The influence of storage and display conditions on the color stability of display-ready pork loin roasts

PII:

SO309-1740(97)00036-3

Meat Science, Vol. 41, No. l/2, l-16, 1997 0 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0309-1740/97 $17.00+0.00

ELSEVIER

The Influence of Storage and Display Conditions on the Color Stability of Display-ready Pork Loin Roasts L. E. Jeremiah & L. L. Gibson Agriculture and Agri-Food Canada Research Centre, Meat Science Section, Lacombe, Alberta. Canada (Received 1 November 1996; revised version received 4 April 1997; accepted 4 April 1997)

ABSTRACT A total of 216 pork loin samples were utilized to examine the color stability of masterpacked, display-ready pork cuts stored in three different atmospheres (100% COz, 100% Nz and 70% Or and 30% CO*) at three storage temperatures (-1.5.2, and S’C) for four day intervals up to 28 days, and then subsequently displayed aerobically for up to 30 hr. The composite results clearly indicated color stability was progressively lost during both chilled storage and subsequent aerobic display. However, retention of color stability was maximized by storage at -1.5”C. In addition, storage in 100% COz tended to maximize retention of color stability, despite the fact samples stored in 70% Oz and 30% CO2 were brighter and redder following storage and prior to subsequent aerobic display. 0 1997 Elsevier Science Ltd

INTRODUCTION Centralized processing and the merchandising of display-ready pork cuts offers considerable economy to the retail sector (Farris et al., 1991), due to reduced labor costs and capital investment requirements. However, for centralized processing and packaging of pork cuts to be effective, the chilled storage life of such cuts must be extended sufficiently to permit their widespread distribution and merchandising. Previous reports have documented the color stability of meat is lost during chilled storage and display (Greer et al., 1993; Gill and Jones, 1994a,6, 1996; Jeremiah et al., 1995Q). However, the effects of chilled storage conditions and duration of subsequent, aerobic display on the color stability of masterpacked, displayready pork cuts remain largely uncharacterized and unquantitated. The present study was designed to investigate the effects of storage atmosphere, storage temperature, and duration of both chilled storage and subsequent aerobic display on color stability.

EXPERIMENTAL Experimental design A total of 216 loin samples from 54 commercial, boneless pork loins, with normal inherent muscle quality, obtained from a local pork slaughter plant were utilized. All loins were

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L. E. Jeremiah, L. L. Gibson

cut into four equal sized subsections, and the subsections were randomly allocated to storage atmospheres, storage temperatures, and storage interval subgroups according to the experimental design grid shown in Table 1. Packaging and storage All subsections were placed on a soaker pad in a hard plastic tray and then were overwrapped with an oxygen permeable film with an oxygen transmission rate of 8000cc/m2/ 24 hr (Vitafilm ‘Choice Wrap’, Goodyear Canada, Ltd.). Two vent holes (3 mm) were then burned through the overwrap film (one at each end of the package) to permit free exchange of atmospheres within the masterpack. The retail-ready packs were then placed into foil-laminate pouches, and the pouches were evacuated, filled with an excess of the designated atmosphere (2 l/kg of gas under pressure), and heat sealed, prior to being placed into storage at the designated temperature. The gas composition was regulated, and the oxygen content was monitored and did not exceed 300ppm in 100% N2 and 100% CO2 packs, after packaging. Muscle surface pH assessment Muscle surface pH was measured prior to packaging, following storage and following 30 hr of subsequent, simulated, aerobic, retail display. Three readings per sample were taken at different anatomical locations using a Horiba pH meter (model number: F-12 Horiba Ltd., Kyoto, Japan) with a flat surface, gel-filled combination electrode (model number: 91-36, Orion Research Inc., Laboratory Products Group, Boston, MA, USA). Retail display conditions Upon removal from the masterpack, following the designated storage interval, the vent holes in each retail pack were immediately covered with adhesive stickers to prevent potential dehydration and contamination and the retail packs were placed into a fan-circulated horizontal type retail display case (Hill Refrigeration of Canada, Ltd., Barrie ON, Canada) with a mean temperature of 6.8”C at the meat surface in a room illuminated with fluorescent lighting and further illuminated with incandescent lighting, to give a constant intensity of 1076 lux at the meat surface for 12 hr per day. TABLE 1 Experimental Design Grid

Atmosphere

100%

Storage temperature

N2

100% co2

70% 02, 30% co2

“Number

of pork loin subsections

-1.5”C 2.O”C 5.O”C -1.5”C 2.O”C 50°C -1.5”C 2.O”C 5JYc per cell.

