc. 34tk ht. Gong. Meut Sci. arzd Teclznology, Brisbane, Australia, p. 621. Taylor, A. A. & Tantikov, M. Z. (1989). Proc. 35th hr. Gong. Meat Sci. and Technology, Copenhagen, Denmark. Taylor, A. A. & Tantikov, M. Z. (1992). Meat Sci., 31, 381. Warkup, C. C., Dilworth, A. W., Kempster, A. J. & Wood, J. D. (1990). him. Prod., 50, 550.
0309-1740(94)E0001
I-T
A. A. Taylor, G. R. Nute Department of Food Animal Science, School of Veterinary Science, University of Bristol, Langford, Bristol, UK, BS18 7DY &
Meat & Livestock Commission, PO Box 44, Winterhill House, Snowdon Drive, Milton Keynes, UK, MK6 1AX
(Received 10 January 1994; revised version received IO March 1994; accepted 22 March 1994)
ABSTRACT Tlze eflect of three dt&rent post-sluughtertreutmentsand subsequent conditioning times on the eating quality of pork was studied, using u totul of 72 pigs (NJ-90 kg live wt). The treatments were: (A) holding in air at >IO”C for 3 h, followed by chilling in air at 1°C; (B) chilling in air at 1°C; (C) high voltage electrical stimulation(ES) ut 20 min post-sluughter, followed by Treatment B. The qualityattributes were measured in M. longissimus thoracis et lumborum (LTL) and in M. semimembranosus CW. There was little dtflerence in cooling rate between the three treatments; the major eflect on quality cume from the use of ES in Treatment C. ES reduced pH at 45 min by approximatelyO-3units, and achieved pH values at 3 h post-slaughter of 5.64 (LTL) and 5.87 (Sm) but did not produce PSE meat. Drip losses were generally low, but were slightly higher with Treatment C. By all three instrumental texture parameters, LTL from Treatment C was significantlymore tender than from A and B at 4, 7 and 12 days post-slaughter, suggesting that either some cold-toughening with A and B was overcome by ES in treatment C or that ES had some other beneJicia1 action. Conditioning at 1°C improved the tenderness of LTL from 4 to 7 days and further to 12 days. Taste panelling of loin chops und
ICP
34u
A. A. Taylor, G. R. Nute, C. C. Warkup Sm roasts confirmed that Treatment C gave significantly more tender meat than A and B, and that ageing from 4 to 7 &ys and further to 12 days significantly improved tenderness. High voltage electrical stimulation at 20 min post-slaughter followed by cooling in air at 1°C (Treatment C) produced loin muscle which was more tender at 4 days than at 12 days with the other treatments.
INTRODUCTION Despite the faster rate of post-mortem glycolysis in pig muscle compared with beef or lamb, cold-toughening in pork has been reported by several workers (Marsh et al., 1972; Gigiel & James, 1984; Dransfield & Lockyer, 1985; BartonGade et al., 1987; Moller & Vestergaard, 1988). In most cases, the toughening has been associated with rapid chilling procedures. Several studies have demonstrated that electrical stimulation (ES) can improve the tenderness of pork, presumably by creating conditions where cold-toughening can not occur, but, on the other hand, some researchers have found no tenderising effect from ES, or even harmful effects such as increased drip loss. Taylor & Tantikov ( 1989, 1992) and Dransfield et al. (1991) have however shown a clear improvement in pork tenderness when high voltage ES was applied to carcasses at 20 min post-slaughter. ES had a tenderising effect not only with carcasses which were rapidly chilled, but also to some extent with carcasses which had been conventionally chilled at 1°C and should therefore have been less prone to cold-toughening. The present study was designed to examine further the effects of ES in pigs, and particularly to determine whether tenderness can be improved in carcasses subjected to slow, conventional chilling at 1°C. The level of tenderness has also been compared with that obtained with an even slower process where chilling was delayed by 3 h.
