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The effects of electrical stimulation, thawing, ageing and packaging on the colour and display life of lamb chops
Meat Science 30 (1991) 131-145
The Effects of Electrical Stimulation, Thawing, Ageing and Packaging on the Colour and Display Life of Lamb Chops V. J. Moore & O. A. Young Meat Industry Research Institute of New Zealand (Inc.), PO Box 617, Hamilton, New Zealand (Received 23 May 1990; accepted 24 August 1990)
A BS TRA C T Electrical stimulation, freezing and thawing, ageing and type of packaging used during ageing are factors which could influence the final colour of meat. The experiment reported here determined the individual and additive effects of these factors in displayed lamb. Initial panel scoresfor colour of chops were increased by electrical stimulation and decreased by thawing and by ageing for up to six weeks. Chopsfrom loins which had been aged in vacuum pack had higher initial scores than those aged in oxygen-permeable film. Hunter colour values for chops from stimulated carcasses were much less variable than those for chops from unstimulated carcasses. Stimulation, therefore, produced a more uniform product. Hunter L, a and b values all declined during display, the greatest decline being in a (redness). Hunter values were not good predictors of initial panel scores but Hunter a and hue ( a / b ) both declined with panel scores. With no ageing, the display life of stimulated chilled lamb was longer than that of unstimulated lamb, but this advantage disappeared after ageing for 2 weeks in a vacuum pack. The display life of thawed unstimulated lamb was only slightly inferior to that of chilled unstimulated lamb when both were vacuum-packed, but thawed stimulated lamb resulted in a much reduced display life. Ageing of loins in o?cygen-permeable film also adversely affected display life of chops. Total drip lossfrom rigor to the end of display was significantly affected by all treatments except stimulation. The dominant cause of drip loss was the freeze-thaw treaonent. In chilled and thawed lamb, ageing increased drip. Packaging had a lesser effect on drip. 131 Meat Science 0309-1740/91/$03-50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
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V. J. Moore, O. A. Young
INTRODUCTION For the New Zealand meat industry to successfully market thawed lamb cuts in the international supermarket trade, it is necessary to know how processing variables, such as electrical stimulation, ageing and packaging during ageing, affect the colour and display life of the thawed meat product. Thawed meat from unaged, unstimulated carcasses had as good a display life as the equivalent chilled product, about three days (Moore, 1990a, b). By contrast, in a preliminary study of stimulated lamb in this laboratory, thawed chops from stimulated carcasses had a very short display life, at most one day, whereas chilled chops had a long display life; seven days if unaged, declining to one day if aged a week at 6°C. The preliminary study also showed that the display life of frozen stimulated chops was shorter than that of frozen unstimulated chops examined in previous work (Moore, 1990c). Collectively, these results suggested that stimulation of lamb may have drawbacks when the meat is frozen. The present study sets out to determine if stimulated lamb can be frozen, thawed, and then held (aged) as a primal cut before successful presentation as a consumer item, in this case chops. Ageing times of up to 6 weeks, at - 1 . 5 ° C , were chosen because vacuum-packed lamb stored at that temperature for that time is well within its storage life (Powell & Eustace, 1981). The effect of packaging materials during ageing (vacuum or oxygenpermeable shrink wrap) was also studied since earlier work (Moore, 1990a) indicated that anaerobically packaged meat held for six weeks had better colour and colour stability than fresh chilled meat. Successful presentation is gauged by display life and this is largely a function of lean meat colour. In the present study lean meat colour was subjectively assessed by a sensory panel to determine display life, and objectively by a Hunter colorimeter.
MATERIALS AND METHODS Meat
Sixteen lamb carcasses were assigned to control (unstimulated) and stimulated groups (8 carcasses each). The latter were electrically stimulated within 30 min of slaughter to the New Zealand AC Specification (90 sec, 1130 V peak, 14.28 pulses/s, Chrystall et al., 1989). Stimulated carcasses were held at 10°C for 5 h before cutting, and control carcasses (unstimulated) were held at 10°C for 24 h, by which time the muscles were in rigor. Long saddles from the 16 carcasses were each cut into six equal portions. Each was systematically assigned to an ageing-packaging combination (see
The colour and display life of lamb chops
133
below), to achieve balance between treatment and position along the loin, since loin position has an effect on colour (Moore & Gill, 1987). Saddle portions were halved through the vertebrae to give a 'chilled' loin and a 'thawed' loin for each ageing-packaging combination. The 'chilled' treatment was as follows. Half the portions were packaged in oxygenpermeable shrink film (see below) and the other half in vacuum bags of low oxygen-permeability. The portions were then aged for 0, 2, 4 and 6 weeks at - 1.5°C. After ageing, drip loss was measured, and 20 m m chops with freshly exposed surfaces were cut from the loin portions. For display, chops were placed directly on white plastic trays and overwrapped with oxygenpermeable film. The 'thawed" treatment was the same as 'chilled' except that the loin portions were wrapped in oxygen-permeable shrink film, frozen and stored at - 2 0 ° C for 1 week, before thawing, subsequent ageing and display. Drip loss was measured after thawing, as well as after ageing and display. Details of freezing and thawing--loins took 14 h to reach - 2 0 ° C and were above 0°C after overnight thawing at 3°C.
Packaging materials The oxygen-permeable shrink film was d-film (W. R. Grace, NZ, Oz permeability > 2000 ml/mZ/24 h/atm at 24°C). The film was shrunk around loin portions and chops with a hot air gun using only enough heat to shrink the film. Vacuum bags were Cryovac barrier bags (W. R. Grace, NZ, O2 permeability 35 ml/mZ/24 h/atm at 24°C).
Display Chops were displayed for 8 h a day at 3°C under Philips 32 Deluxe fluorescent tubes. Chops were assessed by an experienced six member colour panel at the same time each day. The chops were evaluated on a 1 to 5 scale where 1 = poor colour, 2 = fair colour (would buy only if discounted), 3 = good lamb colour, with some browning, 4 = very good lamb colour, 5 = excellent bright red lamb colour. Display life was the number of days the average score for a treatment was 3-0 or higher. When the average panel score for a treatment dropped below 3"0, all chops in that treatment group were removed from display, even though some chops in the group still scored above 3-0.
Colour measurements Hunter L, a, b measurements were made with a Hunter 25D colorimeter immediately after panel evaluation. Hue (a/b) was calculated.
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V.J. Moore, O. A. Young
Drip Percentage drip losses were calculated from the weight losses that occurred after each manipulation; thawing, ageing and display. Total drip was calculated from the individual drip losses.
