Modelling post-mortem tenderisation—I: Texture of electrically stimulated and non-stimulated beef

Meat Science 31 (1992) 57-73

Modelling Post-Mortem Tenderisation I: Texture of Electrically Stimulated and Non-stimulated Beef Eric Dransfield, D i a n e K. Wakefield & I a n D. P a r k m a n Department of Veterinary Medicine,Universityof Bristol, Churchill Building, Langford, Bristol BS18 7DY, UK (Received 20 April 1990; revised version received 15 May 1991; accepted 18 May 1991)

ABSTRA CT Texture in electrically stimulated and non-stimulated beef M Pectoralis profundus, stored under a range of temperatures from 0 to 30°C, while avoiding muscle shortening, was measured from 1 to 21 days after stunning. The pre-rigor temperature (from 0 to 30°C), maintained until the p H had fallen to 6.4 and then held at 15°C, had no effect on the toughness nor on the rate of tenderisation after rigor. Modelling toughness prior to 24 h suggested that toughness of all muscles could be rationalised and that first-order tenderisation began when the muscles reached p H 6.1 when the toughness of all the muscles was projected to be 12.5 kg. After p H 6"1, the rate of tenderisation at 30°C was lO-fold higher than at I°C and was not affected by variations in p H from 6.1 to 5.5. At the higher temperatures, the ultimate toughness of aged meat was slightly higher than at the lower temperatures.

INTRODUCTION The acceptability of meat improves significantly by enzyme tenderisation during storage. Factors which affect the rate and extent of tenderisation have been characterized by measurements of the toughness of meat cooked after the ultimate pH had been reached. Proteolysis, however, occurs soon after stunning (Troy et al., 1986) but it is not known if the proteolysis causes weakening. Direct measurement of toughness of meat 57 Meat Science 0309-1740/92/$03.50 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain

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Eric Dransfield, Diane K. Wakefield, Ian D. Parkman

cooked prior to the attainment of the ultimate pH, is confounded by variations in pH and muscle shortening induced by cooking (Dransfield & Rhodes, 1975). How much, if any, tenderisation occurs prior to the attainment of the ultimate pH therefore remains a matter of speculation. Several workers have stated that little or no tenderisation occurs prior to the attainment of rigor but results, based on the effects of early holding at high temperatures and variations in electrical stimulation on toughness (Marsh, 1983) and the early fragmentation of myofibrils (Koohmaraie et al., 1987), are said to show that the process of tenderisation begins soon after stunning. Determination of the start of tenderisation would enable better prediction of toughness and texture changes and should give important clues to the mechanism and control of tenderisation. The primary aim of this work was to determine the time and temperature dependency for the process of tenderisation from the time of stunning to the completion of ageing. This was achieved by varying the temperature before rigor independently from that after rigor. The method chosen was to store muscles at constant temperature between 0 and 30°C before rigor development and to change the temperature to 15°C during the rapid phase or rigor development (pH 6.4 to pH 6.0) when the muscles were held at a fixed length. After pH 6.0, when in full rigor, the temperature was altered and the toughness monitored throughout storage from 24 h to 21 days. Electrical stimulation was included to increase the rate of rigor development independently of temperature.

EXPERIMENTAL Treatment of muscles and toughness measurements

Fourteen Hereford cross steers, aged 18-20 months, were stunned and slaughtered conventionally. One side of 12 carcasses was electrically stimulated using 500 V at 12.5 or 5Hz for 60s. The pectoralis profundus muscles were removed within 1 h after stunning, packed and placed in air or water at a temperature between 0 and 30°C. The pH was measured hourly for the first 12 h and then at 24 h in homogenates of 2.5 g of meat in 10 ml of 5 mM iodoacetate/150 mM KCI, pH 7.0 and the temperature was logged continuously and recorded every 10 min (Grant Squirrel recorder) with a probe in the centre of the thickest part of the muscle. All muscles were placed at 15°C when the pH was between 6.4 and 6.0 so as to reverse any cold-induced shortening or prevent rigor-induced shortening. When in the relaxed state at 15°C prior to the attainment of

