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Anoxic storage of fresh beef. 2: Colour stability and weight loss
Meat Science 5
(1980-81) 267-281
ANOXIC STORAGE OF FRESH BEEF. 2: COLOUR STABILITY A N D WEIGHT LOSSt
M. O'KEEFFE~& D. E. HOOD
Meat Research Department, The Agricultural Institute, Dunsinea Research Centre, Castleknock, Co. Dublin, Ireland (Received: 3 March, 1980)
SUMMARY Storage variables, such as age of meat, temperature and humidity of storage, and commercial preparation of meat, were evaluated for their effects on the colour stability and weight loss of fresh beef packaged in anoxic gas atmospheres. Apart from age of meat post mortem, increasing storage temperature has the most marked effect on reducing'colour stability and increasing weight loss. Temperature effects on colour stability and weight loss for individual muscles have interesting similarities; the extent of the temperature effect on both these qualities of meat is M. psoas major > M. gluteus medius > M. semimembranosus. The significance of such similarities in terms of differing physical and biochemical characteristics of muscles is discussed.
INTRODUCTION
In a previous paper (O'Keeffe & H o o d , 1980-81) a system for the anoxic storage o f fresh beef was described. This packaging system utilises a catalytic oxygenscavenging mechanism to prevent deterioration o f meat colour during extended storage in nitrogen and/o-r carbon dioxide. The commercial applicability of such a system is dependent largely on the colour stability---determining the shelf-life subsequent to anoxic s t o r a g e - - a n d the water-holding capacity o f the meat. The effect of temperature on meat colour stability is both significant and complex. "~Part 1 of this paper appeared in Vol. 5, No. 1, 1980-81, pp. 27-39. :~Note: Now in the Analytical Services Department, Dunsinea Research Centre. 267 Meat Science 0309-1740/81/0005-0267/$02-50 © Applied Science Publishers Ltd, England, 1981 Printed in Great Britain
268
M. O'KEEFFE, D. E. HOOD
Maximal oxidation ofmyoglobin to metmyoglobin occurs closer to the meat surface with increase in temperature due to the consequent decrease in oxygen solubility (Brooks, 1929; Urbin & Wilson, 1958; Morley, 1971), increase in oxygen-utilising systems in the meat (Urbin & Wilson, 1961; Snyder, 1964; Bendall, 1972) and occurrence of the maximalpO 2 for myoglobin oxidation at a point closer to the meat surface (Ledward, 1970). Increase in storage temperature causes an acceleration in the rate of autoxidation of oxymyoglobin (Brooks, 1931 ; George & Stratmann, 1952; Snyder & Ayres, 1961 ; Brown & Mebine, 1969). The enhanced dissociation of oxygen from its reversible combination with myoglobin, which occurs with increase in temperature, leads to increased autoxidation of the deoxygenated myoglobin produced (Waiters, 1974). Numerous workers have reported that increase in temperature accelerates the rate of discoloration of meat exposed to oxygen (Van den Oord & Wesdorp, 1971; Taylor, 1973; Hood, 1980). However, the effect of temperature on meat colour stability is complicated by the observation that reduction of metmyoglobin increases with temperature (Cutaia & Ordal, 1964, Stewart, et al., 1965). The extent to which temperature influences the colour stability of meat in anoxic storage is investigated. Weight loss, due to evaporation from the exposed surface of meat, increases with temperature (Stringer & Naumann, 1966). The effect of temperature on weight loss from meat stored in anoxic gas atmospheres is investigated. Other aspects included in this work are the effects of age of meat post mortem, length of storage period, humidity and methods of meat preparation for packaging, on the colour stability and water-holding capacity of meat in anoxic storage.
MATERIALS AND METHODS
A description of animals used, preparation of meat, gas-packaging system, colour measurement and statistical analysis has been given in a previous paper (O'Keeffe & Hood, 1980-81). For the experiments described in this paper meat from a total of forty-five heifers was used. In addition to the muscles M. psoas major, M. gluteus medius and M. semimembranosus, the muscles M. bicepsfemoris (from primal cut, outside round) and M. longissimus dorsi (from primal cut, short sirloin) were used in some of the experiments. Commercial meat Meat from a commercial source (an export meat packaging plant) was from heifers 1½- to 2-years-old and in the weight range 300 to 400 kg liveweight, slaughtered on the line according to normal factory procedures. The carcases were aged in the factory chill for.3 to 4 clays (to a deep round muscle temperature of about 5.5 °C) and then broken into primal cuts which were vacuum-packaged and stored at 0 °C. The
ANOXIC STORAGE OF FRESH BEEF: PART
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vacuum-packaged meat was transported to Dunsinea 5 days post mortem and held at 0 °C for an additional 2 days before further processing.
Storage temperature Meat packages used for storage temperature studies were stored either: (1)
in cabinet type deep-freezers, modified for accurate temperature control in the - 5 °C to + 15 °C range (temperature fluctuation of ___1 °C)
or:
(2)
in a 'Fisons' Environmental Cabinet, cooled by an indirect refrigeration system using phenol as coolant, with a temperature range of - 5 ° C to + 30 °C (temperature fluctuation of _+0.25 °C).
Equal temperature was achieved throughout the storage equipment with circulation of the air by fans.
Weight loss The loss in weight of meat samples during storage in gas atmospheres and under vacuum was determined b'y weighing before and after storage. This is expressed as a percentage of the initial weight of the meat. Different groups of animals (four per group) were used to study weight loss during storage in nitrogen and in carbon dioxide. p H of meat A 10-g sample of meat, composed of pieces representative of all parts of the muscle(s), was placed in 100 ml of distilled water and homogenised in a Polytron homogeniser for 30 s. The pH of the homogenate was measured on a Radiometer 22 pH-meter, with expanded scale, at 20°C. Humidity The relative humidity of the inert gas atmospheres, in which trays of meat samples were stored, was controlled by placing relative humidity determinants in a series of desiccators. These relative humidity determinants--silica gel, saturated salt solutions and water--gave relative humidities as shown below (O'Brien, 1948; Young, 1967; Hickman, 1970). Relative Humidi O, (%) at 5°C 0 14 34 76 100
Relative Humidity Determinant Silica gel LiC12.2HzO ] Saturated MgC1 z. 6 H 2 0 ~" salt NaC1 J solution Water
270
O ' K E E F F E , D . E, H O O D RESULTS
Colour stability Age ofmeatpost mortem: The colour stability of fresh meat depends on the age of the meat post mortem. Meat aged for 3-4 weeks has a shorter shelf-life thanlmeat aged for 1 week (normal ageing period) (Table 1). The shelf-life is significantly less TABLE 1 EFFECT OF AGE OF MEAT POST MORTEM ON THE SHELF-LIFE* OF THREE BEEF MUSCLES, DETERMINED DURING STORAGE IN AIR AT 5 o f . N = 5
Muscle
1
Age of meat post mortem (weeks) 3 4 2 weeks vacuum
2 weeks nitrogen
3 weeks vacuum
3 weeks nitrogen
0-89" 4.51 ab 5"63 b
1 "54°b 4'21 b 5-54 ~
1-04 bc 3.61 b 5-63 b
1.18 bc 3'69 b 5-61 b
(ooc)
M. psoas major M. gluteus medius M. semimembranosus
1 "89a 5-33" 8"68"
(ooc)
(ooc)
SE of Significance means of difference ( F test)
(ooc)
0.14t 0-300 0.586
P < 0'001 P < 0"01 P < 0.01
* S h e l f - l i f e - - T i m e ( d a y s ) t o a c c u m u l a t i o n o f 20 ~o m e t m y o g l o b i n , d e t e r m i n e d as A K / S s v 2 / K / S 5 2 s o f 0.16, at meat surface. ~,b,c M e a n s o n t h e s a m e line n o t b e a r i n g c o m m o n s u p e r s c r i p t s differ s i g n i f i c a n t l y ( P < 0-05).
(P < 0.05) for all muscle types aged for 4 weeks compared with meat aged for the normal period of 1 week. Except in the case of M. psoas major meat, aged for 3 weeks, there are no significant" differences in shelf-life due to differing ageing conditions (vacuum or nitrogen) for any muscles for any ageing period. The absolute decrease in shelf-life with age of meat post mortem is dependent on muscle type, viz. the percentage decrease in shelf-life, subsequent to ageing, is generally similar for all muscle types. M. psoas major, M. gluteus medius and M. semimembranosus show decreases in shelf-life of 41 ~o, 32 ~ and 35 ~o, respectively, after ageing for 4 weeks compared with similar meat aged for 1 week. Meat aged for periods shorter than 1 week demonstrates greater colour instability than meat aged for this normal ageing period. The shelf-life of meat aged for only 3 days is significantly shorter (P < 0-05) than that of meat aged for 7 days (Table 2). This effect is recorded both for meat of good colour stability (M. longissimus dorsi) and for meat of poorer colour stability (M. bicepsfemoris). When the meat sampled 3 days post mortem is stored in an anaerobic environment (nitrogen) for 1 week (i.e. 10 days post mortem), its shelf-life is not significantly different from that recorded for meat sampled 7 days post mortem and stored in an anaerobic environment for an additional week (i.e. 14 days post mortem). The observed colour instability of meat aged for 3 days does not persist, therefore, when the meat is held under anaerobic conditions for the normal ageing period. Storage temperature: Storage temperature is of critical importance in
2
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TABLE 2 EFFECT OF AGE OF MEAT POST MORTEM ON THE SHELF-LIVE* OF TWO BEEF MUSCLES, DETERMINED DURING STORAGE IN AIR AT 5 °C. N = 3
Muscle M. biceps femoris
Age of meat post mortem 3 7 10 (N 2 1 week) 14 (N 2 1 week)
M. longissimus dorsi
Mean
SE
t test significance
Mean
SE
t test significance
2.83 4.07 3-73 3-77
0.293 0.493 0.682 0-812
P < 0.05
3.43 5-79 5-94 6.03
0.487 0-400 0-222 0-409
P < 0.05
NS
NS
* Shelf-life--Time (days) to accumulation of 2 0 ~ metmyoglobin, determined as AK/Ss72/K/S525 of 0-16, at meat surface.
