Understanding the action of muscle iron concentration on dark cutting: An important aspect affecting consumer confidence of purchasing meat
Introduction
Product assurance and integrity are two key aspects influencing consumer confidence and retail purchasing power of meat in national and international markets. The prevalence of meat with a dark red colour (dark cutting) upon retail display has been an issue in the red meat industry for the last four decades. Dark cutting causes a significant economic loss to the sheep meat and beef industries (Meat and Livestock Australia, 2018) due to consumers rejecting meat product at the point of purchase. Dark meat is often sold at a discounted price.
Dark meat formation develops from the time of slaughter to time of display and is visually identified immediately when beef and sheep primal cuts are sliced and displayed at the retail counter, for example within 30 min to 1 h of preparation (bloom). Numerous research studies have investigated the effects of on-farm, transport, lairage and post-slaughter conditions on the incidence of dark cutting. Global studies undertaken in Australia (e.g. Shorthose, Harris, Hopkins, & Kingston, 1988), New Zealand (e.g. Purchas, 1989), Ireland (e.g. Tarrant, 1981; Tarrant & Sherington, 1980), Sweden (e.g. Fabiansson, Reuterswfird, & Libelius, 1985), Finland (e.g. Puolanne & Aalto, 1981) and North America (e.g. Grandin, 1980; Grandin & Deesing, 1998) have indicated that dietary background, breed difference, environmental conditions, and pre-slaughter handling are all factors influencing dark meat formation. A recent literature review (Ponnampalam, Hopkins, et al., 2017) covered in detail, the causes, contributing factors and mechanism of dark cutting in sheep and cattle, and suggested some future research directions.
The majority of research investigating the causes of dark cutting concludes that the primary reason for dark cutting is the depletion of muscle glycogen, and this depletion can be mediated by diet, breed, management, environment, human handling or housing. Ultimate pH of meat at 24 h post-slaughter (pH 24 = pHU) is used as a threshold for the assessment of dark cutting in cattle and sheep, since muscle glycogen is related to pHU of meat. Muscle glycogen drives the extent to which the post-mortem muscle acidifies and, below a pH of 5.7, myoglobin maintains its oxygenation and can confer a bright red colour to the meat (Tarrant, 1989). Ponnampalam, Hopkins, et al. (2017) indicated the role of iron concentration in muscle tissues (myoglobin) is likely to contribute to dark meat formation post-slaughter i.e., increased muscle iron concentration may lead to a lower incidence of dark cutting at 1 h display. These authors suggested that this area warrants further investigation. A review by Bekhit and Faustman (2005) proposed that the reducing and oxidising status of myoglobin can determine the brightness and fading of meat colour at display. When sheep meat was displayed under retail conditions for between 72 h to 96 h, muscle iron concentration was associated with increased redness of meat when the vitamin E concentration in muscle was above the threshold of 3.2–3.4 mg/kg meat (Ponnampalam, Butler, McDonagh, Jacobs, & Hopkins, 2012). Polyunsaturated fatty acid (PUFA) concentrations have also been identified as contributors to brownness formation during retail display, at least in some situations (Ponnampalam, Butler, Burnett, Jacobs, & Hopkins, 2013; Ponnampalam, Plozza, et al., 2017).
An experimental study was conducted in pure Merino and Crossbred sheep to understand the effect of camelina (Camelina sativa) forage and camelina meal to a basal forage diet on animal performance, carcass value and meat quality parameters (Ponnampalam et al., 2019). This study produced lamb and yearling carcasses with a wide range in iron concentration and pHU, allowing us to investigate the combined effects of iron and pHU on dark cutting, which has not been reported before. Therefore, this paper investigates the association of muscle iron concentration, in addition to meat pHU, on dark meat formation in the sheep used in the experimental study.
Section snippets
Experimental design
Composite wether lambs based on Coopworth genetics and pure Merino wether yearlings (Merino yearlings) were used in an experiment to examine the effect of diet and breed on meat colour as assessed by dark meat formation (dark cutting) over summer season from January to March 2017. Eighty Composite lambs of 28–38 kg (average 32.8 kg) and 80 Merino yearlings of 37–43 kg (average 39.9 kg) liveweight were selected from the Agriculture Victoria flock, Hamilton Centre, Victoria. Animals were managed
General outcome of dark cutting
Based on meat pHU thresholds of 5.7, 5.8 and 6.0, the percentage of carcasses that would be classified as dark cutting (combination of both breeds) were 25%, 12% and 4%, respectively (Fig. 1). The incidence of dark cutting was much lower in Composite lambs than in Merino yearlings. For example, using pHU of 5.7 as a threshold, 5% of Composite lambs were classified as dark cutting compared with 45% of Merino yearlings. The iron concentration ranged from 10 to 22 mg/kg muscle. There was no close
Discussion
The study confirms the formation of dark meat when muscle cuts are dissected and displayed at simulated display as fresh or vacuum-packed cuts. Both iron concentration and pHU of meat contributed to dark cutting at 1 h display, but in an opposite manner.
A recent industry report (MLA, 2018) indicated that the occurrence of dark cutting is 10% in cattle and costs the Australian beef industry almost $36 million per year. The report estimated the incidence of dark cutting in lamb and sheep
Conclusions
The results clearly show that increased muscle iron concentration was strongly associated with reduced dark cutting in fresh and stored (45 day and 90 day) meat evaluated at 1 h simulated display. This observation was in addition to the influence of pHU on dark cutting and occurred irrespective of storage time. The effect of iron concentration on redness (a*-value) of meat cannot be neglected because meat redness increases by about 0.3 units for each milligram of iron concentration, for animals
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgements
This research was funded by the Victorian Department of Jobs, Precincts and Regions, Australia (Grant number 105393). The authors gratefully acknowledge the technical contributions of Matthew Kerr, Stephanie Muir, Wayne Brown and other staff from the Victorian Department of Jobs, Precincts and Regions. We also thank Hardwicks Pty. Ltd. in Kyneton and their staff for assistance on both slaughter of animals and access to their boning room facility for carcass traits measurement.
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