Storage Interval (Weeks) 0

4

4” 4 4 4 4 4 4 4 4

4 4 444 444 444 444 444 444 444 444

8

12

16

20

24

28

4

4 4 4 4 4 4 4 4 4

4 4 4 4 4 4 4 4 4

4 4 4 4 4 4 4 4 4

4 4 4 4 4 4 4 4 4

Color stability of display-ready pork loin roasts

3

Objective measurement of muscle color and pigments Proportions of the various states of myoglobin (deoxymyoglobin, oxymyoglobin, and metmyoglobin) and ‘L’, ‘a’, and ‘b’ values were assessed by reflectance spectrophotometry using the Macbeth Color Measuring System (Prism Instruments, Whitby, ON, Canada) according to AMSA guidelines (Hunt et al., 1991) prior to packaging, following storage for the designated interval, and during subsequent, simulated, aerobic, retail display at the following time intervals: 1, 2, 4, 6, 24 and 30 hr. Subjective sensory assessment A five-member sensory panel was selected and screened for its ability to perceive color and odor differences. The panel was trained in the use of evaluation procedures and visual and olfactory standards. However, it was not trained in the evaluation of retail appearance. Each panel member had in excess of 200 hr of experience in the evaluation of meat samples for visual and olfactory properties. This panel assessed muscle color [S-point descriptive scale (Agriculture Canada, 1984): 1 = extremely pale; 2 = pale; 3 = normal; 4 = dark; and 5 = extremely dark]; and surface discoloration [7-point descriptive scale (Jeremiah and Greer, 1982): 1 = no surface discoloration; 7 = 100% surface discoloration] prior to packaging, following storage for the designated interval (0, 4, 8, 12, 16, 20, 24, or 28 days) and during subsequent, simulated, aerobic, retail display at the following time intervals: 1, 2, 4, 6, 24, and 30 hr. Microbiology was not included in the present study, since it has been thoroughly documented on similar products from the same source, under identical storage and display conditions and reported elsewhere. Data analysis Data were analyzed using the general linear model of SAS (1985). Meaningful storage atmosphere/storage temperature/storage time interactions were not observed. Therefore, the main effects were evaluated and reported separately. Linear regression was used to detect significant time trends with increasing storage temperature and duration of chilled storage and aerobic display (Puri and Mullen, 1980).

RESULTS Muscle surface pH Muscle surface pH did not differ (p>O.O5) among storage atmospheres or storage temperatures, either prior to subsequent aerobic display or after 30 hr of display. Significant time trends were not observed (p > 0.05) in muscle surface pH values with duration of chilled storage, either following storage and prior to subsequent, aerobic display or following 30 hr of aerobic display. Chemical status of myoglobin Deoxymyoglobin

As expected, samples stored in 100% CO* and 100% N2 had higher proportions of deoxymyoglobin (p > O-05) until after 4 hr of display than samples stored in 70% 02 and 30% CO* [Fig. l(a)]. Samples stored in 100% N2 continued to have higher proportions of deoxymyoglobin (p < O-05) than samples stored in 100% CO2 and 70% 02 and 30% CO*

L. E. Jeremiah, L. L. Gibson Storage Atmosphere

q 90

$60

0

0 t

70

$

60

E 4

60

tW%N, 70%0,30%c0, 1OO%co~

P 40 E g “0 P

30 20 10 0

BP

0

2

1

4

24

6

30

Hours of Retail Display Storage

(b)loo-

EI

60i

2.0°c

q

60-

0

b

70-

[

60-

Ej

60-

0

407

E 8

30

%

20

Temperature -1.5Oc

5.0°G

10 ” BP

2

1

4

24

6

30

Hours of Retail Display Storage Interval (days)

1

2

4

6

24

30

Hours of Retail Display

Fig. 1. The effects of (a) storage atmosphere, (b) storage temperature, and (c) storage interval on the proportions of deoxymyoglobin in retail-ready pork cuts prior to packaging and storage (BP) and after certain aerobic, display intervals.