Statistical analysis Drip data were initially analysed for all treatment effects. However, thawing was a highly significant factor in every drip measurement and formed interactions with all variables. Therefore, a second analysis was carried out in which chilled and thawed chops were analysed separately. The second analysis, which used an angular transformation (arc sinx/(proportion of drip)), sharply defined the effects of the other treatments and is the analysis presented here (Tables 1-3). For Hunter colour values, the percent variance of stimulated chops was significantly lower than that of control chops; it was ten times lower for L values (4-8 versus 48), six times for a values, and four times for b values. Differences in variance for panel scores reflected, but did not equal, the variance differences in Hunter colour data. Thus, in this experiment, stimulation created two colour populations. Analysis of Hunter values and panel scores was, therefore, carried out on control and stimulated carcasses as separate populations. In presentation of data, the effect of a given treatment has usually been shown isolated from other treatments. To do this, data have routinely been averaged across the other treatments. Another aspect of presentation is that where ageing times are examined (Tables 2 and 5) significance values simply show whether ageing has a significant effect, without comparing individual values for each ageing time. Where there is a significant effect the trend is nearly always obvious.
RESULTS
Drip During thawing, loins from control (unstimulated) and stimulated carcasses lost a similar a m o u n t of drip (Table 1). During ageing, chilled loins from stimulated carcasses produced slightly less drip than those from control carcasses (p < 0.05); when thawed loins were aged, the reverse was true. Drip loss during display was small for all treatments. Within the chilled and thawed groups, stimulation had no significant effect on total drip loss.
The colour and display life of lamb chops
135
TABLE 1 The Effect of Electrical Stimulation on the Drip Loss (% Weight Lost) of Lamb Loins and Chops during Thawing, Ageing and Display. Drip Loss is Averaged over Packaging and Ageing Treatments
Treatment
Loins
Chops
Thawing
Ageing
Display
Total
---
1-25 0.94 *
0-31 0-33 NS
1-35 1-11 NS
2-24 3-11
0-30 0-37
*
*
3.97 4-54 NS
Chilled Control Stimulated
Thawed Control Stimulated
1-68 1-56 NS
*, p < 0-05. NS, not significantly different.
TABLE
2
The Effect of Ageing Time on the Drip Loss (% Weight Lost) of Chilled and Thawed Loins and Chops. Drip Loss is Averaged over Stimulation and Packaging Treatments
Ageing time
Thawing
Ageing Display
Total
Chilled and aged for 0 2 4 6
weeks weeks weeks weeks
-0.55 1-23 1.65 ***
0.34 0-28 0-29 0-38 **
0.24 0.84 1.51 2-04 ***
Thawed then aged for 0 2 4 6
weeks weeks weeks weeks
2-24 1-68 1"49 1-38 q~S (Mean = 1.69)
-2-28 2-59 3-13 ***
0"63 0"36 0"32 0-21 **
2"84 4.33 4.34 4-68 NS (0 versus 2--6 weeks, ***)
**, p < 0"01; ***, p < 0-001. NS, not significantly different. (Significance values attached to the data simply show whether ageing has a significant effect on drip, without comparing individual values for each ageing time. Where there is a significant effect the trend is nearly always obvious.)
136
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Fig. 1. Percent drip loss from (O) thawed and (©) chilled lamb. (a) Loss during ageing, (b) total loss from rigor to end of display. Data were averaged over other treatments. The drip from stimulated lamb was clearer than that from control lamb, the latter containing more particulate matter in suspension. The n u m b e r o f weeks that loins were aged at - 1.5°C affected the a m o u n t o f drip (Table 2, Fig. la). During the first two weeks o f ageing, the thawed loins had a large drip loss (2.28%) above that measured during the thawing event (1.69%). Beyond two weeks, drip loss in thawed loins parallelled that in chilled loins (Fig. la). The drip loss o f chops on display was affected by loin ageing time, but the effect was different for the chilled and thawed lamb. With chilled chops, drip during display increased with ageing time, and with thawed chops, it decreased (Table 2). The total drip from chilled lamb increased linearly with ageing time, but with thawed lamb, it increased TABLE 3 Effect of Packaging Material (Oxygen-Permeable or Vacuum) on Drip Loss (% Weight Lost) of Chilled and Thawed Loins and Chops. Drip Loss is Averaged over the Stimulation and Ageing Treatments Thawing
Ageing
Display
Total
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1-20 1-67 NS
2.84 2.48
0"31 0.28
*
*
4.57 4-33 NS
Chilled
0 2 permeable Vacuum Thawed
02 permeable Vacuum
1"62 1.62
*, p < 0-05; ***, p < 0-001. NS, not significantly different.
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137
TABLE 4 Effect of Thawing on Initial Values for Panel, Hunter L, a and b, and Hue of Control and Stimulated Lamb. Values are Averaged over Packaging and Ageing Treatments
Treatment Control Chilled Thawed
Panel scores
L
a
b
Hue
4-0 3"6
28.6 26-0 **
13-6 11-2 **
7-6 5"9 ***
1.82 1'99 NS
27"2 26"3 **
13.7 11-7 ***
7"1 6"3 **
1.97 1-90 NS
*
Stimulated Chilled Thawed
4"5 3"5 ***
*, p < 0-05; **, p < 0-01; ***, p < 0.001. NS, not significantly different.
TABLE 5 Effect o f Ageing Time on Initial Values for Panel, Hunter L, a and b, and Hue. Values are Averaged over Thawing and Packaging Treatments
Ageing time (weeks) Control 0 2 4 6 Stimulated 0 2 4 6
Panel scores
L
a
b
Hue
4-1 3-7 3-9 3-8 NS
25-5 27-7 26-8 27-4 NS
9-4 13-4 12-8 11-0 ***
5-3 7"1 6"7 6'4 NS
1"76 1"94 2-01 1-78 *
4-4 4.2 3-9 3-9 ***
24-6 27"2 26"7 26-3 NS
8-8 13"5 12-3 12-3 ***
4-9 7-4 6"5 6-2 ***
1-80 1-83 1-94 2.04 NS
*, p < 0"05; **, p < 0"01; ***, p < 0"001. NS, not significantly different. (Significance values attached to the data simply show whether ageing has a significant effect on colour, without comparing individual values for each ageing time.)
