Modified Atmosphere Packaging Improves Surface Color of Dark-Cutting Beef

R. M. Mitacek; A. R. English; G. G. Mafi; D. L. VanOverbeke; R. Ramanathan; R. M. Mitacek; A. R. English; G. G. Mafi; D. L. VanOverbeke; R. Ramanathan

doi:10.22175/mmb2017.04.0023

Introduction

Failure to have a characteristic bright-red color at the cut surface of the ribeye muscle during grading results in discounted carcass value. The 2011 National Beef Quality Audit reported that 3% of carcasses sampled were graded as dark-cutters (Moore et al., 2012). Dark-cutting beef is an effect of a metabolic condition(s) due to chronic stress resulting in limited lactic acid formation in post-rigor muscles (Hendrick et al., 1959). A greater pH promotes muscle fibers to hold more water; therefore, less oxygen is diffused deep into the tissue (Lawrie, 1958; Offer and Trinick, 1983). Similarly, elevated muscle pH can enhance mitochondrial activity and deoxymyoglobin formation (Ashmore et al., 1971; Ashmore et al., 1972; Egbert and Cornforth, 1986; Tang et al., 2005; Mancini and Ramanathan, 2014; English et al., 2016a). Therefore, developing strategies to improve redness is essential to maximize the value of beef.

Various organic acids have been used to lower muscle pH and improve surface color of dark-cutting beef (Apple et al., 2011; Sawyer et al., 2009). Introduction of case-ready meat has allowed beef purveyors to modify the gas compositions within packages to improve shelf-life. Use of high-oxygen or carbon monoxide in modified atmosphere packaging has the potential to enhance the surface color of dark-cutting beef. A higher oxygen partial pressure allows oxygen to penetrate deeper into tissue and promote oxymyoglobin formation (Taylor and MacDougall, 1973). Similarly, carbon monoxide has greater affinity for myoglobin than oxygen, and can bind strongly with myoglobin to form a stable bright-red color (Cornforth and Hunt, 2008). Further, a longer aging time can limit mitochondrial activity and alter muscle structure (Nishimura et al., 1998; Lenaz et al., 2002). We recently reported that extended aging of dark-cutting beef decreased muscle oxygen consumption and improved surface reflectance (English et al., 2016a). However, limited published research is currently available on the combined effects of modified atmosphere packaging and aging on surface color of dark-cutting beef. Therefore, the objective of the current study was to determine the effects of high-oxygen and carbon monoxide modified atmosphere packaging (HiOx-MAP and CO-MAP) on the surface color of dark-cutting longissimus lumborum muscle that had been aged for 21 d.

Materials and Methods

Raw materials processing, packaging, and retail display

Ten USDA Choice (normal-pH) and 10 no-roll dark cutting beef carcasses were selected 72 h after slaughter from a commercial packing plant, individually identified and tagged prior to carcasses fabrication. Both normal-pH (IMPS #180; North American Meat Processors Association, 2002) and dark-cutting strip loins (longissimus lumborum) were vacuum packaged and transported on ice to the Robert Kerr Food and Agricultural Products Center at Oklahoma State University. Two 2.5-cm thick steaks were cut from the anterior end of each normal-pH and dark-cutting loin to characterize pH, surface color, and lipid oxidation before aging (no packaging, display, or aging). The remaining loins were vacuum packaged (Prime Source vacuum pouches, 12 × 18 cm, 3 mil high barrier; oxygen partial pressure within package = 0.76%) and aged for 21 d in the dark at 2°C. After 21 d aging, four 2.5-cm thick steaks per loin were cut from the anterior end using a meat slicer (Bizerba USA Inc., Piscataway, NJ), and randomly assigned to 1 of 3 packaging conditions: PVC (oxygen-permeable polyvinyl chloride fresh meat film), HiOx-MAP (80% oxygen and 20% carbon dioxide), and CO-MAP (0.4% carbon monoxide, 69.6% nitrogen, and 30% carbon dioxide). The fourth steak was used to measure pH and lipid oxidation on d 0 of retail display.