Color stability of display-ready pork loin roasts Storage •j

Atmosphere

lOO%N, 70%0,30%c0,

BP

0

1

2

4

6

24

30

Hours of Retail Display

W

Storage 106 60

c D ,o

66

Temperature

q

-l.S°C

q

5.0Dc

70

8 *60 E $60 0 E” 8 ; a.

30 20 10 0 BP

0

1

2

4

6

24

30

Hours of Retell Display Storage Interval Do

n

08 ac

90

0

z

60

E”

70

E 0 E

60

t ii P

(days) El16

4

a

20

q 24

60

40 30 20

BP

0

1

2

4

6

24

30

Hours of Retail Display

Fig. 2. The effects of (a) storage atmosphere, (b) storage temperature, and (c) storage intervals on the proportions of oxymyoglobin in retail-ready pork cuts, prior to packaging and storage (BP) and after certain aerobic display intervals.

6

L. E. Jeremiah, L. L. Gibson

throughout the remainder of the 30 hr aerobic display period [Fig. l(a)]. A significant positive time trend was detected in the proportions of deoxymyoglobin with duration of subsequent, aerobic display in samples stored in 70% O2 and 30% CO2 (p < 0.01, r2 = 0.76), indicating the proportions of deoxymyoglobin increased progressively during aerobic display in these samples [Fig. l(a)]. Following 24 hr of subsequent, aerobic display, samples stored at - 1.5”C had a lower proportion of deoxymyoglobin than samples stored at 2 and 5°C and after 30 hr of subsequent, aerobic display samples stored at -1.5”C had lower proportions of deoxymyoglobin than samples stored at 2°C (pO.O5), the nonsignificant, positive time trends detected in unstored samples and samples stored for up to 12 days with duration of chilled storage indicated the proportions of deoxymyoglobin tended to increase progressively with aerobic display in these samples [Fig. l(c)]. Oxymyoglobin

As would be expected, samples stored in 70% O2 and 30% CO2 had higher proportions of oxymyoglobin (~~0.05) than samples stored in 100% CO2 and 100% N2, prior to subsequent, simulated, aerobic display [Fig. 2(a)]. A significant negative time trend in the proportions of oxymyoglobin with duration of subsequent, aerobic display (p>O.O5, r2=0.61) was detected in samples stored in 70% 02 and 30% CO*, indicating oxymyoglobin was progressively lost from these samples during aerobic display [Fig. 2(a)]. Samples stored at - 1.5”C had higher proportions of oxymyoglobin than samples stored at 5°C following all of the aerobic display intervals evaluated [p < O-05; Fig. 2(b)]. They also had higher proportions of oxymyoglobin (pO.O5) with duration of aerobic display after any of the chilled storage intervals evaluated. Metmyoglobin

Samples stored in 100% N2 had lower proportions of metmyoglobin prior to subsequent, simulated, aerobic display (p < 0.05) than samples stored in 100% CO2 and 70% O2 and 30% CO2 [Fig. 3(a)]. They also had lower proportions of metmyoglobin (p < 0.05) after 1 hr of display than samples stored in 70% 02 and 30% CO2 [Fig. 3(a)]. A significant positive time trend in the proportions of metmyoglobin (p < 0.05, r2-0.55) was observed with duration of subsequent aerobic display in samples stored in 100% N2, indicating a progressive increase in the proportions of metmyoglobin in these samples during aerobic display [Fig. 3(a)]. Samples stored at -1.5”C had lower proportions of metmyoglobin (p < 0.05) than samples stored at 5°C following all of the aerobic display intervals evaluated. They also had lower proportions of metmyoglobin (~~0.05) than samples stored at 2°C prior to aerobic display and after 2, 24 and 30 hr of display [Fig. 3(b)]. Although significant time trends in the proportions of metmyoglobin were not observed in any storage temperature