determining the colour stability o f fresh meat. Substantial differences in colour stability are recorded for meat stored in air at different temperatures (Table 3). There is a significant increase (P < 0.001) in AK/S572/K/Ss2s, representing a m o r e rapid discoloration of meat, with increase in storage temperature from - 1 °C to + 5 °C. The sensitivity 0f temperature control also has an effect on colour stability (Table 3). A slower rate of discoloration is recorded for meat stored in an accurately controlled ( _+0.25 °C) cabinet than for meat stored in a cabinet with temperature fluctuation o f ___1 °C, when both cabinets are set at + 5 °C. TABLE 3 EFFECT OF TEMPERATURE ON COLOUR STABILITY OF THREE BEEF MUSCLES, EXPRESSED AS AK/Ss72/K/Ss25 DURING 4 DAYS" STORAGE IN AIR. N = 3
- l (+_I°C) M. psoas major M. gluteus medius M. semimembranosus
0.20 0-10 0-05
Storage temperature (°C) +2 +5 +5 .(+__I°C) (+_I°C) (+_0.25°C) 0"26 0.11 0.08
0-39 0.19 0-15
0.37 0.18 0.12
SE of means
Significance of difference (F test)
0-014 0"007 0-008
P < 0-001 P < 0'001 P < 0.001
Since the temperature at which meat is held in air affects colour stability, the storage temperature for meat packaged in oxygen-free gas atmospheres might be expected to affect the subsequent rate of discoloration of the meat in air. While a decrease in shelf-life with increase in storage temperature is observed for all muscles, this decrease is significant (P < 0-05) only for M. gluteus medius (Table 4). The effect of temperature on colour stability appears to be related to the intrinsic colour stability properties of the muscles; the increase in storage temperature from 0 °C to 5 °C causes a mean reduction in shelf-life o f 1-16 days (42 9/0) for M. psoas major, o f 1"05 days ( 2 2 ~ ) for M. gluteus medius and of 0-6 days (11 ~o) for M.
semimembranosus.
272
M. O'KEEFFE, D. E. HOOD TABLE 4
EFFECT OF STORAGE TEMPERATURE FOR THREE BEEF MUSCLES, IN NITROGEN ATMOSPHERE FOR 2 WEEKS, ON SUBSEQUENT SHELF-LIFE,* DETERMINED DURING STORAGE IN AIR AT 5 ° C . N = 8
Muscle
M. psoas major M. gluteus medius M. semimembranosus
Storage temperature ( °C) 0 5
SE of means
2-76 4.81 5-55
0-508 0.269 0-281
1-60 3.76 4.95
Significance of difference (F test)
NS P < 0-05 NS
* Shelf-life--Time (days) to accumulation of 20 ~o metmyoglobin, determined as AK/S572/K/S~25 of 0-16, at meat surface. NS = Not significant. I n c r e a s e in the age o f m e a t , as d e t e r m i n e d b y the l e n g t h o f the s t o r a g e p e r i o d in an o x y g e n - f r e e a t m o s p h e r e , a n d in t h e s t o r a g e t e m p e r a t u r e , have a c o m b i n e d effect o n c o l o u r s t a b i l i t y ( T a b l e 5). I n c r e a s i n g the length o f the s t o r a g e p e r i o d f r o m 2 to 4 w e e k s a n d the s t o r a g e t e m p e r a t u r e f r o m 0 °C to 5 °C a n d 10 °C causes a d e c r e a s e in c o l o u r s t a b i l i t y for b o t h muscles. T h e decreases in shelf-life a r e n o t s t a t i s t i c a l l y significant for M . gluteus medius a n d it s h o u l d be n o t e d t h a t the effect o f s t o r a g e p e r i o d is g r e a t e r , b o t h in t e r m s o f a b s o l u t e d e c r e a s e in shelf-life a n d as a p e r c e n t a g e , for M . semirnembranosus t h a n for M . gluteus medius at all s t o r a g e t e m p e r a t u r e s . A t s t o r a g e t e m p e r a t u r e s o f 0 °C, 5 °C a n d 10 °C the d e c r e a s e s in shelf-life after 4 weeks' s t o r a g e , c o m p a r e d with 2 weeks' s t o r a g e , a r e 0.61 (14 ~ ) , 0.05 (2 %) a n d 0-79 (26 %), respectively, for M . gluteus medius a n d 1.16 ( 1 6 % ) , 2.20 ( 3 4 % ) a n d 2.17 ( 4 4 % ) , respectively f o r M . semimembranosus. Source o f meat: T h e effects o f age o f m e a t a n d s t o r a g e t e m p e r a t u r e on the c o l o u r s t a b i l i t y o f c o m m e r c i a l m e a t f r o m a n e x p o r t m e a t p a c k a g i n g p l a n t were d e t e r m i n e d TABLE 5 EFFECT OF STORAGE PERIOD AND STORAGE TEMPERATURE FOR TWO BEEF MUSCLES, IN NITROGEN ATMOSPHERE, ON SUBSEQUENT SHELF-LIFE,* DETERMINED DURING STORAGE IN AIR AT 5 ° C . N = 2
Muscle
M. gluteus medius M. semimembranosus
Storage temperature (°C)
Storage period (Weeks) 2 4
0 5 10 0 5
4.28 3.41 3-05 7"34° 6-40°b
3.67 3-36 2-26 6" 18°b 4"20 cd
I0
5"11 bc
2"94a
SE of means
Significance of difference (F test)
0,557
NS
0"509
P < 0~05
Shelfqife--Time (days) to accumulation of 20 % metmyoglobin, determined as AK/Ss~2/K/S52s of 0"16, on meat surface. ..b,c,aFor M. semimembranosus meat, means not bearing common superscripts differ significantly (P < 0-05). NS = Not significant. *
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A N O X I C S T O R A G E OF FRESH BEEF: P A R T 2
TABLE 6 EFFECT OF STORAGE IN A NITROGEN ATMOSPHERE ON SHELF-LIFE* OF MEAT FROM A COMMERCIAL SOURCE DETERMINED DURING STORAGE IN AIR AT 5 °C. N = 5
Muscle
M. posas major M. gluteus medius
M. semimembranosus
Control
Storage temperature (°C)
2.36 "b
0 5 10
3-40" 1-77 bc 0-93 cd
0 5 10 0 5 10
7-07 a
5.46 Q
Storage period (Weeks) 2 3
SE of means
Significance of difference ( F test)
2-28 b 1 '36 bed 0"68 d
0-370
P < 0.001
4.43 b 3.82 b 2.29 c
4.58 b 2.77 c 1-49 d
0.262
P < 0.001
4"98" 4-40'* 2' 72 ~d
4-89 ° 3.33 bc 1.74 n
0.408
P < 0.001
* S h e l f - l i f e - - T i m e (days) to a c c u m u l a t i o n o f 20 % m e t m y o g l o b i n , d e t e r m i n e d as AK/S572/K/5525 o f 0-16, at m e a t surface. a,b.c,d F o r a single muscle, m e a n v a l u e s not b e a r i n g c o m m o n s u p e r s c r i p t s differ significantly ( P < 0.05).
(Table 6). The shelf-life values recorded, subsequent to storage in oxygen-free gas atmospheres at three storage temperatures and for two storage periods, were compared with the shelf-life values recorded for identical control samples, i.e. commercial meat samples stored only in air at 5 °C. In some respects, the results for this meat differed from those for the experimental meat (cf. Table 1). The better colour stability of M. gluteus medius compared with M. semimembranosus for control samples of the commercial meat contrasts with the colour stability characteristics of these muscles for experimental meat. Whilst all muscles of the experimental meat showed a significant decrease in shelf-life following 3 weeks' storage at 0 °C in an oxygen-free atmosphere, only M. gluteus medius of the commercial meat has a significantly shorter shelf-life after this storage period. Significant differences in colour stability are recorded between temperatures of 0°C and 10°C for all muscles after storage periods of two and three weeks in nitrogen (Table 6). In addition, significant decrease in colour stability is recorded for M. gluteus medius and for M. semimembranosus between storage temperatures of 5 °C and 10 °C after two and three weeks' storage, and significant decrease in colour stability is recorded in both these muscles between temperatures of 0 °C and 5 °C, stored for three weeks. A comparison between the colour stability recorded for commercial meat and meat prepared at Dunsinea after various storage periods in oxygen-free gas atmospheres is shown in Table 7. Although significant differences are recorded in the colour stability of meat from the two sources for particular temperatures, the overall effect of source of meat, taken as the mean o f the shelf-life values for four animals from each source, with three muscles and for three treatments, is not significant. As
274
M. O ' K E E F F E , D. E. H O O D
TABLE 7 EFFECT OF STORAGE IN NITROGEN GAS ATMOSPHERE, AT 0 o f , ON SUBSEQUENTSHELF-LIFE* OF MEAT FROM TWO SOURCES. N = 4 × 2
Muscle
M. posas major M. gluteus medius M. semimembranosus
Source of meat
Dunsinea Commercial Dunsinea Commercial Dunsinea Commercial
Storage period in nitrogen (Weeks) 0 2 3 (Control)
2.93 2.46 5.76 6.82 9.51 5.28
1.60 3.00 6.26 4.36 8.85 4.59
l- 11 2.31 4.05 4.29 6.51 4-77
Analysis of Variance Source of variation Source of meat Muscles Treatments Muscle × source Treatment × source Muscle × treatment
F test significance NS P < 0-001 P < 0.001 P < 0.01 NS NS
* Shelf-life--Time (days) to accumulation of 20 ~ metmyoglobin, determined 0"16, at meat surface. NS = Not significant.