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v. J. Moore, O. A. Young
between 0 and 2 weeks, after which there was little increase (Table 2, Fig. lb). The packaging m e t h o d - - o v e r w r a p in oxygen-permeable film or vacuum package in low oxygen-permeable film--influenced drip loss only during ageing (Table 3). For chilled lamb, the drip in the vacuum pack was significantly (p<0.001) greater than in the oxygen-permeable pack. In thawed lamb, the effect was reversed slightly. Within the chilled and thawed groups, packaging material did not affect total drip. Colour panel assessment A s s e s s m e n t on initial display
Analysis of the pooled data (not shown) demonstrated that stimulation significantly (p < 0.05) increased initial panel scores. Although the data presented were analysed as two populations (control, stimulated) the positive effect of stimulation can still be seen in Table 4, column 1. Whether chops came from stimulated or control (unstimulated) carcasses, thawed chops had a lower initial panel score than chilled chops (Table 4). Ageing did not affect the initial panel scores for control chops, but caused a significant decrease in scores for stimulated chops (Table 5). For both control and stimulated chops, vacuum packaging during ageing resulted in significantly higher initial panel scores on display (Table 6). A s s e s s m e n t &tring display
As expected, the panel scores declined during display. Figure 2 shows the decline averaged across packaging and ageing, and so illustrates the TABLE 6
Effect of Packaging Materials on Initial Values for Panel, Hunter L, a and b and Hue. Valuesare Averaged over Thawing and Ageing Treatments Panel scores
L
a
b
Hue
3-6 4-0
27.4 27-2 NS
12-2 12-6 NS
6.7 6-8 NS
1.91 1-90 NS
3.8 4-2
26.9 26.5 NS
12.9 12-5 NS
6-7 6.7 NS
1.96 1-91 NS
Control
02-permeable Vacuum Stimulated
O2-permeable Vacuum
***, p < 0-001. NS, not significantlydifferent.
139
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Day of display Fig. 2. Change in average panel scores during display of chops cut from loins treated in four ways. (I--1) Stimulated-chilled, (ll) stimulated-thawed, (A) control-chilled and (4,) control-thawed. Data were averaged over ageing and packaging treatments.
stimulation and freeze-thaw effects in isolation. The best treatment was stimulated-chilled and the worst was stimulated-thawed. Table 7 lists the net effect o f the four variables on the display life, defined by the numbers o f days the m e a n panel score exceeded 3"0. The various treatments had dramatic effects on display life.
Objective colour assessment by Hunter values Initial colour values Whether carcasses had been stimulated or not, the chilled lamb chops, when first put on display, were significantly brighter (higher L values), redder TABLE 7 Display Life (in Days) of Chops from Various Stimulation, Thawing, Ageing and Packaging Treatments Packaging and ageing (weeks)
Control
Stimulated
Chilled
Thawed
Chilled
Thawed
0 2
3 4
1 3
? 4
1 1
4 6
3 3
2 2
3 2
1 1
3 1 1 1
1 1 1 1
7 3 1 2
1 1 1 1
Wacuunl
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(higher a values) and yellower (higher b values) than thawed chops (Table 4). Hue was not significantly different. Ageing did not influence initial L values (Table 5), but a and b values were considerably lower for chops from unaged loins than from any aged loins. Ageing for 2 weeks resulted in the highest a and b values. Values decreased thereafter. Packaging did not affect any of the initial Hunter values of chops on display (Table 6).
Colour values during display Hunter values declined during display. When data were averaged across packaging and ageing, the greatest decline was in Hunter a (Fig. 3), b and L being relatively unaffected by display (data not shown). Hue declined for all treatments (Fig. 4). DISCUSSION
Drip losses Drip loss of raw meat from slaughter to cooking can influence profit at all points of the marketing chain because of weight loss and perceived loss of quality. Stimulation had no effect on total drip loss (Table 1), confirming the finding of Jeremiah and Martin (1982) with beef. A greater drip loss might have been expected because the muscles enter rigor at higher temperatures. In pigs, an extreme form of this causes the pale, soft exudative (PSE) condition, where drip losses are extreme. In the present experiment, where lambs were slaughtered under an author's supervision, there was no evidence of a PSE-type condition, consistent with the results in Table 1. There is no obvious explanation for the observed greater clarity of the drip from stimulated lamb.
The colour and display life of lamb chops
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Day of display Fig. 4. Change in Hunter hue values during display. (I-1) Stimulated-chilled, (I) stimulated-thawed, (A) controlled-chilled and (A) control-thawed. Data were averaged over ageing and packaging treatments. Ageing of loins increased drip. The most significant effect was the excessive exudation from thawed meat in the first two weeks of ageing, clearly illustrated in Fig. l(a). It is likely that this loss (2.28%) is a continuation of the loss (1"69%) caused by the freeze-thaw treatment (Table 2). This argument is supported by the higher display loss (0.63%) from thawed chops put on display without ageing (0 weeks) compared with those that were aged (0.36--0.21%). Whatever the subtleties of drip loss during ageing and display, the total drip loss for chilled lamb increased steadily the longer it was aged, but for thawed lamb the effect was obscured by the freeze-thaw treatment (Table 2, Fig. lb). A problem with vacuum packaging is drip loss in the pack (Jeremiah & Jones, 1989), presumably initiated at the time the vacuum is applied. Although researchers have obtained conflicting data on actual, rather than perceived, drip loss during storage (Zarate & Zaritzky, 1985; Weakley et al., 1986), the present data are clear; in Table 3, chilled meat in a vacuum pack lost more drip during ageing than similar meat in oxygen-permeable film (1"33 versus 0.87%). The loss continued to be higher on display, although the difference was not statistically significant. Total drip loss was also slightly higher for vacuum packed meat. Whereas the thawed lamb always suffered more drip loss than chilled lamb (Tables 1-3), vacuum packaging did not exacerbate drip loss from thawed meat (Table 3). (Rather, oxygen-permeable packaging caused a slight increase in drip). The easily removed water may already have been lost during thawing. Effect of stimulation on variability of colour data In contrast to the drip data, the colour data were analysed separately for control and stimulated carcasses because stimulation created a population
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V. J. Moore, O. A. Young
whose colour was much more uniform (variability was much lower for all colour values-except hue which was unaffected--and panel scores). Ledward et al. (1986) similarly found that the variability of L, a, b values for fresh beef was lower for stimulated than for control samples. They further found that on ageing, the differences in variability converged, so finally little difference in variability remained. Reduced variability due to stimulation could in itself be a processing bonus, giving meat of more consistent colour. Colour