EXPERIMENTAL
The experimental design required six groups of 12 pigs, each group consisting of six entire males and six gilts. The pigs were selected to be of bacon weight (80-90 kg live weight) oi Large White-Landrace breeding, low incidence of the halothane gene and with Pz backfat thickness measuring 8-12 mm.
he three post-slaughter treatments examined were: (A) Delay at 10°C until 3 h post-slaughter, before cooling at 1°C until 24 h post-slaughter. ) Cooling at l°C from 45 min until 24 h post-slaughter. (C) High valtage ES and cooling at 1°C from 45 min until 24 h post-slaughter.
Chilling, electrical stimulation and conditioning on pork quality
341
The ES conditions were: 700 V peak, 12.5 Hz, for 90 s, applied at 20 min post-slaughter to whole carcasses. For all treatments the air speeds during chilling were ~0.5 m/s. ES was applied between hook electrodes inserted in the hind legs near the Achilles tendon and in the severed neck muscles. At each slaughter time pigs were allocated at random so that two boars and two gilts were subjected to each of the three treatments. Backfat thickness
This was measured in the P2 position on the hot carcass, using an intrascope. Intramuscular fat
The percentage of fat in a portion of A4.Zongissimus thoracis et fumborum (LTL) caudal to the last rib was estimated by extracting with diethyl ether in a Soxhlet apparatus. Cooling rate
Carcasses were split before chilling and temperatures recorded during the 24 h period by inserting thermocouples into LTL at the last rib and the deepest part of the hind leg of one carcass in each treatment at each slaughter time.
The pH was measured in each carcass in LTL at the 10/l lth rib and in the it4. semimembrunosus (Sm), at 45 min, 3 h and 24 h post-slaughter. For each measurement, 1 g muscle was removed, homogenised in 10 ml iodoacetate solution and the pH measured with a Radiometer p meter and combined electrode. c Colour
The colour of a section of LTL caudal to the last rib was assessed at 24 h postslaughter, using CIELAB L* (lightness), a* and b* colour coordinates, measured on a Minolta Chroma Meter. rip loss
A section of LTL from each carcass adjacent to that used for colour measurement was used for recording drip loss, by suspending a 25 mm thick slice of the muscle inside a plastic bag, so that the drip accumulating over 3 days could be weighed. Inst~~eniaI
tex
A 30 cm section of LTL was removed from one side of each carcass at 24 h and divided to give three portions (one forward from the last rib and two rear of the last rib) which were individually vacuum packed and aged at l°C for assessment at 4, 7 and 12 days post-slaughter. The Sm was removed and treated similarly to provide a sample at 4 days. The muscle samples were cooked in a water bath at
342
A. A. Taylor, 6. R. Nute, C. C Warkup
80°C to a centre temperature of 78°C and cooled overnight. Then six blocks (10 x 10 X 20 mm) were cut from each muscle sample, with the longest side
parallel to the fibre direction, and sheared using Volodkevitch-type jaws on a Stevens CR Analyser, three measurements were recorded during shearing: yield force (kg), compression force (kg) and total work done (J X lO-2). Taste panelling From the opposite side of each carcass, three 10 cm long sections of LTL were removed for instrumental texture measurement at 24 h post-slaughter, vacuum packed and aged at 1°C until 4, 7 and 12 days after slaughter. After ageing, the vacuum packed samples were blast frozen and stored at -20°C until required for assessment, at which time they were thawed and cut into 25 mm thick steaks. These were cooked on a pre-heated griddle, turning every 3 min until the centre temperature was 80°C. The steaks were cut into 2 X 3 cm blocks and submitted, hot, to 10 panellists. At each session, assessors received samples of each of the three treatments and two ageing times which they evaluated on an eight-point scale ranging from ‘extremely tough’ (1) to ‘extremely tender’ (8). From the same side of each carcass, the Sm was removed at 24 h post-slaughter, vacuum packed and aged at 1°C until 4 days post-slaughter, The muscle samples were roasted to an internal temperature of 8O”C, and panellists given I cm thick slices to evaluate. Analysis of data The data from boars and gilts were subjected to analysis of variance to examine the effects of sex and post-slaughter treatments and possible interactions. There was no effect of sex, and therefore the data from boars and gilts were combined. Treatment differences were tested for significance at the 5% level, based on the standard error of the difference between means obtained from the analysis of variance.