as A K / S s v 2 / K / S 5 2 ~
of
expected, b o t h muscle and treatment effects are highly significant (P < 0.001), and the significance o f the muscles x source term (P < 0.01) confirms that the c o l o u r stability characteristics are different for muscles f r o m the two sources. The shelf-life values recorded f r o m M . psoas m a j o r and M . s e m i m e m b r a n o s u s a c c o u n t for this source effect on muscle c o l o u r stability. M . psoas major, f r o m the commercial source, has comparatively g o o d c o l o u r stability and the shelf-life values recorded after b o t h storage periods in nitrogen are substantially better t h a n the shelf-life values recorded for M . psoas m a j o r f r o m Dunsinea. In contrast with this pattern, on the other hand, the c o l o u r stability o f M . s e m i m e m b r a n o s u s , f r o m Dunsinea, is consistently better than that o f M . s e m i m e m b r a n o s u s f r o m the commercial source. H u m i d i t y o f storage atmosphere: The shelf-life values recorded for meat, subsequent to storage in nitrogen a t m o s p h e r e at different relative humidities for 1 week at 5°C, are n o t dependent on the relative humidity o f the a t m o s p h e r e (Table 8). The very dry a p p e a r a n c e o f the meat samples, on withdrawal f r o m atmospheres o f low relative humidity, is unsatisfactory, but ' b l o o m i n g ' o f the meat is n o t prevented. T h e effects o f extremes o f humidity on meat colour, reported for meat stored in air (Brooks, 1931), d o n o t apply to meat stored in an oxygen-free gas a t m o s p h e r e where the m y o g l o b i n is in the reduced f o r m and n o t subject to oxidation. The relative h u m i d i t y o f the oxygen-free gas a t m o s p h e r e does not affect subsequent c o l o u r stability o f the meat in air. U n d e r n o r m a l p a c k a g i n g conditions, the a t m o s p h e r e in
ANOXIC STORAGE OF FRESH BEEF: PART
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275
TABLE 8 RELATIVE HUMIDITY IN NITROGEN ATMOSPHERE, FOR 1 WEEK AT 5 °C, ON THE SHELF-LIFE* OF TWO BEEF MUSCLES, DETERMINED DURING SUBSEQUENT STORAGE IN AIR AT 5 o f . N = 2
Muscle
Control 0
M. posas major M. semimembranosus
1-06 4.52
1-09 3-86
Relative humidity (%) 14 34 76
0-98 5-18
1-18 4.37
100
1.06 4.35
1-08 3-40
* Shelf-life--Time (days) to accumulation of 20 ~ metmyoglobin, determined as AK/Ss72/K/S525 of 0"16, at meat surface. h i g h l y i m p e r m e a b l e flexible m e a t p a c k a g e s w o u l d have relative h u m i d i t y values close to 1 0 0 K . Weight loss Weight loss in gas atmospheres and in vacuum: S a m p l e s o f m e a t p a c k a g e d in n i t r o g e n o r c a r b o n d i o x i d e have significantly less weight loss d u r i n g s t o r a g e t h a n those p a c k a g e d u n d e r v a c u u m ( T a b l e 9). S a m p l e s o f m e a t , s t o r e d in v a c u u m , lose between 1 1 ~ a n d 22 ~ o f weight ( m e a n values), in the f o r m o f d r i p , d u r i n g a s t o r a g e p e r i o d o f 1 week at 0 °C. In c o n t r a s t , s a m p l e s o f m e a t , s t o r e d in n i t r o g e n o r c a r b o n d i o x i d e a t m o s p h e r e s , lose o n l y 2 ~ to 7 ~ o f weight ( m e a n values) over the s a m e s t o r a g e period. T h e weight loss r e c o r d e d for the t r e a t m e n t s varies with muscle. T h e weight loss r e c o r d e d for M . psoas major, s t o r e d u n d e r v a c u u m , is significantly g r e a t e r t h a n t h a t r e c o r d e d for M . s e m i m e m b r a n o s u s u n d e r v a c u u m . H o w e v e r , with s t o r a g e in n i t r o g e n , the'weight loss r e c o r d e d for M . psoas major is significantly less t h a n t h a t r e c o r d e d for M . s e m i m e m b r a n o s u s u n d e r n i t r o g e n . F o r the c o m p a r i s o n between m e a t s t o r e d u n d e r v a c u u m a n d in c a r b o n d i o x i d e , a similar p a t t e r n is observed. T h e greatest weight loss u n d e r v a c u u m is r e c o r d e d for M . psoas major, being j u s t less t h a n significantly different, at the P < 0.05 level, f r o m
TABLE 9 WEIGHT LOSS (PERCENTAGE OF INITIAL WEIGHT) FROM THREE BEEF MUSCLES WlTH STORAGE UNDER VACUUM AND IN NITROGEN AND CARBON DIOXIDE GAS ATMOSPHERES FOR l WEEK AT 0 °C. N ~-- 4
Treatment
(i) Vacuum Nitrogen (ii) Vacuum
M. psoas t test M. gluteus t test M. semit test major significance medius significancemembranosus significance Mean (SE) Mean (SE) Mean (SE) 18-3a (1-20) 2.0" (0.24) 21.4a (3-45)
12.2"b (2-31) P < 0.001
3-1b (0.27) 16-0" (2-56)
P<0.01 Carbon dioxide
5.9° (1-80)
11-3~ (0-60) P < 0-01
3-7b (0-45) 13.3° (1-38)
P<0-05 6.7a (2-36)
P < 0.001 P<0.01
5-4° (1-49)
a,bMeans on the same line not bearing common superscripts differ significantly (P < 0,05).
276
M. O'I,ZEEFrE, D. E. HOOD
,i 1
! 2
Storage period in N 2 (weeks) Fig, 1.
Effect of length of storage period in nitrogen on weight loss from four bovine muscles, (N = 6).
~ - - M . biceps femoris; I - - M . psoas major; A - - M , gluteus medius; O--M. semimembranosus. a,b,c: Values not bearing common superscripts differ significantly (P < 0.05).
that recorded for M. semimembranosus. The weight losses recorded for these three muscles on storage in carbon dioxide are not significantly different. Length of storage period: Weight loss from meat in gas atmosphere storage increases with the length of the storage period (Fig. 1). M. bicepsfemoris has less weight loss after either one or two weeks' storage in nitrogen than the other muscles. After both storage periods in nitrogen, the weight loss recorded for M. semimembranosus is significantly greater than the weight loss recorded for M. psoas major or M. biceps femoris. M. gluteus medius, with an increase in weight loss of only 0.5 ~o (15 % increase) between one and two weeks' storage, is affected less than the other muscles by longer storage periods; the increases in weight loss recorded for M, psoas major, M. biceps femoris and M. semimembranosus, between one and two weeks' storage in nitrogen atmosphere, are 0-9 % (43 % increase), 0.7% (39% increase) and 1.5 ~ (42 % increase), respectively. The weight losses recorded after a two-week storage period are significantly greater than the weight losses recorded after one week's storage for M. psoas major and M. semimembranosus.
A N O X I C S T O R A G E OF F R E S H B E E F : P A R T 2
277
d cd
bc/ q~
m m 0 n
I
I
I
0
5
10
Temperature (°C) Fig. 2. Effect of storage temperature on weight loss from three bovine muscles during storage in nitrogen for 2 weeks. (N = 5). • - - M. psoas major; • - - M. gluteus medius; • - - M . semimembranosus. a,b,c: Values not bearing common superscripts differ significantly (P < 0-05).
Temperature o f storage: The amount o f ' d r i p ' which exudes from meat during storage in a gas atmosphere is greatly influenced by the storage temperature (Fig. 2). An increase in storage temperature from 0 °C to 10 °C causes a significant increase in weight loss from all three muscles during storage in nitrogen for 2 weeks. At 0 °C the mean weight loss recorded for M. psoas major is significantly less than that recorded for M. semimembranosus, and at 5 °C the weight loss recorded for M. psoas major is significantly less than the weight losses recorded for M. gluteus medius and M. semimembranosus. However, there are no significant differences between the weight losses recorded for the three muscles when stored at 10 °C. Weight loss increases more rapidly with temperature from 5 °C to I 0 °C than from 0 °C to 5 °C. The effect is most marked for M. psoas major: weight losses recorded for this muscle at 10°C are similar to those recorded for the other muscles at this temperature. Source o f meat: The weight losses recorded for two groups o f animals, one from a commercial source and one from Dunsinea, are compared in Table 10. For all
278
M. O ' K E E F F E , D , E. H O O D
T A B L E 10 WEIGHT LOSS (PERCENTAGE OF INITIAL WEIGHT) FROM MEAT FROM TWO SOURCES WITH STORAGE IN NITROGEN AT 0 ° C FOR 2 WEEKS. N = 5 x 2
Muscle
Source
Mean weight loss
SE
t test significance
M. psoas major
Commercial Dunsinea
2-40 3.18
0-21 0.25
P < 0.05
M. gluteus medius
Commercial Dunsinea
3'48 4.54
0.38 0-62
NS
M. semimembranosus
Commercial Dunsinea
4.16 4-76
0.35 0-51
NS
N S = N o t significant.
muscles studied, the weight losses recorded for meat from the commercial source are lower than those recorded for meat from Dunsinea, although the differences between the means are only significant for M. psoas major.
