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The rapid browning of lamb meat on retail display reduces its appeal to consumers and thus the marketability of lamb meat. Predicting the rate of meat browning on retail display would allow retailers to effectively manage this issue. The ability of bloomed meat color at the start of retail display to predict meat browning over subsequent retail display was investigated in lamb loin meat. Mixed breed lambs (
Consumers demand that displayed lamb meat has a bright red color. The rapid change in lamb meat color from red to brown on retail display reduces its appeal to consumers and thus the marketability of lamb meat (
Lamb meat develops a bright red color with blooming, as oxygen binds with myoglobin to form the red pigment oxymyoglobin. Though oxidative metabolism in muscle postmortem is limited, it has important effects on meat color. Meat with high proportions of oxidative muscle fibers contain more mitochondria that compete with myoglobin for oxygen, reducing the depth of oxygen penetration with blooming (
Accounting for variation in animal factors that influence myoglobin oxidation in meat may improve the ability of bloomed meat color to predict color stability on retail display. Increasing ultimate meat pH (pHu), intramuscular fat and markers of muscle oxidative capacity [muscle isocitrate dehydrogenase (ICDH) activity, myoglobin and iron concentration] have been associated with increased browning of lamb meat on retail display (
The extent of meat browning may be inferred using spectrophotometric light reflectance to measure the redness or red: brown (R630/R580) of a meat surface (
This study was approved by the Department of Primary Industries and Regional Development, Animal Ethics Committee (No: 1–10–1), Western Australia.
The Sheep Cooperative Research Center produced 4404 lambs at 5 sites across Australia between 2007 and 2011 as part of the information nucleus flock experiment, which has been comprehensively described previously (
At each production site lambs were consigned to smaller groups to be killed on the same day (slaughter groups). Lambs were designated to slaughter groups based on their live weights, with the aim to achieve an average carcass weight of 22 to 23 kg. A total of 73 groups of lambs were slaughtered. The group size ranged from 23 to 131 lambs, containing 60 lambs on average. Each site produced 2 to 5 slaughter groups each year, most commonly 3 to 4. The time of year that lambs were slaughtered varied considerably between slaughter groups and between sites and years of production.
The average age of the lambs in a slaughter group varied substantially between groups. Lambs were aged 8 mo (or 248 d) at slaughter on average, though ranged from 134 to 503 d old. In contrast the range of lambs ages within slaughter groups was small, as lambs were bred via artificial insemination and therefore were born within a short period of time. The age range of lambs within slaughter groups was 11 d on average, and varied from as little as 5 d and up to 36 d age difference.
The day prior to slaughter, lambs were yarded and fasted for 6 h before being weighed and transported to a commercial abattoir. The distance from the site of production to the abattoir ranged from 12 to 430 km, though was consisent for each site. At the abattoir, lambs were held in lairage overnight with access to water only before slaughter the following day. Lamb carcasses were electrically stimulated on medium voltage (
Lambs were measured and sampled for a wide range of carcass and muscle traits (
The mean, standard deviation and range of R630/R580 at 0, 24, 48, and 72 h of simulated retail display and for the carcass covariates tested
Trait | Number | Mean | St. dev | Range |
R630/R5801: 0 h | 4537 | 5.38 | 1.07 | 2.44 – 15.9 |
24 h | 4538 | 4.22 | 1.06 | 2.00 – 11.6 |
48 h | 4538 | 3.52 | 0.85 | 1.87 – 8.24 |
72 h | 4404 | 3.04 | 0.68 | 1.81 – 8.05 |
