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The effects of electrical stimulation and ageing on beef tenderness
Meat Science 12 (1985) 243-251
The Effects of Electrical Stimulation and Ageing on Beef Tenderness
D. G. Taylor & J. G. Cornell Department of Animal Science, Queensland Agricultural College, Lawes, Queensland 4343, Australia (Received: 22 March, 1984)
SUMMA RY Seventeen beef carcasses f r o m cattle with a range o f breeds, ages and body conditions were used in this trial. The .[our treatments applied to each carcass were control (C), electrical stimulation (ES), ageingjbr 28 days (A) and electrical stimulation plus ageing Jor 28 days (ES + A). Post-mortem muscle" p H was measured at O, 0"5, 4 and 24h poststimulation. Significantly lower muscle p H vahws (P <0"01) were achieved by the stimulated carcass side compared to the unstimulated side at 0"5 (pH6.47 vs. 6.91) and 4h (pH5.96 vs. 6.44) post-stimulation. Warner-Bratzler shear and taste panel methods were used to assess the tenderness oJ Longissimus dorsi muscle sample's f r o m each o f the Jour treatments. The ES, A and ES + A treatments were significantly more tender (P <0"01) than the control treatment. The ES and the A treatments resulted in a similar improvement in tenderness compared to the control. The ES + A treatment was significantly more tender (P < 0.01) than the ES treatment alone, but there was no significant difference in tenderness between the A and the ES + A treatments.
INTRODUCTION The effects of electrical stimulation on beef and sheep carcasses has received considerable research and reporting over the last six years (e.g. Savell et al., 1978b; Bendall, 1980). This process has been shown to hasten 243 Meat Science 0309-1740/85/$03-30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
244
D. G. Taylor, J. G. Cornell
the time of onset, and increase the rate, of post-mortem glycolysis and rigor mortis, and to improve the tenderness of meat. It is apparent that the mechanisms which bring about this tenderness improvement are still not completely understood (Moiler et al., 1983), although there have been a number of proposals including reduction in cold-shortening toughening (Chrystall & Hagyard, 1976), disruption of myofibrillar structure (Savell et al., 1978a) and increased activity of proteolytic enzymes (Dutson et al., 1980). Notwithstanding the above uncertainty in scientific circles on the mechanism of tenderness improvement, electrical stimulation has been introduced into many commercial abattoirs in Australia. Savell et al. (1981) claimed that the process of ageing meat for tenderization is probably the most widely used tenderizing process in the meat industry. In Australia, vacuum packaging and ageing of primal cuts of beef is receiving increasing usage for both domestic and some export markets. The mechanism by which ageing improves the tenderness of meat appears to be through post-rigor enzymic activity resulting in myofibrillar breakdown at the Z-line (Davey & Gilbert, 1976). Comparisons of the tenderness improvement produced by electrical stimulation compared to the improvement from ageing has received only limited reporting in the literature. The results of Savell et al. (1978b) indicate no significant difference in tenderness between muscle samples electrically stimulated and aged for 7 days compared to those unstimulated and aged for 21 days. Bowles Axe et al. (1983) reported no significant difference in tenderness of Longissimus dorsi and Semimembranosus muscle samples which received either electrical stimulation or 6-day ageing treatment. Lyon et al. (1983) detected no significant difference in tenderness of Triceps branchii and Psoas major muscle samples between electrically stimulated/hot boned treatment and 6-day ageing treatment. The additive tenderizing effects which might accrue from applying both electrical stimulation and ageing treatments to meat samples has received some research attention. Savell et aL (1978b), George et aL (1980) and Savell et aL (1981) reported that electrical stimulation accelerates the post-mortem ageing of beef, but concluded that the extent of ultimate tenderization depended on the "inherent tenderness of the beef' (Savell et al., 1981). Babiker & Lawrie (1983) have demonstrated that beef tenderness was significantly enhanced following electrical stimulation and ageing for 5 h at 30 or 40°C. The objective of this study was to investigate beef tenderness produced
E/Ji,cts o f electrical stimulation and ageing on bee] tenderness
245
by electrical stimulation, ageing or a combination of electrical stimulation and ageing when all treatments were applied to muscle samples from the same carcass.