DISCUSSION
Age of meat post mortem Apart from the poor colour stability of very fresh meat, beef aged for about one week has the best colour stability, and meat held in an anaerobic environment for longer has more rapid discoloration when exposed to air. This effect is independent of the intrinsic colour stability of the meat, the decrease in shelf-life, relative to the control meat, being proportionately the same (in most cases) for all muscles. While colour stability decreases with age of meat, this is not an important problem for centralised prepackaging. As has been found already with vacuum-packaged primal cuts of meat, the increased control of the display time in air, which is possible with anaerobically packaged meat, means that a shorter shelf-life is required. Prepackaged retail cuts of fresh meat, stored in oxygen-free gas atmospheres, need to be exposed to air for only a short time before being sold, because no cutting or packaging operations are involved. Even very fresh meat (e.g. three days post mortem) could be centrally prepackaged, as the additional ageing required for good colour stability would take place during storage in the oxygen-free gas atmosphere. Temperature of storage The storage temperature, both while the meat is in an oxygen-free gas atmosphere and during subsequent holding in air, has a marked effect on colour stability. When the storage temperature is raised above - 1 to 0 °C more rapid discoloration of the meat surface occurs. The results of this work show that, during Storage in air, the increased rate of discoloration caused by higher storage temperature is proportionately the same for all muscles, or is greater for muscles of more stable
ANOXIC STORAGE OF FRESH BEEF: PART 2
279
c o l o u r - - - e . g . M , semimembranosus. For meat stored in an oxygen-free gas atmosphere, the effect of temperature on colour stability, determined during subsequent storage in air, is quite different (Table 4). Here, the effect o f increased storage temperature is greater for muscles with intrinsically less stable colour--e.g. M. psoas major. The similar effect of temperature on rate of discoloration and weight loss for each muscle is interesting, M. semimembranosus, which shows the least decrease in colour stability with increase in temperature (Table 4), shows also the least increase in weight loss with increase in temperature (Fig. 2). M. psoas major shows the greatest decrease in colour stability and increase in weight loss with increase in temperature, whilst the effect of temperature on these two properties for M. gluteus medius is intermediate. These results suggest the possibility o f a common temperature effect influencing both colour stability and water-holding capacity.
Weight loss during storage A major problem with any centralised packaging system for retail cuts o f fresh meat is drip. Apart from the weight loss involved, drip has an adverse effect on the appearance and acceptability of the meat. By comparison with vacuum-packaging, the amount of drip recorded for retail cuts of meat stored in an oxygen-free gas atmosphere is considerably less, and, by using a good meat tray in conjunction with a blotter to absorb the drip, this may be kept to an acceptable level. Low temperature storage (0 °C) restricts the amount o f drip to approximately 3 ~o-5 ~ , over a 2-week storage period, depending on the particular muscle. The wide variation in water-holding capacity which occurs between muscles is a reflection of their differing physical and biochemical characteristics. These differences in water-holding capacity occur even when the rate and extent o f p H fall are identical, suggesting that there are different types o f protein present (Lockett et al., 1962). An interesting observation is that the muscle with the greatest tendency to discoloration, M. psoas major, has the least weight loss in an oxygen-free gas atmosphere, but loses the most weight in vacuum (Table 9). M. psoas major is the most tender muscle (Ramsbottom & Strandine, 1948), a property associated with high water-holding capacity and a iloosening ~o f the network of the protein ge! (Hamm, 1974). Under the pressure in a vacuum package, the moisture is easily forced out from this muscle, but in a gaseous environment the loose protein network would allow the meat to retain moisture. By comparison with M. psoas major, M. gluteus medius and M. semimembranosus are less tender and so have lower waterholding capacities which could account both for the greater moisture loss recorded in gas atmosphere storage and the lesser moisture loss recorded in vacuum for these two muscles. Howard et al. (1960) reported that drip from M. longissimus dorsi is generally twice as great as that from M. psoas major, which is a more tender muscle than M. longissimus dorsi. High pH has been found to reduce the amount of'drip' from meat (Lawrie, 1979).
280
M. O'KEEFFE, D. E. HOOD
T A B L E 11 ULTIMATE pH (pHu) FOR MEATFROM TWO SOURCES, COMPARED WITH WEIGHT LOSS(PERCENTAGEOF INITIAL WEIGHT) ON STORAGE OF STEAKS IN NITROGEN AT 0° C FOR 2 WEEKS. N ~ 5 x 2
Muscle
Source
Mean pH,,
SE
Mean weight loss
M. psoas major
Commercial Dunsinea
5.63 5.70
0.022 0-024
2.40 3.18
M. gluteus medius
Commercial Dunsinea
5.46 5.52
0-024 0.011
3.48 4.54
M. semimembranosus
Commercial Dunsinea
5-49 5-56
0.027 0.020
4-16 4.76
In this connection, the effect of carbon dioxide enriched atmospheres on meat is to cause a significant lowering of pH (Ledward, 1970) which results in isoelectric precipitation of some of the sarcoplasmic proteins (Stewart et al., 1965) with consequent reduction in water-holding capacity. Such an effect may explain the considerably greater weight loss from meat stored under carbon dioxide compared with meat stored under nitrogen (Table 9). The consistently higher 'drip' recorded for Dunsinea steaks than for commercial steaks during storage in nitrogen cannot be explained by pH differences. All meat types from the Dunsinea source record slightly higher ultimate pH values (pI-~) than meat from the commercial source (Table 11). When commercial meat was being cut into steaks, it was noticed that it had a 'drier' appearance than Dunsinea meat. The longer holding period of commercial meat in carcass form in the chill--3-4 days compared with 2 days for Dunsinea meat--may have an effect on the level of'drip' recorded subsequently for steaks stored in nitrogen. The higher weight loss from carcass meat than from vacuum-packaged cuts could result in the commercial meat losing more of its 'available drip' during the 7-day ageing period. Commercial meat would then have less tendency to lose 'drip' during gas storage.
ACKNOWLEDGEMENTS
The technical assistance of Mr M. Murray, Mr S. Schwer and Mr J. Dalton is gratefully acknowledged. We thank Mr J. Sherington, Statistics Department, The Agricultural Institute, for the statistical analyses and Ms B. Griffiths for typing the manuscript. Mr P. Doyle, Irish Meat Producers Ltd, is thanked for arranging the provision of commercial meat samples.
REFERENCES BENDALL, J. R. (1972). J. Sci. Fd. Agric., 23, 61-72. BROOKS, J. (1929). Biochem. J., 23, 1391.
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BROOKS, J. (1931). Ann. Rept. Fd. Invest. Bd., 36--41. BROWN, W. D. & MEBINE, L. B. (1969). J. Biol. Chem., 244, 6696. CUTAIA, A. J. & ORDAL, Z. J. (1964). Fd. Technol., 757, 163-6. GEORGE, P. & STRATMANN, C. J. (1952). Biochem. J., 51, 418-25. HAMM, R. (1974). Water-holding capacity of meat. In Meat (Cole, D. J. A. & Lawrie, R. A. (Eds)), Butterworths, London, 321-38. HICKMAN, M. J. (1970). Nat. Phy. Lab. Notes on Appl. Sci, No. 4 (4th edn), 1-33. HOOD, D. E. (1980). Meat Science, 4, 247-65. HOWARD, A., LEE, C. A. & WEBSTER, H. L. (1960). CSIROtDiv. Fd. Pres. Tech. Paper No. 21. LAWRIE, R. A. (1979). Meat science (3rd edn), Pergamon Press, Oxford, 229. LEDWARD, D. A. (1970). J. Fd. Science, 35, 33-6. LOCI~ETT, C., SWIFT, C. E. & SULZBACr~R, W. L. (1962). J. Fd. Sci., 27, 36. MOrtLEe, M. J. (1971). J. Fd. Technol., 6, 371-82~ O'BRtEN, F. E. M. (1948). J. Sci. Instrum., 25, 73-6. O'KEEFFE, M. & HOOD, D. E. (1980-81). Meat Science, 5, 27-39. RAMSBOTTOM, J. M. & STRAr~DINE, E. J. (1948). Fd. Res., 13, 315. SNYDER, H. E. (1964). J. Fd. Sci., 29, 535-9. SNYDER, H. E. & AYR~, J. C. (1961). J. Fd, Sci., 26, 469-74. STEWART, M. R., HUTCm~S, B., ZIPSER, M. & WATTS, B. (1965). J. Fd. Sci., 30, 487-91. STRINGER, W. C. & NAUMANN, H. D . (1966). Fd. Technol., 20, 108-10. TAYLOR, A. A. (1973). Inst. Meat Bull., 79, 26-32. URBIN, M. C. & WILSON, G. D. (1958). Proe. lOth Meat Ind. Conf. A M I F , 13-20. URBIN, M. C. & WILSON, G. D. (1961). J. Fd. Sci., 26, 314-17. VAN DEN OORD, A. H. A. & WESDORP, J. J. (1971). J. Fd. TeehnoL, 6, 15-20. WALTEaS, C. L. (1974). Meat colour: The importance of haem chemistry. In Meat (Cole, D. J. A. & Lawrie, R. A. (Eds)), Butterworths, London, 385-401. YOUNG, J. F. (1967). J. AppL Chem., 17, 241-4.