Modelling post-mortem tenderisation--I

59

pH 6-0, their slack length, in the direction of the muscle fibres, was determined and then the muscles were stretched to 1.2 times their slack length. The muscles were maintained stretched until the pH reached 6.0 when some muscles were changed to 1 or to 30°C whilst others were maintained at 15°C until 24 h after stunning. Treatments were coded, for example, 15/24/1, indicating that the muscle was held at 15°C for 24 h and then held at I°C. The code 0/pH/15, indicates that the muscle was held at 0°C until the pH was 6.4 and then maintained at 15°C. After 24 h, the muscles were cut into sections (about 300g), packed under vacuum and stored at a constant temperature of I, 15 or 30°C. At times up to 21 days, the sections were heated in water at 80°C for 45 min, cooled in running tap-water and then overnight at 1°C. On average, each muscle was heated at 7 storage times and the toughness (first yield force, 6 replicates per sample) was measured at room temperature on the day following heating (Dransfield et al., 1981).

Modelling the changes in toughness An exponential model was used to determine the rate and extent of tenderisation in meat cooked 24 h or more after stunning when the temperature was maintained constant and the meat was at the ultimate pH. The different temperatures employed allowed calculation of the temperature dependency of the rate and extent of tenderisation. A similar model was then used to predict the toughness during the first 24 h from the parameters determined post-rigor and incorporating the temperature and pH profiles measured during the first 24 h. Toughness at the ultimate p H Variations in the toughness of meat cooked after 24 h at the ultimate pH and stored at constant temperature were analysed using an exponential decay equation to determine the parameters of tenderisation. The rate constant, the theoretical toughness at the time of stunning (F0) and that at infinite time (F..) at 1, 15 and 30°C were calculated for each muscle from the toughness determined at 7 storage times fitted to eqn (1): F t = F . + (F0 - F.) e-k'

(1)

where Ft is the toughness at time t hours, k is the rate constant and t, the time (h). This type of model has been used to determine the temperature dependence for various muscles (Dransfield et al., 1981) and a similar equation has been used to determine the storage changes in mechanical properties under sinusoidal compression (Kamoun & Culioli, 1989). The

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Eric Dransfield, Diane K. Wakefield, lan D. Parkman

refinement used by those workers by normalising F~. values was not included in this work since it is important to quantify and compare the (residual) toughness values at infinite time. Comparisons of the treatments were performed by analysis of variance of the parameters for displacement (F0 and F**) and the rate constants. These were derived for each curve (muscle) separately on about 45 determinations and then for all data (up to 16 muscles) combined into one curve (up to 840 determinations). This hierarchical structure gives an estimate of the variance due to different muscles and within each muscle. Where there were no significant differences between muscles, the decay curves were pooled and the common rates derived by parallel curve analysis (Ross, 1980). Calculation of the common rate maintains the variation in toughness values between muscles (treatments) and gives the best value for the rate constant among all muscles. The temperature dependency of the common rate constants was analysed according to the Arrhenius equation by plotting In(rate constant), determined post-rigor at 1, 15 and 30°C, against 1/T, where T is the absolute temperature. The plot gives a straight line of slope -E/R, where E is the activation energy and R the gas constant.

Prediction of early toughness Prediction of toughness values during 24 h post mortem was performed using one of three models: (i) temperature alone, (ii) temperature and increasing tenderisation rate with increasing pH, (iii) temperature and decreasing tenderisation rate with increasing pH. Calculations for temperature alone (i) were performed as follows: for each muscle, the predicted values at 24 h and at infinite time were calculated from eqn (1). The average muscle temperature over the 23 to 24-h period was used to calculate the rate constant of tenderisation (Arrhenius equation) over this period. This rate constant, together with the previously determined F24 and F. values, were used to calculate the new value for F0 (using eqn (1)). The new F 0, F~ and rate constant were then used to calculate F23 (the predicted toughness at 23 h) using eqn (1). The whole procedure was repeated successively for each hour back to the time of stunning (t = 0) in order to construct the curve predicting the toughness (equivalent to that at the ultimate pH) from the time of stunning to 24 h. Calculations were also modelled to include temperature and pH ((ii) and (iii) above). These were performed similarly but the rate constant at each hourly step was calculated according to the temperature and incorporating one of two additional factors. One equation (neutral model)

