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The importance of chill rate when characterising colour change of lamb meat during retail display
Meat Science 90 (2012) 478–484
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The importance of chill rate when characterising colour change of lamb meat during retail display R.H. Jacob ⁎, K.L. Thomson Department of Food and Agriculture WA, Baron Hay Court, South Perth WA 6151, Australia
a r t i c l e
i n f o
Article history: Received 11 May 2011 Received in revised form 14 September 2011 Accepted 14 September 2011 Keywords: Lamb Meat Colour stability Retail display Oxy/met
a b s t r a c t An experiment was conducted to compare the effect of two chilling rates (Con and Fast) on colour change of lamb meat during simulated retail display. Measurements were made on 3 muscles; LD (m. longisimuss dorsi), SM (m semimembranosus) and ST (m. semitendinous). Meat samples from 32 Merino crossbred lambs were vacuum packed and stored for 5 days at 2 °C, then cut and overwrapped in polyvinyl chloride film on black polystyrene trays, stored in a display cabinet at 4 °C with lights on and measured twice daily for 4 days, using a Hunterlab minilab 45/20 L D65, aperture 10°. Sarcomere length was shorter, shear force higher and colour change greater in meat from the Fast treatment compared to the Con treatment. Colour differences between treatments were likely due to oxygenation (bloom) as well as oxidation effects. Chill rate is important when characterising colour change during display and should be considered in measurement protocols. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction The surface of meat changes in hue of colour from red to brown during retail display, due to the formation of metmyoglobin (Faustman, 1990). Colour is an important visual cue denoting freshness and quality to consumers who prefer to purchase meat that is red rather than brown in colour. Findings that colour stability is likely to be heritable (King et al., 2010), has created an interest in creating a colour stability trait that could be used for animal breeding purposes (Mortimer et al., 2010). Spectrophotometer measurement has the advantage of being cheap and easy to do when compared to chemical analyses. However, the technical specifications of the instruments used to measure colour can alter the magnitude of colour estimates (Yancey & Kropf, 2008) and a need to standardise colour measurement techniques was recently highlighted by Tapp, Yancey, and Apple (2011). Furthermore, the potential for processing conditions to affect colour has been demonstrated (Hildrum, Nilsen, Bekken, & Naes, 2000; Ledward, 1985). To add further complication, different authors have applied the term colour stability to different colour parameters, meat processing and display scenarios. These range from a change in colour after freezing and thawing (Farouk & Lovatt, 2000) to a change in colour during simulated retail display when either exposed to (Jacob, D'Antuono, Smith, Pethick, & Warner, 2007), or not exposed to light (Johnson, McLean, Bain, Young, & Campbell, 2008).
⁎ Corresponding author. Tel.: + 61 8 93683470; fax: + 61 8 93682905. E-mail address: [email protected] (R.H. Jacob). 0309-1740/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.meatsci.2011.09.011
Standardising a definition and measurement protocol for colour stability would allow closer comparison between studies and would be useful for specific purposes such as calculating animal breeding values. Without standardisation, the various methods might represent different components of the relevant biochemical pathways, associated with meat colour and colour change during display (Faustman, 1990). Oxygen consumption due to mitochondria remaining active post mortem influences the depth from the surface of the oxymyoglobin layer, hence blooming time and the depth from the surface at which metmyoglobin forms (Bendall, 1972a). Oxygen consumption also generates free radicals that favour lipid per-oxidation (Bendall, 1972a; Tang et al., 2005), which recently was linked to myoglobin oxidation (Faustman, Mancini, Sun, & Suman, 2010). Antioxidants particularly vitamin E reduce the rates of lipid and myoglobin oxidation. The enzyme metmyoglobin reductase can also reduce metmyoglobin concentration once formed (Faustman, Chan, Schaefer, & Havens, 1998). In a commercial scenario, factors causal to colour change during retail display operate at different parts of the lamb meat supply chain yet potentially still interact with one another. For example, colour may be associated with rigor temperature when meat is fresh but not after an extended ageing period (Bendall, 1972a; Rosenvold & Wiklund, 2011). The requirement for vitamin E in animal diets (Faustman et al., 1998) depends on the meat ageing period (Jose, Pethick, Gardner, & Jacob, 2008) and packaging system. Factors that operate in one part of a supply chain, such as chilling regime, could potentially confound comparisons of factors in another part of a supply chain, such as animal genotype. This experiment compared colour change in lamb meat from 3 muscles; m. longissimus dorsi (LD), m. semimembranosus (SM) and m. semitendinous (ST) during simulated retail display, using two different chilling rates, conventional (Con) and fast (Fast).
R.H. Jacob, K.L. Thomson / Meat Science 90 (2012) 478–484
2. Method 2.1. Experimental design A chiller at a commercial abattoir was used for the Con treatment, and a prototype chiller designed to achieve a very fast chilling temperature time profile of 0 °C in 5 h (Sheridan, 1990), was used for the Fast treatment. The design consisted of a comparison between two post slaughter chilling treatments; conventional (Con) and fast (Fast) and consisted of 8 replicates of 4 carcases, consisting of 2 carcases per treatment, selected from 8 different consignments of cross bred merino lambs at a commercial abattoir, all from different vendors. Three replicates were conducted on one day and then another 5 replicates on the day following. The lambs were 5–8 months of age and the hot carcase weight was 20.7 ± 0.60 kg (mean ± SEM) at the time of slaughter. Lambs (n = 32) within a replicate were stratified according to carcase weight to low (19.2 ± 0.65 kg) and high (22.2 ± 0.88 kg) categories then assigned randomly to each treatment within these categories. 2.2. Slaughter details Lambs from both treatments were electrically stimulated immediately post dressing, about 25 min post stunning, using a portable medium voltage electrical stimulation unit supplied by Applied Sorting Technologies, 5 Kim Close, Bulleen, Victoria, 3105. Each carcase was stimulated individually by applying one electrode to the right front leg and the other electrode to the rear left leg. The setting used was a frequency of 15 Hz, current 1A, and pulse width of 2.5 μs; stimulation was applied for a period of 40 s. Carcases were placed in the chilling treatments immediately after electrical stunning was completed. Con carcases were chilled in a commercial chiller for 24 h (mean air temperature 6.8 ± 0.83 °C), and Fast carcases were placed in the prototype chiller for 3 h (mean air temperature − 10.2 ± 0.28 °C), then a commercial chiller for 21 h (mean air temperature 1.9 ± 0.11 °C) thereafter. 2.3. Measurements Carcases from both treatments were weighed post dressing (HSCW), then 3 h and 24 h after the commencement of chilling. Muscle pH was measured using a TPS W-80 metre (http://www.tps.com. au) attached to a Mettler Toledo LoT406-M6-DXK-S7/25 glass probe electrode and calibrated with standard buffers. Acidity (pH) was measured 0.4 and 0.5 h post slaughter to validate the effectiveness of electrical stimulation that occurred in between these times. Acidity (pH) and temperature of the short loin were also monitored during cooling to compare temperatures at rigor, although the time intervals were slightly different between treatments due to practical difficulties associated with measuring the Fast carcases. Con carcases were measured at 3.4, 6.5, 10.1, 13.2, and 21.3 h and the Fast carcases at 6.9, 18.15 and 19.7 h post slaughter. Low temperature of carcasses in the Fast treatment made prediction of temperature when the pH reached 6 difficult, so a prediction of the pH when the temperature reached 18 °C was used as an alternative. Ultimate pH (pHu) was measured at 6 days post slaughter at the time samples were processed for other meat quality measurements. Muscle temperature was measured in all carcases at an interval of 1 min for 24 h post slaughter. Muscle temperatures were monitored using Hobo U12-006 4 station external data loggers attached to TMC6-HC 100 mm stainless steel probes. The probes were placed in the loin and hind leg of each carcase with 4 probes and one logger per carcase. For the hind leg the probes were placed parallel to the surface at a depth of 5 mm (surface) and orthogonal to the surface at a depth of 100 mm in the SM, about 10 cm distal to the anus (deep). For the LD the probes were placed at the level of the