Stimulation resulted in slightly higher initial panel scores (see Table 4), confirming the findings of Miller et aL (1987) for beef and one study of two by Riley et aL (1980a, b) for lamb. However, any colour advantage conferred by stimulation can be very short-lived, depending on subsequent treatment of the meat. The freeze-thaw treatment adversely affected the colour of lamb (Table 4), but it was on stimulated lamb where this effect was worse, resulting in lower panel scores initially and a much reduced display life (Table 7). The display life data for stimulated thawed lamb examined in the preliminary study (Introduction) have, therefore, been corroborated by the data in Table 7. Thawed-stimulated chops had a display life of one day (Table 7). The question naturally arises as to why electrical stimulation has such undesirable effects on the colour of lamb after it has been fro7en and thawed. There is no doubt that electrical stimulation can cause tissue damage (George et aL, 1980; Sorinmade et al., 1982), and this might be made worse by freezing and thawing. However, this explanation would not explain the long display life of chilled stimulated lamb (Table 7). Without ageing, chilled stimulated lamb had a display life of seven days, a finding also obtained by Renerre and Dantchau (1987). Miller et al. (1987) suggested that the (initial) brighter colour of stimulated beef may be due to a looser structure of the muscle fibres, allowing for deeper penetration of oxygen and thus higher oxymyoglobin levels. However, this would not explain the longer display life of the chilled stimulated meat, for if more oxymyoglobin were formed, one would expect a greater potential for subsequent metmyoglobin formation and so a shorter display life. Ageing also had an adverse effect on the initial panel score of stimulated lamb (Table 5). While thawed stimulated chops had a very short display life, regardless of ageing (Table 7), ageing of their chilled equivalents also took a toll on display life. After two weeks ageing in vacuum the display life was about the same as if the meat were unstimulated, and continued to decline. Not only did the packaging used during ageing affect initial panel scores
The colour and display life of lamb chops
143
(Table 6), it also affected display life (Table 7). Together, the data show that if meat is aged in a vacuum-pack rather than oxygen-permeable film, the display life is better. The major effect was on control (unstimulated) lamb. For example, display life was dramatically reduced when chilled control chops were aged in oxygen-permeable film, being only one day rather than three. Data in the first column of Table 7 (control-chilled) indicate that packaging used during ageing was a more important factor than ageing time on maintaining colour stability during display. In Table 7 oxygen-permeable data, the greater colour stability of stimulated-chilled chops compared with their control equivalents is a puzzle. No benefit of electrical stimulation on controlling microbial growth has been demonstrated (Gill, 1980; Mrigadat et al., 1980; Nortje et al., 1986). It is possible that the better colour stability of electrically stimulated meat reflects the shorter post-mortem holding time before packaging (see Materials and Methods); microorganisms would have had less time to colonise the meat. Although stimulation had little effect on initial Hunter colour values, the freeze-thaw treatment significantly affected them (Table 4); all values were lower except that of hue. Low Hunter values reflected the lower panel scores for thawed chops. When the data were analysed to test the overall effect of ageing (Table 5), any ageing at all caused a decrease in initial panel score. But in contrast to the data in Table 4, where high panel scores were reflected in high Hunter values, chops from any aged loins had lower panel scores but higher Hunter values. This was true whether carcasses were stimulated or not. These differences cast doubt on the value of initial Hunter colour values in predicting initial colour scores. Packaging materials likewise significantly influenced panel scores, but not Hunter values (Table 6). Although initial Hunter colour values did not appear to be good predictors of initial panel score, Hunter a and hue values showed clear trends • during the display. Moreover, Hunter a values especially, declined in concert with panel scores (Figs 2 and 3). Restated, redness declined with panel scores. Consumers select red meat on the basis of redness, with the belief that if it is red it is safe to eat, and that if not red, is of dubious quality. This study illustrated the fallacy of that belief; when the chilled loins in this study had been aged for four weeks in oxygen-permeable film, many had developed a mould on the cut surfaces of lean tissue, and off odours were detectable. By six weeks, the loins were obviously spoiled. Yet when the end chop was discarded, and a chop with a fresh surface was cut and displayed, the newlycut surface bloomed well. The off odour could not be detected through the display film. The only factor which detracted from the appearance was a barely visible discolouration of the membrane on the outside of the chop,
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v. J. Moore, O. A. Young
and a slight yellowing of the fat. It might be argued that such clearly spoiled chops should not have been used for panel assessment, but their assessment preserved the balance in the experiment. The results clearly show that selection o f meat on the basis of colour is not a very valid method o f selection. Nevertheless old habits die hard, and the consumer will undoubtedly continue to use colour as his guide to selection, and scientists will try to preserve the red colour for as long as possible. The main findings about display life in this experiment (Table 7) can be summarised as follows. The display life o f stimulated-chilled lamb during display was better than that o f unstimulated (control) lamb. This advantage disappeared after 2 weeks ageing in a vacuum pack at - 1-5°C. Packaging loins in oxygen-permeable film during ageing greatly reduced the display life o f the unstimulated chops, compared with vacuum packaging. The display life o f thawed control lamb was only slightly inferior to the chilled equivalent, when both were vacuum packed. The display life o f chops from thawed stimulated loins, one day for all treatments, was very poor.
A C K N O W L E D G E M ENTS The authors wish to thank M I R I N Z staffwho served on colour panels, and D r John Waller, R u a k u r a Agricultural Research Centre, for carrying out the statistical analysis.
REFERENCES Chrystall, B. B., Devine, C. E., Longdill, G. R., Gill, C. O., Swan, J. E. & Peterson, G. V. (1989). In Meat Production and Processing, ed. R. W. Purchas, B. W. Butler-Hogg & A. S. Davies. N.Z. Soc. Anim. Prod. Occas. Publ. No. 11, Hamilton, New Zealand. George, A. R., Bendall, J. R. & Jones R. C. D. (1980). Meat Sci., 4, 51. Gill, C. O. (1980). J. Food Protect., 43, 190. Jeremiah, L. E. & Jones, S. D. M. (1989). J. Food Protect., 52, 473. Jeremiah, L. E. & Martin, A. H. (1982). J. Food QuaL, 4, 175. Ledward, D. A., Dickinson, R. F., Powell, V. H. & Shorthose, W. R. (1986). Meat Sci., 16, 245. Miller, M. F., Cross, H. R., Buyck, M. J. & Crouse, J. D. (1987). Meat Sci., 19, 253. Moore, V. J. (1990a). Meat Sci., 28, 9. Moore, V. J. (1990b). Int. J. Food Sci. & Technol., 25. Moore, V. J. (1990c). Meat Sci., 27, 91. Moore, V. J. & Gill, C. O. (1987). N . Z . J . Agr. Res., 30, 449. Mrigadat, B., Smith, G. C., Dutson, T. R., Hall, L. C., Hanna, M. O. & Vanderzant, C. (1980). J. Food Protect., 43, 686.
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Nortje, G. L., Naumann. H. D., Naude, R. T., Oosthuizen, W., Jordaan, E. & Grobler, I. (1986). J. Food Sci., 51, 12. Powell, V. H. & Eustace, I. J. (1981). Chem. Australia, 48, 351. Renerre, M. & Dantchau. S. (1987). Sciences des aliments, 7, 535. Riley, R. R., Savell, J. W., Smith, G. C. & Shelton, M. (1980a). J. FoodSci., ,15, 119. Riley, R. R., Savell, J. W. & Smith, G. C. (1980b). J. FoodSci., 45, 1101. Sorinmade, S. O., Cross, H. R., Ono, K. & Wergin, W. P. (1982). Meat Sci., 6, 71. Weakley, D. F., McKeith, F. K., Bechtel, P. J,, Martin, S. E. & Thomas, D. L. (1986). J. Food Sci., 51,281. Zarate, J. R. & Zaritzky, N. E. (1985). J. Food Sci., 50, 155.