EX PERI M E N T A L Experimental design The four treatments applied to each carcass were: (1) (2) (3) (4)
Control (C). Electrical stimulation (ES). Ageing (A). Electrical stimulation plus ageing (ES + A).
By electrically stimulating the right side of each carcass and leaving the left side as an unstimulated control, and by ageing the muscle sample from the hindquarter of each carcass side, the four treatments were quite simply applied to all carcasses in the trial. Two Longissimus dorsi muscle samples were taken from the 12th/13th rib section of each carcass side to make up the four treatment samples from each carcass. Animals and carcass preparation Seventeen cattle representing a range of breeds, ages, weights and body conditions were used. The cattle were stunned, bled, skinned, eviscerated and split into sides using conventional procedures at the abattoir of the Meat Technology Complex at Queensland Agricultural College. All these procedures were completed within 30 min of stunning.
Electrical stimulation The right side of each carcass was electrically stimulated at 30 min poststunning using the low voltage procedure previously described by Taylor & Marshall (1980). The direct current was increased stepwise from 0 to 32 V over 90 s, and was applied to the carcass side through two electrodes, one positioned into the distal end of the Bicepsfemoris muscle and the other into the Brachiocephalicus muscle.
246
D. G. Taylor, J. G. Cornell
Both sides of each carcass were placed in a chiller at 1 °C approximately 1 h after stunning.
Muscle pH measurements As a means of monitoring rigor mortis changes following electrical stimulation, muscle pH readings were taken prior to stimulation and at 0-5, 4 and 24 h post-stimulation. These measurements were taken using a Townson meat pH-meter with the probe inserted deep into the Longissimus dorsi muscle over the 13th rib.
Muscle sample preparation and ageing Both sides of each carcass were chilled at 1 °C for 48 h post-mortem. After this time two 4cm thick muscle samples were removed from the Longissimus dorsi over the 12th/13th rib of each carcass side. One of these muscle samples was wrapped in plastic and frozen at - 2 0 ° C until required for tenderness evaluation. The other muscle sample from each side was vacuum packaged in a Cryovac Barrier Bag and aged by storing at I°C for 28 days; it was then frozen at - 2 0 ° C until required for tenderness evaluation.
Tenderness evaluation The four muscle samples from one carcass were subjected to tenderness evaluation at the same time. The frozen muscle samples were thawed at room temperature for 24 h, then deep fried to an internal temperature of 80°C and allowed to cool. Square cross-sectional cores, with dimensions of 1-4 x 1.4 x 4 cm, were cut from half of the cooked muscle sample using a double-bladed scalpel, and ensuring that the muscle fibres were lying parallel to the long axis of the core. Six cores were taken from each muscle sample and subjected to Warner-Bratzler shear force determination, with the mean of the six shear force values being the value for that sample. The other half of the cooked muscle sample was used for subjective tenderness evaluation by taste panel. The taste panel members (10 males and 2 females) were presented with a cooked slice of muscle approximately 5 x 3 x l cm. Each member was requested to rate the tenderness of each sample on a 10-point descriptive scale, 10 being extremely tender and 0 being extremely tough.
E-[Jects oJ electrical stimulation and ageing on bee[ tenderness
247
Statistical analysis An analysis of variance was carried out on the results to test for significance of treatment effects, and to determine the least significant difference between treatment means at P = 0.05 and P = 0.01. RESULTS Effects of electrical stimulation on carcass and muscle The passage of the electrical current through the beef side during stimulation resulted in pronounced muscular contraction and distortion of the side. This distortion became more pronounced as the voltage was increased to 32 V, and at this voltage rigid extension of the fore and hind limbs and bending of the split vertebral column occurred. When the current was switched off at 90 s the carcass reverted to its normal posture. That the electrical stimulation treatment had a marked influence on post-mortem muscle glycolysis is demonstrated in Fig. 1.