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thoracolumbar junction, parallel to the surface at a depth of 5 mm (surface) and on an acute angle such that the tip was located at a depth of 50 mm approximately. Ambient air temperature was measured every 15 min for each Fast replicate (n = 8) and with 1 logger continuously on each of the 2 days (n = 2) for the Con treatment. Samples from 3 muscles (LD, SM and ST), were collected one day after slaughter for measurement of shear force, sarcomere length, drip loss and colour stability. At the time of collection they were sliced into appropriate sizes as detailed later, weighed and packed in a plastic vacuum bag. Shear force samples were cut into 65 g blocks and frozen on day 1 and day 6 after slaughter accordingly. Shear force was measured with a Lloyd (www.lloyd-instruments.co.uk.) TAPlus Texture Analyser (1 kN), using the method described in Pearce (2008). Samples were thawed, cooked in a water bath at 70 °C for 30 min, cut into strips and sheared with an inverted V shaped blade passed through muscle orthogonal to the direction of muscle fibres. Sarcomere length samples were frozen one day after slaughter then measured with a laser diffraction technique described by Bouton, Harris, Ratcliff, and Roberts (1978). Drip loss was measured one day after slaughter by suspending 40 g (approximate) samples in a plastic bag for 24 h at 2 °C then reweighed. Drip loss was expressed as a percentage of the weight change between the initial and final weighing. Samples for colour measurement were stored at 2 °C for 6 days after slaughter then re-sliced to provide a fresh surface for measurement. Samples were allowed to bloom for 30 min at 2 °C before wrapping with polyvinyl chloride cling wrap (Resinite “DHW” Meat AEP, 3 μ thickness, oxygen transmission rate of 2300–3000 cm 3/100 sq in/24 h) on a black polystyrene foam tray and exposed to simulated retail display for 4 days in a cool room with air temperature kept in the range of − 2 to 6 °C. During this time the samples were exposed constantly to an overhead light source provided by 58 W Nelson Fluorescent Meat Display BRB Tubes of 1520 mm in length. This light source was suspended above the table at a sufficient height to provide a light intensity of 1000 lx at the table level. Light intensity was measured with a Dick Smith Electronics Light Meter Q1367. Colour measurements were conducted in the cool room used for display and samples were left wrapped during measurement. At each reading the measurement was replicated after rotating the spectrophotometer 90° in the horizontal plane. A Hunter Lab Mini Scan (tm) XE Plus (Cat. No. 6352, model No. 45/0-L, reading head diameter 37 mm) was used to measure light reflectance. The light source was set at “D65” with aperture set to 10°. The instrument was calibrated on a black glass then a white enamel tile as directed by the manufacturer's specifications. Oxy/met was calculated by dividing the percentage of light reflectance at 630 nm by the percentage of light reflectance at 580 nm (Hunt et al., 1991). Luminescence (L*), red green (a*), and blue yellow (b*) colour measurements were made with the same Hunterlab Mini Scan XE Plus instrument using the colour programme. Psychometric hue angle (h) and psychometric chroma (C) were calculated using the equations; Psychometric chroma C = (a 2 + b 2) 0.5, Psychometric hue h = tan −1 (b/a) (Hunt et al., 1991). 2.4. Statistical analyses Genstat version 12 was used for all statistical analyses (http:// www.vsni.co.uk). Analysis of variance was used to compare treatments (Con, Fast) for carcase weight, shear force, sarcomere length, pH at the time when temperature was 18 °C, and drip loss; with blocking for replicate within kill day. The pH at 18 °C was predicted using the method described in the Sheep CRC operational protocol (Pearce, 2008). For carcase weight the effects of time, treatment (Con, Fast) and the interactions between time and treatment were compared with blocking for replicate within kill day and initial weight as a covariate.
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Analyses of colour data (oxy/met, L*, a*, b*, h and C) were done using REML. A repeated measures model was fitted with carcases as subjects, retail display time, treatment (Con, Fast), and muscle (LD, SM) as fixed effects using a power correlation and no heterogeneity. 3. Results The results reconfirmed that processing conditions can influence colour change in lamb meat during retail display and the importance of this in relation to standardising the definition of meat colour stability is discussed.
Table 1 Temperature (°C) of m. longissimus dorsi (LD) and m. semimembranosus (SM) during chilling for Con and Fast treatments at the surface and deep within the muscle (values are means, T1h is temperature at 1 h, and T3h is temperature at 3 h, min is minimum and max is maximum temperature). Measure point
SM
LD
Surface
Deep
Con Max T 1h T 3h Min
26.82 20.7 18.21 4.42
37.28 31.93 23.12 5.51
Fast Max T1h T3h Min
28.88 4.66 − 1.98 − 2.05
37.62 23.09 5.08 0.13
Mean surface and deep
Surface
Deep
Mean surface and deep
32.05 26.31 20.67 4.97
26.81 22.37 17.87 4.06
36.67 28.59 21.23 5.31
31.74 25.48 19.55 4.69
33.25 13.87 1.55 − 0.96
27.67 8.19 − 1.86 − 1.91
37.04 20.8 3.84 0.37
32.36 14.5 0.99 − 0.77
3.1. Temperature The ambient air temperature in the Con treatment reached a minimum of 2.75 °C in 14 h and 15 min on day 1 and 1.25 °C in 22 h on day 2. By comparison the mean ambient air temperature in the Fast treatment was −10.2 ± 0.29 °C and the minimum temperature was reached in 45 min (Fig. 1). Temperatures of meat held in the Fast treatment reached subzero levels at the surface but not at deep levels for both LD and SM (Table 1). Mean temperatures were lower in LD than SM (Fig. 2) but remained above 0 °C for both muscles, such that the very fast chill criteria of 0 °C within 5 h (Joseph, 1996) was approached but not achieved. 3.2. Carcase yield There was an effect of time on carcase weight and an interaction between time and treatment (Fig. 3). This suggests that the yield loss may have occurred earlier in the chilling period for the Fast treatment, but there was no difference at the end of chilling. 3.3. Meat quality The mean pH reduced from 6.83 before to 6.58 (LSD,=0.08) immediately after electrical stimulation suggesting that electrical stimulation was effective. There was a significant effect (Pb 0.001) of treatment (Table 2) on the predicted pH at the time the temperature reached 18 °C, the pH at 24 h and pHu . For the LD pH was higher for Fast than for the Con carcases at 24 h post mortem, for pHu and when the temperature was 18 °C. The rate of pH decline with time in the LD appeared to be slower for Fast than Con carcases (Fig. 4) although data limitations make this comparison difficult. There was a significant effect (Pb 0.01) of muscle and treatment
Fig. 1. Ambient air temperatures (°C) for Con and Fast treatments over the 24 h chilling period (values are means).
and an interaction (P=0.03) between muscle and treatment for sarcomere length (Fig. 5). Sarcomere length was longer in the ST than the SM and longer in SM than LD. Sarcomere length was shorter in Fast compared to Con carcases and this effect was greatest in the ST. There was an effect of treatment and ageing period (P b 0.01) and no interaction between treatment and ageing period (P = 0.25) for shear force of LD (Fig. 6). Shear force was higher on day 1 than day 6 and higher for Fast than Con. LD aged at the same rate in both treatments but Con LD failed to reach the benchmark value for tenderness of 40 N (Bickerstaffe, Bekhit, Robertson, Roberts, & Geesink, 2001) by day 6. A similar effect of treatment was seen in SM at day 1, but SM was not measured for shear force at day 6. The mean shear force for Fast was 82.2 N compared to 60.4 N for Con (LSD for comparing treatments in SM = 7.08, P b 0.05). There was an effect of muscle (P b 0.01) but no effect of treatment (P = 0.86) on drip loss (Fig. 7). Drip loss was greater in the LD than the SM and greater in the SM than the ST. There were no effects of muscle or treatment on cooking loss (P N 0.05). The mean cooking loss across all samples was 20.53%. There was a trend for cooking loss to be greater for day 6 compared to day 1 (P = 0.093). 3.4. Meat colour There were main effects of time (P b 0.01) and muscle (P b 0.01) but not treatment on L* value (Fig. 8), with interactions between
Fig. 2. Muscle temperature (°C) of m. longissimus dorsi (LD) and m. semimembranosus (SM) during chilling for Con and Fast treatments (values are means of surface and deep probes for 8 animals).
