Muscle Metabolism and Myoglobin

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Chapter 2

Abstract

Color development is largely driven by pH and temperature declines postmortem. Accelerated postmortem metabolism results in rapid pH decline at a high carcass temperature. Together, this denatures muscle proteins, decreasing water holding capacity, which alters light refraction, thus, producing an undesirable meat color. We hypothesized changes in harvesting practices, such as, shortening scald time to improve color development by reducing carcass temperature earlier postmortem. Length of scald was investigated by thirty-two carcasses undergoing either a 4 (n=16) or 8 min (n=16) scald time at 60°C. Samples were collected before or after scalding and at 24 hrs. Interestingly, muscle temperature decreased after scalding regardless of scald time (p < 0.05). Although no changes in muscle temperature were detected, a 50% reduction in scald time resulted in lighter color (L*) muscle early post-mortem (p < 0.0001), while the 8 min scald treatment was darker (p < 0.005) at 24 hrs. Although differences in pH (p < 0.0001) and color were observed, carcasses undergo changes that cannot be attributed to carcass temperature as a result of scald time. To that end, we tested this hypothesis in a processing plant setting to further understand the effects of scald time on ultimate color. Two-hundred carcasses were assigned treatments of 6.5 (n=100) or 7.5 (n=100) min scald times, and SM muscle samples were collected at 24 hrs. A shorter scald time resulted in (p < 0.05) a lighter color, contradicting our first study. To study this issue further, we uncoupled scald from the dehairing process. To achieve this goal, twenty-four carcasses were assigned to either an 8 or 16 min time to dehair, with or without scalding (n=6 per treatment). Protracted time to dehair resulted in a higher ultimate pH (p < 0.005) and less color variation across the muscle (p < 0.05). Though a color gradient remains, the variation across the muscle was reduced by increasing time to dehair. These data show that time to dehair impacts pork quality development and suggest that delaying time to physical manipulation of the carcass may improve pork color.

Introduction

In addition to price, consumer purchases are largely driven by meat appearance with an emphasis on product color and consistency (Hutchings, 1977; Troy & Kerry, 2010). As consumers often associate consistency and color with freshness and palatability, color variations both within and between products deter consumers, costing the pork industry billions annually (Van Laack, Kauffman, & Polidori, 1995). Changes in practices and operations to alleviate meat color defects have been the focus of meat science research for several decades. While the prevalence of some defects such as PSE have been decreased from over 15% in 2003 to less than 2% in 2017 (National Pork Board, 2017), variations in fresh lean color still exist. Of particular concern, the phenomenon of two-toning results in variable color both between adjacent muscles and within the same muscle, giving the product a halo appearance (King, Shackelford, Schnell, Pierce, & Wheeler, 2018). Though less frequent in the loin, ham muscles are more susceptible to the condition (King et al., 2018) and this defect becomes more exacerbated in cured products (McDonagh, Troy, Kerry, & Mullen, 2005). Therefore, understanding the factors that influence this color phenomenon are needed. Two-toning and color variations can be partially attributed to products being derived from muscles that have inherently different muscle characteristics, which is a result of functional disparities. Muscles closer to the bone, such as those towards the center of hams, are fatigue resistant and function to stabilize joints, whereas, superficial muscles, those found on the outside of the ham, are purposed for generating quick, powerful movements. These functional differences, coupled with underlying biochemistry during the conversion of muscle to meat, contribute to color defects; however, other practices can alleviate or exacerbate pork color defects. While two-toning was once thought to be associated with specific genetic lines (Wilson, Ginger, Schweigert, & Aunan, 1959) or ante-mortem handling (Moss, 1984), King and Pierce (2015) reported two-toning in pork muscles from various suppliers and genetic lines. Additionally, muscles excised immediately post-exsanguination display two-toning, suggesting the defect is present in live muscle (King, Shackelford, Schnell, Pierce, Wheeler, 2018). Although a color and pH gradient exist across the semimembranosus muscle at 24 hrs postmortem, the abundance of myoglobin or myosin heavy change type 1 isoform did not vary (Stufft, Elgin, Patterson, Matarneh, Preisser, Shi, England, Scheffler, Mills, Gerrard, 2017). Collectively, this suggests that while muscle characteristics may contribute to ham two-toning, other factors are at play early postmortem. Different stages of harvesting have been heavily studied in our attempts to optimize meat quality. Carbon dioxide stunning has become increasingly prevalent in industry replacing electrical stimulation, to reduce animal stress (Channon, Payne, & Warner, 2002) and muscle contractions that would otherwise increase glycolytic flux early postmortem and increase the rate of pH decline (Sante-Lhoutellier & Monin, 2014). Further, dehairing methods have come into question for the potential to elevate or elongate periods of high carcass temperature. In animals exposed to ante-mortem stressors or genetic mutations, the rise in metabolic rate increases carcass temperature and accelerates glycolytic flux and pH decline. This combination of high temperature and rapid pH decline denatures muscle proteins, which disrupts water holding capacity (WHC). Reductions in WHC decrease moisture retention, producing a meat surface that refracts rather than absorbs light. Improvements in WHC and meat color are observed when the carcass is cooled quickly to avoid situations of high temperature and rapid pH decline. It was thought longer scald times would negatively impact meat quality as a result of prolonged high carcass temperatures. However, longer scald time resulted in only minor changes on muscle temperature decline and meat quality (Van der Wal, Van Beek, Veerkamp, & Wijngaards, 1993), whereas, carcasses that alternatively had the hide removed showed an improvement in WHC and darker meat color (Maribo, Olsen, Barton-Gade, & Møller, 1998). Skinned carcasses have a lower temperature for the first hour postmortem compared to scalded carcasses (Maribo et al., 1998) and demonstrates the importance of heat dissipation early post mortem. Although hide removal may improve meat quality attributes, this method is not used often because of the economic loss to the industry. Nonetheless, regulation of carcass temperature has been exploited in industry to improve quality, specifically the implementation of blast chillers. This improved rate of temperature decline influences glycolytic flux with a faster rate of temperature decline resulting in a higher pH, less protein denaturation, and ultimately improves meat color (D’souza, Dunshea, Warner, & Leury, 1998; Gardner, Huff-Lonergan, Rowe, Schultz-Kaster, & Lonergan, 2006; Goldspink & McLoughlin, 1964). Though carcass temperature does not begin to significantly drop until evisceration (Maribo, Olsen, Barton-Gade, Møller, & Karlsson, 1998), the defect is detected early postmortem (King et al., 2018) suggesting a process early in harvesting is the primary driver of pork qualty. Therefore, we investigated how changes in carcass temperature, as mediated by scald can impact fresh pork color in the SM muscle.