7.0
6.5
MEAN pH
6.0
5.5
1
2
3
4
24 TIME
Fig. I.
(h)
Mean rate of pH fall in L. dorsi muscle for electrically stimulated and
unstimulated control beef sides. Upper line. unstimulated: lower line. stimulated; ]'. standard error of mean.
248
D. G. Taylor, J. G. Cornell
TABLE 1 M e a n s , S t a n d a r d Deviations (SD) a n d Differences between M e a n s Showing Levels of Significance for Each of the F o u r T r e a t m e n t s w h e n Assessed by S h e a r Force ~ Treatment
Control ES A ES + A
Mean b
7.94 5.80 5.38 4-03
SD
2-79 2.04 1.84 1-46
Differences between treatment means Control
ES
A
ES + A
-2' 14"* 2.56** 3.91"*
2.14"* -0.42 1-77"
2-56** 0.42 -1-35
3-91"* 1.77" 1.35 --
F o r 1"4 x l ' 4 c m square cross-section core. b Lower m e a n values indicate greater tenderness. Least significant difference (LSD) P < 0.05 * = 1-43; LSD P < 0.01 ** = 1.91.
The mean muscle pH values for the stimulated carcass sides were significantly lower (P < 0.01) than the mean muscle pH values for the unstimulated sides at both 0-5 and 4h post stimulation, but not significantly different at 24 h. Warner-Bratzler shear force
The results of objective tenderness assessment by Warner-Bratzler shear force are presented in Table 1. The significant improvement in tenderness resulting from ES, A or TABLE 2 Means, S t a n d a r d Deviations (SD) a n d Differences between M e a n s Showing Levels of Significance for Each of the F o u r T r e a t m e n t s when Assessed by Taste Panel Treatment
Control ES A ES + A
Mean a
3"85 5" 13 6"02 6-19
SD
1.17 1.37 1.39 0-96
DiJJerence between treatment means Control
ES
A
ES + A
-1.28** 2.17"* 2.34**
1.28"* -0.89* 1-06"
2.17"* 0-89* -0-17
2-34** 1-06" 0"17 --
O n a 10-point scale: 0 being extremely tough, 10 being extremely tender. LSD P < 0 - 0 5 * = 0 . 8 5 ; LSD P < 0 - 0 1 * * = 1-13.
Effects of electrical stimulation and ageing on bee/ tenderness
249
ES + A treatments compared to the untreated control is evident. The significant difference ( P < 0 . 0 5 ) between ES and ES + A treatments should be noted, as should the lack of a significant difference between the ES and the A treatments and the A and ES + A treatments.
Taste panel The results for the subjective tenderness assessment by taste panel are presented in Table 2, and demonstrate the same order of significant differences between treatments as those indicated for objective tenderness assessment. The only discrepancy between these results and those in Table 1 is the significant difference (P < 0-05) between the ES and the A treatments which just achieved significance in this case.