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Fig. 3. Carcase weight loss (%) for Con and Fast treatments (values are means, LSD = least significant difference for comparing treatments at the same time, P b 0.05).
muscle and time (P = 0. 011), and treatment and time (P = 0.016). L* value increased with time between 0 and 0.5 days then remained constant until increasing again between 3 and 3.5 days. L* value was higher for LD than SM and for Con than Fast but at day 0 only for both comparisons. There were interactions between muscle and time (P = 0.011), and treatment and time (P = 0.016) for chroma (Fig. 9). Chroma increased between days 0 and 0.5; thereafter decreased such that day 0 was different to all display times except 3.5. Chroma was higher for LD than SM except for times 0, 2 and 2.5 days and was higher for Con than Fast except at time 3 and 3.5 days. For hue there were interactions between time and muscle, time and treatment, and muscle and treatment (Fig. 10). Hue increased with time, was lower for SM than LD at 0 days only, was lower for Con than Fast for all times except 0 days and was higher for SM than LD in Con but not Fast. For oxy/met there were interactions between time and muscle; time and treatment (Fig. 11). There was no difference between LD and SM at 0 and 0.5 days; thereafter the value was lower for SM than LD. Oxy/met was higher for Con than Fast until 3 days of display time. Furthermore oxy/met decreased between 0 and 0.5 days for Fast but not Con. There were no three way interactions between time, muscle and treatment for any of the colour parameters; L*, chroma, hue or oxy/met.
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Fig. 4. The pH of the m. longissimus dorsi (LD) at various times during chilling for carcases in Con and Fast treatments.
there was no effect of treatment on carcass yield loss, drip loss or cooking loss there appears to have been no effect of treatment on water holding capacity despite the pH being higher in the Fast treatment (Huff-Lonergan & Lonergan, 2005). Other effects such as sarcomere shortening may have influenced this result (Pearce, Rosenvold, Andersen, & Hopkins, 2011). 4.1. Definition of colour stability The effect of treatment on colour was profound with the differences between the mean values of all time points for both chroma and oxy/met (Figs. 9, 11) being greater between treatments than muscles. Importantly with this study both treatments were electrically stimulated and the difference between treatments was due to chilling rate alone. The change in meat colour during simulated display time (P b 0.001) depended on the parameter and muscle measured as well as the treatment. This highlights the importance of colour stability being clearly defined if used for purposes such as animal breeding. Oxy/met, being the ratio of reflectance values of light at 630 and 580 nm, represents a relative colour change due to either oxygenation (blooming) or oxidation (browning) because 580 nm and 630 nm are absorption maxima for oxymyoglobin and metmyoglobin
4. Discussion Although the temperature profiles with time were clearly different between treatments, the very fast chilling criterion of 0 °C within 5 h (Joseph, 1996) was not reached for Fast. Shorter sarcomere lengths (Fig. 5) and higher shear force values (Fig. 6) for treatment Fast relative to Con were consistent with this treatment inducing cold shortening (Locker & Hagyard, 1963), and such changes may not have occurred if the very fast chilling profile was achieved. As
Table 2 pH of the m. longissimus dorsi (LD) when temperature reached 18 °C (pH@18), at 24 h (pH24) and 6 days (pHu) post slaughter for Con and Fast treatment (values are means, values with different superscripts within a row are different (P b 0.05). Measure point
Treatment Con
Fast
pH @18 pH24 pHu
5.93a 5.63a 5.54a
6.35b 5.75b 5.63b
LSD (P b 0.05) 0.16 0.036 0.031
Fig. 5. The sarcomere length (μ) of m. longissimus dorsi (LD), m. semimembranosus (SM) and m. semitendinosus (ST) for Con and Fast treatments (values are means, bar is least significant difference for the interaction between muscle and treatment, P b 0.05).
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Fig. 6. The shear force (N) of the m. longissimus dorsi (LD) for Con and Fast treatments after 1 and 6 days ageing (values are means, bar is least significant difference for the interaction between ageing time and treatment, P b 0.05).
respectively. Fast meat reached a benchmark value for oxy/met of 3.5, below which consumers perceive meat to be brown in colour (Khliji, van de Vend, Lamba, Lanzac, & Hopkins, 2010; Morrissey, Jacob, & Pluske, 2008), about 1 day before the Con meat for SM and LD. A difference of 1 day in shelf life represents a commercially valuable period of time under retail conditions, so the magnitude of this effect is meaningful in a commercial sense. By comparison the hue and chroma was more stable over time and L* values no different for Fast compared to Con. Notably the change in hue occurred mostly within the first day for both treatments. Oxy/met therefore seems more useful for defining colour stability than L*, chroma or hue. 4.2. Colour change due to oxygenation There was a difference at time 0 between Con and Fast for L*, chroma and oxy/met, and this may have been influenced by the difference in pH between treatments. However, Hopkins and Fogarty (1998) concluded that the association between pHu and colour was not as strong for lamb as beef. The present result was similar to that reported by (Farouk & Lovatt, 2000) although they also observed a difference between rigor temperatures with hue. The difference in the present study between treatments for oxy/met at time 0 suggests either more metmyoglobin for Fast or alternatively more oxymyoglobin (or less deoxymyoglobin) for Con at this time. If the difference
Fig. 7. The drip loss (%) of the m. longissimus dorsi (LD), m. semimembranosus (SM) and m. semitendinosus (ST) for Con and Fast treatments.
Fig. 8. The lightness of colour (L*) m. longissimus dorsi (LD) and m. semimembranosus (SM) for Con and Fast treatments during the display period (values are means, bar is least significant difference for the interaction between display time and treatment, P b 0.05).
was due to more oxymyoglobin then this represents a difference in blooming characteristics. The day 0 measurements were done about 30 min after slicing so may have been too soon for metmyoglobin formation to account for the magnitude of the difference seen. Metmyoglobin formation occurs at the junction of oxygenated and deoxygenated layers, after slicing and exposure of the resulting surface to air (Faustman, 1990). The lower chroma at time 0 for Fast compared to Con is consistent with the oxy/met difference. Young, Priolo, Simmons, and West (1999) proposed that chroma was a better indicator of blooming than a* value because both a* and b* values increase during this period. The values for chroma and L* reached maximum values at 0.5 days for both treatments suggesting that blooming times were similar and continued for at least 12 h after slicing, for both treatments. A period of 30 min is often used to allow blooming prior to colour measurement (Pearce, 2008), as several studies particularly with beef have shown that blooming is complete in this time (Lee, Apple, Yancey, Sawyer, & Johnson, 2008). Tapp et al. (2011) reported that most papers do not report bloom time but those
Fig. 9. The chroma of m. longissimus dorsi (LD) and m. semimembranosus (SM) for Con and Fast treatments during the display period (values are means, bar is least significant difference for the interaction between display time and treatment, P b 0.05).
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4.3. Colour change due to oxidation
Fig. 10. The hue of m. longissimus dorsi (LD) and m. semimembranosus (SM) for Con and Fast treatments during the display period (values are means, bar is least significant difference for the interaction between display time and treatment, P b 0.05).
that do commonly report 30-60min. By contrast Farouk and Lovatt (2000) used 4 h in dark conditions. However, a period of 12 h is within the time indicated by Young et al. (1999) of 36 h required for completion of the blooming process in lamb meat. In the present study the reduction in mean oxy/met values between day 0 and 0.5 suggests that blooming was complete for the Fast treatment at the time of the day 0 measurement or at least before the measurement at 0.5 days. The lower oxy/met and chroma value at day 0 and this immediate reduction in oxy/met between day 0 and 0.5 for Fast but not Con, together may suggest that blooming time and depth of the oxygenated layer was influenced by treatment, being relatively shorter and shallower for Fast compared to Con. Unfortunately this was not confirmed with direct measurement of the depth of the oxygenated layer; but would be consistent with lower temperature pre-rigor increasing the oxygen consumption rate for the Fast treatment post rigor. When mitochondria remain active post mortem oxygen consumption is increased and the depth of the oxygenated layer reduced (Bendall, 1972b; Tang et al., 2005). There were differences in pH as well as temperature between treatments that might favour mitochondrial activity.
Fig. 11. The oxy/met ratio of m. longissimus dorsi (LD) and m. semimembranosus (SM) for Con and Fast treatments during the display period (values are means, bar is least significant difference for the interaction between display time and treatment, P b 0.05).