Covariates, units: | ||||
pH24 | 4393 | 5.66 | 0.15 | 5.23 – 6.67 |
Myoglobin, mg/g wet muscle | 2623 | 6.43 | 1.65 | 2.15 – 15.6 |
Iron mg/100 g wet muscle | 2625 | 20.0 | 3.39 | 8.12 – 45.1 |
Isocitrate dehydrogenase activity, µmol/min/g of wet muscle | 1804 | 5.03 | 1.56 | 1.43 – 11.3 |
Zinc mg/100 g wet muscle | 2625 | 24.1 | 4.50 | 12.0 – 44.9 |
Intramuscular fat, % | 2628 | 4.10 | 1.01 | 1.59 – 9.59 |
Hot carcass weight, kg | 4355 | 22.7 | 3.37 | 12.5 – 36.0 |
Shortloin muscle weight, g | 4400 | 360 | 95.6 | 157 – 1110 |
Shortloin fat weight, g | 4397 | 171 | 76.0 | 10.0 – 590 |
Age d | 4403 | 248 | 76.0 | 134 – 503 |
1Higher R630/R580 represents a redder meat color and lower R630/R580 represents a browner meat color. R630/R580 < 3.3 is considered too brown for consumer approval of lamb meat color (
Loin muscle temperature and pH (pH24) were measured in the center of the loin muscle, adjacent to the 12th rib. A TPS WP-80 pH and temperature meter (with a Mettler Toledo puncture pH probe- LoT406-M6-DXK-S7/25) was calibrated at pH 4 and 7 within the chiller before insertion into the muscle, as further described by
ICDH activity was measured in the loin muscle of lambs produced in 2007 and 2008 (
Intramuscular fat, myoglobin, iron and zinc concentrations were measured in the loin muscle of lambs produced from 2007 to 2009 (
Small muscle portions were sampled from the caudal section of the loin and frozen for measurement of myoglobin, iron and zinc concentrations. Myoglobin concentration was determined using a 1-g sample excised, finely diced and stored at –20°C until analysis using the method of
After excision of the loin muscle at around 24 h postmortem, a full cross-section at least 50 mm in length was cut from the cranial aspect of the loin muscle. This portion of muscle was originally located directly caudal to the 12th rib and was cut perpendicular to the long axis of the loin. The width of the sample was trimmed to 50 mm, while the depth of the muscle sample was determined by the depth of the whole loin muscle (around 30 mm). Samples were individually vacuum packaged using an 11 L/min vacuum (Orved Eco Vacuum pro), clear gas-impermeable packaging (20/80 to 100 microns, transparent polyamide air impenetrable exterior, polyethylene food approved interior, water vapor transmission rate measured at 23°C and 85% R.H– 2.6 gr/mq– 24hr, oxygen permeability measured at 23°C and 0% R.H– 50 cm3/mq– 24hr– bar) and were stored within a dark chiller at 3 to 4°C for 5 d.
The muscle samples were briefly removed from the chiller for re-slicing, re-packaging and blooming within a boning room at 18 to 20°C, mimicking standard retail preparation of lamb meat. The loin muscle samples were removed from their vacuum packaging and re-sliced using a butterfly cut (perpendicular to the long axis of the loin muscle) to create a fresh meat surface at least 30 mm in length. The freshly sliced sample was re-packaged on black Styrofoam trays (12 × 12 cm) and wrapped with oxygen-permeable polyvinal chloride film (Resinite “DHW” Meat AEP, 15 µm thickness, oxygen transmission rate of 35,650 – 46,500 cc/m2 per 24 hr). The freshly cut meat surface was facing upward and in full contact with the overwrapped film. The samples were bloomed for 30 min before being placed under simulated retail display for 72 h. For the simulated display, samples were displayed on a flat horizontal surface in a walk-in chiller (3.8 × 3 x 4 m) set to 4°C with no defrost cycle. Temperatures within the chiller fluctuated between 2 and 6°C on these settings. An overhead light was suspended 1.5 m above the samples to provide a light intensity of 1,000 Lux, as measured with a Dick Smith Electronics Light meter Q1367. This light source consisted of 8 Nelson Fluorescent Meat Display BRB Tubes, 58 W and 1520 mm in length. A diffuser was fitted to the lights to achieve an even distribution of light and an actual color temperature of 4,000 K.