Both HiOx-MAP and CO-MAP were performed using a MAP system utilizing Rock-Tenn DuraFresh rigid trays sealed with clear, multi-layer barrier film (LID 1050 film, Cryovac Sealed Air, Duncan, SC) in a Mondini semi-automatic tray-sealing machine (Model CV/VG-5, G. Mondini S.P.A. Cologne, Italy) and certified gas blends (Stillwater Steel and Welding Supply, Stillwater, OK). For PVC packaging, steaks were placed onto foam trays with absorbent pads, and over-wrapped with a PVC film (15,500 to 16,275 cm³ O₂/m²/24 h at 23°C, E-Z Wrap Crystal Clear Polyvinyl Chloride Wrapping Film, Koch Supplies, Kansas City, MO). A headspace analyzer (Bridge 900131 O₂/CO₂/CO, Illinois Instruments, Ingleside, IL), was used to determine the percentage oxygen, carbon dioxide, and carbon monoxide in HiOx- and CO-MAP. Gas compositions were determined after 24 h of packaging using extra steaks not used in the study by a needle pierced into package. The average gas compositions in HiOx-MAP were 78 to 80% oxygen and 18 to 20% carbon dioxide and 0.3 to 0.4% carbon monoxide in CO-MAP. After packaging, steaks were placed in a coffin-style open display case maintained at 2°C ± 1 under continuous lighting (1612 to 2152 lx, Philips Delux Warm White Fluorescent lamps; Andover, MA; color rendering index = 86; color temperature = 3000 K). All packages were rotated daily to minimize variances in light intensity or temperature caused by the location.

Characterizing muscle pH, instrumental color, and lipid oxidation prior to aging

Normal-pH and dark-cutting steaks were over-wrapped with PVC film and allowed to bloom at 4°C for 2 h. The pH, instrumental color, and lipid oxidation of bloomed steaks were determined to characterize the muscle quality prior to aging.

The pH values of normal- and dark-cutting steaks were obtained by using an Accumet combination glass electrode connected to an Accumet 50 pH meter (Fisher Scientific, Fairlawn, NJ). Samples were blended in a Sorvall Omni tabletop mixer (Newton, CT). The pH was determined by combining 10-g sample with 100 mL of deionized water and homogenized for 30 s. Prior to pH measurements, the pH electrode was standardized with pH 4 and 7 buffers.

Surface color prior to aging was measured after 2 h of bloom at 4°C using a HunterLab MiniScan XE Plus spectrophotometer (Model 45/0 LAV, 2.5-cm diameter aperture, illuminant A, 10° observer; HunterLab, Reston, VA). The color measurements of the steaks packaged in PVC film were taken at three locations, and the subsamples were averaged for statistical analyses. The CIE L*, a*, and chroma values were used to characterize steak surface color (American Meat Science Association, 2012).

Lipid oxidation was measured according to the procedure of Witte et al. (1970). From each steak, 5 g of the sample that contained both interior and surface exposed to air was blended with 25 mL of 20% trichloroacetic acid (TCA) solution. Samples were homogenized using a Sorvall Omni mixer (Newton, CT) for 1 min and filtered through a Whatman (#1) filter paper. One mL of filtrate was mixed with 1 mL of 20 mM thiobarbituric acid (TBA) solution and incubated at 100°C in a boiling water bath for 10 min. After incubation, samples were cooled to room temperature and absorbance was measured at 532 nm using a Shimadzu UV-2600 PC spectrophotometer. The blank consisted of 1 mL TCA solution and 1 mL TBA solution. Lipid oxidation values were reported as mg malonaldehyde/kg meat using a validated equation (Section X, American Meat Science Association, 2012).

Muscle pH, color, and lipid oxidation of aged and modified atmosphere packaged steaks

After 21 d aging, each loin section was cut into 4 steaks. The first steak was used to determine pH and lipid oxidation prior to display (no packaging effect), and remaining 3 steaks were randomly packaged in PVC, HiOx-MAP, and CO-MAP. The steaks were displayed for 6 d and the surface color was evaluated using a HunterLab MiniScan XE Plus spectrophotometer every 24 h of display. For the steaks packaged in MAP, precautions were taken as recommended in the AMSA guidelines (Section VIII, American Meat Science Association, 2012). Following surface color measurement on d 6, the steaks were used to determine pH and lipid oxidation as described in the previous section.

Visual panel

A trained panel (n = 6) conducted daily visual color evaluations. All panelists passed the Farnsworth Munsell 100-hue test. Panelists were selected and trained according to the American Meat Science Association (1991) guidelines. Panelists scored each steak every 24 h of display to assess muscle color using a 7-point scale (1- no darkening, 2- little darkening, 3- slight darkening, 4- modest darkening, 5- moderate darkening, 6- extensive darkening, and 7- extreme darkening) and discoloration using a 7-point scale [1- no discoloration (0% metmyoglobin), 2- minimal discoloration (1 to 20%), 3- slight discoloration (11 to 20%), 4- small discoloration (21 to 40%), 5- modest discoloration (41 to 60%), 6- moderate discoloration (61 to 80%), and 7- extensive discoloration (81 to 100%)].