(1980-81) 267-281
ANOXIC STORAGE OF FRESH BEEF. 2: COLOUR STABILITY A N D WEIGHT LOSSt
M. O'KEEFFE~& D. E. HOOD
Meat Research Department, The Agricultural Institute, Dunsinea Research Centre, Castleknock, Co. Dublin, Ireland (Received: 3 March, 1980)
SUMMARY Storage variables, such as age of meat, temperature and humidity of storage, and commercial preparation of meat, were evaluated for their effects on the colour stability and weight loss of fresh beef packaged in anoxic gas atmospheres. Apart from age of meat post mortem, increasing storage temperature has the most marked effect on reducing'colour stability and increasing weight loss. Temperature effects on colour stability and weight loss for individual muscles have interesting similarities; the extent of the temperature effect on both these qualities of meat is M. psoas major > M. gluteus medius > M. semimembranosus. The significance of such similarities in terms of differing physical and biochemical characteristics of muscles is discussed.
INTRODUCTION
In a previous paper (O'Keeffe & H o o d , 1980-81) a system for the anoxic storage o f fresh beef was described. This packaging system utilises a catalytic oxygenscavenging mechanism to prevent deterioration o f meat colour during extended storage in nitrogen and/o-r carbon dioxide. The commercial applicability of such a system is dependent largely on the colour stability---determining the shelf-life subsequent to anoxic s t o r a g e - - a n d the water-holding capacity o f the meat. The effect of temperature on meat colour stability is both significant and complex. "~Part 1 of this paper appeared in Vol. 5, No. 1, 1980-81, pp. 27-39. :~Note: Now in the Analytical Services Department, Dunsinea Research Centre. 267 Meat Science 0309-1740/81/0005-0267/$02-50 © Applied Science Publishers Ltd, England, 1981 Printed in Great Britain
268
M. O'KEEFFE, D. E. HOOD
Maximal oxidation ofmyoglobin to metmyoglobin occurs closer to the meat surface with increase in temperature due to the consequent decrease in oxygen solubility (Brooks, 1929; Urbin & Wilson, 1958; Morley, 1971), increase in oxygen-utilising systems in the meat (Urbin & Wilson, 1961; Snyder, 1964; Bendall, 1972) and occurrence of the maximalpO 2 for myoglobin oxidation at a point closer to the meat surface (Ledward, 1970). Increase in storage temperature causes an acceleration in the rate of autoxidation of oxymyoglobin (Brooks, 1931 ; George & Stratmann, 1952; Snyder & Ayres, 1961 ; Brown & Mebine, 1969). The enhanced dissociation of oxygen from its reversible combination with myoglobin, which occurs with increase in temperature, leads to increased autoxidation of the deoxygenated myoglobin produced (Waiters, 1974). Numerous workers have reported that increase in temperature accelerates the rate of discoloration of meat exposed to oxygen (Van den Oord & Wesdorp, 1971; Taylor, 1973; Hood, 1980). However, the effect of temperature on meat colour stability is complicated by the observation that reduction of metmyoglobin increases with temperature (Cutaia & Ordal, 1964, Stewart, et al., 1965). The extent to which temperature influences the colour stability of meat in anoxic storage is investigated. Weight loss, due to evaporation from the exposed surface of meat, increases with temperature (Stringer & Naumann, 1966). The effect of temperature on weight loss from meat stored in anoxic gas atmospheres is investigated. Other aspects included in this work are the effects of age of meat post mortem, length of storage period, humidity and methods of meat preparation for packaging, on the colour stability and water-holding capacity of meat in anoxic storage.
MATERIALS AND METHODS
A description of animals used, preparation of meat, gas-packaging system, colour measurement and statistical analysis has been given in a previous paper (O'Keeffe & Hood, 1980-81). For the experiments described in this paper meat from a total of forty-five heifers was used. In addition to the muscles M. psoas major, M. gluteus medius and M. semimembranosus, the muscles M. bicepsfemoris (from primal cut, outside round) and M. longissimus dorsi (from primal cut, short sirloin) were used in some of the experiments. Commercial meat Meat from a commercial source (an export meat packaging plant) was from heifers 1½- to 2-years-old and in the weight range 300 to 400 kg liveweight, slaughtered on the line according to normal factory procedures. The carcases were aged in the factory chill for.3 to 4 clays (to a deep round muscle temperature of about 5.5 °C) and then broken into primal cuts which were vacuum-packaged and stored at 0 °C. The
ANOXIC STORAGE OF FRESH BEEF: PART
2
269
vacuum-packaged meat was transported to Dunsinea 5 days post mortem and held at 0 °C for an additional 2 days before further processing.
Storage temperature Meat packages used for storage temperature studies were stored either: (1)
in cabinet type deep-freezers, modified for accurate temperature control in the - 5 °C to + 15 °C range (temperature fluctuation of ___1 °C)
or:
(2)
in a 'Fisons' Environmental Cabinet, cooled by an indirect refrigeration system using phenol as coolant, with a temperature range of - 5 ° C to + 30 °C (temperature fluctuation of _+0.25 °C).
Equal temperature was achieved throughout the storage equipment with circulation of the air by fans.
Weight loss The loss in weight of meat samples during storage in gas atmospheres and under vacuum was determined b'y weighing before and after storage. This is expressed as a percentage of the initial weight of the meat. Different groups of animals (four per group) were used to study weight loss during storage in nitrogen and in carbon dioxide. p H of meat A 10-g sample of meat, composed of pieces representative of all parts of the muscle(s), was placed in 100 ml of distilled water and homogenised in a Polytron homogeniser for 30 s. The pH of the homogenate was measured on a Radiometer 22 pH-meter, with expanded scale, at 20°C. Humidity The relative humidity of the inert gas atmospheres, in which trays of meat samples were stored, was controlled by placing relative humidity determinants in a series of desiccators. These relative humidity determinants--silica gel, saturated salt solutions and water--gave relative humidities as shown below (O'Brien, 1948; Young, 1967; Hickman, 1970). Relative Humidi O, (%) at 5°C 0 14 34 76 100
Relative Humidity Determinant Silica gel LiC12.2HzO ] Saturated MgC1 z. 6 H 2 0 ~" salt NaC1 J solution Water
270
O ' K E E F F E , D . E, H O O D RESULTS
Colour stability Age ofmeatpost mortem: The colour stability of fresh meat depends on the age of the meat post mortem. Meat aged for 3-4 weeks has a shorter shelf-life thanlmeat aged for 1 week (normal ageing period) (Table 1). The shelf-life is significantly less TABLE 1 EFFECT OF AGE OF MEAT POST MORTEM ON THE SHELF-LIFE* OF THREE BEEF MUSCLES, DETERMINED DURING STORAGE IN AIR AT 5 o f . N = 5
Muscle
1
Age of meat post mortem (weeks) 3 4 2 weeks vacuum
2 weeks nitrogen
3 weeks vacuum
3 weeks nitrogen
0-89" 4.51 ab 5"63 b
1 "54°b 4'21 b 5-54 ~
1-04 bc 3.61 b 5-63 b
1.18 bc 3'69 b 5-61 b
(ooc)
M. psoas major M. gluteus medius M. semimembranosus
1 "89a 5-33" 8"68"
(ooc)
(ooc)
SE of Significance means of difference ( F test)
(ooc)
0.14t 0-300 0.586
P < 0'001 P < 0"01 P < 0.01
* S h e l f - l i f e - - T i m e ( d a y s ) t o a c c u m u l a t i o n o f 20 ~o m e t m y o g l o b i n , d e t e r m i n e d as A K / S s v 2 / K / S 5 2 s o f 0.16, at meat surface. ~,b,c M e a n s o n t h e s a m e line n o t b e a r i n g c o m m o n s u p e r s c r i p t s differ s i g n i f i c a n t l y ( P < 0-05).
(P < 0.05) for all muscle types aged for 4 weeks compared with meat aged for the normal period of 1 week. Except in the case of M. psoas major meat, aged for 3 weeks, there are no significant" differences in shelf-life due to differing ageing conditions (vacuum or nitrogen) for any muscles for any ageing period. The absolute decrease in shelf-life with age of meat post mortem is dependent on muscle type, viz. the percentage decrease in shelf-life, subsequent to ageing, is generally similar for all muscle types. M. psoas major, M. gluteus medius and M. semimembranosus show decreases in shelf-life of 41 ~o, 32 ~ and 35 ~o, respectively, after ageing for 4 weeks compared with similar meat aged for 1 week. Meat aged for periods shorter than 1 week demonstrates greater colour instability than meat aged for this normal ageing period. The shelf-life of meat aged for only 3 days is significantly shorter (P < 0-05) than that of meat aged for 7 days (Table 2). This effect is recorded both for meat of good colour stability (M. longissimus dorsi) and for meat of poorer colour stability (M. bicepsfemoris). When the meat sampled 3 days post mortem is stored in an anaerobic environment (nitrogen) for 1 week (i.e. 10 days post mortem), its shelf-life is not significantly different from that recorded for meat sampled 7 days post mortem and stored in an anaerobic environment for an additional week (i.e. 14 days post mortem). The observed colour instability of meat aged for 3 days does not persist, therefore, when the meat is held under anaerobic conditions for the normal ageing period. Storage temperature: Storage temperature is of critical importance in
2
ANOXIC STORAGE OF FRESH BEEF: PART
271
TABLE 2 EFFECT OF AGE OF MEAT POST MORTEM ON THE SHELF-LIVE* OF TWO BEEF MUSCLES, DETERMINED DURING STORAGE IN AIR AT 5 °C. N = 3
Muscle M. biceps femoris
Age of meat post mortem 3 7 10 (N 2 1 week) 14 (N 2 1 week)
M. longissimus dorsi
Mean
SE
t test significance
Mean
SE
t test significance
2.83 4.07 3-73 3-77
0.293 0.493 0.682 0-812
P < 0.05
3.43 5-79 5-94 6.03
0.487 0-400 0-222 0-409
P < 0.05
NS
NS
* Shelf-life--Time (days) to accumulation of 2 0 ~ metmyoglobin, determined as AK/Ss72/K/S525 of 0-16, at meat surface.