Color stability of display-ready pork loin roasts

Storage a

Atmosphere

lW%N, 70% 0,30%

EI

co,

loo%CO1

40

E

I

z 30 a 20 10 0 BP

0

2

1

4

6

30

24

Hours of Retail Display (‘4

StorageTemperature &I

lOO-

-1.5cc 2.0nc

90E * 2 g E ‘, =

•3

5.0°c

60706060-

E40 ti

5”

O20 10 0 BP

0

2

1

4

6

30

24

Hours of Retail Display Storage

Interval

El0 100 90 !

n

60

cl6

fzI 4

16 20 24

12

70

(days)

8

26

60 50 40 30 20

10 0

1

2

4

6

Hours of Retail Display

Fig. 3. The effects of (a) storage atmosphere, (b) storage temperature, and (c) storage interval on the proportions of metmyoglobin in retail-ready pork cuts, prior to packaging and storage (BP) and after certain aerobic display intervals.

8

L. E. Jeremiah, L. L. Gibson

group (p > 0.05)the apparent positive time trends in the proportions of metmyoglobin with duration of subsequent, aerobic display observed in all storage temperature groups evaluated, indicated the proportions of metmyoglobin increased progressively during aerobic display, irrespective of storage temperature. Significant positive time trends in the proportions of metmyoglobin were observed with duration of chilled storage following all aerobic display intervals evaluated (p < 0.01; r* = 0.86 to O-96) indicating the proportions of metmyoglobin increased progressively during chilled storage [Fig. 3(c)]. Although significant time trends in the proportions of metmyoglobin with duration of subsequent, aerobic display were generally not observed (p > 0.05), the apparent positive time trends observed in the proportions of metmyoglobin with duration of subsequent, aerobic display suggested the proportions of metmyoglobin increased progressively during aerobic display. Muscle color co-ordinates Samples stored in 100% N2 were darker and had lower ‘L*’ values than samples stored in 70% O2 and 30% CO2 when placed on simulated aerobic display. They were also darker and had lower ‘L*’ values (p < 0.05) than samples stored in 100% CO2 or 70% O2 and 30% CO2 when displayed aerobically for 1 to 30 hr (Fig. 4). Samples stored in 70% O2 and 30% CO2 had higher ‘a*’ and ‘b*’ values (were redder and yellower, respectively, p < 0.05) than samples stored in 100% CO2 and 100% NZ, prior to subsequent, simulated, aerobic display [Figs 5(a) and 61. They also had higher ‘a*’ values and were redder during the early and late stages of display (1, 2, 24 and 30 hr) and had higher ‘b*’ values throughout display than samples stored in 100% N2 [p < 0.05, Figs 5(a) and 6(a)]. A significant negative time trend (p < 0.01, r* = 0.84) was observed in the ‘a*’ values (redness) of samples stored in 100% N2, indicating a progressive loss of redness from these samples during aerobic display [Fig. 5(a)]. In addition, significant positive time trends in ‘b*’ values (yellowness) were observed in samples stored in 100% Nz and 100% CO2 (p < 0.05, r* = 0.55 and p < 0.01, r* = 0.76, respectively), indicating yellowness increased progressively in these samples during aerobic display (Fig. 6).

65

Storage ff$i

64

Atmosphere 100%

N 2

70% 0,

63

EZI

30% co,

lco%CO,

62 61 60 56 56 Sf 56 55 BP

0

1

2

4

6

24

Hours of Retail Display

Fig. 4. The effects of storage atmosphere on the ‘L’ values of retail-ready pork cuts, prior to packaging and storage (BP) and after certain aerobic, display intervals.