The color of each meat surface was measured within the chiller using a Hunterlab spectrophotometer XE Plus (Cat. No. 6352, model 45/0-L, aperture of 3.18 cm) held flush with each overwrapped meat surface. The light source was set at D65, the observer was set to 10° and the instrument was calibrated using black glass and white ceramic tiles according to Hunterlab (Hunter Associates Laboratory, Inc.) directions. The redness or red:brown color of a meat surface (R630/R580) is determined by the % of light reflectance at 630 nm/% reflectance at 580 nm (
Correlations between R630/R580 values at 0, 24, 48, and 72 h of simulated retail display were analyzed in SAS (SAS Version 9.1, SAS Inst. Inc., Cary, NC). Simple correlations were estimated using the PROC CORR command, and partial correlations were estimated using a multivariate analysis. In the multivariate analysis the correlations between R630/R580 values at different time points were corrected for fixed effects including site, year, slaughter group within site by year, and sex and dam breed within sire type. The same data set was used to calculate simple and partial correlations.
The correlations between R630/R580 time point measures were then tested in a multivariate analysis with different phenotypic covariates (pH24, myoglobin, iron, ICDH activity, zinc, intramuscular fat, HCWT, relative shortloin muscle and fat weight, lamb age) accounted for in the model. Each phenotypic covariate was tested separately in each model, along with their squared term and interactions with fixed effects, before nonsignificant terms (
R630/R580 values at each time point (0, 24, 48, and 72 h) were then analyzed separately using linear mixed effects models. Fixed effects for site, year, slaughter group within site by year, sex and dam breed within sire type were included in these models along with random terms for sire and dam by year. Nonsignificant (
Linear mixed effects models were then used to assess the associations between R630/R580 values at different time points of simulated retail display. In these models fixed terms for production site, year and slaughter group within site by year were not included, given that this information is very unlikely to be known to a retailer interested in predicting lamb meat browning on display. R630/R580 at 0 h was incorporated as a covariate into the base models for R630/R580 at 24 and 72 h, along with its squared term and any interactions with sex and dam breed within sire type, with the same random effects and process of step-wise removal of nonsignificant effects (
These same models were then used to test the influence of phenotypic covariates (pH24, myoglobin, iron, ICDH activity, zinc, intramuscular fat, HCWT, shortloin muscle and fat weight, lamb age) on the associations between R630/R580 at different time points. The covariates were incorporated one at a time into each model along with their squared term and interaction with sex and dam breed within sire type. Nonsignificant (
Given the large range in R630/R580 values measured during display (
The mean, standard deviation and range in lamb carcass and muscle traits including loin R630/R580 at 0, 24, 48, and 72 h of simulated retail display are shown in
Lamb loin R630/R580 measured at 0, 24, 48, and 72 h of display were positively correlated. However, loin R630/R580 at 0 h had simple correlation coefficients of only 0.20, 0.18 and 0.10 with R630/R580 at 24, 48, and 72 h respectively (
Simple (upper, above the diagonal) and partial (lower, below the diagonal) correlation coefficients between R630/R580 measures at each time point of simulated retail display
R630/R580 | 0 h | 24 h | 48 h | 72 h |
0 h | – | 0.20 | 0.18 | 0.10 |
24 h | 0.40 | – | 0.82 | 0.81 |
48 h | 0.41 | 0.83 | – | 0.85 |
72 h | 0.32 | 0.80 | 0.87 | – |
Numerator and denominator degrees of freedom (NDF, DDF), F-values and
F-values | |||||
Effect | NDF, DDF | 0 h | 24 h | 48 h | 72 h |
Site | 4, 776 | 23.7 | 88.4 | 153.7 | 86.1 |
Year | 4, 3106 | 94.1 | 39.5 | 6.5 | 16.7 |
Slaughter group (site × year) | 62, 776 | 58.2 | 49.1 | 40.5 | 43.1 |