DISCUSSION The reported observations on the extreme muscular contraction during electrical stimulation and the rapid muscle pH fall following stimulation (Fig. 1) demonstrate that the low voltage stimulation system used in this trial had a pronounced effect on the physical and biochemical aspects of rigor mortis. The muscular contraction during stimulation reported here was of the same order as that reported previously by Taylor & Marshall (1980) using a low voltage, and by Bendall (1980) using higher voltages. The rapid fall of muscle pH to below 6 by 4 h post-stimulation is similar to that reported by Davey & Gilbert (1976) using 1600V, Shaw & Walker (1977) using 110 V and Bowles Axe et al. (1983) using 400 V. The relationship between the voltage and type of current used for electrical stimulation and the effect on muscle biochemistry has been reviewed by Bendall (1980). The significant improvement in tenderness of meat samples following electrical stimulation of the carcass is demonstrated by the results for objective (Table I) and subjective (Table 2) tenderness assessment. Numerous previous workers have reported a similar effect (e.g. Chrystall & Hagyard, 1967; Bendall, 1980; Savell et al., 1981), although Bowles Axe et al. (1983) detected no consistent tenderness difference between muscle samples from stimulated and unstimulated carcasses. That ageing of meat results in a significant improvement in tenderness
250
D. G. Taylor, J. G. Cornell
has also been widely reported (e.g. Smith et al., 1978; Savell et al., 1981) and is supported by the results of this study (Tables 1 and 2). The tenderness of muscle following stimulation compared to the tenderness following ageing for 28 days shows an interesting result. The Warner-Bratzler shear force results (Table 1) show no significant difference in tenderness between these two treatments, whereas the mean taste panel ratings (Table 2) show a difference which is just statistically significant. In both cases the mean rating for the ageing treatment indicates greater tenderness than the rating for the electrically stimulated samples. The lack of a substantially significant difference in tenderness between muscle which has received either the electrical stimulation or the ageing treatment supports the reports of Savell et al. (1978b), Bowles Axe et al. (1983) and Lyon et al. (1983). The results displayed in Tables 1 and 2 show that the ES + A treatment produced a significant (P < 0.05) improvement in tenderness over ES alone. The results reported by Savell et al. (1981), although using different stimulation and ageing conditions, showed a similar trend with 10- and 14-day aged and stimulated muscle samples giving significantly lower shear force values than those stimulated and aged for 2 or 6 days. Bendall (1980) presented results which showed that mean shear force values for stimulated plus 7-, 14- or 21-day aged meat samples were substantially lower than the shear force values of the unstimulated and 1- or 2-day aged meat samples, although the statistical analysis had not been done on this comparison. The combined effect of electrical stimulation and ageing has been described as an acceleration of post-mortem ageing by electrical stimulation treatment (Savell et al., 1978b, 1981; George et al,, 1980). However, Moiler et al. (1983) working with ovine muscle concluded that there was no evidence to suggest that stimulation increased the rate of ageing tenderization. It should be noted that neither objective nor subjective methods of tenderness assessment could detect a significant difference in tenderness between the A treatment and the combined ES + A treatment. In conclusion it can be stated that either ES or A treatments result in a significant improvement in tenderness compared to an untreated control. The improvement in tenderness produced by either of these treatments is of about the same order, with a possible slight advantage for 4-week ageing over low voltage electrical stimulation. The combined effect of ES + A results is a significant improvement in tenderness over ES alone, but not over A alone. Thus in abattoirs where electrical stimulation is
Effects of electrical stimulation and ageing on beef tenderness
251
carried out as a routine procedure, there is an additional improvement in tenderness possible if ageing is also carried out. However, if electrical stimulation is not a routine procedure, an improvement in tenderness comparable to that resulting from stimulation plus ageing can be achieved by ageing alone.