Color stability of display-ready pork loin roasts

Storage &j

Atmosphere

lOO%N, 70% 0 z 30% co 2

c3

16 0 cl

15

?

14

100%

co >

lb 13 12 11 10 BP

0

2

1 Hours

(b’

4

of

Retail

6

Display

17

30

fB

Storage

1

18

24

Temperature

-1 5-c

2.o”c

16

BP

0

2

1 Hours

of

4 Retail

6

24

30

Display Storage

(C)

Interval

q

QO

q

4

q

(days) 16 20

16 2 ?

15 14

(D 13

BP

0

1

4

2 Hours

of

Retail

6

24

30

Display

Fig. 5. The effects of (a) storage atmosphere, (b) storage temperature and (c) storage interval on ‘a’ values (muscle color redness) of retail-ready pork cuts, prior to packaging and storage (BP) and after certain aerobic, display intervals.

10

L. E. Jeremiah, L. L. Gibson Storage

12

&!

1

BP

0

1

2

4

Atmosphere

lOO%N z

6

24

30

Hours of Retail Display

The effects of storage atmosphere on ‘b’ values (muscle color yellowness) of retail-ready pork cuts, prior to packaging and storage (BP) and after certain aerobic, display intervals.

Fig. 6.

The only difference in ‘L*’ values attributable to storage temperature occurred after 1 hr of aerobic display, when samples stored at -1+5”C were darker and had lower ‘L*’ values (p < 0.05) than samples stored at 5°C. Significant time trends in ‘L*’ values with extension of subsequent aerobic display were not observed (p > 0.05) in any of the storage temperature groups following any of the chilled storage intervals evaluated. Samples stored at - 1.5”C were redder and had higher ‘a*’ values throughout subsequent aerobic display than samples stored at 5°C (p < 0.05) [Fig. 5(b)]. They were also redder and had higher (p O-05) in ‘a*’ values in any of the storage temperature groups, following any of the chilled storage intervals evaluated with duration of subsequent aerobic display, the apparent negative time trends observed suggested redness tended to be lost from all samples during aerobic

Storage

5

ti?j

Atmosphere

lOO%N, 70% 0,30%

1

BP

0

1

2

4

6

24

co,

30

Hours of Retail Display

Fig. 7.

The effects of storage atmosphere on subjective muscle color scores of retail-ready pork cuts, prior to packaging and storage (BP) and after certain aerobic, display intervals.

Color stability of display-ready pork loin roasts

11

display, irrespective of storage temperature [Fig. 5(b)]. Significant differences in ‘b*’ values were not observed (p > 0.05) among storage temperatures following any of the aerobic display intervals evaluated, and significant time trends in ‘b*’ values with duration of subsequent aerobic display were not observed (p>O.O5), irrespective of storage temperature. Significant time trends in lightness (‘L*‘) values were not observed (~~0.05) with duration of either chilled storage or subsequent aerobic display. Significant negative time trends (p < 0.01; r2 = O-71 to 0.92) were observed in redness (‘a*‘) values with extension of chilled storage, indicating redness was lost as chilled storage was prolonged [Fig. 5(c)]. However, significant time trends in ‘a*’ values were not observed with duration of subsequent aerobic display (p > O-05). Significant time trends in ‘b*’ values were not observed with either duration of chilled storage or aerobic display (p > 0.05). Subjective muscle color Samples stored in 100% Nz were perceived to have a darker color than samples stored in 70% 02 and 30% CO2 until after 24 hr of aerobic display (p0.05). Subjective muscle color scores did not differ due to storage temperature following any of the aerobic display intervals evaluated (p > 0.05). Significant time trends were not observed in subjective muscle color scores with extension of subsequent, aerobic display (p > 0.05) in any of the storage temperature groups, following any of the chilled storage intervals evaluated. Significant time trends in subjective muscle color scores were not observed with duration of either chilled storage or subsequent, aerobic display (p > 0.05). Surface discoloration Despite the differences observed in the proportions of metmyoglobin, subjective surface discoloration scores were not influenced (p>O.O5) by storage atmosphere. However, significant positive time trends were observed in surface discoloration scores with duration of subsequent, aerobic display in all storage atmosphere groups (p < 0.05, r2 = 0.62 to 0.78), indicating sample surfaces became progressively discolored during aerobic display, irrespective of the storage atmosphere utilized. Samples stored at -1.5°C displayed less surface discoloration (p < O-05) than samples stored at 5°C after all of the aerobic display intervals evaluated [Fig. 8(a)]. They also exhibited less surface discoloration than samples stored at 2°C following 1, 2, 4, 24, and 30 hr of aerobic display. However, differences in surface discoloration were not observed among storage temperatures prior to subsequent, aerobic display (p > O-05). Moreover, significant, positive time trends were observed in surface discoloration scores with duration of aerobic display in all storage temperature groups evaluated (p < 0.05; r2 = 0.63 to 0.72) indicating surface discoloration increased progressively during aerobic display, irrespective of storage temperature. Significant positive time trends were observed in surface discoloration scores with duration of chilled storage, following all subsequent, aerobic display intervals (p < 0.05; r2 = 0.63 to 0.72) indicating samples discolored progressively during chilled storage [Fig. 8(b)]. Significant positive time trends in surface discoloration scores were also detected with duration of aerobic display following all chilled storage intervals (p < O-05; r* = 0.52 to 0.94) indicating samples discolored progressively during aerobic display.