Sex Dam breed (sire type) | 5, 776 | 13.9 | 35.5 | 35.8 | 33.4 |
1All effects have a
Incorporating each carcass trait (
The influence of production effects (site and year of lamb production, slaughter group within site and year, lamb sex, sire type and dam breed) on loin R630/R580 after 72 h of retail display has been previously published (
The models described in
Association between loin R630/R580 (at 0, 24, 48, and 72 h of display) and loin muscle covariates: pH24, myoglobin concentration (mg/g wet muscle), iron concentration (mg/100g wet muscle), ICDH or isocitrate dehydrogenase activity (µmol/min/g of wet muscle) and IMF or intramuscular fat %. Solid lines within figures represent the predicted means, dotted lines represent the standard error of the mean (often too small to distinguish dotted lines clearly). The number above each figure represents the unit change in R630/R580 across a covariate range of 2 standard deviations from the mean. All associations represented in this figure are statistically significant (
Association between loin R630/R580 (at 0, 24, 48, and 72 h of display) and lamb carcass covariates: HCWT or hot carcass weight (kg), shortloin muscle weight (g) relative to carcass weight (representing carcass muscling), shortloin fat weight (g) relative to carcass weight (representing carcass fatness), and lamb age in days. Solid lines within figures represent the predicted means, dotted lines represent the standard error of the mean (often too small to distinguish dotted lines clearly). The number above each figure represents the unit change in R630/R580 across a covariate range of 2 standard deviations from the mean. All associations represented in this figure are statistically significant (
The iron concentration of the loin muscle was associated with R630/R580 only at 0 (
Increasing lamb HCWT from 16.0 to 29.5 kg increased loin R630/R580 by a similar magnitude throughout the simulated retail display (
The associations between (a) R630/R580 at 0 and 72 h, (b) R630/R580 at 0 and 24 h, (c) R630/R580 at 24 and 72 h, and (d) R630/R580 at 48 and 72 h are shown in
Associations between loin R630/R580 at a) 0 and 72 h of display; b) 0 and 24 h display; c) 24 and 72 h display and d) 48 and 72 h display. Solid lines represent predicted means, dashed lines represent the standard error from the predicted mean (standard error too small to distinguish dotted lines clearly) and individual points represent the residuals values of the lambs (difference of raw value from the predicted mean). The coefficient of determination (R-Square or
The influence of carcass traits on the coefficient of determination (R-square) and root mean square error (RMSE) of the associations between R630/R580 at different time points on simulated retail display
R630/R580 at 0 and 72 h | R630/R580 at 0 and 24 h | R630/R580 at 24 and 72 h | R630/R580 at 48 and 72 h | |||||
R630/R580 association: | 0.10 | 0.64 | 0.11 | 1.00 | 0.67 | 0.39 | 0.74 | 0.35 |
Carcass trait incorporated: | ||||||||
pH24 | 0.17 | 0.62 | 0.18 | 0.96 | 0.67 | 0.39 | 0.74 | 0.35 |
Myoglobin,mg/g muscle | 0.18 | 0.64 | 0.15 | 1.01 | 0.71 | 0.38 | 0.78 | 0.33 |
Iron, mg/100g muscle | 0.18 | 0.64 | 0.14 | 1.02 | 0.71 | 0.38 | 0.77 | 0.34 |
Isocitrate dehydrogenase activity, µmol/min/g1 | 0.22 (0.20) | 0.66 (0.66) | 0.16 (0.15) | 1.02 (1.03) | 0.72 (0.71) | 0.39 (0.40) | 0.78 (0.77) | 0.35 (0.36) |
Zinc, mg/100g muscle | 0.17 | 0.65 | 0.14 | 1.02 | 0.67* | 0.39* | 0.77 | 0.34 |
Intramuscular fat, % | 0.18 | 0.64 | 0.15 | 1.01 | 0.71 | 0.38 | 0.77 | 0.34 |
Hot carcass weight, kg | 0.13 | 0.63 | 0.16 | 0.97 | 0.68 | 0.38 | 0.74 | 0.35 |
Shortloin muscle weight, g2 | 0.14 | 0.63 | 0.18 | 0.96 | 0.68 | 0.38 | 0.74 | 0.35 |
Shortloin fat weight, g2 | 0.20 | 0.61 | 0.22 | 0.94 | 0.68 | 0.38 | 0.74 | 0.35 |
Age, d | 0.13 | 0.63 | 0.13 | 0.99 | 0.69 | 0.38 | 0.78 | 0.32 |
*Zinc did not impact the association between R630/R580 at 24 and 72 h (
1Isocitrate dehydrogenase activity was measured in only 1,804 lambs in this study. R-square values and RSME of the association between R630/R580 measures within this subset of lambs are shown in brackets.