The Effects of Electrical Stimulation, Thawing, Ageing and Packaging on the Colour and Display Life of Lamb Chops V. J. Moore & O. A. Young Meat Industry Research Institute of New Zealand (Inc.), PO Box 617, Hamilton, New Zealand (Received 23 May 1990; accepted 24 August 1990)
A BS TRA C T Electrical stimulation, freezing and thawing, ageing and type of packaging used during ageing are factors which could influence the final colour of meat. The experiment reported here determined the individual and additive effects of these factors in displayed lamb. Initial panel scoresfor colour of chops were increased by electrical stimulation and decreased by thawing and by ageing for up to six weeks. Chopsfrom loins which had been aged in vacuum pack had higher initial scores than those aged in oxygen-permeable film. Hunter colour values for chops from stimulated carcasses were much less variable than those for chops from unstimulated carcasses. Stimulation, therefore, produced a more uniform product. Hunter L, a and b values all declined during display, the greatest decline being in a (redness). Hunter values were not good predictors of initial panel scores but Hunter a and hue ( a / b ) both declined with panel scores. With no ageing, the display life of stimulated chilled lamb was longer than that of unstimulated lamb, but this advantage disappeared after ageing for 2 weeks in a vacuum pack. The display life of thawed unstimulated lamb was only slightly inferior to that of chilled unstimulated lamb when both were vacuum-packed, but thawed stimulated lamb resulted in a much reduced display life. Ageing of loins in o?cygen-permeable film also adversely affected display life of chops. Total drip lossfrom rigor to the end of display was significantly affected by all treatments except stimulation. The dominant cause of drip loss was the freeze-thaw treaonent. In chilled and thawed lamb, ageing increased drip. Packaging had a lesser effect on drip. 131 Meat Science 0309-1740/91/$03-50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
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V. J. Moore, O. A. Young
INTRODUCTION For the New Zealand meat industry to successfully market thawed lamb cuts in the international supermarket trade, it is necessary to know how processing variables, such as electrical stimulation, ageing and packaging during ageing, affect the colour and display life of the thawed meat product. Thawed meat from unaged, unstimulated carcasses had as good a display life as the equivalent chilled product, about three days (Moore, 1990a, b). By contrast, in a preliminary study of stimulated lamb in this laboratory, thawed chops from stimulated carcasses had a very short display life, at most one day, whereas chilled chops had a long display life; seven days if unaged, declining to one day if aged a week at 6°C. The preliminary study also showed that the display life of frozen stimulated chops was shorter than that of frozen unstimulated chops examined in previous work (Moore, 1990c). Collectively, these results suggested that stimulation of lamb may have drawbacks when the meat is frozen. The present study sets out to determine if stimulated lamb can be frozen, thawed, and then held (aged) as a primal cut before successful presentation as a consumer item, in this case chops. Ageing times of up to 6 weeks, at - 1 . 5 ° C , were chosen because vacuum-packed lamb stored at that temperature for that time is well within its storage life (Powell & Eustace, 1981). The effect of packaging materials during ageing (vacuum or oxygenpermeable shrink wrap) was also studied since earlier work (Moore, 1990a) indicated that anaerobically packaged meat held for six weeks had better colour and colour stability than fresh chilled meat. Successful presentation is gauged by display life and this is largely a function of lean meat colour. In the present study lean meat colour was subjectively assessed by a sensory panel to determine display life, and objectively by a Hunter colorimeter.
MATERIALS AND METHODS Meat
Sixteen lamb carcasses were assigned to control (unstimulated) and stimulated groups (8 carcasses each). The latter were electrically stimulated within 30 min of slaughter to the New Zealand AC Specification (90 sec, 1130 V peak, 14.28 pulses/s, Chrystall et al., 1989). Stimulated carcasses were held at 10°C for 5 h before cutting, and control carcasses (unstimulated) were held at 10°C for 24 h, by which time the muscles were in rigor. Long saddles from the 16 carcasses were each cut into six equal portions. Each was systematically assigned to an ageing-packaging combination (see
The colour and display life of lamb chops
133
below), to achieve balance between treatment and position along the loin, since loin position has an effect on colour (Moore & Gill, 1987). Saddle portions were halved through the vertebrae to give a 'chilled' loin and a 'thawed' loin for each ageing-packaging combination. The 'chilled' treatment was as follows. Half the portions were packaged in oxygenpermeable shrink film (see below) and the other half in vacuum bags of low oxygen-permeability. The portions were then aged for 0, 2, 4 and 6 weeks at - 1.5°C. After ageing, drip loss was measured, and 20 m m chops with freshly exposed surfaces were cut from the loin portions. For display, chops were placed directly on white plastic trays and overwrapped with oxygenpermeable film. The 'thawed" treatment was the same as 'chilled' except that the loin portions were wrapped in oxygen-permeable shrink film, frozen and stored at - 2 0 ° C for 1 week, before thawing, subsequent ageing and display. Drip loss was measured after thawing, as well as after ageing and display. Details of freezing and thawing--loins took 14 h to reach - 2 0 ° C and were above 0°C after overnight thawing at 3°C.
Packaging materials The oxygen-permeable shrink film was d-film (W. R. Grace, NZ, Oz permeability > 2000 ml/mZ/24 h/atm at 24°C). The film was shrunk around loin portions and chops with a hot air gun using only enough heat to shrink the film. Vacuum bags were Cryovac barrier bags (W. R. Grace, NZ, O2 permeability 35 ml/mZ/24 h/atm at 24°C).
Display Chops were displayed for 8 h a day at 3°C under Philips 32 Deluxe fluorescent tubes. Chops were assessed by an experienced six member colour panel at the same time each day. The chops were evaluated on a 1 to 5 scale where 1 = poor colour, 2 = fair colour (would buy only if discounted), 3 = good lamb colour, with some browning, 4 = very good lamb colour, 5 = excellent bright red lamb colour. Display life was the number of days the average score for a treatment was 3-0 or higher. When the average panel score for a treatment dropped below 3"0, all chops in that treatment group were removed from display, even though some chops in the group still scored above 3-0.
Colour measurements Hunter L, a, b measurements were made with a Hunter 25D colorimeter immediately after panel evaluation. Hue (a/b) was calculated.
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V.J. Moore, O. A. Young
Drip Percentage drip losses were calculated from the weight losses that occurred after each manipulation; thawing, ageing and display. Total drip was calculated from the individual drip losses.