REFERENCES Babiker, S. A. & Lawrie, R. A. (1983). Meat Sci., 8, 1. Bendall, J. R. (1980). In: Developments in meat scienee--I (Lawrie, R. A. (Ed.)). Applied Science Publishers, London, p. 37. Bowles Axe, J. E., Kastner, G. L., Dikeman, M. E., Hunt, M. C., Kropf, D. H. & Milliken, G. A. (1983). J. F d S c . , 48, 332. Chrystall, B. B. & Hagyard, C. J. (1976). N . Z . J . Agric. Res., 19, 7. Davey, C. L. & Gilbert, K. V. (1976). J. Sci. Fd Agric., 27, 244. Dutson, T. R., Smith, G. C. & Carpenter, Z. L. (1980). J. Fd Sci., 45, 1097. George, A. R., Bendall, J. R. & Jones, R. C. D. (1980). Meat Sci., 4, 51. Lyon, M., Kastner, C. L., Dikeman, M. E., Hunt, M. C., Kropf, D. H. & Schwenke, J. R. (1983). J. FdSci., 48, 131. Moiler, A. J., Bouton, P. E., Harris, P. V. & Jones, P. N. (1983). J. FdSci., 48, 874. Savell, J. W., Dutson, T. R., Smith, G. C. &Carpenter, Z. L. (1978a). J. FdSci., 43, 1606. Savell, J. W., Smith, G. C. & Carpenter, Z. L. (1978b). J. FdSci., 43, 1666. Savell, J. W., McKeith, F. K. & Smith, G. C. (1981). J. FdSci., 46, 1777. Shaw, F. D. & Walker, D. J. (1977). J. Fd Sci., 42, 1140. Smith, G. C., Culp, G. R. & Carpenter, Z. L. (1978). J. FdSci., 43, 823. Taylor, D. G. & Marshall, A. R. (1980). J. Fd Sci., 45, 144.
The Effects of Electrical Stimulation and Ageing on Beef Tenderness
D. G. Taylor & J. G. Cornell Department of Animal Science, Queensland Agricultural College, Lawes, Queensland 4343, Australia (Received: 22 March, 1984)
SUMMA RY Seventeen beef carcasses f r o m cattle with a range o f breeds, ages and body conditions were used in this trial. The .[our treatments applied to each carcass were control (C), electrical stimulation (ES), ageingjbr 28 days (A) and electrical stimulation plus ageing Jor 28 days (ES + A). Post-mortem muscle" p H was measured at O, 0"5, 4 and 24h poststimulation. Significantly lower muscle p H vahws (P <0"01) were achieved by the stimulated carcass side compared to the unstimulated side at 0"5 (pH6.47 vs. 6.91) and 4h (pH5.96 vs. 6.44) post-stimulation. Warner-Bratzler shear and taste panel methods were used to assess the tenderness oJ Longissimus dorsi muscle sample's f r o m each o f the Jour treatments. The ES, A and ES + A treatments were significantly more tender (P <0"01) than the control treatment. The ES and the A treatments resulted in a similar improvement in tenderness compared to the control. The ES + A treatment was significantly more tender (P < 0.01) than the ES treatment alone, but there was no significant difference in tenderness between the A and the ES + A treatments.
INTRODUCTION The effects of electrical stimulation on beef and sheep carcasses has received considerable research and reporting over the last six years (e.g. Savell et al., 1978b; Bendall, 1980). This process has been shown to hasten 243 Meat Science 0309-1740/85/$03-30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
244
D. G. Taylor, J. G. Cornell
the time of onset, and increase the rate, of post-mortem glycolysis and rigor mortis, and to improve the tenderness of meat. It is apparent that the mechanisms which bring about this tenderness improvement are still not completely understood (Moiler et al., 1983), although there have been a number of proposals including reduction in cold-shortening toughening (Chrystall & Hagyard, 1976), disruption of myofibrillar structure (Savell et al., 1978a) and increased activity of proteolytic enzymes (Dutson et al., 1980). Notwithstanding the above uncertainty in scientific circles on the mechanism of tenderness improvement, electrical stimulation has been introduced into many commercial abattoirs in Australia. Savell et al. (1981) claimed that the process of ageing meat for tenderization is probably the most widely used tenderizing process in the meat industry. In Australia, vacuum packaging and ageing of primal cuts of beef is receiving increasing usage for both domestic and some export markets. The mechanism by which ageing improves the tenderness of meat appears to be through post-rigor enzymic activity resulting in myofibrillar breakdown at the Z-line (Davey & Gilbert, 1976). Comparisons of the tenderness improvement produced by electrical stimulation compared to the improvement from ageing has received only limited reporting in the literature. The results of Savell et al. (1978b) indicate no significant difference in tenderness between muscle samples electrically stimulated and aged for 7 days compared to those unstimulated and aged for 21 days. Bowles Axe et al. (1983) reported no significant difference in tenderness of Longissimus dorsi and Semimembranosus muscle samples which received either