2Shortloin muscle and fat weights were analyzed with hot carcass weight accounted for in the model to examine the influence of relative shortloin muscle weight (muscling) and relative shortloin fat weight (fatness) on R630/R580.
Contrary to our hypothesis, measuring the bloomed color (R630/R580) of loin meat at the start of retail display could not accurately predict meat browning after 3 d of retail display, even when used in combination with measurement of lamb age, loin pH24 and intramuscular fat concentration. This was shown in a number of ways. First through the poor correlations between lamb loin R630/R580 measured at the start of display with all subsequent R630/R580 measures across the 3 d simulated display (
In contrast to this, there were strong correlations between R630/R580 at 24, 48, and 72 h display, that varied little when corrected for fixed production factors or carcass phenotype measurements. Furthermore R630/R580 at both 24 and 48 h were strongly associated with R630/R580 at 72 h (
The ability of 24 h meat color rather than initial bloomed color to predict retail meat browning suggests that key factors determining retail meat browning are not manifesting their effects until some point between initial meat blooming and 24 h post-oxygenation. Generally the greatest difference in individual covariate effects on R630/R580 at 0, 24, 48, and 72 h were observed between 0 and 24 h of display (
The effects of muscle myoglobin, iron and ICDH activity on R630/R580 across display may also relate to the different mechanisms underpinning bloomed meat color development and subsequent meat browning on retail display. The substantial effect of myoglobin concentration on R630/R580 at 0 h aligns with other work (
Meat color measured shortly after blooming and 24 h later representing different meat color traits may account for the weak correlations between R630/R580 at 0 h and all subsequent measures of R630/R580, and the strong correlations between R630/R580 at 24, 48, and 72 h of display. Based on these results, measuring meat color at 24 h display could provide an excellent prediction of meat browning in the subsequent days of retail display. However, measuring meat color after 24 h of display is unrealistic in a commercial retail setting. Having only measured R630/R580 at 24 h intervals in this study, the possibility remains that R630/R580 measured earlier in retail display could better predict subsequent retail meat browning. For example, measuring R630/R580 6 h post-blooming may prove more feasible in a retail setting. Until work has been undertaken to determine if R630/R580 measured between 0 and 24 h could accurately predict meat browning, the value of R630/R580 may be limited to the use of 24 h measures in driving genetic improvement of lamb meat color stability via the development of a retail color breeding value for lamb meat.
Despite common mechanisms influencing bloomed lamb meat color and its stability over retail display, initial bloomed loin color was poorly correlated with subsequent meat color measures over a 72 h simulated retail display. Accounting for key muscle traits influencing meat color such as pH24, myoglobin, iron or intramuscular fat concentration did not substantially increase the correlations between bloomed meat color and subsequent meat color measured over retail display. The influence of carcass traits on meat color differed over the simulated retail display, initial bloomed meat color being principally determined by the concentration of myoglobin pigment present, while meat color after 72 h display was influenced by factors such as pH and ICDH activity, presumably due to their effects on myoglobin oxidation. Measuring bloomed meat color along with key carcass traits is therefore unlikely to be a useful tool for predicting color stability in a commercial application. In contrast, meat color measured at 24 h display can accurately predict the extent of lamb meat browning at 2 to 3 d of retail display. While measuring meat color after 24 h on display is not practical in a retail setting, these measures may prove valuable to the lamb industry in the development of a retail color breeding value to reduce the rate of lamb meat browning on retail display.
The authors gratefully acknowledge the financial support of the Cooperative Research Council for Sheep Industry Innovation. The authors also wish to thank the many people who assisted with site supervision, sample collection and measurement as well as abattoir staff. For the New South Wales sites; D. Hopkins, G. Refshauge, D. Stanley, S. Langfield, T. Bird-Gardiner, T. Lamb, E. Toohey and M. Kerr. For the Victorian sites; E. Ponnampalam, W. Brown, A Naththarampatha and M. Kerr. For the Western Australian sites; Kelly Pearce, K. Hart, M. Boyce and A. Williams.