Modelling post-mortem tenderisation--I

61

increased the rate of tenderisation 3.6-fold for an increase of 1 unit of pH. The factor of '3.6', over the range pH 5.5 to 7.0, was that of calpain I isolated from rat skeletal muscle using casein as substrate (Kawashima et al., 1986). The other factor (acidic model) increased the rate of tenderisation by a factor of 16 for a decrease of 1 pH unit over the range pH 7.0 to 5.5. The latter factor was that for cathepsin B isolated from rabbit lung and its digestion of histones (Chatterjee et al., 1986).

RESULTS

Temperature and pH profiles Typical temperature profiles for some of the treatments are shown in Fig. 1. Muscles 30/pH/15 were cooled to about 25°C during the initial trimming and fat removal and, after placing in air at 30°C, then rose to almost 30°C where they were maintained until pH 6.4 (about 5 h). They were then cooled at 15°C and reached 15°C in about 12 h after stunning when the pH had fallen to 6.1. Muscles 15/pH/1 which had been electrically stimulated were held at 15°C and then cooled at I°C after 3 h (when the pH was 6.4) and reached 5°C, 8 h after stunning. Muscles cooled at 15°C reached 15°C after about 7 h. Muscles 0/pH/15 were

30=

20-

== E 10 m

5

10 Time A f t e r

25 Stunning

30

(hours)

Fig. 1. Temperature profiles during muscle storage. The curves represent the average temperatures for musclesheld at 15°C(non-labelled)and for the other treatments(labelled).

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Eric Dransfield, Diane K. Wakefield, Ian D. Parkman

6.5

P~.

~ ~ ~ ~ t 5 ,

C

*

I5 I 10 Time After Stunning(hours~

I 15

Fig. 2. The pH profiles during muscle storage. The pH of the muscles was measured hourly; the curves represent the average pH values for control (6 muscles), electrically stimulated (6 muscles) and muscles initially held at 0°C (2 muscles)and typical variations (+SD) given at two times. cooled at 0°C and reached 4°C after 3 h (when the pH was 6.4) causing muscle shortening. The muscles were then warmed in water at about 20°C for 15 min when the muscles relaxed and were stretched and placed in air at 15°C and the centre temperature reached 15°C, 7 h after stunning. Muscles 15/24/30 reached 30°C at about 29 h after stunning and muscles 15/24/1 reached I°C after 33 h. The pH values (Fig. 2) differed mainly between control and electrically stimulated muscles. At 15°C, control muscles reached pH 6.5 at 8 h and pH 6.0, 13 h after stunning. Electrically stimulated muscles reached pH 6-5 after 1 h and pH 6.0, 5 h after stunning. Muscles 0/pH/15 had an intermediate rate of reduction of pH, reaching 6.5 after 3 h and pH 6.0 after 8 h.

Variations in pH and toughness between treatments The average times to reach pH 6.2 and the toughness for the control and their electrically stimulated paired muscles are given in Table 1. Applying 12.5 Hz stimulation reduced the time to reach pH 6-2 from about 10 h in control (15°C) to about 6 h. Decreasing the frequency to 5 Hz reduced the time to reach pH 6-2 to 2 h. The ultimate pH was unaffected by stimulation or by temperature treatments and averaged 5-48.