Statistical analysis

A split-plot design was used to evaluate the effects of packaging on dark-cutting beef color. Within the whole plot, 10 normal-pH longissimus lumborum and 10 dark-cutting longissimus lumborum muscles were considered experimental units (N = 10 for each muscle and N = 20 total muscles). Within the subplot, each longissimus muscle was divided into steaks and packaged in PVC, HiOx-MAP, or CO-MAP (sub-plot experimental unit). The fixed effects included muscle type, packaging, and their interactions. For the split-plot, random effects included loin, loin × whole plot treatments (Error A), and residual error (Error B). The most appropriate structure was determined using Akaike’s information criterion and Sawa’s Bayesian information criterion output. The repeated option in the Mixed Procedure of SAS (SAS Inst. Inc., Cary, NC) was used to assess covariance–variance structure among the repeated measures for instrumental and visual color data. Least squares means were separated using a pairwise t test (PDIFF option). Day 0 surface color and pH (prior to aging) was utilized only to characterize normal-pH and dark-cutting beef. Therefore, aging time was not included in the model.

Results

pH and surface color prior to aging

There were significant differences in pH and color between muscle types before aging (Table 1). As expected, dark-cutting beef had greater (P < 0.0001) pH and lower L*, a*, and chroma values than normal-pH beef.

Table 1.

Least squares means for pH, color, and lipid oxidation between muscle types prior to aging

	Normal-pH	Dark-cutting	SE¹	P-value
Parameter	Normal-pH	Dark-cutting	SE¹	P-value
pH	5.56	6.40	0.03	0.0001
L* value	38.2	25.2	0.60	0.0001
a* value	28.4	18.4	0.80	0.0001
Chroma	34.5	25.4	1.00	0.0001
Lipid oxidation (mg MDA/kg meat)	0.18	0.17	0.06	0.0600

¹SE = standard error.

pH and L* values of steaks aged for 21 d

Following 21 d aging, dark-cutting steaks had greater pH than normal-pH steaks (the average pHs of normal and dark-cutting beef were 5.50 and 6.36, respectively; SE = 0.03). There was a packaging × muscle type interaction for L* values (Fig. 1). Packaging in HiOx-MAP and CO-MAP increased lightness of dark-cutting steaks compared with dark-cutting steaks in PVC packaging (HiOx-MAP = CO-MAP > PVC; P < 0.001).

Figure 1.

Least squares means of muscle type × packaging interaction for L* value. Standard error bars are indicated at each time point. ^a–dLeast square means with different letters are different (P < 0.05).

a* values and chroma

A packaging × muscle type × display time interaction resulted for a* values (redness) and chroma (red intensity; Table 2). On d 0 of the display, dark-cutting steaks in HiOx-MAP had greater a* values than dark-cutting PVC and CO-MAP. However, normal-pH and dark-cutting steaks in HiOx-MAP had identical (P = 0.35) redness values on d 3. By d 6 of the display, dark-cutting steaks had greater (P < 0.0001) redness values in HiOx-MAP and CO-MAP than dark-cutting steaks in PVC packaging. Compared to normal-pH steaks in PVC and CO-MAP, normal-pH steaks in HiOx-MAP were less red (P < 0.05) by d 4 of display. Both dark-cutting and normal-pH steaks in CO-MAP had greater a* values by d 4 of display than other packaging types.

Table 2.

Least squares means for a* and chroma (packaging × muscle type × day interaction) of steaks displayed for 6 d

			Display (d)
Parameter	Packaging	Muscle type	0	1	2	3	4	5	6
a* value	PVC	Normal-pH	29.7^a,u	23.4^b,w	24.7^c,v	22.9^b,w	23.7^bc,w	21.09^d,w	19.7^e,w
	PVC	Dark-cutting	20.6^ab,y	21.5^a,x	20.5^ab,x	19.9^bc,x	18.8^c,z	17.64^d,x	16.8^d,x
SE = 0.5	HiOx-MAP	Normal-pH	30.4^a,u	28.4^b,u	26.4^c,wu	23.5^d,w	20.4^e,y	18.4^f,x	15.4^g,y
	HiOx-MAP	Dark-cutting	25.4^b,v	27.9^a,u	25.4^b,vw	23.5^c,w	22.6^cd,x	21.5^de,w	20.5^e,w
	CO-MAP	Normal-pH	24.5^d,v	25.6^d,v	27.5^c,u	28.4^bc,u	28.6^abc,u	29.4^ab,u	29.7^a,u
	CO-MAP	Dark-cutting	22.4^e,x	23.5^e,w	24.5^cd,v	25.4^bc,v	26.5^ab,v	26.4^ab,v	26.8^a,v
Chroma	PVC	Normal-pH	36.7^a,v	30.4^b,w	30.2^b,x	29.9^b,w	30.7^b,v	28.1^c,w	26.7^d,w
	PVC	Dark-cutting	27.0^a,y	28.5^b,x	27.4^a,y	26.5^ac,x	26.0^ac,x	25.5^cd,x	24.5^d,x
SE = 0.6	HiOx-MAP	Normal-pH	38.4^a,u	36.4^b,u	34.4^c,u	31.5^d,v	28.4^e,w	26.4^f,x	23.4^g,x
	HiOx-MAP	Dark-cutting	32.4^b,w	34.9^a,v	32.4^b,vw	30.5^c,w	29.6^cd,vw	28.5^de,w	27.5^e,w
	CO-MAP	Normal-pH	30.5^d,x	31.6^d,v	33.5^c,uv	34.4^bc,u	34.6^abc,u	35.4^ab,u	35.7^a,u
	CO-MAP	Dark-cutting	29.4^e,x	30.5^de,vw	31.5^cd,w	32.4^bc,v	33.5^ab,u	33.4^ab,v	34.2^a,v