12

L. E. Jeremiah, L. L. Gibson (a)

Storage

7

@ $’

6

Temperature -l.S°C 2.0°c

2

1

4

6

24

30

Hours of Retail Display Storage Interval (days)

(W

7 2 I u)

EZIO

El

16

n

ip

20

q

24

q

26

4

06 6

12

BP

0

1

2

Hours of Retail

4

6

24

30

Display

Fig. 8. The effects of (a) storage temperature and (b) duration of chilled storage and subsequent aerobic display on surface discoloration scores of retail-ready pork cuts, prior to packaging and storage (BP) and after certain aerobic, display intervals.

DISCUSSION The main factor determining consumer acceptance during the selection of meat purchases is muscle color (Jeremiah, 1982). Consequently, desirable color must be maintained during chilled storage, distribution and subsequent retail display, if preservative packaging systems are to be effective. Previous observations have indicated the color stability of meat gradually deteriorates as chilled storage is prolonged and may markedly reduce the subsequent retail case-life attainable as well (Greer et al., 1993; Jeremiah et al., 1992a, 1995a,b). However, to date, the deterioration in pork color stability has not been documented, quantitated, and characterized. It has been documented, however, the display life of pork is generally limited by deterioration in product color, which occurs substantially before microbiological spoilage (Greer et al., 1993; Jeremiah et al., 1995a,b). Such color deterioration presently severely restricts possibilities for widespread distribution of