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Modelling post-mortem tenderisation--I TABLE 1

Variations in pH and Toughnessof M Pectoralis profundus~ Treatment

n

Time (h) to pH 6.2

Ultimate pH

Toughness at 24 h

Ultimate toughness

C ES

15/24/1 15/24/1

2 2

8.4 3.0

5.48 5.58

7.75 6.80

3.93 4.03

C ES

15/pH/l 15/pH/1

2 2

11.0 3-5

5.63 5.54

9.95 8-90

3.78 4.10

C 12.5ES 5ES

30/pH/15 6 30/pH/l5 2 30/pH/15 4

8.2 5.7 2.1

5.41 5-43 5.41

6.70 7.63 6.87

4.39 4.38 3.95

C ES

15/24/30 2 15/pH/30 2

11.1 2.0

5.59 5.48

7.30 5.15

4.43 4-18

C C

0/pH/15 2 30/pH/15 2

6.6 10.7

5-40 5.41

7.40 7.58

3.79 4.30

a Values are the mean of pH and toughness (kg) of the number (n) of muscles in each treatment group of control (C) and electricallystimulated (ES) muscles. The time to pH 6.2 was estimated graphically from measurementstaken hourly and the ultimate toughness estimated using an exponential decay equation. Toughness at 24 h

When temperature was maintained at 15°C for 24 h (15/24/1), electrical stimulation reduced the toughness at 24 h from 7-8 to 6.8 kg demonstrating the tenderising effect of stimulation in the absence of shortening. When altered to I°C at about pH 6.0 (15/pH/1), tougher meat resulted but the difference between the control (9.95 kg) and the stimulated (8.9) was maintained. The temperature after the attainment of pH 6 was therefore important in determining the toughness and, when cooled similarly, early rigor development, produced by stimulation, gave more tender meat than control at 24 h. Lower frequency stimulation tended to give more effective reduction in pH and produced slightly more tender meat than that at the higher frequency. Stimulated (12-5 Hz) meat held at 30°C for about 4 h (30/pH/15) gave similar toughness at 24 h to non-stimulated meat. Changing the temperature to 30°C after the attainment of pH 6 (15/pH/30) in stimulated meat produced the most tender meat at 24 h. The temperature later during rigor development therefore had a greater effect than it did at earlier times soon after slaughter. Cooling at 0°C (0/pH/15) gave fast reduction in pH, reaching pH 6.2

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Eric Dransfieid, Diane K. Wakefield, Ian D. Parkman

in 6.6 h compared with about 11 h at 15°C and 30°C (30/pH/15) and may have been due to muscle shortening at low temperature. Cooling at 0°C, until the pH reached 6.4 and then at 15°C (0/pH/15), however, had no effect on toughness at 24 h, enforcing the previous conclusion that early temperature has little effect on toughness.

Toughness at infinite time From toughness measurements after 24 h (at a constant pH of 5-48), the exponential decay model was applied to each muscle separately and the average common toughness values at infinite time for each treatment group are given in Table 1. There were only small differences between treatments, the F. values ranging from 3.78 to 4.43 kg. Twelve paired comparisons of electrically stimulated and control showed a difference (control-stimulated) of 0-12 (SE 0.10) kg which was not significantly different from zero and the authors cannot prove that electrical stimulation gave any permanent tenderisation. The toughest meat at infinite time was produced when meat was held at 30°C post-rigor (15/24/30). Post-rigor temperature therefore appeared to affect the ultimate toughness (higher temperatures producing tougher meat), but the effect was small relative to the changes which could be produced by storage. Temperature and tenderisation Although it was evident, from the toughness data at 24 h, that the amount of tenderisation which had taken place varied among the treatments, the rate constants and the projected toughness values at infinite time do not depend on having data from the time of the start of the tenderisation process and therefore the parameters, even from incomplete curves, can be compared among treatments. Typical data for control muscles aged at 15°C are given in Fig. 3, which shows the replicate toughness values and the fitted curve and the determined parameters. Among the 28 muscles, 3 constant temperatures (1, 15 and 30°C) were used after 24 h. Pooling data from all treatments into their temperature group allows comparisons of the muscles within each temperature. The comparisons are given in Table 2 in the form of analysis of variance. Thus at I°C there were 8 muscles and a total of 296 toughness values (from about 7 times and about 6 replicates at each time) including 2 cooling treatments (at 15°C until pH 6.4 or 24 h) in both ES and control muscles with 2 replicates (Table 1). The analysis of variance shows that the displacements (F0 - F . and F.) and the rate constants did not differ significantly between the 8 curves (muscles) and therefore a common rate