RESULTS
Hot carcass weight, P2 backfat thickness and intramuscular fat expressed as a percentage of wet weight, are shown in Table 1. The only treatment difference wzs in intramuscular fat where the mean value for carcasses in Treatment A was slightly higher than in B and C (P = 0.025). This was largely because two of the pigs in this group had fat levels above 2.0%. Cooling rate The delayed chilling of Treatment A reduced the temperature of deep LTL to 10°C by 6.2 h after slaughter, and the deep leg to 10°C by 9.4 h. With Treatments B and C where there was no delay before chilling, the temperature in LTL fell to 10°C in 5.6 h and in deep leg in 9.0 h. Despite the delay with Treatment A, there was only a slight difference in cooling rate in the deep muscle.
Chilling, electrical stimulation and conditioning on pork quality
343
TABLE I Carcass Characteristics. Mean Values of 24 Pigs per Treatment Treatment A Hot carcass weight (kg) Backfat thickness (P2) (mm) Intramuscular fat (%)
71.93 9.62 1*14a
B
c
s.e.d.
Significance
72.55 9.57 0.87b
72.38 9.58 0.83b
1.230 0.572 0.123
NS NS co.05
Means with different superscripts are significantly different (P < O-05).
pH changes changes in pH in LTL and Sm are shown in Table 2. The The post-slaughter effect of ES in Treatment C is clearly shown in the pH measured at 45 min (after ES) and at 3 h, the values being significantly (P < 0.001) lower than in the unstimulated carcasses. There was no significant difference with treatment in ultimate pH, which was approximately 5.5 for both muscles.
Colour The mean of the CIELAB colour coefficients L* (lightness), a* (green-red) and b* (blue-yellow) are shown in Table 3, along with the calculated values for Saturation and Hue. Only in the case of lightness was there any significant difference with Treatment C, where ES had been used, producing slightly lighter meat. There was no evidence of PSE meat.
pH at 45 min, 3 h and 24 h Post-slaughter in M. longissimus thoracis et lumborum (LTL) and h4. semimembranosus (Sm) Muscles. Mean Values of 24 pigs per Treatment Treatment
LTL 45 min 3h 24 h Sm 45 min 3h 24 h
s. e.d.
Significance
A
B
6.63” 6a25a 5.51
6.50” 6.20” 5.51
6.22b 564b 5.46
0.070 0,087 0,025
6~65~ 6.43” 5.53
6.53b 6.25b 5.53
6.26’ 5.87’ 5.51
0.063 0.079 0,018
co.00 1 <0*001 NS
Means with different superscripts are significantly different (P < 0.05).
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A. A. Taylor. G. R. Nufe, C. C. Warkup
TABLE 3 Colour (L*, a*, b*), Saturation and Hue at 24 h Post-slaughter in LTL and Drip Loss (% by wt) over 72 h at 1°C from LTL and Sm. Mean Value!: of 24 pigs per Treatment Treatment
Colour, in LTL L* a* b+ Saturation Hue Drip loss LTL Sm
A
B
c
s.e.d.
Sign$cance
52.58” 6.05 4.80 7-74 38.3 1
52-68a 6.24 4.74 7.85 37.61
54.65b 6.21 5.15 8.08 39.68
0.657 O-387 o-305 0.463 l-256
3.24 1.89
3.63 2.25
4.18 2.23
O-387 O-228
NS NS
Means with different superscripts are significantly different (P < 0.05).
TABLE 4 Instrumental Texture (Yf, Cf, W) of LTL at 4, 7 and 12-days Post-slaughter, and Sm at 4 days post-slaughter. Mean Values of 24 pigs per Treatment Treatment A
Yield force (Yf) (kg) LTL 4 days 7 days 12 days Sm 4 days CO;~~ force (Cf) (kg)
B
C
s.e.d.