Statistical analysis Drip data were initially analysed for all treatment effects. However, thawing was a highly significant factor in every drip measurement and formed interactions with all variables. Therefore, a second analysis was carried out in which chilled and thawed chops were analysed separately. The second analysis, which used an angular transformation (arc sinx/(proportion of drip)), sharply defined the effects of the other treatments and is the analysis presented here (Tables 1-3). For Hunter colour values, the percent variance of stimulated chops was significantly lower than that of control chops; it was ten times lower for L values (4-8 versus 48), six times for a values, and four times for b values. Differences in variance for panel scores reflected, but did not equal, the variance differences in Hunter colour data. Thus, in this experiment, stimulation created two colour populations. Analysis of Hunter values and panel scores was, therefore, carried out on control and stimulated carcasses as separate populations. In presentation of data, the effect of a given treatment has usually been shown isolated from other treatments. To do this, data have routinely been averaged across the other treatments. Another aspect of presentation is that where ageing times are examined (Tables 2 and 5) significance values simply show whether ageing has a significant effect, without comparing individual values for each ageing time. Where there is a significant effect the trend is nearly always obvious.
RESULTS
Drip During thawing, loins from control (unstimulated) and stimulated carcasses lost a similar a m o u n t of drip (Table 1). During ageing, chilled loins from stimulated carcasses produced slightly less drip than those from control carcasses (p < 0.05); when thawed loins were aged, the reverse was true. Drip loss during display was small for all treatments. Within the chilled and thawed groups, stimulation had no significant effect on total drip loss.
The colour and display life of lamb chops
135
TABLE 1 The Effect of Electrical Stimulation on the Drip Loss (% Weight Lost) of Lamb Loins and Chops during Thawing, Ageing and Display. Drip Loss is Averaged over Packaging and Ageing Treatments
Treatment
Loins
Chops
Thawing
Ageing
Display
Total
---
1-25 0.94 *
0-31 0-33 NS
1-35 1-11 NS
2-24 3-11
0-30 0-37
*
*
3.97 4-54 NS
Chilled Control Stimulated
Thawed Control Stimulated
1-68 1-56 NS
*, p < 0-05. NS, not significantly different.
TABLE
2
The Effect of Ageing Time on the Drip Loss (% Weight Lost) of Chilled and Thawed Loins and Chops. Drip Loss is Averaged over Stimulation and Packaging Treatments
Ageing time
Thawing
Ageing Display
Total
Chilled and aged for 0 2 4 6
weeks weeks weeks weeks
-0.55 1-23 1.65 ***
0.34 0-28 0-29 0-38 **
0.24 0.84 1.51 2-04 ***
Thawed then aged for 0 2 4 6
weeks weeks weeks weeks
2-24 1-68 1"49 1-38 q~S (Mean = 1.69)
-2-28 2-59 3-13 ***
0"63 0"36 0"32 0-21 **
2"84 4.33 4.34 4-68 NS (0 versus 2--6 weeks, ***)
**, p < 0"01; ***, p < 0-001. NS, not significantly different. (Significance values attached to the data simply show whether ageing has a significant effect on drip, without comparing individual values for each ageing time. Where there is a significant effect the trend is nearly always obvious.)
136
V. J. Moore, O. A. Young
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Fig. 1. Percent drip loss from (O) thawed and (©) chilled lamb. (a) Loss during ageing, (b) total loss from rigor to end of display. Data were averaged over other treatments. The drip from stimulated lamb was clearer than that from control lamb, the latter containing more particulate matter in suspension. The n u m b e r o f weeks that loins were aged at - 1.5°C affected the a m o u n t o f drip (Table 2, Fig. la). During the first two weeks o f ageing, the thawed loins had a large drip loss (2.28%) above that measured during the thawing event (1.69%). Beyond two weeks, drip loss in thawed loins parallelled that in chilled loins (Fig. la). The drip loss o f chops on display was affected by loin ageing time, but the effect was different for the chilled and thawed lamb. With chilled chops, drip during display increased with ageing time, and with thawed chops, it decreased (Table 2). The total drip from chilled lamb increased linearly with ageing time, but with thawed lamb, it increased TABLE 3 Effect of Packaging Material (Oxygen-Permeable or Vacuum) on Drip Loss (% Weight Lost) of Chilled and Thawed Loins and Chops. Drip Loss is Averaged over the Stimulation and Ageing Treatments Thawing
Ageing
Display
Total
0-87 1-33 ***
0"30 0-34 NS
1-20 1-67 NS
2.84 2.48
0"31 0.28
*
*
4.57 4-33 NS
Chilled
0 2 permeable Vacuum Thawed
02 permeable Vacuum
1"62 1.62
*, p < 0-05; ***, p < 0-001. NS, not significantly different.
The colour and display life o f lamb chops
137
TABLE 4 Effect of Thawing on Initial Values for Panel, Hunter L, a and b, and Hue of Control and Stimulated Lamb. Values are Averaged over Packaging and Ageing Treatments
Treatment Control Chilled Thawed
Panel scores
L
a
b
Hue
4-0 3"6
28.6 26-0 **
13-6 11-2 **
7-6 5"9 ***
1.82 1'99 NS
27"2 26"3 **
13.7 11-7 ***
7"1 6"3 **
1.97 1-90 NS
*
Stimulated Chilled Thawed
4"5 3"5 ***
*, p < 0-05; **, p < 0-01; ***, p < 0.001. NS, not significantly different.
TABLE 5 Effect o f Ageing Time on Initial Values for Panel, Hunter L, a and b, and Hue. Values are Averaged over Thawing and Packaging Treatments
Ageing time (weeks) Control 0 2 4 6 Stimulated 0 2 4 6
Panel scores
L
a
b
Hue
4-1 3-7 3-9 3-8 NS
25-5 27-7 26-8 27-4 NS
9-4 13-4 12-8 11-0 ***
5-3 7"1 6"7 6'4 NS
1"76 1"94 2-01 1-78 *
4-4 4.2 3-9 3-9 ***
24-6 27"2 26"7 26-3 NS
8-8 13"5 12-3 12-3 ***
4-9 7-4 6"5 6-2 ***
1-80 1-83 1-94 2.04 NS
*, p < 0"05; **, p < 0"01; ***, p < 0"001. NS, not significantly different. (Significance values attached to the data simply show whether ageing has a significant effect on colour, without comparing individual values for each ageing time.)