Modelling post-mortem tenderisation--I

65

10" o

~

o

A

m Q o I-o

8"

Fo = 18.46 (2.72) kg

~o

FaD

= 4.09 (0.15) k g

k = 0.635 (0.009) h - I

fn

6' o

4"

.e

0-

8--

o o

100

200

i

t

Time after stunning (h)

Fig. 3. Parameters for tenderisation. The toughness data (6 replicates) for one control muscle held at 15°C, the fitted exponential curve and the parameters (standard error) are given.

constant (0.0133 h-l) and a common toughness at infinite time (3-96 kg) were derived. Similar calculations were done for the 16 muscles stored at 15°C and the 4 muscles stored at 30°C (treatments given in Table 1). At these two temperatures also, the displacements of the curves and the rate of tenderisation did not vary significantly between the muscles. TABLE 2

Variability of Tenderisation at pH 5.5a Source

Storage temperature (°C) 1

15

30

df

ms

df

ms

df

ms

Displacement Rate Within curves

14 7 275

9.9 3-0 0.4

30 15 791

6.3 1.3 0.3

6 3 96

10.5 2.3 0.3

Common rate constant (h -1) Toughness at infinite time (kg)

0-013 3 (0.002 3) 0-055 6 (0.013 4) 0.129 6 (0-026 8) 3-96 (O.ll) 4.13 (0.10) 4.30 (0.15)

Comparisons of the tcnderisation during storage were made using an exponential model. Analyses of variance were performed on all individual toughness values within each temperature and the degrees of freedom (df) and mean squares (ms) reported. Common rate constants (SE in parentheses) were derived for each temperature and their mean toughness (SE in parentheses) at infinite time calculated and averaged across muscles.

66

Eric Dransfield, Diane K. Wakefield, lan D. Parkman

m 0 o

-3

o

-4

-5 3.2

T

!

!

I

3.3

3.4

3.5

3.6

lIT

3.7

xl000

Fig. 4. Temperature dependence of the rate of tenderisation. The rate constants (Table 2) for the tenderisation at 3 temperatures are shown in the Arrhenius form together with the regression equation.

The common rate constants were derived for each of the 3 temperatures and showed that the rate at 15°C was 4 times, and the rate at 30°C, ten times that at 1°C (Table 2) which was typical for beef (Dransfield et al., 1981). The rates were plotted against the temperature in the form of the Arrhenius curve and showed that E / R = 6.5 (Fig. 4). The extent of tenderisation (F 0 - F . . ) varied between treatments within each temperature group but this is of little significance because the F0 values are those predicted at stunning and not those at the start of the tenderisation process. At infinite time, average toughness values predicted using the common rates increased consistently from 3-96 at I°C to 4-30 at 30°C. The start of tenderisation

The simplest model for tenderisation is that tenderisation occurs by the same process(es) in all treatments. When all other contributors to toughness are constant, as they should be for these muscles of similar sarcomere length and composition, the toughness of these unaged muscles should then be a constant. It was clear that the toughness at 24 h varied considerably among treatments and therefore the process resulting in tenderisation must have started within 24 h. Also, since the very early (pre-rigor) temperature had no effect on toughness at 24 h nor on the predicted final toughness, the process must have started significantly after stunning. The start of the

Modellingpost-mortem tenderisation--I

67

process can be predicted by modelling the changes in toughness with time after stunning when, at the start of tenderisation, all muscles should have the same toughness value. Electrical stimulation produced more tender meat than control at 24 h but not after ageing which suggests that the start of the tenderisation process correlated with rigor development and therefore the modelling of toughness against time must be performed with respect to rigor development. Muscles were pooled into three rigor groups which were considered separately: one (slow glycolytic) group in which the muscle took more than 7 h to reach pH 6-2, one which took 4-7 h (intermediate) and one (fast glycolytic) group which took only 1-4 h to reach pH 6.2. A predicted toughness curve from stunning to 24 h should contain a toughness value for unaged meat and, within each glycolytic group, the curves from muscles stored at different temperatures should coincide at this toughness.