SignjJicance
7.86” 7.29” 6.77”
8.00” 7.41” 6.92”
8-34
7.92
7.57
0.377
7.06” 6.68” 6.18”
5,60b 4~81~ 4.61’
O-532 0.485 0.478
744
7.28
0.388
NS
43-04” 41.39” 40.74”
36.41b 350lb 34*79b
2-i i7 1a993 1.976
43.63
43.06
2-l 17
NS
4 days 7.18” 7 days 6.54a 12 days 6.06” Sm 4 days 8.10 Work done (W) (J x lo-“) LTL 4 days 42-59” 7 days 41.74” 12 days 38.99” Sm 4 days 44.80
O-466 0,418 0.422
Means with different superscripts are significantly different (P < 0.05).
co.0 1
NS
co-0 1 co*00 1
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345
Drip loss The mean values for drip loss over 3 days from LTL and Sm are shown in Table 3. There was no significant treatment difference for either muscle although LTL samples in Treatment C tended to lose slightly more drip than those from the other two treatments. In this experiment, the drip losses were generally low.
Table 4 shows the mean values for the three texture parameters measured in LTL at 4, 7 and 12 days after slaughter and in Sm at 4 days. For LTL, Treatment C gave significantly more tender meat, expressed by all three parameters, than A and B. There was no significant difference between Treatments A and B. These results were true for all ageing times. The yield force (Yf) was significantly lower in Treatment C by about 1.5 kg per sample after 4 days (P = ONll), 7 days (P < 0.001) and 12 days (P = 0.002). The compression force (Cf) was significantly lower in Treatment C by more than 1.5 kg after 4 days (P = 0.006), 7 days (P c 0.001) and 12 days (P = O-01). Superimposed on the effect of treatment, an improvement in tenderness with ageing was observed in all parameters after 7 and 12 days. However, as measured by all three texture parameters, ageing up to 12 days after Treatments A and B did not achieve the degree of tenderness achieved in 4 days with Treatment C. The Sm muscle was measured only at 4 days post-slaughter and, although there was no significant overall treatment effect on texture parameters, samples from Treatment C tended to be less tough than the others in terms of yie!d, compression and work done. Taste panelling Panel scores for the tenderness of griddled loin slices and roast Sm joints are shown in Table 5. The mean values show quite clearly that Treatment C produced samples of LTL which at 4 days and 7 days were significantly more tender
TABLES Taste Panel Assessment of Tenderness of Griddled Loin Slices at 4, 7 and 12 days Postslaughter, and of Roast Sm at 4 days Post-slaughter. Eight Point Rating Scales from ‘Extremely Tough’ (1) to ‘Extremely Tender’(8). Mean Values of 24 Pigs per Treatment Treatment
LTL 4 days 7 days 12 days Sm 4 days
A
B
c
s. e. d.
Significance
3.64” 4-W 4.26
3~83~ 4*00” 4-32
4-28b 4-59b 4.65
0.203 o-192 O-214
co*01 CO.0i NS
4@Ia
4-2gb
4.w
0.119
Means with different superscripts are significantly different (P < O-05).