Color stability of display-ready pork loin roasts

13

display-ready pork cuts, since this deteriorative process commences immediately upon exposure of the meat surface to oxygen (Jeremiah et al., 19926). In addition, the stability of meat color declines with lower oxygen tension until very low oxygen concentrations are attained. However, all color deterioration is apparently prevented in the absence of oxygen (Gill, 1989, 1990; Jeremiah et al., 1992b). Consequently, proper use of anoxic masterpacking systems for aggregated display-ready product may permit the distribution of such product over relatively long distances using surface transportation. Such an approach has been adopted in commercial practice to extend the storage life of chilled lamb being shipped from the southern hemisphere to very distant northern hemisphere markets by sea. Both modified and controlled atmospheres have been proposed as potential techniques for extending the storage life of centrally processed display ready pork cuts. However, to date, very little knowledge is available regarding the use of different atmospheres for extending storage life under varying storage conditions, particularly with display-ready packages of pork. Presently, the pack conditions required to maintain product color indefinitely can be achieved using specifically designed equipment (Gill, 1989, 1990). However, the functional parameters of the equipment and the atmosphere necessary for maintenance of color stability have been ascertained only by ad-hoc experimentation. If controlled atmosphere packaging technology and equipment are to be improved and applied more widely and with greater economic efficiency, it will be necessary to establish a qualitative understanding of the effects of pack atmosphere composition on meat color. Such understanding would greatly simplify development of appropriate master and retail packagings and facilitate development of varied and less costly packaging systems for preserving meat color during storage and distribution to allow trading in branded, display-ready products. The advantages of merchandising such products appear obvious, since consumer products with assured quality, clearly differentiated from commodity products, provide greater monetary returns and are more resistant to market fluctuations than poorly differentiated commodities. Modified atmospheres containing oxygen (02) carbon dioxide (CO*), and possibly some nitrogen (Nz) to prevent package collapse have been commonly used in retail packs (Smith et al., 1988). However, to maintain an adequate preservative composition, an excessive gas to meat volume ratio is required (Holland, 1980). Such oversized packs increase per unit distribution and display costs and are generally unfavorably received by consumers (Renerre, 1989). Consequently, in commercial practice, modified atmosphere packs are often suboptimal and confer only modest extension of product storage life. Problems encountered with oversized retail packs can be avoided by utilizing a masterpackaging system, in which several retail packs are sealed in a master pack containing a preservative atmosphere. Since the product will bloom (oxygenate) when exposed to oxygen upon opening the master pack, anoxic as well as oxygen rich atmospheres can be utilized in master-packing systems (Gill, 1990). In general, the display life of red meats is normally limited by the time required for the proportion of metmyoglobin in the surface layers of muscle, to reach levels perceived by consumers to be unacceptable, due to the brownish appearance. The rate at which metmyoglobin accumulates in the surface layers of muscle varies considerably, depending upon the muscle and storage conditions. In general, the rate at which metmyoglobin accumulates is proportional to the storage temperature (Gill, 1990) and any metmyoglobin present before display will reduce the time a desirable red color can be maintained during display. Moreover, prolonged storage prior to display in atmospheres which restrict metmyoglobin formation still reduce the display life attainable, since the metmyoglobin reduction capacity of the muscle deteriorates during storage and display (Gill, 1990).