electrical stimulation or 6-day ageing treatment. Lyon et al. (1983) detected no significant difference in tenderness of Triceps branchii and Psoas major muscle samples between electrically stimulated/hot boned treatment and 6-day ageing treatment. The additive tenderizing effects which might accrue from applying both electrical stimulation and ageing treatments to meat samples has received some research attention. Savell et aL (1978b), George et aL (1980) and Savell et aL (1981) reported that electrical stimulation accelerates the post-mortem ageing of beef, but concluded that the extent of ultimate tenderization depended on the "inherent tenderness of the beef' (Savell et al., 1981). Babiker & Lawrie (1983) have demonstrated that beef tenderness was significantly enhanced following electrical stimulation and ageing for 5 h at 30 or 40°C. The objective of this study was to investigate beef tenderness produced
E/Ji,cts o f electrical stimulation and ageing on bee] tenderness
245
by electrical stimulation, ageing or a combination of electrical stimulation and ageing when all treatments were applied to muscle samples from the same carcass.
EX PERI M E N T A L Experimental design The four treatments applied to each carcass were: (1) (2) (3) (4)
Control (C). Electrical stimulation (ES). Ageing (A). Electrical stimulation plus ageing (ES + A).
By electrically stimulating the right side of each carcass and leaving the left side as an unstimulated control, and by ageing the muscle sample from the hindquarter of each carcass side, the four treatments were quite simply applied to all carcasses in the trial. Two Longissimus dorsi muscle samples were taken from the 12th/13th rib section of each carcass side to make up the four treatment samples from each carcass. Animals and carcass preparation Seventeen cattle representing a range of breeds, ages, weights and body conditions were used. The cattle were stunned, bled, skinned, eviscerated and split into sides using conventional procedures at the abattoir of the Meat Technology Complex at Queensland Agricultural College. All these procedures were completed within 30 min of stunning.
Electrical stimulation The right side of each carcass was electrically stimulated at 30 min poststunning using the low voltage procedure previously described by Taylor & Marshall (1980). The direct current was increased stepwise from 0 to 32 V over 90 s, and was applied to the carcass side through two electrodes, one positioned into the distal end of the Bicepsfemoris muscle and the other into the Brachiocephalicus muscle.
246
D. G. Taylor, J. G. Cornell
Both sides of each carcass were placed in a chiller at 1 °C approximately 1 h after stunning.
Muscle pH measurements As a means of monitoring rigor mortis changes following electrical stimulation, muscle pH readings were taken prior to stimulation and at 0-5, 4 and 24 h post-stimulation. These measurements were taken using a Townson meat pH-meter with the probe inserted deep into the Longissimus dorsi muscle over the 13th rib.
Muscle sample preparation and ageing Both sides of each carcass were chilled at 1 °C for 48 h post-mortem. After this time two 4cm thick muscle samples were removed from the Longissimus dorsi over the 12th/13th rib of each carcass side. One of these muscle samples was wrapped in plastic and frozen at - 2 0 ° C until required for tenderness evaluation. The other muscle sample from each side was vacuum packaged in a Cryovac Barrier Bag and aged by storing at I°C for 28 days; it was then frozen at - 2 0 ° C until required for tenderness evaluation.
Tenderness evaluation The four muscle samples from one carcass were subjected to tenderness evaluation at the same time. The frozen muscle samples were thawed at room temperature for 24 h, then deep fried to an internal temperature of 80°C and allowed to cool. Square cross-sectional cores, with dimensions of 1-4 x 1.4 x 4 cm, were cut from half of the cooked muscle sample using a double-bladed scalpel, and ensuring that the muscle fibres were lying parallel to the long axis of the core. Six cores were taken from each muscle sample and subjected to Warner-Bratzler shear force determination, with the mean of the six shear force values being the value for that sample. The other half of the cooked muscle sample was used for subjective tenderness evaluation by taste panel. The taste panel members (10 males and 2 females) were presented with a cooked slice of muscle approximately 5 x 3 x l cm. Each member was requested to rate the tenderness of each sample on a 10-point descriptive scale, 10 being extremely tender and 0 being extremely tough.