determining the colour stability o f fresh meat. Substantial differences in colour stability are recorded for meat stored in air at different temperatures (Table 3). There is a significant increase (P < 0.001) in AK/S572/K/Ss2s, representing a m o r e rapid discoloration of meat, with increase in storage temperature from - 1 °C to + 5 °C. The sensitivity 0f temperature control also has an effect on colour stability (Table 3). A slower rate of discoloration is recorded for meat stored in an accurately controlled ( _+0.25 °C) cabinet than for meat stored in a cabinet with temperature fluctuation o f ___1 °C, when both cabinets are set at + 5 °C. TABLE 3 EFFECT OF TEMPERATURE ON COLOUR STABILITY OF THREE BEEF MUSCLES, EXPRESSED AS AK/Ss72/K/Ss25 DURING 4 DAYS" STORAGE IN AIR. N = 3
- l (+_I°C) M. psoas major M. gluteus medius M. semimembranosus
0.20 0-10 0-05
Storage temperature (°C) +2 +5 +5 .(+__I°C) (+_I°C) (+_0.25°C) 0"26 0.11 0.08
0-39 0.19 0-15
0.37 0.18 0.12
SE of means
Significance of difference (F test)
0-014 0"007 0-008
P < 0-001 P < 0'001 P < 0.001
Since the temperature at which meat is held in air affects colour stability, the storage temperature for meat packaged in oxygen-free gas atmospheres might be expected to affect the subsequent rate of discoloration of the meat in air. While a decrease in shelf-life with increase in storage temperature is observed for all muscles, this decrease is significant (P < 0-05) only for M. gluteus medius (Table 4). The effect of temperature on colour stability appears to be related to the intrinsic colour stability properties of the muscles; the increase in storage temperature from 0 °C to 5 °C causes a mean reduction in shelf-life o f 1-16 days (42 9/0) for M. psoas major, o f 1"05 days ( 2 2 ~ ) for M. gluteus medius and of 0-6 days (11 ~o) for M.
semimembranosus.
272
M. O'KEEFFE, D. E. HOOD TABLE 4
EFFECT OF STORAGE TEMPERATURE FOR THREE BEEF MUSCLES, IN NITROGEN ATMOSPHERE FOR 2 WEEKS, ON SUBSEQUENT SHELF-LIFE,* DETERMINED DURING STORAGE IN AIR AT 5 ° C . N = 8
Muscle
M. psoas major M. gluteus medius M. semimembranosus
Storage temperature ( °C) 0 5
SE of means
2-76 4.81 5-55
0-508 0.269 0-281
1-60 3.76 4.95
Significance of difference (F test)
NS P < 0-05 NS
* Shelf-life--Time (days) to accumulation of 20 ~o metmyoglobin, determined as AK/S572/K/S~25 of 0-16, at meat surface. NS = Not significant. I n c r e a s e in the age o f m e a t , as d e t e r m i n e d b y the l e n g t h o f the s t o r a g e p e r i o d in an o x y g e n - f r e e a t m o s p h e r e , a n d in t h e s t o r a g e t e m p e r a t u r e , have a c o m b i n e d effect o n c o l o u r s t a b i l i t y ( T a b l e 5). I n c r e a s i n g the length o f the s t o r a g e p e r i o d f r o m 2 to 4 w e e k s a n d the s t o r a g e t e m p e r a t u r e f r o m 0 °C to 5 °C a n d 10 °C causes a d e c r e a s e in c o l o u r s t a b i l i t y for b o t h muscles. T h e decreases in shelf-life a r e n o t s t a t i s t i c a l l y significant for M . gluteus medius a n d it s h o u l d be n o t e d t h a t the effect o f s t o r a g e p e r i o d is g r e a t e r , b o t h in t e r m s o f a b s o l u t e d e c r e a s e in shelf-life a n d as a p e r c e n t a g e , for M . semirnembranosus t h a n for M . gluteus medius at all s t o r a g e t e m p e r a t u r e s . A t s t o r a g e t e m p e r a t u r e s o f 0 °C, 5 °C a n d 10 °C the d e c r e a s e s in shelf-life after 4 weeks' s t o r a g e , c o m p a r e d with 2 weeks' s t o r a g e , a r e 0.61 (14 ~ ) , 0.05 (2 %) a n d 0-79 (26 %), respectively, for M . gluteus medius a n d 1.16 ( 1 6 % ) , 2.20 ( 3 4 % ) a n d 2.17 ( 4 4 % ) , respectively f o r M . semimembranosus. Source o f meat: T h e effects o f age o f m e a t a n d s t o r a g e t e m p e r a t u r e on the c o l o u r s t a b i l i t y o f c o m m e r c i a l m e a t f r o m a n e x p o r t m e a t p a c k a g i n g p l a n t were d e t e r m i n e d TABLE 5 EFFECT OF STORAGE PERIOD AND STORAGE TEMPERATURE FOR TWO BEEF MUSCLES, IN NITROGEN ATMOSPHERE, ON SUBSEQUENT SHELF-LIFE,* DETERMINED DURING STORAGE IN AIR AT 5 ° C . N = 2
Muscle
M. gluteus medius M. semimembranosus
Storage temperature (°C)
Storage period (Weeks) 2 4
0 5 10 0 5
4.28 3.41 3-05 7"34° 6-40°b
3.67 3-36 2-26 6" 18°b 4"20 cd
I0
5"11 bc
2"94a
SE of means
Significance of difference (F test)
0,557
NS
0"509
P < 0~05
Shelfqife--Time (days) to accumulation of 20 % metmyoglobin, determined as AK/Ss~2/K/S52s of 0"16, on meat surface. ..b,c,aFor M. semimembranosus meat, means not bearing common superscripts differ significantly (P < 0-05). NS = Not significant. *
273
A N O X I C S T O R A G E OF FRESH BEEF: P A R T 2
TABLE 6 EFFECT OF STORAGE IN A NITROGEN ATMOSPHERE ON SHELF-LIFE* OF MEAT FROM A COMMERCIAL SOURCE DETERMINED DURING STORAGE IN AIR AT 5 °C. N = 5
Muscle
M. posas major M. gluteus medius
M. semimembranosus
Control
Storage temperature (°C)
2.36 "b
0 5 10
3-40" 1-77 bc 0-93 cd
0 5 10 0 5 10
7-07 a
5.46 Q
Storage period (Weeks) 2 3
SE of means
Significance of difference ( F test)
2-28 b 1 '36 bed 0"68 d
0-370
P < 0.001
4.43 b 3.82 b 2.29 c
4.58 b 2.77 c 1-49 d
0.262
P < 0.001
4"98" 4-40'* 2' 72 ~d
4-89 ° 3.33 bc 1.74 n
0.408
P < 0.001
* S h e l f - l i f e - - T i m e (days) to a c c u m u l a t i o n o f 20 % m e t m y o g l o b i n , d e t e r m i n e d as AK/S572/K/5525 o f 0-16, at m e a t surface. a,b.c,d F o r a single muscle, m e a n v a l u e s not b e a r i n g c o m m o n s u p e r s c r i p t s differ significantly ( P < 0.05).
(Table 6). The shelf-life values recorded, subsequent to storage in oxygen-free gas atmospheres at three storage temperatures and for two storage periods, were compared with the shelf-life values recorded for identical control samples, i.e. commercial meat samples stored only in air at 5 °C. In some respects, the results for this meat differed from those for the experimental meat (cf. Table 1). The better colour stability of M. gluteus medius compared with M. semimembranosus for control samples of the commercial meat contrasts with the colour stability characteristics of these muscles for experimental meat. Whilst all muscles of the experimental meat showed a significant decrease in shelf-life following 3 weeks' storage at 0 °C in an oxygen-free atmosphere, only M. gluteus medius of the commercial meat has a significantly shorter shelf-life after this storage period. Significant differences in colour stability are recorded between temperatures of 0°C and 10°C for all muscles after storage periods of two and three weeks in nitrogen (Table 6). In addition, significant decrease in colour stability is recorded for M. gluteus medius and for M. semimembranosus between storage temperatures of 5 °C and 10 °C after two and three weeks' storage, and significant decrease in colour stability is recorded in both these muscles between temperatures of 0 °C and 5 °C, stored for three weeks. A comparison between the colour stability recorded for commercial meat and meat prepared at Dunsinea after various storage periods in oxygen-free gas atmospheres is shown in Table 7. Although significant differences are recorded in the colour stability of meat from the two sources for particular temperatures, the overall effect of source of meat, taken as the mean o f the shelf-life values for four animals from each source, with three muscles and for three treatments, is not significant. As
274
M. O ' K E E F F E , D. E. H O O D
TABLE 7 EFFECT OF STORAGE IN NITROGEN GAS ATMOSPHERE, AT 0 o f , ON SUBSEQUENTSHELF-LIFE* OF MEAT FROM TWO SOURCES. N = 4 × 2
Muscle
M. posas major M. gluteus medius M. semimembranosus
Source of meat
Dunsinea Commercial Dunsinea Commercial Dunsinea Commercial
Storage period in nitrogen (Weeks) 0 2 3 (Control)
2.93 2.46 5.76 6.82 9.51 5.28
1.60 3.00 6.26 4.36 8.85 4.59
l- 11 2.31 4.05 4.29 6.51 4-77
Analysis of Variance Source of variation Source of meat Muscles Treatments Muscle × source Treatment × source Muscle × treatment
F test significance NS P < 0-001 P < 0.001 P < 0.01 NS NS
* Shelf-life--Time (days) to accumulation of 20 ~ metmyoglobin, determined 0"16, at meat surface. NS = Not significant.