The reduction in oxy/met between time 0 and 0.5 days for Fast but not Con, might be interpreted as being due to oxidation of myoglobin commencing in the Fast before the Con treatment. However, because oxy/met is a relative measure only, the difference between Fast and Con treatments at day 0.5 was effectively confounded by the difference between treatments at day 0. If the oxygenated layer was wider in one treatment, the relative width of the metmyoglobin layer would be less as a proportion of the width of the oxymyoglobin layer for any given width of the metmyoglobin layer. Using oxy/met alone to compare rates of oxidation between treatments is therefore difficult, particularly if the treatment also affects oxygenation. This delay in oxy/met change of 0.5 days at the commencement of simulated display accounted for part but not all of the difference between treatments, for the time taken to reach the oxy/met threshold of 3.5, being 1 day longer for Con. Between 0.5 and 3.5 days the greater rate of change in oxy/met for the Fast treatment occurred when oxy/met was declining for both treatments, so was likely due to the rate of metmyoglobin formation being greater for the Fast treatment for this period. A higher oxygen consumption rate increases the rate of auto oxidation (Ledward, 1985) so may be associated with the changes at the beginning of the display period. The concentrations of antioxidants such as vitamin E are not likely to have changed directly by rigor temperature. Warner et al. (2010) also found a correlation between oxy/met value after 3 days of simulated retail display and temperature at pH 6 consistent with low temperature at the time of rigor causing meat to change from red to brown at a fast rate, when lamb meat was aged for 5 days post slaughter. So whilst the Fast treatment represents an extreme pH temperature profile, the effect may still be seen under commercial conditions. An effect on blooming time and depth may also explain the difference between the present study and the results of Rosenvold and Wiklund (2011). In the latter, a clear effect of temperature at rigor was observed on colour at 2 days but not after 7 weeks of anaerobic storage, except for at a very high rigor temperature of 42 °C. Mitochondrial activity is likely to cease after 6 weeks of storage (Bendall, 1972b) so temperature at the time of rigor may not influence blooming time and depth when sliced 7 weeks after vacuum packing. The effect of high rigor temperature was attributed to denaturation of myoglobin reductase. 4.4. Importance to measurement protocols Standardising measurement protocols for oxy/met is important because at any particular time during simulated display, oxy/met is likely to be a function of oxygenation as well as oxidation of myoglobin. Implicit in some current measurement protocols (Pearce, 2008) is an assumption that oxygenation is complete within a short period of slicing and independent of treatment once this period has elapsed. If this were not the case then oxygenation differences might influence oxy/met values early in the display period in the way speculated in the present study. Subsequently oxygenation time and depth may confound interpretations of colour change due to metmyoglobin formation, if colour stability is taken to be the change in oxy/met from the starting point of display. A simple way to adjust for this may be to measure the depth of the oxygenated layer at the time of colour measurement, or take factors such as temperature at rigor into account during statistical analyses. Further investigation is required to determine if this would account for differences seen at the commencement of simulated retail display and whether this is a practical solution in commercial scenarios. The practical implication of our findings is that colour change measured during simulated display conditions may be altered by chill rate which is a component of meat processing conditions. Comparisons between factors such as lamb genotype, a component of
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the animal production part of the supply chain, may need to consider chill rate either in the measurement protocol or during data analysis. Development of a more specific measure of colour stability such as oxy/met at a given time point may also be worthy of consideration. 5. Conclusions The nature of colour stability depends on the parameters measured and the temperature conditions under which the meat is processed. Fast chill rate reduced colour stability, represented by oxy/met value measured at a set time during simulated retail display, of lamb meat aged for 5 days. This effect could have been due to colour changes associated with blooming (oxygenation) at the beginning as well as differences in the rate of metmyoglobin formation (oxidation) during simulated retail display. Standardisation of chilling rate either directly or by use of statistical adjustments may be useful to standardise comparisons of colour stability in addition to standardisation of measuring techniques. Poor colour stability together with toughening, appears to be associated with temperatures at rigor that are low enough to cause sarcomere shortening, but above the target for very fast chilling. Acknowledgements The authors thank Meat and Livestock Australia and Food Processing Equipment for providing financial support for the project. Mr. Chris Sentence assisted with project design and operation. Staff in the DAFWA meat quality laboratory provided much appreciated technical support and Mr. Mario D'Antuono provided statistical advice. References Bendall, J. (1972). Consumption of oxygen by the muscle of beef animals and related species, and its effect on the colour of meat I. Oxygen consumption in pre-rigor muscle. Journal of the Science of Food and Agriculture, 23(1), 61–72. Bendall, J. (1972). Consumption of oxygen by the muscle of beef animals and related species, and its effect on the colour of meat II. Consumption of oxygen by postrigor muscle. Journal of the Science of Food and Agriculture, 23, 707–719. Bickerstaffe, R., Bekhit, A. E. D., Robertson, L. J., Roberts, N., & Geesink, G. H. (2001). Impact of introducing specifications on the tenderness of retail meat. Meat Science, 59, 303–315. Bouton, P., Harris, P., Ratcliff, D., & Roberts, D. (1978). Shear force measurements on cooked meat from sheep of various ages. Journal of Food Science, 43, 1038–1039. Farouk, M., & Lovatt, S. (2000). Initial chilling rate of pre-rigor beef muscles as an indicator of colour of thawed meat. Meat Science, 56, 139–144. Faustman, C. (1990). The biochemical basis for discolouration in fresh meat: A review. Journal of Muscle Foods, 1, 217–243. Faustman, C., Chan, W., Schaefer, D., & Havens, A. (1998). Beef color update: The role of vitamin E. Journal of Animal Science, 76, 1019–1026. Faustman, C., Mancini, R., Sun, Q., & Suman, S. (2010). Myoglobin and lipid oxidation: Mechanistic basis and control. In S. K. Lee (Ed.), International Congress of Meat Science and Technology Jeju, Korea.