Temperature-only model Using the temperature-only model, which assumes that the toughness is dependent only on temperature, toughness values equivalent to those at pH 5-5 were derived by stepwise back calculation (see experimental section) and plotted against time after stunning (Fig. 5). In the slow glycolytic group (Fig. 5(A)), two temperatures (I°C and 15°C) were used and the predicted toughness curves coincided at 13.2 kg. In the intermediate group, only one temperature was used (Fig. 5(B)) and the data cannot be used to give information on the toughness of unaged meat but a toughness of 13.2 kg was projected to occur at about 5 h after stunning. In the rapid glycolytic group (mainly stimulated muscles), three temperatures (Fig. 5(C)) were applied and the curves coincided at 10.6 kg (l°C and 15°C curves) at about 8 h and 14.2 kg (15°C and 30°C curves) at about 2 h. Taking all coincident points, the average value (weighted on the number of muscles) for the coincident toughness was 12-5 kg. The conclusion is that all unaged muscles could be predicted to have a toughness value (equivalent to that at pH 5.5) of 12.5 kg prior to tenderisation. Furthermore, the time at which this toughness occurred varied from 2 h in rapidly glycolysing muscle to 7 h in slow glycolysing muscles. The average pH at these times was 6-1 +0.1(Table 3). Using the temperatureonly model the best estimate for a single common process is that tenderisation would start, on average, at pH 6.1 when the toughness was equivalent in cooked meat to 12.5 kg. Temperature and pH models Similar calculations to those done for temperature alone were performed for the two other models in which variations in both temperature (Fig. 1)

68

Eric Dransfield, Diane K. Wakefield, Ian D. Parkman

15,,,,

A Ioc

10,-

~.

~5oc --15

Predicted

Toughness (Kgf) B 1

15°C

'°I

~30oC

5

10

10

I

15

Time After Stunning (hours)

Fig. 5. Estimating the unaged toughness of cooked muscle. The curves were derived from stepwise prediction of the toughness from the measured toughness after 24 h and the temperature profile during the first 24 h after stunning for the slow (A), fast glycolising (C) muscles (left axis) and for intermediate (B) glycolysing muscle (right axis). and p H (Fig. 2) were taken into account. The neutral model gave a slightly lower initial and m o r e variable toughness value but a start p H similar to that obtained in the temperature only model (Table 3). The acidic model gave a very low initial value (lower than some measured values at 24 h) but a similar start pH.

Prediction of toughness T h e start o f tenderisation for the temperature-only model was projected to occur at p H 6.1 when the toughness was 12.5 kg (Fig. 5, Table 3).

Modelling post-mortem tenderisation--I

69

TABLE 3 Determining the Start of Tenderisation and Prediction of Toughnessa n

Model Temperature

Neutral

Acidic

Toughness pH

25 28

12.48 (0.28) 6.12 (0.08)

11.60 (0.42) 5.99 (0.06)

8.67 (0.24) 6.12 (0.15)

Correlation Slope Intercept

196 196 196

0.91 0.93 0.40

0.86 0.78 0.88

0-23 0.17 4.10

Toughness curves were predicted, for 3 models from temperature and pH, from stunning to 24 h. Curves for individual treatments converged. The mean (SE in parentheses) of the toughness and pH at the points of convergence are given. These, together with the ultimate toughness (4.12 kg) were used to calculate the toughness at the measured postrigor at times up to 21 days using temperature-only, neutral and acidic models. Their effectiveness in predicting the observed toughness pooling all treatments is given in the form of regression parameters for observed (y) and predicted (x) values.