as A K / S s v 2 / K / S 5 2 ~
of
expected, b o t h muscle and treatment effects are highly significant (P < 0.001), and the significance o f the muscles x source term (P < 0.01) confirms that the c o l o u r stability characteristics are different for muscles f r o m the two sources. The shelf-life values recorded f r o m M . psoas m a j o r and M . s e m i m e m b r a n o s u s a c c o u n t for this source effect on muscle c o l o u r stability. M . psoas major, f r o m the commercial source, has comparatively g o o d c o l o u r stability and the shelf-life values recorded after b o t h storage periods in nitrogen are substantially better t h a n the shelf-life values recorded for M . psoas m a j o r f r o m Dunsinea. In contrast with this pattern, on the other hand, the c o l o u r stability o f M . s e m i m e m b r a n o s u s , f r o m Dunsinea, is consistently better than that o f M . s e m i m e m b r a n o s u s f r o m the commercial source. H u m i d i t y o f storage atmosphere: The shelf-life values recorded for meat, subsequent to storage in nitrogen a t m o s p h e r e at different relative humidities for 1 week at 5°C, are n o t dependent on the relative humidity o f the a t m o s p h e r e (Table 8). The very dry a p p e a r a n c e o f the meat samples, on withdrawal f r o m atmospheres o f low relative humidity, is unsatisfactory, but ' b l o o m i n g ' o f the meat is n o t prevented. T h e effects o f extremes o f humidity on meat colour, reported for meat stored in air (Brooks, 1931), d o n o t apply to meat stored in an oxygen-free gas a t m o s p h e r e where the m y o g l o b i n is in the reduced f o r m and n o t subject to oxidation. The relative h u m i d i t y o f the oxygen-free gas a t m o s p h e r e does not affect subsequent c o l o u r stability o f the meat in air. U n d e r n o r m a l p a c k a g i n g conditions, the a t m o s p h e r e in
ANOXIC STORAGE OF FRESH BEEF: PART
2
275
TABLE 8 RELATIVE HUMIDITY IN NITROGEN ATMOSPHERE, FOR 1 WEEK AT 5 °C, ON THE SHELF-LIFE* OF TWO BEEF MUSCLES, DETERMINED DURING SUBSEQUENT STORAGE IN AIR AT 5 o f . N = 2
Muscle
Control 0
M. posas major M. semimembranosus
1-06 4.52
1-09 3-86
Relative humidity (%) 14 34 76
0-98 5-18
1-18 4.37
100
1.06 4.35
1-08 3-40
* Shelf-life--Time (days) to accumulation of 20 ~ metmyoglobin, determined as AK/Ss72/K/S525 of 0"16, at meat surface. h i g h l y i m p e r m e a b l e flexible m e a t p a c k a g e s w o u l d have relative h u m i d i t y values close to 1 0 0 K . Weight loss Weight loss in gas atmospheres and in vacuum: S a m p l e s o f m e a t p a c k a g e d in n i t r o g e n o r c a r b o n d i o x i d e have significantly less weight loss d u r i n g s t o r a g e t h a n those p a c k a g e d u n d e r v a c u u m ( T a b l e 9). S a m p l e s o f m e a t , s t o r e d in v a c u u m , lose between 1 1 ~ a n d 22 ~ o f weight ( m e a n values), in the f o r m o f d r i p , d u r i n g a s t o r a g e p e r i o d o f 1 week at 0 °C. In c o n t r a s t , s a m p l e s o f m e a t , s t o r e d in n i t r o g e n o r c a r b o n d i o x i d e a t m o s p h e r e s , lose o n l y 2 ~ to 7 ~ o f weight ( m e a n values) over the s a m e s t o r a g e period. T h e weight loss r e c o r d e d for the t r e a t m e n t s varies with muscle. T h e weight loss r e c o r d e d for M . psoas major, s t o r e d u n d e r v a c u u m , is significantly g r e a t e r t h a n t h a t r e c o r d e d for M . s e m i m e m b r a n o s u s u n d e r v a c u u m . H o w e v e r , with s t o r a g e in n i t r o g e n , the'weight loss r e c o r d e d for M . psoas major is significantly less t h a n t h a t r e c o r d e d for M . s e m i m e m b r a n o s u s u n d e r n i t r o g e n . F o r the c o m p a r i s o n between m e a t s t o r e d u n d e r v a c u u m a n d in c a r b o n d i o x i d e , a similar p a t t e r n is observed. T h e greatest weight loss u n d e r v a c u u m is r e c o r d e d for M . psoas major, being j u s t less t h a n significantly different, at the P < 0.05 level, f r o m
TABLE 9 WEIGHT LOSS (PERCENTAGE OF INITIAL WEIGHT) FROM THREE BEEF MUSCLES WlTH STORAGE UNDER VACUUM AND IN NITROGEN AND CARBON DIOXIDE GAS ATMOSPHERES FOR l WEEK AT 0 °C. N ~-- 4
Treatment
(i) Vacuum Nitrogen (ii) Vacuum
M. psoas t test M. gluteus t test M. semit test major significance medius significancemembranosus significance Mean (SE) Mean (SE) Mean (SE) 18-3a (1-20) 2.0" (0.24) 21.4a (3-45)
12.2"b (2-31) P < 0.001
3-1b (0.27) 16-0" (2-56)
P<0.01 Carbon dioxide
5.9° (1-80)
11-3~ (0-60) P < 0-01
3-7b (0-45) 13.3° (1-38)
P<0-05 6.7a (2-36)
P < 0.001 P<0.01
5-4° (1-49)
a,bMeans on the same line not bearing common superscripts differ significantly (P < 0,05).
276
M. O'I,ZEEFrE, D. E. HOOD
,i 1
! 2
Storage period in N 2 (weeks) Fig, 1.
Effect of length of storage period in nitrogen on weight loss from four bovine muscles, (N = 6).
~ - - M . biceps femoris; I - - M . psoas major; A - - M , gluteus medius; O--M. semimembranosus. a,b,c: Values not bearing common superscripts differ significantly (P < 0.05).
that recorded for M. semimembranosus. The weight losses recorded for these three muscles on storage in carbon dioxide are not significantly different. Length of storage period: Weight loss from meat in gas atmosphere storage increases with the length of the storage period (Fig. 1). M. bicepsfemoris has less weight loss after either one or two weeks' storage in nitrogen than the other muscles. After both storage periods in nitrogen, the weight loss recorded for M. semimembranosus is significantly greater than the weight loss recorded for M. psoas major or M. biceps femoris. M. gluteus medius, with an increase in weight loss of only 0.5 ~o (15 % increase) between one and two weeks' storage, is affected less than the other muscles by longer storage periods; the increases in weight loss recorded for M, psoas major, M. biceps femoris and M. semimembranosus, between one and two weeks' storage in nitrogen atmosphere, are 0-9 % (43 % increase), 0.7% (39% increase) and 1.5 ~ (42 % increase), respectively. The weight losses recorded after a two-week storage period are significantly greater than the weight losses recorded after one week's storage for M. psoas major and M. semimembranosus.
A N O X I C S T O R A G E OF F R E S H B E E F : P A R T 2
277
d cd
bc/ q~
m m 0 n
I
I
I
0
5
10
Temperature (°C) Fig. 2. Effect of storage temperature on weight loss from three bovine muscles during storage in nitrogen for 2 weeks. (N = 5). • - - M. psoas major; • - - M. gluteus medius; • - - M . semimembranosus. a,b,c: Values not bearing common superscripts differ significantly (P < 0-05).
Temperature o f storage: The amount o f ' d r i p ' which exudes from meat during storage in a gas atmosphere is greatly influenced by the storage temperature (Fig. 2). An increase in storage temperature from 0 °C to 10 °C causes a significant increase in weight loss from all three muscles during storage in nitrogen for 2 weeks. At 0 °C the mean weight loss recorded for M. psoas major is significantly less than that recorded for M. semimembranosus, and at 5 °C the weight loss recorded for M. psoas major is significantly less than the weight losses recorded for M. gluteus medius and M. semimembranosus. However, there are no significant differences between the weight losses recorded for the three muscles when stored at 10 °C. Weight loss increases more rapidly with temperature from 5 °C to I 0 °C than from 0 °C to 5 °C. The effect is most marked for M. psoas major: weight losses recorded for this muscle at 10°C are similar to those recorded for the other muscles at this temperature. Source o f meat: The weight losses recorded for two groups o f animals, one from a commercial source and one from Dunsinea, are compared in Table 10. For all
278
M. O ' K E E F F E , D , E. H O O D
T A B L E 10 WEIGHT LOSS (PERCENTAGE OF INITIAL WEIGHT) FROM MEAT FROM TWO SOURCES WITH STORAGE IN NITROGEN AT 0 ° C FOR 2 WEEKS. N = 5 x 2
Muscle
Source
Mean weight loss
SE
t test significance
M. psoas major
Commercial Dunsinea
2-40 3.18
0-21 0.25
P < 0.05
M. gluteus medius
Commercial Dunsinea
3'48 4.54
0.38 0-62
NS
M. semimembranosus
Commercial Dunsinea
4.16 4-76
0.35 0-51
NS
N S = N o t significant.
muscles studied, the weight losses recorded for meat from the commercial source are lower than those recorded for meat from Dunsinea, although the differences between the means are only significant for M. psoas major.