Hildrum, K., Nilsen, B., Bekken, A., & Naes, T. (2000). Effects of chilling rate and low voltage electrical stimulation on sensory properties of ovine M. Longissimus. Journal of Muscle Foods, 11, 85–98. Hopkins, D., & Fogarty, N. (1998). Diverse lamb genotypes-2. Meat pH, colour and tenderness. Meat Science, 49, 477–488. Huff-Lonergan, E., & Lonergan, S. (2005). Mechanisms of water holding capacity: The role of postmortem biochemical and structural changes. Meat Science, 71, 194–204. Hunt, M., Acton, J., Benedict, R., Calkins, C., Cornforth, D., Jeremiah, L., et al. (1991). Guidelines for meat color evaluation (pp. 1–17). Kansas State University, Manhattan, KS: American Meat Science Association. Jacob, R., D'Antuono, M., Smith, G., Pethick, D., & Warner, R. (2007). The effect of age and electrical stimulation on the colour stability of fresh lamb meat. Australian Journal of Agricultural Research, 58, 374–382. Johnson, P., McLean, N., Bain, W., Young, E., & Campbell, A. (2008). Factors affecting colour stability of fresh chilled lamb meat. In D. Barber (Ed.), New Zealand Society of Animal Production, Vol. 68. (pp. 164–165) Brisbane, Australia. Jose, C., Pethick, D., Gardner, G., & Jacob, R. (2008). The colour stability of aged lamb benefits from vitamin E supplementation. In R. Venter (Ed.), International Congress of Meat Science and Technology. Capetown, South Africa: University of Stellenbosch. Joseph, R. (1996). Very fast chilling of beef and tenderness— A report from an EU concerted action. Meat Science, 43, S217–S227. Khliji, S., van de Vend, R., Lamba, T., Lanzac, M., & Hopkins, D. (2010). Relationship between consumer ranking of lamb colour and objective measures of colour. Meat Science, 85, 224–229. King, D., Shackleford, S., Kuehn, L., Kemp, C., Rodriguez, A., Thallman, R., et al. (2010). Contribution of genetic influences to animal to animal variation in myoglobin content and beef lean color stability. Journal of Animal Science, 88, 1160–1167. Ledward, D. (1985). Post-slaughter influences on the formation of metmyoglobin in beef muscles. Meat Science, 15, 149–171. Lee, M., Apple, J., Yancey, J., Sawyer, J., & Johnson, Z. (2008). Influence of wet ageing on bloom development in the longisimuss thoracis. Meat Science, 80, 703–707. Locker, R., & Hagyard, C. (1963). A cold shortening effect in beef muscles. Journal of the Science of Food and Agriculture, 14, 787–793. Morrissey, E., Jacob, R., & Pluske, J. (2008). Perception of red brown colour by consumers. In R. Venter (Ed.), International Congress of Meat Science and Technology Cape Town SA. Mortimer, S., van der Werf, J., Jacob, R., Pethick, D., Pearce, K., Warner, R., et al. (2010). Preliminary estimates of genetic parameters for carcasss and meat quality traits in Australian sheep. Animal Production Science, 50, 1135–1144. Pearce, K. (2008). (1 ed.). Sheep CRC Program 3: Next Generation Meat Quality Project 3.1.1 Phenotyping the Information Nucleus Flocks: Operational Protocol Series, Vol. 1. (pp. 107). Perth, Western Australia: Murdoch University. Pearce, K., Rosenvold, K., Andersen, H., & Hopkins, D. (2011). Water distribution and mobility in meat during the conversion of muscle to meat and ageing and the impacts on fresh meat quality attributes— A review. Meat Science, 89, 111–124. Rosenvold, K., & Wiklund, E. (2011). Retail colour display life of chilled lamb as affected by processing conditions and storage temperature. Meat Science, 88, 354–360. Sheridan, J. J. (1990). The ultra-rapid chilling of lamb carcasses. Meat Science, 28, 31–50. Tang, J., Faustman, C., Hoagland, T., Mancini, R., Seyfert, M., & Hunt, M. (2005). Post mortem oxygen consumption by mitochondria and its effects on myoglobin form and stability. Journal of Agricultural and Food Chemistry, 53, 1223–1230. Tapp, W., Yancey, J., & Apple, J. (2011). How is the instrumental color of meat measured. Meat Science, 89, 1–5. Warner, R., Jacob, R., Edwards, J. H., McDonagh, M., Pearce, K., Geesink, G., et al. (2010). Quality of lamb meat from the Information Nucleus Flock. Animal Production Science, 50, 1–12. Yancey, J., & Kropf, D. (2008). Instrumental reflectance values of fresh pork are dependent on aperture size. Meat Science, 79, 734–739. Young, O., Priolo, A., Simmons, N., & West, J. (1999). Effects of rigor attainment temperature on meat blooming and colour on display. Meat Science, 52, 47–56.
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The importance of chill rate when characterising colour change of lamb meat during retail display R.H. Jacob ⁎, K.L. Thomson Department of Food and Agriculture WA, Baron Hay Court, South Perth WA 6151, Australia
a r t i c l e
i n f o
Article history: Received 11 May 2011 Received in revised form 14 September 2011 Accepted 14 September 2011 Keywords: Lamb Meat Colour stability Retail display Oxy/met
a b s t r a c t An experiment was conducted to compare the effect of two chilling rates (Con and Fast) on colour change of lamb meat during simulated retail display. Measurements were made on 3 muscles; LD (m. longisimuss dorsi), SM (m semimembranosus) and ST (m. semitendinous). Meat samples from 32 Merino crossbred lambs were vacuum packed and stored for 5 days at 2 °C, then cut and overwrapped in polyvinyl chloride film on black polystyrene trays, stored in a display cabinet at 4 °C with lights on and measured twice daily for 4 days, using a Hunterlab minilab 45/20 L D65, aperture 10°. Sarcomere length was shorter, shear force higher and colour change greater in meat from the Fast treatment compared to the Con treatment. Colour differences between treatments were likely due to oxygenation (bloom) as well as oxidation effects. Chill rate is important when characterising colour change during display and should be considered in measurement protocols. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction The surface of meat changes in hue of colour from red to brown during retail display, due to the formation of metmyoglobin (Faustman, 1990). Colour is an important visual cue denoting freshness and quality to consumers who prefer to purchase meat that is red rather than brown in colour. Findings that colour stability is likely to be heritable (King et al., 2010), has created an interest in creating a colour stability trait that could be used for animal breeding purposes (Mortimer et al., 2010). Spectrophotometer measurement has the advantage of being cheap and easy to do when compared to chemical analyses. However, the technical specifications of the instruments used to measure colour can alter the magnitude of colour estimates (Yancey & Kropf, 2008) and a need to standardise colour measurement techniques was recently highlighted by Tapp, Yancey, and Apple (2011). Furthermore, the potential for processing conditions to affect colour has been demonstrated (Hildrum, Nilsen, Bekken, & Naes, 2000; Ledward, 1985). To add further complication, different authors have applied the term colour stability to different colour parameters, meat processing and display scenarios. These range from a change in colour after freezing and thawing (Farouk & Lovatt, 2000) to a change in colour during simulated retail display when either exposed to (Jacob, D'Antuono, Smith, Pethick, & Warner, 2007), or not exposed to light (Johnson, McLean, Bain, Young, & Campbell, 2008).
⁎ Corresponding author. Tel.: + 61 8 93683470; fax: + 61 8 93682905. E-mail address: [email protected] (R.H. Jacob). 0309-1740/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.meatsci.2011.09.011
Standardising a definition and measurement protocol for colour stability would allow closer comparison between studies and would be useful for specific purposes such as calculating animal breeding values. Without standardisation, the various methods might represent different components of the relevant biochemical pathways, associated with meat colour and colour change during display (Faustman, 1990). Oxygen consumption due to mitochondria remaining active post mortem influences the depth from the surface of the oxymyoglobin layer, hence blooming time and the depth from the surface at which metmyoglobin forms (Bendall, 1972a). Oxygen consumption also generates free radicals that favour lipid per-oxidation (Bendall, 1972a; Tang et al., 2005), which recently was linked to myoglobin oxidation (Faustman, Mancini, Sun, & Suman, 2010). Antioxidants particularly vitamin E reduce the rates of lipid and myoglobin oxidation. The enzyme metmyoglobin reductase can also reduce metmyoglobin concentration once formed (Faustman, Chan, Schaefer, & Havens, 1998). In a commercial scenario, factors causal to colour change during retail display operate at different parts of the lamb meat supply chain yet potentially still interact with one another. For example, colour may be associated with rigor temperature when meat is fresh but not after an extended ageing period (Bendall, 1972a; Rosenvold & Wiklund, 2011). The requirement for vitamin E in animal diets (Faustman et al., 1998) depends on the meat ageing period (Jose, Pethick, Gardner, & Jacob, 2008) and packaging system. Factors that operate in one part of a supply chain, such as chilling regime, could potentially confound comparisons of factors in another part of a supply chain, such as animal genotype. This experiment compared colour change in lamb meat from 3 muscles; m. longissimus dorsi (LD), m. semimembranosus (SM) and m. semitendinous (ST) during simulated retail display, using two different chilling rates, conventional (Con) and fast (Fast).
