Authors: Grace C. Johnson, Kara A. Reynolds, Graduate Students, Animal Science, Lincoln; Mariane Beline, Postdoc, Animal Science, Virginia Tech; Leila G. Venzor, Research Technician, Animal Science, Lincoln; David E. Gerrad, Professor, Animal Sciencces, Virginia Tech; Gary A. Sullivan, Associate Professor; Jordan C. Wicks, Assistant Professor, Animal Science, Lincoln.
Summary with Implications
This study investigated the relationship between carcass weight and the development of atypical dark cutting (ATDC) beef by examining differences in chilling rates, muscle metabolism, and meat quality traits. Forty-four beef carcasses (heavy weight, HW, 510.17 ± 4.6 lb.; light weight, LW, 399.21 ± 4.6 lb.) were monitored postmortem for temperature, pH, and metabolite levels. LW carcasses cooled faster from 6 to 24 hours postmortem and exhibited higher pH at 12 hours, darker lean, and lower tenderness compared to HW carcasses. Elevated levels of glycogen and glucose-6-phosphate were observed in LW carcasses, suggesting that faster chilling may alter muscle metabolism and contribute to ATDC beef characteristics. The findings show carcass weight influences chilling dynamics thus generating both metabolic changes and disparities in quality between HW and LW carcasses. Still more research is needed to better understand to what extent chilling would need to be modified in order to alleviate dark lean in LW carcasses.
Introduction
Over the past decade, beef carcass size has increased by nearly 100 lb. Curiously, and at the same time, atypical dark cutting (ATDC) beef has emerged as a notable quality defect in the beef industry. Unlike traditional dark, firm, and dry (DFD) beef, which lacks sufficient glycogen for pH decline, atypical dark cutters have ample glycogen yet still exhibit a slightly elevated ultimate pH and result in darker lean. Regardless of the mechanism, ATDC still has the same financial discounts of that of traditional DFD, costing producers and packers nearly 30% of market value. While the metabolic state of tissue of ATDC beef is known, the exact cause remains unclear. Previous literature suggests that lighter-weight carcasses may be more prone ATDC, possibly as a result of faster chilling rates. However, this has yet to be evaluated. Therefore, this study aimed to compare heavy (HW) and light weight (LW) carcass cooling rates to assess their response to standard harvesting and chilling practices, providing insight into the factors contributing to atypical dark cutting beef.
Procedure
Forty-four half carcasses were randomly selected from contemporaneous commercial cross-bred steers of similar physiological maturity that were harvested under inspection at the University of Nebraska-Lincoln Loeffel Meat Laboratory. Carcass sides were allocated to a classification of either HW (n=22; HCW= 510.17 ± 4.6 lbs.), or LW (n=22; HCW = 399.21 ± 4.6 lbs.). Carcasses were subjected to similar standard harvesting procedures and temperatures were continuously monitored using probes inserted approximately 4 inches into the longissimus lumborum (LL) between the 12th and 13th rib. Carcass sides entered a conventional chilling cooler (35 ± 1 °F) at approximately 60 min postmortem. Additionally, tissue from the LL was excised immediately following exsanguination (0 min) as well as at 3, 6, 12, and 24 h postmortem. Samples were diced, snap-frozen in liquid nitrogen and stored until analyses which included muscle pH and metabolite levels of glycogen, glucose, and glucose-6-phosphate. Following 30 h chilling period (35 ± 1°F) carcasses were ribbed between the 12th and 13th rib for carcass evaluation, and carcass maturity and marbling scores were used to determine carcass quality grade. Instrumental color was also evaluated following a 1 h bloom time and color values were expressed as L* (lightness), a* (redness), and b* (yellowness). Finally, two 1-inch-thick steaks were collected from the LL and subjected to Warner-Bratzler shear force (WBSF) analysis. Data were analyzed as a completely randomized design using SAS Software (SAS 9.4, SAS Institute INC, Cary, NC, USA). Carcass data, color, and shear force were analyzed using the MIXED procedure, considering the fixed effect of carcass weight (HW versus LW) and harvest day as a random effect. Temperature, pH, and metabolites were analyzed as repeated measures considering the treatment, time, and their interactions as fixed effects and harvest day as a random effect. Covariance structures were tested for each characteristic and the best fit was used. Differences were considered statistically significant when P ≤ 0.05 unless otherwise noted.
Results
Carcass data indicated that HW and LW carcasses differ in carcass composition (Figure 1). Specifically, HW carcasses had increased HCW (P < 0.001),ribeye area (REA) (P < 0.001), 12th rib fat thickness (P < 0.001), and percent kidney, pelvic, heart fat (KPH) (P < 0.001). Subsequently, yield grade was greater (P < 0.001) in HW compared to LW carcasses, as was marbling score (P < 0.001). Moreover, cooling rates of LW and HW carcasses differed with LW carcasses (P < 0.01) chilling at a faster rate than HW carcasses, specifically from 6 to 24 h postmortem. Even so, all carcasses reached similar temperatures at 30 h postmortem when carcass evaluation and objective color was evaluated. The rate of pH decline was not different at 0, 3, 6 or 24 h postmortem. However, LW carcasses had an elevated (P < 0.05) pH at 12 h compared to that of HW carcasses. Although there was no interaction between carcass weight and time postmortem, both glycogen (6 h, P < 0.05) and glucose-6-phosphate (6 h, 24 h, P < 0.01 and P < 0.05. respectively) were elevated in LW carcasses. Still, glucose was not different between treatments. Finally, our data shows LW carcasses produced darker (L*, P < 0.05), less red (a*, P < 0.001) lean than that of HW carcasses and resulted in less tender steaks 1 d postmortem (P < 0.01) as indicated by WBSF values. There was also a trend for LW carcasses to produce a less tender steaks following a 14-d aging period (P = 0.08).
Conclusions
These data suggest that carcass weight alters chilling rate and changes meat quality traits through changing carbohydrate metabolism in muscle postmortem and support our hypothesis that LW carcasses, due to their faster chilling rate and metabolic differences, may be more susceptible to ATDC beef characteristics (darker, tougher). Still, the exact mechanisms remain unclear and argue more work is needed to better understand the metabolic state of muscle in LW carcasses and its response to advanced chilling systems used in commercial abattoirs.
Acknowledgment
This project was funded in part of ARD HATCH Multistate.
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