DISCUSSION
Age of meat post mortem Apart from the poor colour stability of very fresh meat, beef aged for about one week has the best colour stability, and meat held in an anaerobic environment for longer has more rapid discoloration when exposed to air. This effect is independent of the intrinsic colour stability of the meat, the decrease in shelf-life, relative to the control meat, being proportionately the same (in most cases) for all muscles. While colour stability decreases with age of meat, this is not an important problem for centralised prepackaging. As has been found already with vacuum-packaged primal cuts of meat, the increased control of the display time in air, which is possible with anaerobically packaged meat, means that a shorter shelf-life is required. Prepackaged retail cuts of fresh meat, stored in oxygen-free gas atmospheres, need to be exposed to air for only a short time before being sold, because no cutting or packaging operations are involved. Even very fresh meat (e.g. three days post mortem) could be centrally prepackaged, as the additional ageing required for good colour stability would take place during storage in the oxygen-free gas atmosphere. Temperature of storage The storage temperature, both while the meat is in an oxygen-free gas atmosphere and during subsequent holding in air, has a marked effect on colour stability. When the storage temperature is raised above - 1 to 0 °C more rapid discoloration of the meat surface occurs. The results of this work show that, during Storage in air, the increased rate of discoloration caused by higher storage temperature is proportionately the same for all muscles, or is greater for muscles of more stable
ANOXIC STORAGE OF FRESH BEEF: PART 2
279
c o l o u r - - - e . g . M , semimembranosus. For meat stored in an oxygen-free gas atmosphere, the effect of temperature on colour stability, determined during subsequent storage in air, is quite different (Table 4). Here, the effect o f increased storage temperature is greater for muscles with intrinsically less stable colour--e.g. M. psoas major. The similar effect of temperature on rate of discoloration and weight loss for each muscle is interesting, M. semimembranosus, which shows the least decrease in colour stability with increase in temperature (Table 4), shows also the least increase in weight loss with increase in temperature (Fig. 2). M. psoas major shows the greatest decrease in colour stability and increase in weight loss with increase in temperature, whilst the effect of temperature on these two properties for M. gluteus medius is intermediate. These results suggest the possibility o f a common temperature effect influencing both colour stability and water-holding capacity.
Weight loss during storage A major problem with any centralised packaging system for retail cuts o f fresh meat is drip. Apart from the weight loss involved, drip has an adverse effect on the appearance and acceptability of the meat. By comparison with vacuum-packaging, the amount of drip recorded for retail cuts of meat stored in an oxygen-free gas atmosphere is considerably less, and, by using a good meat tray in conjunction with a blotter to absorb the drip, this may be kept to an acceptable level. Low temperature storage (0 °C) restricts the amount o f drip to approximately 3 ~o-5 ~ , over a 2-week storage period, depending on the particular muscle. The wide variation in water-holding capacity which occurs between muscles is a reflection of their differing physical and biochemical characteristics. These differences in water-holding capacity occur even when the rate and extent o f p H fall are identical, suggesting that there are different types o f protein present (Lockett et al., 1962). An interesting observation is that the muscle with the greatest tendency to discoloration, M. psoas major, has the least weight loss in an oxygen-free gas atmosphere, but loses the most weight in vacuum (Table 9). M. psoas major is the most tender muscle (Ramsbottom & Strandine, 1948), a property associated with high water-holding capacity and a iloosening ~o f the network of the protein ge! (Hamm, 1974). Under the pressure in a vacuum package, the moisture is easily forced out from this muscle, but in a gaseous environment the loose protein network would allow the meat to retain moisture. By comparison with M. psoas major, M. gluteus medius and M. semimembranosus are less tender and so have lower waterholding capacities which could account both for the greater moisture loss recorded in gas atmosphere storage and the lesser moisture loss recorded in vacuum for these two muscles. Howard et al. (1960) reported that drip from M. longissimus dorsi is generally twice as great as that from M. psoas major, which is a more tender muscle than M. longissimus dorsi. High pH has been found to reduce the amount of'drip' from meat (Lawrie, 1979).
280
M. O'KEEFFE, D. E. HOOD
T A B L E 11 ULTIMATE pH (pHu) FOR MEATFROM TWO SOURCES, COMPARED WITH WEIGHT LOSS(PERCENTAGEOF INITIAL WEIGHT) ON STORAGE OF STEAKS IN NITROGEN AT 0° C FOR 2 WEEKS. N ~ 5 x 2
Muscle
Source
Mean pH,,
SE
Mean weight loss
M. psoas major
Commercial Dunsinea
5.63 5.70
0.022 0-024
2.40 3.18
M. gluteus medius
Commercial Dunsinea
5.46 5.52
0-024 0.011
3.48 4.54
M. semimembranosus
Commercial Dunsinea
5-49 5-56
0.027 0.020
4-16 4.76
In this connection, the effect of carbon dioxide enriched atmospheres on meat is to cause a significant lowering of pH (Ledward, 1970) which results in isoelectric precipitation of some of the sarcoplasmic proteins (Stewart et al., 1965) with consequent reduction in water-holding capacity. Such an effect may explain the considerably greater weight loss from meat stored under carbon dioxide compared with meat stored under nitrogen (Table 9). The consistently higher 'drip' recorded for Dunsinea steaks than for commercial steaks during storage in nitrogen cannot be explained by pH differences. All meat types from the Dunsinea source record slightly higher ultimate pH values (pI-~) than meat from the commercial source (Table 11). When commercial meat was being cut into steaks, it was noticed that it had a 'drier' appearance than Dunsinea meat. The longer holding period of commercial meat in carcass form in the chill--3-4 days compared with 2 days for Dunsinea meat--may have an effect on the level of'drip' recorded subsequently for steaks stored in nitrogen. The higher weight loss from carcass meat than from vacuum-packaged cuts could result in the commercial meat losing more of its 'available drip' during the 7-day ageing period. Commercial meat would then have less tendency to lose 'drip' during gas storage.
ACKNOWLEDGEMENTS
The technical assistance of Mr M. Murray, Mr S. Schwer and Mr J. Dalton is gratefully acknowledged. We thank Mr J. Sherington, Statistics Department, The Agricultural Institute, for the statistical analyses and Ms B. Griffiths for typing the manuscript. Mr P. Doyle, Irish Meat Producers Ltd, is thanked for arranging the provision of commercial meat samples.
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BROOKS, J. (1931). Ann. Rept. Fd. Invest. Bd., 36--41. BROWN, W. D. & MEBINE, L. B. (1969). J. Biol. Chem., 244, 6696. CUTAIA, A. J. & ORDAL, Z. J. (1964). Fd. Technol., 757, 163-6. GEORGE, P. & STRATMANN, C. J. (1952). Biochem. J., 51, 418-25. HAMM, R. (1974). Water-holding capacity of meat. In Meat (Cole, D. J. A. & Lawrie, R. A. (Eds)), Butterworths, London, 321-38. HICKMAN, M. J. (1970). Nat. Phy. Lab. Notes on Appl. Sci, No. 4 (4th edn), 1-33. HOOD, D. E. (1980). Meat Science, 4, 247-65. HOWARD, A., LEE, C. A. & WEBSTER, H. L. (1960). CSIROtDiv. Fd. Pres. Tech. Paper No. 21. LAWRIE, R. A. (1979). Meat science (3rd edn), Pergamon Press, Oxford, 229. LEDWARD, D. A. (1970). J. Fd. Science, 35, 33-6. LOCI~ETT, C., SWIFT, C. E. & SULZBACr~R, W. L. (1962). J. Fd. Sci., 27, 36. MOrtLEe, M. J. (1971). J. Fd. Technol., 6, 371-82~ O'BRtEN, F. E. M. (1948). J. Sci. Instrum., 25, 73-6. O'KEEFFE, M. & HOOD, D. E. (1980-81). Meat Science, 5, 27-39. RAMSBOTTOM, J. M. & STRAr~DINE, E. J. (1948). Fd. Res., 13, 315. SNYDER, H. E. (1964). J. Fd. Sci., 29, 535-9. SNYDER, H. E. & AYR~, J. C. (1961). J. Fd, Sci., 26, 469-74. STEWART, M. R., HUTCm~S, B., ZIPSER, M. & WATTS, B. (1965). J. Fd. Sci., 30, 487-91. STRINGER, W. C. & NAUMANN, H. D . (1966). Fd. Technol., 20, 108-10. TAYLOR, A. A. (1973). Inst. Meat Bull., 79, 26-32. URBIN, M. C. & WILSON, G. D. (1958). Proe. lOth Meat Ind. Conf. A M I F , 13-20. URBIN, M. C. & WILSON, G. D. (1961). J. Fd. Sci., 26, 314-17. VAN DEN OORD, A. H. A. & WESDORP, J. J. (1971). J. Fd. TeehnoL, 6, 15-20. WALTEaS, C. L. (1974). Meat colour: The importance of haem chemistry. In Meat (Cole, D. J. A. & Lawrie, R. A. (Eds)), Butterworths, London, 385-401. YOUNG, J. F. (1967). J. AppL Chem., 17, 241-4.