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A. A. Taylor, G. R. Nute, C. C. Warkup
(P < O-01) than the other treatments. By 12 days the treatment differences were no longer significant. At 4 days, Treatment C produced Sm which was significantly more tender (P c 0.001) than the other treatments, and Treatment A gave Sm which was significantly tougher than the other treatments. The effect of ageing is seen in the LTL samples, where the tenderness, with all treatments, improved with ageing from 4 days to 7 days and 12 days. DISCUSSION Carcass characteristics, with the exception of intramuscular fat, were similar across all three treatments. The slightly higher intramuscular fat levels of carcasses in Treatment A might have been expected to enhance tenderness to a slight extent, but in fact Treatment A samples were still among the toughest. The 3 h delay before chilling in Treatment A had only a small effect on the cooling rate of LTL or the deeper Sm compared to carcasses where chilling commenced 45 min after slaughter. Even so, the deepest part of LTL had cooled to 10°C in little more than 6 h. In this case therefore, as with the earlier cooling in the other treatments, a certain amount of cold-toughening might be expected in some of the muscle. The similarity in cooling rates across treatments meant that the greatest effect on quality parameters was likely to come from the electrical stimulation in Treatment C. This was clearly demonstrated by the rapid pH fall in Treatment C, where values were approximately O-3 units lower at 45 min after slaughter, and approaching the ultimate pH by 3 h. The rapid pH fall was the likely cause of the slightly lighter colour of LTL with stimulation. This increased lightness was however not sufficiently pronounced to be visible to the eye. Although the general level of drip loss was low across all treatments, the slightly higher loss in Treatment C was probably due to ES. This tendency towards higher drip losses with ES was noted in earlier studies (Taylor & Tantikov, 1989, 1992) where it was also shown that it, could be avoided by faster chilling. The clearest effect of treatment was on meat tenderness measured both instrumentally and by taste panel. These results are consistent with those in the earlier studies and demonstrated the tenderising effect of ES. The improvement in tenderness in Treatment C by all instrumental parameters was considerable, compared with the other two treatments which did not incorporate ES. Treatment C with ES produced LTL which was more tender 4 days after slaughter than meat produced by the other two (non-ES) treatments after 12 days ageing. The improvement in tenderness with Treatment C was maintained throughout ageing, so that by 12 days, the yield and compression forces were still about l-5 kg lower than in Treatments A and B. The Sm was less affected by treatment, possibly because there was less likelihood of cold shortening. Nevertheless, Treatment C samples were the most tender by all three texture parameters, and slowly cooled samples from Treatment A the toughest. The instrumental texture results were strongly supported by taste panel assessment, with significantly higher ratings for Treatment C samples, not only for LTL but also for Sm where instrumental measurements were less discriminating between treatments. The magnitude of the improved tenderness judged by taste panels at 4 days was equal to, or greater than, the O-5 scale points improvement
Chilling. electrical stimulation and conditioning on pork quality
347
in panel rating of tenderness attributed by Warkup eCal. (1990) to rapid growth caused by ad iibitum feeding. The overall improvement in tenderness of LTL with ageing was also clearly demonstrated, so that by 12 days the ES effect was no longer significant. These results suggest t.5 at, cvcn *under the conventional chilling conditions used in this study, either some cold-toughening occurs in pig carcasses which can be effectively avoided by ES or the ES has some other beneficial effect. Delaying the onset of chilling by 3 h gave no improvement in tenderness, but all samples became more tender with ageing up to 12 days. The greatest improvement in tenderness however, came from applying high voltage ES at 20 min after slaughter. ACKNOWLEDGEMENTS The authors wish to thank Miss A. M. Perry, Mr S. P. Johnson and Mrs A. Baker for their expert technical assistance in the experimental work, and Mr G. A. Fursey and Mrs J. Bayntun for their help and advice in analysing the data. This research was part of a programme funded by the Meat and Livestock Commission.
REFERENCES Barton-Gade, P., Bejerholm, C. & Borup. V. (1987). Proc. 33rd ht. Cong. Meat Sci. and Technology, Helsinki, Finland, p_ 18I. Dransfield, E. & Lockyer, D. K. (1985). Meat Sci., 13, 19. Dransfield, E., Ledwith, M, J. & Taylor, A. A. (199 1). Mear Sci., 29, 129. Gigiel, A. J. & James, S. J. ( 1984). Meat Sci., 11, I. Marsh, B. B., Cassens, R. G., Kaufman, R. G. & Briskey, E. J. (1972). J. Food SC+., 37, 179.
Moller, A. J. & Vestergaard, T. (1988). Proc. 34tk ht. Gong. Meut Sci. arzd Teclznology, Brisbane, Australia, p. 621. Taylor, A. A. & Tantikov, M. Z. (1989). Proc. 35th hr. Gong. Meat Sci. and Technology, Copenhagen, Denmark. Taylor, A. A. & Tantikov, M. Z. (1992). Meat Sci., 31, 381. Warkup, C. C., Dilworth, A. W., Kempster, A. J. & Wood, J. D. (1990). him. Prod., 50, 550.