14

L. E. Jeremiah, L. L. Gibson

Both the intensity and stability of meat color is enhanced by oxygen-rich environments since the high oxygen tension increases the depth of the oxygenated surface layer, thereby increasing the amount of oxymyoglobin perceptible to consumers and more importantly prolongs the time it will take the metmyoglobin layer to become visible. On the other hand, low oxygen tensions result in rapid metmyoglobin formation at meat surfaces. Anoxic environments generally preclude metmyoglobin formation during storage and allow metmyoglobin, present before packaging, to be reconverted to deoxymyoglobin, depending upon the metmyoglobin reduction capacity of the muscle tissue. Therefore, when meat is removed from anoxic environments most of the myoglobin is present as deoxymyoglobin, which oxygenates rapidly upon exposure to air and produces a bright red color (Moore and Gill, 1987). Present findings indicate storage in 100% N2 produced a darkening and dulling of muscle color, which did not improve during aerobic display. In fact, the color became more yellow but less red in these samples as aerobic display was prolonged. Present findings also indicate samples stored in 70% O2 and 30% CO* came out of storage with a bright red color which deteriorated progressively throughout aerobic display. Although the duration of chilled storage and subsequent, aerobic display exerted little influence on muscle surface pH values, subjective muscle color scores and muscle color lightness (‘L*‘) values were affected. However, all subjective color ratings were within the normal range. Consequently, the influences of duration of chilled storage and aerobic display on subjective ratings of muscle color would appear to have little practical importance. The magnitudes of differences in ‘L*’ values attributable to either duration of chilled storage or aerobic display also make them of little practical importance. However, ‘a*’ values decreased progressively during both chilled storage and subsequent aerobic display, indicating muscle color redness was progressively lost throughout storage and display. As expected, samples stored in 100% CO* and 100% N2 had higher proportions of deoxymyoglobin until after 4 hr of subsequent, aerobic display than samples stored in 70% O2 and 30% CO*. Samples stored in 100% Nz continued to have higher proportions of deoxymyoglobin than samples stored in 100% CO2 and 70% 02 and 30% CO2 throughout the remainder of the 30 hr aerobic display period. The proportion of deoxymyoglobin increased progressively during aerobic display in samples stored in 70% 02 and 30% CO*, and reached 9% after 30 hr of display. Samples stored in 70% 02 and 30% COz had higher proportions of oxymyoglobin than samples stored in 100% CO1 and 100% N2 prior to subsequent aerobic display. They also had higher proportions of oxymyoglobin than samples stored in 100% N2 until after 2 hr of aerobic display. However, samples stored in 70% O2 and 30% CO* progressively lost oxymyoglobin as aerobic display was prolonged, but oxymyoglobin was still 57% after 30 hr of display. Such findings indicate samples stored in 100% N2 did not bloom or oxygenate to the same extent as samples stored in 100% CO* and 70% 02 and 30% CO2 when exposed to aerobic conditions. They also indicate samples stored in 70% 02 and 30% CO2 progressively lost bloom or oxygenation, and oxymyoglobin was converted to deoxymyoglobin in these samples during aerobic display (deoxymyoglobin increased from 5% prior to display to 9% after 30 hr of display and oxymyoglobin decreased from 67% prior to display to 57% after 30 hr of display). Samples stored in 100% N2 had lower proportions of metmyoglobin prior to aerobic display than samples stored in 100% CO2 and 70% 02 and 30% COz. They also had lower proportions of metmyoglobin after 1 hr of aerobic display than samples stored in 70% 02 and 30% CO*, but the proportion of metmyoglobin increased progressively in samples stored in 100% Nz during aerobic display. However, subjective surface discoloration scores were not influenced by storage atmosphere. Such findings indicate

Color stability of display-ready pork loin roasts

15

myoglobin is not oxidized to the same extent in 100% N2 as in 100% CO* or 70% 02 and 30% COz. However, oxidation of myoglobin increased progressively during aerobic display in samples stored in 100% Nz. Samples stored at -1.5”C generally had lower proportions of deoxymyoglobin and higher proportions of oxymyoglobin than samples stored at higher temperatures, after 24 hr of subsequent aerobic display and throughout aerobic display, respectively. Samples stored at -1.5°C also generally had lower proportions of metmyoglobin than samples stored at higher temperatures throughout aerobic display. Such findings clearly demonstrate the importance of subzero storage temperatures to pork color stability. Oxymyoglobin, and therefore, muscle color redness, was progressively lost during both chilled storage and subsequent aerobic display. In contrast metmyoglobin increased progressively during both chilled storage and subsequent aerobic display. The fact surface discoloration scores were not influenced by storage atmosphere may indicate the differences in metmyoglobin formation observed are of little practical importance. However, it should be noted the majority of samples stored in all of the atmospheres evaluated had a slight amount of surface discoloration after storage. Moreover, samples stored at - 1.5”C generally exhibited less surface discoloration than samples stored at 2 and 5°C throughout subsequent, aerobic display, despite the fact differences in surface discoloration attributable to storage temperature were not apparent following storage and prior to aerobic display. Such findings clearly demonstrate the importance of subzero storage temperatures for the retention of color stability. Irrespective of storage atmosphere or storage temperature, surface discoloration increased progressively during both chilled storage and subsequent retail display. Consequently, the composite results of the present study clearly indicated color stability was progressively lost during both chilled storage and subsequent aerobic display. However, retention of color stability was maximized by storage at subzero temperatures. In addition, storage in 100% CO2 tended to maximize retention of color stability, despite the fact samples stored in 70% O2 and 30% CO;! were redder and brighter following storage and prior to subsequent aerobic display.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support and assistance of the Alberta Agricultural Research Institute, the technical assistance of Jane Trace, the dedicated service of the sensory panel, and the typing assistance of Loree Verquin and Jennifer Johnson in preparing the manuscript.

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