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Materials and Methods

Exp. 1 Scald time affects two-toning independent of temperature.
Thirty-two market weight pigs were raised at a commercial farm, transported to the Virginia Tech Meat Center, and harvested under normal operating procedures with the following exceptions. Pigs were randomly assigned to either a 4 (n=16) or 8 min (n=16) scald time. Animals were humanely harvested by electrical stunning (Model ES Best and Donovan, Blue As, OH, USA), exsanguinated, and samples were collected before scalding (pre-scald) or immediately after scalding (post-scald) from the semimembranosus muscle (SM). Temperature of the scald tank (Boss Machinery, Cincinnati, OH) remained constant at 60°C throughout process for all treatments. Either pre- or post-scald, the entire SM was excised from the carcass. A center slice of the SM was cut, approximately 5 cm thick, and was then separated into four zones (A, B, C, D) from the most cranial part of the muscle being zone A and the most caudal part being zone D (Figure 1). Zones A-C were sectioned in equal parts, while the most caudal tip contained zone D. Muscle samples from each zone were diced, snap frozen in liquid nitrogen, and stored in at -80°C until further analysis. Additionally, 24 hr SM muscle samples were then taken using the opposite ham of each carcass as described for initial sample collection and stored at -80°C until further analysis. Color Analysis The SM muscle was collected as previously mentioned, and color measurements were taken from each zone (A, B, C, D) in triplicates, averaged, and recorded. The Minolta CR300 colorimeter (Ramsey, NJ, USA), Illuminant D, 0o observer angle was used and color measurements were taken in areas where there were no visible color defects. Colors were expressed as Commission Internationale de l’Éclairage (CIE) L* (lightness). Temperature Temperature measurements were collected using a ThermoPro Digital Food Thermometer (Model No.: TP-16, Dultuh, GA). Measurements were taken from the SM muscle, while still in the carcass, before scald (pre-scald), after scald (post-scald), 45 min, 180 min, and at 24 hrs. Three temperature measurements were then taken from the center slice, excised muscle at Zone A, Middle (Zone B and C), and Zone D to determine temperature changes across muscle. Statistical Analysis Continuous variables, including temperature, color, pH, lactate, and glycogen were all analyzed by ANOVA using PROC MIXED procedure using SAS version 9.3 (SAS Institute Inc., Cary, NC). Carcass was the experimental unit and the model included the scalding treatment, zones and time of sampling as fixed effects. Means were compared using Tukey-Kramer Multiple Comparison Test if a significant effect was detected. Data on graphs are least square means ±standard error means (SEM), and differences were considered significant at p < 0.05.
Exp. 2 In-plant study
To validate the findings of the pilot plant, two hundred market weight pigs were raised and harvested by an industry partner. Carcasses received either a 6.5 (n=100) or 7.5 (n=100) min scald treatments. Animals were humanely harvested under normal processing procedures with the exception of length of scald. Color was then taken at 24 hrs using a Minolta Colorimeter using the same technique as the initial study.
Exp. 3 Adjusting time to dehairer decreases two-toning in selected ham muscle Twenty-four market weight pigs of similar genetics of experiments outlined above were raised at a commercial farm and transported to the Virginia Tech Meat Center for harvesting. Pigs were randomly assigned to either an 8 (n=12) or 16 (n=12) min time to dehair, with (n = 6) or without (n = 6) scalding. Pigs were harvested as outlined in Exp 1 except that all carcasses were then subjected to 1 min of dehairing then dehided. Samples were collected from the semimembranosus muscle (SM). Muscle samples were collected and stored in the same manner as the previous experiment. Temperature measurements were taken using the same thermometer as the previous experiment. Measurements were taken from the SM muscle, while still in the carcass, before scald, after dehairing, 45 min, 180 min, and at 24 hrs. Temperature was collected in the same locations as outlined above. Color, pH, and lactate were collected and analyzed using the same techniques as in experiment 1.

Chapter 1
Introduction
Pale, Soft, and Exudative (PSE)
Two-Toning
Muscle Metabolism and Myoglobin
Postmortem Metabolism
Harvesting techniques effect on pork quality
Stunning Methods
Scalding and Dehairing Methods
Carcass Chilling
Conclusion
References
Chapter 2
Abstract
Materials and Methods
Results and Discussion
Conclusion
References
Chapter 3
Future Implications

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Early postmortem harvest practices influence pork color

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