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2. Method 2.1. Experimental design A chiller at a commercial abattoir was used for the Con treatment, and a prototype chiller designed to achieve a very fast chilling temperature time profile of 0 °C in 5 h (Sheridan, 1990), was used for the Fast treatment. The design consisted of a comparison between two post slaughter chilling treatments; conventional (Con) and fast (Fast) and consisted of 8 replicates of 4 carcases, consisting of 2 carcases per treatment, selected from 8 different consignments of cross bred merino lambs at a commercial abattoir, all from different vendors. Three replicates were conducted on one day and then another 5 replicates on the day following. The lambs were 5–8 months of age and the hot carcase weight was 20.7 ± 0.60 kg (mean ± SEM) at the time of slaughter. Lambs (n = 32) within a replicate were stratified according to carcase weight to low (19.2 ± 0.65 kg) and high (22.2 ± 0.88 kg) categories then assigned randomly to each treatment within these categories. 2.2. Slaughter details Lambs from both treatments were electrically stimulated immediately post dressing, about 25 min post stunning, using a portable medium voltage electrical stimulation unit supplied by Applied Sorting Technologies, 5 Kim Close, Bulleen, Victoria, 3105. Each carcase was stimulated individually by applying one electrode to the right front leg and the other electrode to the rear left leg. The setting used was a frequency of 15 Hz, current 1A, and pulse width of 2.5 μs; stimulation was applied for a period of 40 s. Carcases were placed in the chilling treatments immediately after electrical stunning was completed. Con carcases were chilled in a commercial chiller for 24 h (mean air temperature 6.8 ± 0.83 °C), and Fast carcases were placed in the prototype chiller for 3 h (mean air temperature − 10.2 ± 0.28 °C), then a commercial chiller for 21 h (mean air temperature 1.9 ± 0.11 °C) thereafter. 2.3. Measurements Carcases from both treatments were weighed post dressing (HSCW), then 3 h and 24 h after the commencement of chilling. Muscle pH was measured using a TPS W-80 metre (http://www.tps.com. au) attached to a Mettler Toledo LoT406-M6-DXK-S7/25 glass probe electrode and calibrated with standard buffers. Acidity (pH) was measured 0.4 and 0.5 h post slaughter to validate the effectiveness of electrical stimulation that occurred in between these times. Acidity (pH) and temperature of the short loin were also monitored during cooling to compare temperatures at rigor, although the time intervals were slightly different between treatments due to practical difficulties associated with measuring the Fast carcases. Con carcases were measured at 3.4, 6.5, 10.1, 13.2, and 21.3 h and the Fast carcases at 6.9, 18.15 and 19.7 h post slaughter. Low temperature of carcasses in the Fast treatment made prediction of temperature when the pH reached 6 difficult, so a prediction of the pH when the temperature reached 18 °C was used as an alternative. Ultimate pH (pHu) was measured at 6 days post slaughter at the time samples were processed for other meat quality measurements. Muscle temperature was measured in all carcases at an interval of 1 min for 24 h post slaughter. Muscle temperatures were monitored using Hobo U12-006 4 station external data loggers attached to TMC6-HC 100 mm stainless steel probes. The probes were placed in the loin and hind leg of each carcase with 4 probes and one logger per carcase. For the hind leg the probes were placed parallel to the surface at a depth of 5 mm (surface) and orthogonal to the surface at a depth of 100 mm in the SM, about 10 cm distal to the anus (deep). For the LD the probes were placed at the level of the
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thoracolumbar junction, parallel to the surface at a depth of 5 mm (surface) and on an acute angle such that the tip was located at a depth of 50 mm approximately. Ambient air temperature was measured every 15 min for each Fast replicate (n = 8) and with 1 logger continuously on each of the 2 days (n = 2) for the Con treatment. Samples from 3 muscles (LD, SM and ST), were collected one day after slaughter for measurement of shear force, sarcomere length, drip loss and colour stability. At the time of collection they were sliced into appropriate sizes as detailed later, weighed and packed in a plastic vacuum bag. Shear force samples were cut into 65 g blocks and frozen on day 1 and day 6 after slaughter accordingly. Shear force was measured with a Lloyd (www.lloyd-instruments.co.uk.) TAPlus Texture Analyser (1 kN), using the method described in Pearce (2008). Samples were thawed, cooked in a water bath at 70 °C for 30 min, cut into strips and sheared with an inverted V shaped blade passed through muscle orthogonal to the direction of muscle fibres. Sarcomere length samples were frozen one day after slaughter then measured with a laser diffraction technique described by Bouton, Harris, Ratcliff, and Roberts (1978). Drip loss was measured one day after slaughter by suspending 40 g (approximate) samples in a plastic bag for 24 h at 2 °C then reweighed. Drip loss was expressed as a percentage of the weight change between the initial and final weighing. Samples for colour measurement were stored at 2 °C for 6 days after slaughter then re-sliced to provide a fresh surface for measurement. Samples were allowed to bloom for 30 min at 2 °C before wrapping with polyvinyl chloride cling wrap (Resinite “DHW” Meat AEP, 3 μ thickness, oxygen transmission rate of 2300–3000 cm 3/100 sq in/24 h) on a black polystyrene foam tray and exposed to simulated retail display for 4 days in a cool room with air temperature kept in the range of − 2 to 6 °C. During this time the samples were exposed constantly to an overhead light source provided by 58 W Nelson Fluorescent Meat Display BRB Tubes of 1520 mm in length. This light source was suspended above the table at a sufficient height to provide a light intensity of 1000 lx at the table level. Light intensity was measured with a Dick Smith Electronics Light Meter Q1367. Colour measurements were conducted in the cool room used for display and samples were left wrapped during measurement. At each reading the measurement was replicated after rotating the spectrophotometer 90° in the horizontal plane. A Hunter Lab Mini Scan (tm) XE Plus (Cat. No. 6352, model No. 45/0-L, reading head diameter 37 mm) was used to measure light reflectance. The light source was set at “D65” with aperture set to 10°. The instrument was calibrated on a black glass then a white enamel tile as directed by the manufacturer's specifications. Oxy/met was calculated by dividing the percentage of light reflectance at 630 nm by the percentage of light reflectance at 580 nm (Hunt et al., 1991). Luminescence (L*), red green (a*), and blue yellow (b*) colour measurements were made with the same Hunterlab Mini Scan XE Plus instrument using the colour programme. Psychometric hue angle (h) and psychometric chroma (C) were calculated using the equations; Psychometric chroma C = (a 2 + b 2) 0.5, Psychometric hue h = tan −1 (b/a) (Hunt et al., 1991). 2.4. Statistical analyses Genstat version 12 was used for all statistical analyses (http:// www.vsni.co.uk). Analysis of variance was used to compare treatments (Con, Fast) for carcase weight, shear force, sarcomere length, pH at the time when temperature was 18 °C, and drip loss; with blocking for replicate within kill day. The pH at 18 °C was predicted using the method described in the Sheep CRC operational protocol (Pearce, 2008). For carcase weight the effects of time, treatment (Con, Fast) and the interactions between time and treatment were compared with blocking for replicate within kill day and initial weight as a covariate.
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Analyses of colour data (oxy/met, L*, a*, b*, h and C) were done using REML. A repeated measures model was fitted with carcases as subjects, retail display time, treatment (Con, Fast), and muscle (LD, SM) as fixed effects using a power correlation and no heterogeneity. 3. Results The results reconfirmed that processing conditions can influence colour change in lamb meat during retail display and the importance of this in relation to standardising the definition of meat colour stability is discussed.
Table 1 Temperature (°C) of m. longissimus dorsi (LD) and m. semimembranosus (SM) during chilling for Con and Fast treatments at the surface and deep within the muscle (values are means, T1h is temperature at 1 h, and T3h is temperature at 3 h, min is minimum and max is maximum temperature). Measure point
SM
LD
Surface
Deep
Con Max T 1h T 3h Min
26.82 20.7 18.21 4.42
37.28 31.93 23.12 5.51
Fast Max T1h T3h Min
28.88 4.66 − 1.98 − 2.05
37.62 23.09 5.08 0.13
Mean surface and deep
Surface
Deep
Mean surface and deep
32.05 26.31 20.67 4.97
26.81 22.37 17.87 4.06
36.67 28.59 21.23 5.31
31.74 25.48 19.55 4.69
33.25 13.87 1.55 − 0.96
27.67 8.19 − 1.86 − 1.91
37.04 20.8 3.84 0.37
32.36 14.5 0.99 − 0.77
3.1. Temperature The ambient air temperature in the Con treatment reached a minimum of 2.75 °C in 14 h and 15 min on day 1 and 1.25 °C in 22 h on day 2. By comparison the mean ambient air temperature in the Fast treatment was −10.2 ± 0.29 °C and the minimum temperature was reached in 45 min (Fig. 1). Temperatures of meat held in the Fast treatment reached subzero levels at the surface but not at deep levels for both LD and SM (Table 1). Mean temperatures were lower in LD than SM (Fig. 2) but remained above 0 °C for both muscles, such that the very fast chill criteria of 0 °C within 5 h (Joseph, 1996) was approached but not achieved. 3.2. Carcase yield There was an effect of time on carcase weight and an interaction between time and treatment (Fig. 3). This suggests that the yield loss may have occurred earlier in the chilling period for the Fast treatment, but there was no difference at the end of chilling. 3.3. Meat quality The mean pH reduced from 6.83 before to 6.58 (LSD,=0.08) immediately after electrical stimulation suggesting that electrical stimulation was effective. There was a significant effect (Pb 0.001) of treatment (Table 2) on the predicted pH at the time the temperature reached 18 °C, the pH at 24 h and pHu . For the LD pH was higher for Fast than for the Con carcases at 24 h post mortem, for pHu and when the temperature was 18 °C. The rate of pH decline with time in the LD appeared to be slower for Fast than Con carcases (Fig. 4) although data limitations make this comparison difficult. There was a significant effect (Pb 0.01) of muscle and treatment
Fig. 1. Ambient air temperatures (°C) for Con and Fast treatments over the 24 h chilling period (values are means).