138
v. J. Moore, O. A. Young
between 0 and 2 weeks, after which there was little increase (Table 2, Fig. lb). The packaging m e t h o d - - o v e r w r a p in oxygen-permeable film or vacuum package in low oxygen-permeable film--influenced drip loss only during ageing (Table 3). For chilled lamb, the drip in the vacuum pack was significantly (p<0.001) greater than in the oxygen-permeable pack. In thawed lamb, the effect was reversed slightly. Within the chilled and thawed groups, packaging material did not affect total drip. Colour panel assessment A s s e s s m e n t on initial display
Analysis of the pooled data (not shown) demonstrated that stimulation significantly (p < 0.05) increased initial panel scores. Although the data presented were analysed as two populations (control, stimulated) the positive effect of stimulation can still be seen in Table 4, column 1. Whether chops came from stimulated or control (unstimulated) carcasses, thawed chops had a lower initial panel score than chilled chops (Table 4). Ageing did not affect the initial panel scores for control chops, but caused a significant decrease in scores for stimulated chops (Table 5). For both control and stimulated chops, vacuum packaging during ageing resulted in significantly higher initial panel scores on display (Table 6). A s s e s s m e n t &tring display
As expected, the panel scores declined during display. Figure 2 shows the decline averaged across packaging and ageing, and so illustrates the TABLE 6
Effect of Packaging Materials on Initial Values for Panel, Hunter L, a and b and Hue. Valuesare Averaged over Thawing and Ageing Treatments Panel scores
L
a
b
Hue
3-6 4-0
27.4 27-2 NS
12-2 12-6 NS
6.7 6-8 NS
1.91 1-90 NS
3.8 4-2
26.9 26.5 NS
12.9 12-5 NS
6-7 6.7 NS
1.96 1-91 NS
Control
02-permeable Vacuum Stimulated
O2-permeable Vacuum
***, p < 0-001. NS, not significantlydifferent.
139
The colour and display life o f lamb chops
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Day of display Fig. 2. Change in average panel scores during display of chops cut from loins treated in four ways. (I--1) Stimulated-chilled, (ll) stimulated-thawed, (A) control-chilled and (4,) control-thawed. Data were averaged over ageing and packaging treatments.
stimulation and freeze-thaw effects in isolation. The best treatment was stimulated-chilled and the worst was stimulated-thawed. Table 7 lists the net effect o f the four variables on the display life, defined by the numbers o f days the m e a n panel score exceeded 3"0. The various treatments had dramatic effects on display life.
Objective colour assessment by Hunter values Initial colour values Whether carcasses had been stimulated or not, the chilled lamb chops, when first put on display, were significantly brighter (higher L values), redder TABLE 7 Display Life (in Days) of Chops from Various Stimulation, Thawing, Ageing and Packaging Treatments Packaging and ageing (weeks)
Control
Stimulated
Chilled
Thawed
Chilled
Thawed
0 2
3 4
1 3
? 4
1 1
4 6
3 3
2 2
3 2
1 1
3 1 1 1
1 1 1 1
7 3 1 2
1 1 1 1
Wacuunl
O,.-permeable 0 2 4 6
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(higher a values) and yellower (higher b values) than thawed chops (Table 4). Hue was not significantly different. Ageing did not influence initial L values (Table 5), but a and b values were considerably lower for chops from unaged loins than from any aged loins. Ageing for 2 weeks resulted in the highest a and b values. Values decreased thereafter. Packaging did not affect any of the initial Hunter values of chops on display (Table 6).
Colour values during display Hunter values declined during display. When data were averaged across packaging and ageing, the greatest decline was in Hunter a (Fig. 3), b and L being relatively unaffected by display (data not shown). Hue declined for all treatments (Fig. 4). DISCUSSION
Drip losses Drip loss of raw meat from slaughter to cooking can influence profit at all points of the marketing chain because of weight loss and perceived loss of quality. Stimulation had no effect on total drip loss (Table 1), confirming the finding of Jeremiah and Martin (1982) with beef. A greater drip loss might have been expected because the muscles enter rigor at higher temperatures. In pigs, an extreme form of this causes the pale, soft exudative (PSE) condition, where drip losses are extreme. In the present experiment, where lambs were slaughtered under an author's supervision, there was no evidence of a PSE-type condition, consistent with the results in Table 1. There is no obvious explanation for the observed greater clarity of the drip from stimulated lamb.
The colour and display life of lamb chops
141
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Day of display Fig. 4. Change in Hunter hue values during display. (I-1) Stimulated-chilled, (I) stimulated-thawed, (A) controlled-chilled and (A) control-thawed. Data were averaged over ageing and packaging treatments. Ageing of loins increased drip. The most significant effect was the excessive exudation from thawed meat in the first two weeks of ageing, clearly illustrated in Fig. l(a). It is likely that this loss (2.28%) is a continuation of the loss (1"69%) caused by the freeze-thaw treatment (Table 2). This argument is supported by the higher display loss (0.63%) from thawed chops put on display without ageing (0 weeks) compared with those that were aged (0.36--0.21%). Whatever the subtleties of drip loss during ageing and display, the total drip loss for chilled lamb increased steadily the longer it was aged, but for thawed lamb the effect was obscured by the freeze-thaw treatment (Table 2, Fig. lb). A problem with vacuum packaging is drip loss in the pack (Jeremiah & Jones, 1989), presumably initiated at the time the vacuum is applied. Although researchers have obtained conflicting data on actual, rather than perceived, drip loss during storage (Zarate & Zaritzky, 1985; Weakley et al., 1986), the present data are clear; in Table 3, chilled meat in a vacuum pack lost more drip during ageing than similar meat in oxygen-permeable film (1"33 versus 0.87%). The loss continued to be higher on display, although the difference was not statistically significant. Total drip loss was also slightly higher for vacuum packed meat. Whereas the thawed lamb always suffered more drip loss than chilled lamb (Tables 1-3), vacuum packaging did not exacerbate drip loss from thawed meat (Table 3). (Rather, oxygen-permeable packaging caused a slight increase in drip). The easily removed water may already have been lost during thawing. Effect of stimulation on variability of colour data In contrast to the drip data, the colour data were analysed separately for control and stimulated carcasses because stimulation created a population
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V. J. Moore, O. A. Young
whose colour was much more uniform (variability was much lower for all colour values-except hue which was unaffected--and panel scores). Ledward et al. (1986) similarly found that the variability of L, a, b values for fresh beef was lower for stimulated than for control samples. They further found that on ageing, the differences in variability converged, so finally little difference in variability remained. Reduced variability due to stimulation could in itself be a processing bonus, giving meat of more consistent colour. Colour