E-[Jects oJ electrical stimulation and ageing on bee[ tenderness
247
Statistical analysis An analysis of variance was carried out on the results to test for significance of treatment effects, and to determine the least significant difference between treatment means at P = 0.05 and P = 0.01. RESULTS Effects of electrical stimulation on carcass and muscle The passage of the electrical current through the beef side during stimulation resulted in pronounced muscular contraction and distortion of the side. This distortion became more pronounced as the voltage was increased to 32 V, and at this voltage rigid extension of the fore and hind limbs and bending of the split vertebral column occurred. When the current was switched off at 90 s the carcass reverted to its normal posture. That the electrical stimulation treatment had a marked influence on post-mortem muscle glycolysis is demonstrated in Fig. 1.
7.0
6.5
MEAN pH
6.0
5.5
1
2
3
4
24 TIME
Fig. I.
(h)
Mean rate of pH fall in L. dorsi muscle for electrically stimulated and
unstimulated control beef sides. Upper line. unstimulated: lower line. stimulated; ]'. standard error of mean.
248
D. G. Taylor, J. G. Cornell
TABLE 1 M e a n s , S t a n d a r d Deviations (SD) a n d Differences between M e a n s Showing Levels of Significance for Each of the F o u r T r e a t m e n t s w h e n Assessed by S h e a r Force ~ Treatment
Control ES A ES + A
Mean b
7.94 5.80 5.38 4-03
SD
2-79 2.04 1.84 1-46
Differences between treatment means Control
ES
A
ES + A
-2' 14"* 2.56** 3.91"*
2.14"* -0.42 1-77"
2-56** 0.42 -1-35
3-91"* 1.77" 1.35 --
F o r 1"4 x l ' 4 c m square cross-section core. b Lower m e a n values indicate greater tenderness. Least significant difference (LSD) P < 0.05 * = 1-43; LSD P < 0.01 ** = 1.91.
The mean muscle pH values for the stimulated carcass sides were significantly lower (P < 0.01) than the mean muscle pH values for the unstimulated sides at both 0-5 and 4h post stimulation, but not significantly different at 24 h. Warner-Bratzler shear force
The results of objective tenderness assessment by Warner-Bratzler shear force are presented in Table 1. The significant improvement in tenderness resulting from ES, A or TABLE 2 Means, S t a n d a r d Deviations (SD) a n d Differences between M e a n s Showing Levels of Significance for Each of the F o u r T r e a t m e n t s when Assessed by Taste Panel Treatment
Control ES A ES + A
Mean a
3"85 5" 13 6"02 6-19
SD
1.17 1.37 1.39 0-96
DiJJerence between treatment means Control
ES
A
ES + A
-1.28** 2.17"* 2.34**
1.28"* -0.89* 1-06"
2.17"* 0-89* -0-17
2-34** 1-06" 0"17 --
O n a 10-point scale: 0 being extremely tough, 10 being extremely tender. LSD P < 0 - 0 5 * = 0 . 8 5 ; LSD P < 0 - 0 1 * * = 1-13.
Effects of electrical stimulation and ageing on bee/ tenderness
249
ES + A treatments compared to the untreated control is evident. The significant difference ( P < 0 . 0 5 ) between ES and ES + A treatments should be noted, as should the lack of a significant difference between the ES and the A treatments and the A and ES + A treatments.
Taste panel The results for the subjective tenderness assessment by taste panel are presented in Table 2, and demonstrate the same order of significant differences between treatments as those indicated for objective tenderness assessment. The only discrepancy between these results and those in Table 1 is the significant difference (P < 0-05) between the ES and the A treatments which just achieved significance in this case.