a

These two parameters and the average F~ (4.12 kg) were then used to predict the toughness. F o r each muscle, the time at p H 6.1 was estimated graphically and the temperature (and pH) at this time were used to calculate the rate constant. The toughness was then calculated at 1 h later and the process repeated each h o u r until 21 days. The effectiveness o f prediction was m a d e by comparisons o f the predicted and the observed data using linear regression analysis for all 196 (28 muscles x 7 storage times) m e a n toughness values. Temperature-only model The predicted values accounted for 83% (r = 0.91, Table 3) o f the observed variation in toughness measured from 24 h to 21 days. The slope o f the regression line was close to unity and the origin close to zero (Table 3). O f the 196 m e a n values which were measured, 95% o f them fell within 1.2 kg o f the predicted values. There was no apparent treatm e n t bias in the predictions. Temperature and p H models The neutral model was less effective (r = 0.86, Table 3) in predicting the observed toughness than the temperature-only model, probably due to the high variability in the initial predicted toughness value (Table 3). The acidic model was a poor predictor o f observed toughness (r = 0.23, Table 3) due mainly to the low initial predicted toughness (Table 3).

70

Eric Dransfield, Diane K. Wakefield, Ian D. Parkman

DISCUSSION The temperature and rate of rigor development during the first 24 h after slaughter were shown to be very important in determining the toughness of beef pectoralis muscle at 24 h. Depending on the conditions pertaining during the 24 h after stunning, the toughness of meat at 24 h varied from 5 to 10 kg and therefore as much as 86% or as little as 8% of the maximum tenderisation due to ageing had occurred during the first 24 h.

Early post-mortemtemperature In non-ES muscles, pre-rigor temperature had no effect on toughness or the rate of tenderisation in post-rigor meat. This shows that tenderisation started some time after stunning. If the tenderisation had started at the time of stunning, and was influenced by temperature in the same way as in post-rigor meat, the effect of holding at 30°C (see Fig. 1) would be to increase the rate of tenderisation by a factor of about 5 during the initial 5-h period compared with that held at 0°C, i.e. tenderisation equivalent to a difference of 1 day's storage at about 15°C and should have been, but was not, detected in cooked meat. Empirically, early rigor in carcasses can be useful in the prediction and selection for tough meat (Khan & Lentz, 1973). Also, by empirical examination, Lochner et al. (1980) showed that tenderness had the highest statistical correlation to the temperature at 2 h p o s t mortem and suggested that an unidentified mechanism was operating in which elevated temperatures during the first few hours promoted tenderness. In their work, lean and fat beef sides were cooled either at -2°C in air at 1.5 m s -I or at 9°C for 7-5 h in static air and then at -2°C. Sides were then transferred to 3°C at 24 h. Unfortunately pH was not reported but the higher prerigor temperature in the fat carcasses is likely to have caused more rapid reduction in pH (Buts et al., 1986; Tornberg & Larsson, 1986) and the fat carcasses would have entered rigor earlier than the leaner carcasses. The earlier start and prevailing higher temperature post-rigor in the fatter carcasses would produce more initial tenderisation which could account for their tenderness after 4 days. When considered alone, the fat carcasses, chilled slowly or rapidly, had similar temperature profiles, and showed no relationship between tenderness and temperature at 2 h post mortem. This explanation would be consistent with the observations in this study in which there was no unique effect of early post-mortem temperature on texture.