and an interaction (P=0.03) between muscle and treatment for sarcomere length (Fig. 5). Sarcomere length was longer in the ST than the SM and longer in SM than LD. Sarcomere length was shorter in Fast compared to Con carcases and this effect was greatest in the ST. There was an effect of treatment and ageing period (P b 0.01) and no interaction between treatment and ageing period (P = 0.25) for shear force of LD (Fig. 6). Shear force was higher on day 1 than day 6 and higher for Fast than Con. LD aged at the same rate in both treatments but Con LD failed to reach the benchmark value for tenderness of 40 N (Bickerstaffe, Bekhit, Robertson, Roberts, & Geesink, 2001) by day 6. A similar effect of treatment was seen in SM at day 1, but SM was not measured for shear force at day 6. The mean shear force for Fast was 82.2 N compared to 60.4 N for Con (LSD for comparing treatments in SM = 7.08, P b 0.05). There was an effect of muscle (P b 0.01) but no effect of treatment (P = 0.86) on drip loss (Fig. 7). Drip loss was greater in the LD than the SM and greater in the SM than the ST. There were no effects of muscle or treatment on cooking loss (P N 0.05). The mean cooking loss across all samples was 20.53%. There was a trend for cooking loss to be greater for day 6 compared to day 1 (P = 0.093). 3.4. Meat colour There were main effects of time (P b 0.01) and muscle (P b 0.01) but not treatment on L* value (Fig. 8), with interactions between
Fig. 2. Muscle temperature (°C) of m. longissimus dorsi (LD) and m. semimembranosus (SM) during chilling for Con and Fast treatments (values are means of surface and deep probes for 8 animals).
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Fig. 3. Carcase weight loss (%) for Con and Fast treatments (values are means, LSD = least significant difference for comparing treatments at the same time, P b 0.05).
muscle and time (P = 0. 011), and treatment and time (P = 0.016). L* value increased with time between 0 and 0.5 days then remained constant until increasing again between 3 and 3.5 days. L* value was higher for LD than SM and for Con than Fast but at day 0 only for both comparisons. There were interactions between muscle and time (P = 0.011), and treatment and time (P = 0.016) for chroma (Fig. 9). Chroma increased between days 0 and 0.5; thereafter decreased such that day 0 was different to all display times except 3.5. Chroma was higher for LD than SM except for times 0, 2 and 2.5 days and was higher for Con than Fast except at time 3 and 3.5 days. For hue there were interactions between time and muscle, time and treatment, and muscle and treatment (Fig. 10). Hue increased with time, was lower for SM than LD at 0 days only, was lower for Con than Fast for all times except 0 days and was higher for SM than LD in Con but not Fast. For oxy/met there were interactions between time and muscle; time and treatment (Fig. 11). There was no difference between LD and SM at 0 and 0.5 days; thereafter the value was lower for SM than LD. Oxy/met was higher for Con than Fast until 3 days of display time. Furthermore oxy/met decreased between 0 and 0.5 days for Fast but not Con. There were no three way interactions between time, muscle and treatment for any of the colour parameters; L*, chroma, hue or oxy/met.
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Fig. 4. The pH of the m. longissimus dorsi (LD) at various times during chilling for carcases in Con and Fast treatments.
there was no effect of treatment on carcass yield loss, drip loss or cooking loss there appears to have been no effect of treatment on water holding capacity despite the pH being higher in the Fast treatment (Huff-Lonergan & Lonergan, 2005). Other effects such as sarcomere shortening may have influenced this result (Pearce, Rosenvold, Andersen, & Hopkins, 2011). 4.1. Definition of colour stability The effect of treatment on colour was profound with the differences between the mean values of all time points for both chroma and oxy/met (Figs. 9, 11) being greater between treatments than muscles. Importantly with this study both treatments were electrically stimulated and the difference between treatments was due to chilling rate alone. The change in meat colour during simulated display time (P b 0.001) depended on the parameter and muscle measured as well as the treatment. This highlights the importance of colour stability being clearly defined if used for purposes such as animal breeding. Oxy/met, being the ratio of reflectance values of light at 630 and 580 nm, represents a relative colour change due to either oxygenation (blooming) or oxidation (browning) because 580 nm and 630 nm are absorption maxima for oxymyoglobin and metmyoglobin
4. Discussion Although the temperature profiles with time were clearly different between treatments, the very fast chilling criterion of 0 °C within 5 h (Joseph, 1996) was not reached for Fast. Shorter sarcomere lengths (Fig. 5) and higher shear force values (Fig. 6) for treatment Fast relative to Con were consistent with this treatment inducing cold shortening (Locker & Hagyard, 1963), and such changes may not have occurred if the very fast chilling profile was achieved. As
Table 2 pH of the m. longissimus dorsi (LD) when temperature reached 18 °C (pH@18), at 24 h (pH24) and 6 days (pHu) post slaughter for Con and Fast treatment (values are means, values with different superscripts within a row are different (P b 0.05). Measure point
Treatment Con
Fast
pH @18 pH24 pHu
5.93a 5.63a 5.54a
6.35b 5.75b 5.63b
LSD (P b 0.05) 0.16 0.036 0.031
Fig. 5. The sarcomere length (μ) of m. longissimus dorsi (LD), m. semimembranosus (SM) and m. semitendinosus (ST) for Con and Fast treatments (values are means, bar is least significant difference for the interaction between muscle and treatment, P b 0.05).
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Fig. 6. The shear force (N) of the m. longissimus dorsi (LD) for Con and Fast treatments after 1 and 6 days ageing (values are means, bar is least significant difference for the interaction between ageing time and treatment, P b 0.05).
respectively. Fast meat reached a benchmark value for oxy/met of 3.5, below which consumers perceive meat to be brown in colour (Khliji, van de Vend, Lamba, Lanzac, & Hopkins, 2010; Morrissey, Jacob, & Pluske, 2008), about 1 day before the Con meat for SM and LD. A difference of 1 day in shelf life represents a commercially valuable period of time under retail conditions, so the magnitude of this effect is meaningful in a commercial sense. By comparison the hue and chroma was more stable over time and L* values no different for Fast compared to Con. Notably the change in hue occurred mostly within the first day for both treatments. Oxy/met therefore seems more useful for defining colour stability than L*, chroma or hue. 4.2. Colour change due to oxygenation There was a difference at time 0 between Con and Fast for L*, chroma and oxy/met, and this may have been influenced by the difference in pH between treatments. However, Hopkins and Fogarty (1998) concluded that the association between pHu and colour was not as strong for lamb as beef. The present result was similar to that reported by (Farouk & Lovatt, 2000) although they also observed a difference between rigor temperatures with hue. The difference in the present study between treatments for oxy/met at time 0 suggests either more metmyoglobin for Fast or alternatively more oxymyoglobin (or less deoxymyoglobin) for Con at this time. If the difference
Fig. 7. The drip loss (%) of the m. longissimus dorsi (LD), m. semimembranosus (SM) and m. semitendinosus (ST) for Con and Fast treatments.