DISCUSSION The reported observations on the extreme muscular contraction during electrical stimulation and the rapid muscle pH fall following stimulation (Fig. 1) demonstrate that the low voltage stimulation system used in this trial had a pronounced effect on the physical and biochemical aspects of rigor mortis. The muscular contraction during stimulation reported here was of the same order as that reported previously by Taylor & Marshall (1980) using a low voltage, and by Bendall (1980) using higher voltages. The rapid fall of muscle pH to below 6 by 4 h post-stimulation is similar to that reported by Davey & Gilbert (1976) using 1600V, Shaw & Walker (1977) using 110 V and Bowles Axe et al. (1983) using 400 V. The relationship between the voltage and type of current used for electrical stimulation and the effect on muscle biochemistry has been reviewed by Bendall (1980). The significant improvement in tenderness of meat samples following electrical stimulation of the carcass is demonstrated by the results for objective (Table I) and subjective (Table 2) tenderness assessment. Numerous previous workers have reported a similar effect (e.g. Chrystall & Hagyard, 1967; Bendall, 1980; Savell et al., 1981), although Bowles Axe et al. (1983) detected no consistent tenderness difference between muscle samples from stimulated and unstimulated carcasses. That ageing of meat results in a significant improvement in tenderness
250
D. G. Taylor, J. G. Cornell
has also been widely reported (e.g. Smith et al., 1978; Savell et al., 1981) and is supported by the results of this study (Tables 1 and 2). The tenderness of muscle following stimulation compared to the tenderness following ageing for 28 days shows an interesting result. The Warner-Bratzler shear force results (Table 1) show no significant difference in tenderness between these two treatments, whereas the mean taste panel ratings (Table 2) show a difference which is just statistically significant. In both cases the mean rating for the ageing treatment indicates greater tenderness than the rating for the electrically stimulated samples. The lack of a substantially significant difference in tenderness between muscle which has received either the electrical stimulation or the ageing treatment supports the reports of Savell et al. (1978b), Bowles Axe et al. (1983) and Lyon et al. (1983). The results displayed in Tables 1 and 2 show that the ES + A treatment produced a significant (P < 0.05) improvement in tenderness over ES alone. The results reported by Savell et al. (1981), although using different stimulation and ageing conditions, showed a similar trend with 10- and 14-day aged and stimulated muscle samples giving significantly lower shear force values than those stimulated and aged for 2 or 6 days. Bendall (1980) presented results which showed that mean shear force values for stimulated plus 7-, 14- or 21-day aged meat samples were substantially lower than the shear force values of the unstimulated and 1- or 2-day aged meat samples, although the statistical analysis had not been done on this comparison. The combined effect of electrical stimulation and ageing has been described as an acceleration of post-mortem ageing by electrical stimulation treatment (Savell et al., 1978b, 1981; George et al,, 1980). However, Moiler et al. (1983) working with ovine muscle concluded that there was no evidence to suggest that stimulation increased the rate of ageing tenderization. It should be noted that neither objective nor subjective methods of tenderness assessment could detect a significant difference in tenderness between the A treatment and the combined ES + A treatment. In conclusion it can be stated that either ES or A treatments result in a significant improvement in tenderness compared to an untreated control. The improvement in tenderness produced by either of these treatments is of about the same order, with a possible slight advantage for 4-week ageing over low voltage electrical stimulation. The combined effect of ES + A results is a significant improvement in tenderness over ES alone, but not over A alone. Thus in abattoirs where electrical stimulation is
Effects of electrical stimulation and ageing on beef tenderness
251
carried out as a routine procedure, there is an additional improvement in tenderness possible if ageing is also carried out. However, if electrical stimulation is not a routine procedure, an improvement in tenderness comparable to that resulting from stimulation plus ageing can be achieved by ageing alone.
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