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Eiectrieal stimulation Clearly the temperature effect cannot be considered in isolation from pH or rigor development and electrical stimulation has been used to study temperature independent of rigor development. Electrical stimulation produced more tender meat than non-stimulated meat at 24 h but the difference was eliminated after completion of ageing. When the temperature was lowered to I°C soon after the muscle reached pH 6.0, ES meat was tougher at 24 h than the control meat held at 15°C but again gave similar toughness to the control meat after storage. Stimulation therefore gave only transient changes in tenderness which could be accounted for by temperature differences. When chilled similarly, ES temporarily improves tenderness but may be prevented by cooling soon after rigor. Most evidence, based on similar cooling of control and ES sides, shows a tenderisation following stimulation (Cross, 1979; Savell et al., 1977, 1981) but others show no effects on texture (Crouse et al., 1985; Hawrysch & Wolfe, 1985; Jeremiah et al., 1985) and recent work suggests a possible toughening. The tenderisation in carcass meats may have been due to the prevention or reduction in cold shortening toughness (Carse, 1973) but, even in the absence of cold shortening, stimulation makes meat more tender initially (Savell et al., 1978) as was the case in this work. The mechanism of the tenderisation has been the topic of research by several investigators. Fibre rupture to produce weak, stretched or fractured zones in the muscle, which may be accompanied with super-stretching, tearing and the loss of the Z-lines (Savell et al., 1978; Sorinmade et al., 1982), has been suggested as the cause of tenderisation but no modelling has been done to show how cracks or other histologically-observed changes relate to strength in raw or cooked meat. It is also unclear if the disruption occurs directly as a result of the contraction caused by the stimulation or by the earlier and enhanced proteolysis (Sorinmade et al., 1982). Irregular contraction bands in bovine longissimus dorsi at 24 h (Savell et al., 1989) were due to denaturation and not to muscle damage and it is doubtful if these changes were the cause of tenderisation (George et al., 1980; Fabiansson & Libelius, 1985). The present work suggests enhanced proteolysis alone could account for the tenderisation since the same model predicts texture in both control and stimulated meats. Recent conclusions on the toughening effect (Marsh et al., 1987) were derived from a series of experiments (Takahashi et al., 1984; Uruh et al., 1984, 1986) in which stimulation reduced the pH to 5.4 in 4 h whilst the temperature was 33°C. The meat was therefore exposed to conditions which can induce rigor shortening and, in turn, cause toughening (Dransfield & Etherington, 1989) and cannot be used to propose a

72

Eric Dransfield, Diane K. Wakefield, lan D. Parkman

mechanism for ageing in the absence of shortening. Holding carcasses from feed-lot cattle at 35°C for 3 h, even with normal rates of glycolysis in the absence of stimulation, can produce rigor shortening and toughening (Lee & Ashmore, 1985). Clearly the beneficial effects of prevention of cold toughening by electrical stimulation must be weighed against the possibility of heat (rigor) shortening (Pommier et al., 1987). Start of tenderisation

Determination of the start of tenderisation has been hitherto problematical for the following reasons: (i) Tenderisation is determined in cooked meat but heating pre-rigor meat induces shortening and reduces the pH. Although these effects on toughness of cooked meat are additive (Dransfield & Rhodes, 1975), measurements are sufficiently variable to induce large errors in any modelling. (ii) We do not know the structural changes responsible for the enzyme mechanisms inducing tenderisation. Although changes in the myofibrillar proteins occur soon after death (Troy et al., 1986) their importance to changes in toughness have yet to be determined before they can provide information on the tenderisation. (iii) Characterising the mechanical properties of raw muscle is in its infancy and measurements need careful interpretation. The fragmentation of myofibrils during homogenisation increased during rigor development and has been used to suggest that tenderisation starts soon after stunning (Koohmaraie et al., 1987). However, the fragmentation will be affected by the shearing stresses on the myofibrils which are determined by stiffness of the myofibrils. As the stiffness increases during rigor development more fragmentation would be expected, even in the absence of weakening due to ageing. Weakening in raw meat, determined by tensile testing at low stresses, has also been modelled and suggests that tenderisation started at maximum stiffness (Dransfield et al., 1986), in broad agreement with the present findings that the process started at pH 6.1 following a significant lag phase after stunning. REFERENCES Buts, B., Claeys, E. & Demeyer, D. (1986). Proc. 32nd Meeting Europ. Meat Res. Workers, State University of Ghent, Ghent, Belgium, p. 131. Carse, W. A. (1973). J. Fd Technol., 8, 163. Chatterjee, R., Lones, M. & Kalnitsky, G. (1986). Cysteine Proteinases and Their Inhibitors. Proc. Int. Symp. Sept. 1985 Port-aroz, Yugoslavia, ed. Vigo Turk. Walter de Gruyter, Berlin, p. 97.

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