Stimulation resulted in slightly higher initial panel scores (see Table 4), confirming the findings of Miller et aL (1987) for beef and one study of two by Riley et aL (1980a, b) for lamb. However, any colour advantage conferred by stimulation can be very short-lived, depending on subsequent treatment of the meat. The freeze-thaw treatment adversely affected the colour of lamb (Table 4), but it was on stimulated lamb where this effect was worse, resulting in lower panel scores initially and a much reduced display life (Table 7). The display life data for stimulated thawed lamb examined in the preliminary study (Introduction) have, therefore, been corroborated by the data in Table 7. Thawed-stimulated chops had a display life of one day (Table 7). The question naturally arises as to why electrical stimulation has such undesirable effects on the colour of lamb after it has been fro7en and thawed. There is no doubt that electrical stimulation can cause tissue damage (George et aL, 1980; Sorinmade et al., 1982), and this might be made worse by freezing and thawing. However, this explanation would not explain the long display life of chilled stimulated lamb (Table 7). Without ageing, chilled stimulated lamb had a display life of seven days, a finding also obtained by Renerre and Dantchau (1987). Miller et al. (1987) suggested that the (initial) brighter colour of stimulated beef may be due to a looser structure of the muscle fibres, allowing for deeper penetration of oxygen and thus higher oxymyoglobin levels. However, this would not explain the longer display life of the chilled stimulated meat, for if more oxymyoglobin were formed, one would expect a greater potential for subsequent metmyoglobin formation and so a shorter display life. Ageing also had an adverse effect on the initial panel score of stimulated lamb (Table 5). While thawed stimulated chops had a very short display life, regardless of ageing (Table 7), ageing of their chilled equivalents also took a toll on display life. After two weeks ageing in vacuum the display life was about the same as if the meat were unstimulated, and continued to decline. Not only did the packaging used during ageing affect initial panel scores
The colour and display life of lamb chops
143
(Table 6), it also affected display life (Table 7). Together, the data show that if meat is aged in a vacuum-pack rather than oxygen-permeable film, the display life is better. The major effect was on control (unstimulated) lamb. For example, display life was dramatically reduced when chilled control chops were aged in oxygen-permeable film, being only one day rather than three. Data in the first column of Table 7 (control-chilled) indicate that packaging used during ageing was a more important factor than ageing time on maintaining colour stability during display. In Table 7 oxygen-permeable data, the greater colour stability of stimulated-chilled chops compared with their control equivalents is a puzzle. No benefit of electrical stimulation on controlling microbial growth has been demonstrated (Gill, 1980; Mrigadat et al., 1980; Nortje et al., 1986). It is possible that the better colour stability of electrically stimulated meat reflects the shorter post-mortem holding time before packaging (see Materials and Methods); microorganisms would have had less time to colonise the meat. Although stimulation had little effect on initial Hunter colour values, the freeze-thaw treatment significantly affected them (Table 4); all values were lower except that of hue. Low Hunter values reflected the lower panel scores for thawed chops. When the data were analysed to test the overall effect of ageing (Table 5), any ageing at all caused a decrease in initial panel score. But in contrast to the data in Table 4, where high panel scores were reflected in high Hunter values, chops from any aged loins had lower panel scores but higher Hunter values. This was true whether carcasses were stimulated or not. These differences cast doubt on the value of initial Hunter colour values in predicting initial colour scores. Packaging materials likewise significantly influenced panel scores, but not Hunter values (Table 6). Although initial Hunter colour values did not appear to be good predictors of initial panel score, Hunter a and hue values showed clear trends • during the display. Moreover, Hunter a values especially, declined in concert with panel scores (Figs 2 and 3). Restated, redness declined with panel scores. Consumers select red meat on the basis of redness, with the belief that if it is red it is safe to eat, and that if not red, is of dubious quality. This study illustrated the fallacy of that belief; when the chilled loins in this study had been aged for four weeks in oxygen-permeable film, many had developed a mould on the cut surfaces of lean tissue, and off odours were detectable. By six weeks, the loins were obviously spoiled. Yet when the end chop was discarded, and a chop with a fresh surface was cut and displayed, the newlycut surface bloomed well. The off odour could not be detected through the display film. The only factor which detracted from the appearance was a barely visible discolouration of the membrane on the outside of the chop,
144
v. J. Moore, O. A. Young
and a slight yellowing of the fat. It might be argued that such clearly spoiled chops should not have been used for panel assessment, but their assessment preserved the balance in the experiment. The results clearly show that selection o f meat on the basis of colour is not a very valid method o f selection. Nevertheless old habits die hard, and the consumer will undoubtedly continue to use colour as his guide to selection, and scientists will try to preserve the red colour for as long as possible. The main findings about display life in this experiment (Table 7) can be summarised as follows. The display life o f stimulated-chilled lamb during display was better than that o f unstimulated (control) lamb. This advantage disappeared after 2 weeks ageing in a vacuum pack at - 1-5°C. Packaging loins in oxygen-permeable film during ageing greatly reduced the display life o f the unstimulated chops, compared with vacuum packaging. The display life o f thawed control lamb was only slightly inferior to the chilled equivalent, when both were vacuum packed. The display life o f chops from thawed stimulated loins, one day for all treatments, was very poor.
A C K N O W L E D G E M ENTS The authors wish to thank M I R I N Z staffwho served on colour panels, and D r John Waller, R u a k u r a Agricultural Research Centre, for carrying out the statistical analysis.
REFERENCES Chrystall, B. B., Devine, C. E., Longdill, G. R., Gill, C. O., Swan, J. E. & Peterson, G. V. (1989). In Meat Production and Processing, ed. R. W. Purchas, B. W. Butler-Hogg & A. S. Davies. N.Z. Soc. Anim. Prod. Occas. Publ. No. 11, Hamilton, New Zealand. George, A. R., Bendall, J. R. & Jones R. C. D. (1980). Meat Sci., 4, 51. Gill, C. O. (1980). J. Food Protect., 43, 190. Jeremiah, L. E. & Jones, S. D. M. (1989). J. Food Protect., 52, 473. Jeremiah, L. E. & Martin, A. H. (1982). J. Food QuaL, 4, 175. Ledward, D. A., Dickinson, R. F., Powell, V. H. & Shorthose, W. R. (1986). Meat Sci., 16, 245. Miller, M. F., Cross, H. R., Buyck, M. J. & Crouse, J. D. (1987). Meat Sci., 19, 253. Moore, V. J. (1990a). Meat Sci., 28, 9. Moore, V. J. (1990b). Int. J. Food Sci. & Technol., 25. Moore, V. J. (1990c). Meat Sci., 27, 91. Moore, V. J. & Gill, C. O. (1987). N . Z . J . Agr. Res., 30, 449. Mrigadat, B., Smith, G. C., Dutson, T. R., Hall, L. C., Hanna, M. O. & Vanderzant, C. (1980). J. Food Protect., 43, 686.
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Nortje, G. L., Naumann. H. D., Naude, R. T., Oosthuizen, W., Jordaan, E. & Grobler, I. (1986). J. Food Sci., 51, 12. Powell, V. H. & Eustace, I. J. (1981). Chem. Australia, 48, 351. Renerre, M. & Dantchau. S. (1987). Sciences des aliments, 7, 535. Riley, R. R., Savell, J. W., Smith, G. C. & Shelton, M. (1980a). J. FoodSci., ,15, 119. Riley, R. R., Savell, J. W. & Smith, G. C. (1980b). J. FoodSci., 45, 1101. Sorinmade, S. O., Cross, H. R., Ono, K. & Wergin, W. P. (1982). Meat Sci., 6, 71. Weakley, D. F., McKeith, F. K., Bechtel, P. J,, Martin, S. E. & Thomas, D. L. (1986). J. Food Sci., 51,281. Zarate, J. R. & Zaritzky, N. E. (1985). J. Food Sci., 50, 155.