Fig. 8. The lightness of colour (L*) m. longissimus dorsi (LD) and m. semimembranosus (SM) for Con and Fast treatments during the display period (values are means, bar is least significant difference for the interaction between display time and treatment, P b 0.05).
was due to more oxymyoglobin then this represents a difference in blooming characteristics. The day 0 measurements were done about 30 min after slicing so may have been too soon for metmyoglobin formation to account for the magnitude of the difference seen. Metmyoglobin formation occurs at the junction of oxygenated and deoxygenated layers, after slicing and exposure of the resulting surface to air (Faustman, 1990). The lower chroma at time 0 for Fast compared to Con is consistent with the oxy/met difference. Young, Priolo, Simmons, and West (1999) proposed that chroma was a better indicator of blooming than a* value because both a* and b* values increase during this period. The values for chroma and L* reached maximum values at 0.5 days for both treatments suggesting that blooming times were similar and continued for at least 12 h after slicing, for both treatments. A period of 30 min is often used to allow blooming prior to colour measurement (Pearce, 2008), as several studies particularly with beef have shown that blooming is complete in this time (Lee, Apple, Yancey, Sawyer, & Johnson, 2008). Tapp et al. (2011) reported that most papers do not report bloom time but those
Fig. 9. The chroma of m. longissimus dorsi (LD) and m. semimembranosus (SM) for Con and Fast treatments during the display period (values are means, bar is least significant difference for the interaction between display time and treatment, P b 0.05).
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4.3. Colour change due to oxidation
Fig. 10. The hue of m. longissimus dorsi (LD) and m. semimembranosus (SM) for Con and Fast treatments during the display period (values are means, bar is least significant difference for the interaction between display time and treatment, P b 0.05).
that do commonly report 30-60min. By contrast Farouk and Lovatt (2000) used 4 h in dark conditions. However, a period of 12 h is within the time indicated by Young et al. (1999) of 36 h required for completion of the blooming process in lamb meat. In the present study the reduction in mean oxy/met values between day 0 and 0.5 suggests that blooming was complete for the Fast treatment at the time of the day 0 measurement or at least before the measurement at 0.5 days. The lower oxy/met and chroma value at day 0 and this immediate reduction in oxy/met between day 0 and 0.5 for Fast but not Con, together may suggest that blooming time and depth of the oxygenated layer was influenced by treatment, being relatively shorter and shallower for Fast compared to Con. Unfortunately this was not confirmed with direct measurement of the depth of the oxygenated layer; but would be consistent with lower temperature pre-rigor increasing the oxygen consumption rate for the Fast treatment post rigor. When mitochondria remain active post mortem oxygen consumption is increased and the depth of the oxygenated layer reduced (Bendall, 1972b; Tang et al., 2005). There were differences in pH as well as temperature between treatments that might favour mitochondrial activity.
Fig. 11. The oxy/met ratio of m. longissimus dorsi (LD) and m. semimembranosus (SM) for Con and Fast treatments during the display period (values are means, bar is least significant difference for the interaction between display time and treatment, P b 0.05).
The reduction in oxy/met between time 0 and 0.5 days for Fast but not Con, might be interpreted as being due to oxidation of myoglobin commencing in the Fast before the Con treatment. However, because oxy/met is a relative measure only, the difference between Fast and Con treatments at day 0.5 was effectively confounded by the difference between treatments at day 0. If the oxygenated layer was wider in one treatment, the relative width of the metmyoglobin layer would be less as a proportion of the width of the oxymyoglobin layer for any given width of the metmyoglobin layer. Using oxy/met alone to compare rates of oxidation between treatments is therefore difficult, particularly if the treatment also affects oxygenation. This delay in oxy/met change of 0.5 days at the commencement of simulated display accounted for part but not all of the difference between treatments, for the time taken to reach the oxy/met threshold of 3.5, being 1 day longer for Con. Between 0.5 and 3.5 days the greater rate of change in oxy/met for the Fast treatment occurred when oxy/met was declining for both treatments, so was likely due to the rate of metmyoglobin formation being greater for the Fast treatment for this period. A higher oxygen consumption rate increases the rate of auto oxidation (Ledward, 1985) so may be associated with the changes at the beginning of the display period. The concentrations of antioxidants such as vitamin E are not likely to have changed directly by rigor temperature. Warner et al. (2010) also found a correlation between oxy/met value after 3 days of simulated retail display and temperature at pH 6 consistent with low temperature at the time of rigor causing meat to change from red to brown at a fast rate, when lamb meat was aged for 5 days post slaughter. So whilst the Fast treatment represents an extreme pH temperature profile, the effect may still be seen under commercial conditions. An effect on blooming time and depth may also explain the difference between the present study and the results of Rosenvold and Wiklund (2011). In the latter, a clear effect of temperature at rigor was observed on colour at 2 days but not after 7 weeks of anaerobic storage, except for at a very high rigor temperature of 42 °C. Mitochondrial activity is likely to cease after 6 weeks of storage (Bendall, 1972b) so temperature at the time of rigor may not influence blooming time and depth when sliced 7 weeks after vacuum packing. The effect of high rigor temperature was attributed to denaturation of myoglobin reductase. 4.4. Importance to measurement protocols Standardising measurement protocols for oxy/met is important because at any particular time during simulated display, oxy/met is likely to be a function of oxygenation as well as oxidation of myoglobin. Implicit in some current measurement protocols (Pearce, 2008) is an assumption that oxygenation is complete within a short period of slicing and independent of treatment once this period has elapsed. If this were not the case then oxygenation differences might influence oxy/met values early in the display period in the way speculated in the present study. Subsequently oxygenation time and depth may confound interpretations of colour change due to metmyoglobin formation, if colour stability is taken to be the change in oxy/met from the starting point of display. A simple way to adjust for this may be to measure the depth of the oxygenated layer at the time of colour measurement, or take factors such as temperature at rigor into account during statistical analyses. Further investigation is required to determine if this would account for differences seen at the commencement of simulated retail display and whether this is a practical solution in commercial scenarios. The practical implication of our findings is that colour change measured during simulated display conditions may be altered by chill rate which is a component of meat processing conditions. Comparisons between factors such as lamb genotype, a component of
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the animal production part of the supply chain, may need to consider chill rate either in the measurement protocol or during data analysis. Development of a more specific measure of colour stability such as oxy/met at a given time point may also be worthy of consideration. 5. Conclusions The nature of colour stability depends on the parameters measured and the temperature conditions under which the meat is processed. Fast chill rate reduced colour stability, represented by oxy/met value measured at a set time during simulated retail display, of lamb meat aged for 5 days. This effect could have been due to colour changes associated with blooming (oxygenation) at the beginning as well as differences in the rate of metmyoglobin formation (oxidation) during simulated retail display. Standardisation of chilling rate either directly or by use of statistical adjustments may be useful to standardise comparisons of colour stability in addition to standardisation of measuring techniques. Poor colour stability together with toughening, appears to be associated with temperatures at rigor that are low enough to cause sarcomere shortening, but above the target for very fast chilling. Acknowledgements The authors thank Meat and Livestock Australia and Food Processing Equipment for providing financial support for the project. Mr. Chris Sentence assisted with project design and operation. Staff in the DAFWA meat quality laboratory provided much appreciated technical support and Mr. Mario D'Antuono provided statistical advice. References Bendall, J. (1972). Consumption of oxygen by the muscle of beef animals and related species, and its effect on the colour of meat I. Oxygen consumption in pre-rigor muscle. Journal of the Science of Food and Agriculture, 23(1), 61–72. Bendall, J. (1972). Consumption of oxygen by the muscle of beef animals and related species, and its effect on the colour of meat II. Consumption of oxygen by postrigor muscle. Journal of the Science of Food and Agriculture, 23, 707–719. Bickerstaffe, R., Bekhit, A. E. D., Robertson, L. J., Roberts, N., & Geesink, G. H. (2001). Impact of introducing specifications on the tenderness of retail meat. Meat Science, 59, 303–315. Bouton, P., Harris, P., Ratcliff, D., & Roberts, D. (1978). Shear force measurements on cooked meat from sheep of various ages. Journal of Food Science, 43, 1038–1039. Farouk, M., & Lovatt, S. (2000). Initial chilling rate of pre-rigor beef muscles as an indicator of colour of thawed meat. Meat Science, 56, 139–144. Faustman, C. (1990). The biochemical basis for discolouration in fresh meat: A review. Journal of Muscle Foods, 1, 217–243. Faustman, C., Chan, W., Schaefer, D., & Havens, A. (1998). Beef color update: The role of vitamin E. Journal of Animal Science, 76, 1019–1026. Faustman, C., Mancini, R., Sun, Q., & Suman, S. (2010). Myoglobin and lipid oxidation: Mechanistic basis and control. In S. K. Lee (Ed.), International Congress of Meat Science and Technology Jeju, Korea.
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