^a–fLeast square means with different letters within a row are different (P < 0.05).
^u–zLeast square means with different letters within a column are different (P < 0.05).

Dark-cutting steaks in HiOx- and CO-MAP increased chroma (red intensity) compared with dark-cutting steaks in PVC on d 0 of display. Dark-cutting steaks attained the highest numerical chroma values on d 1 and 6 for HiOx- and CO-MAP, respectively compared with PVC dark-cutting steaks. By the end of display, both CO-MAP and HiOx-MAP steaks had greater chroma values compared with dark-cutting steaks in PVC.

Visual color

There was a packaging × muscle type × display time interaction (P < 0.0001) for trained color panel scores on muscle darkening and surface discoloration (Table 3). On d 0, dark-cutting steaks in HiOx-MAP had lower (P < 0.0001) muscle darkening scores than dark-cutting steaks in PVC and CO-MAP. By the end of display, both dark-cutting steaks in CO-MAP and HiOx-MAP had less muscle darkening than PVC dark-cutting steaks. On d 2 of display, dark-cutting steaks in HiOx- and CO-MAP had less surface discoloration compared with dark-cutting steaks in PVC packaging. CO-MAP had the least (P < 0.001) amount of surface discoloration by the end of display than PVC and HiOx-MAP for both muscle types.

Table 3.

Least squares means for muscle darkening¹ and surface discoloration² (packaging × muscle type × day interaction) for beef steaks displayed for 6 d

			Display, d
Parameter	Packaging	Muscle type	0	1	2	3	4	5	6
Muscle darkening	PVC	Normal-pH	1.45^d,y	1.59^d,y	1.79^cd,y	2.46^b,y	2.25^bc,x	2.56^b,x	4.13^a,w
Muscle darkening	PVC	Dark-cutting	5.03^e,v	5.38^de,v	5.63^cd,v	5.91^bc,v	6.33^ab,v	6.66^a,v	6.70^a,v
	HiOx-MAP	Normal-pH	1.35^cd,y	1.24^cd,y	1.20^d,z	2.06^b,y	1.74^bc,xy	1.76^bc,y	3.07^a,y
SE = 0.26	HiOx-MAP	Dark-cutting	4.15^a,w	3.62^bc,w	3.83^ab,x	4.33^a,x	3.24^c,w	3.63^bc,w	3.85^ab,wx
	CO-MAP	Normal-pH	2.98^a,x	2.42^b,x	1.88^cd,y	2.33^bc,y	1.31^e,y	1.34^e,y	1.54^de,z
	CO-MAP	Dark-cutting	5.50^a,v	5.02^ab,v	4.90^b,w	4.86^b,w	3.51^c,w	3.66^c,w	3.45^c,xy
Surface discoloration	PVC	Normal-pH	1.00^e,w	1.29^e,w	2.30^d,w	2.42^d,w	3.28^c,w	4.10^b,w	4.96^a,x
	PVC	Dark-cutting	1.00^d,w	1.00^d,w	1.34^d,x	1.33^d,xy	2.56^c,x	3.49^b,x	4.92^a,x
	HiOx-MAP	Normal-pH	1.00^e,w	1.10^e,w	1.35^de,x	1.69^d,x	2.03^c,y	3.81^b,wx	5.58^a,w
SE = 0.19	HiOx-MAP	Dark-cutting	1.00^b,w	1.03^b,w	1.13^b,x	1.18^b,y	1.16^b,z	1.23^b,y	1.80^a,y
	CO-MAP	Normal-pH	1.00^a,w	1.06^a,w	1.17^a,x	1.23^a,y	1.17^a,z	1.23^a,y	1.13^a,z
	CO-MAP	Dark-cutting	1.00^a,w	1.02^a,w	1.03^a,x	1.10^a,y	1.10^a,z	1.16^a,y	1.10^a,z

^a–dLeast square means with different letters within a row, within a parameter, are different (P < 0.05).
^v–zLeast square means with different letters within a column, within a parameter, are different (P < 0.05).
¹1 = no darkening, 3 = slight darkening, 5 = moderately dark, and 7 = very dark.
²1 = no discoloration, 2 = slight discoloration, 3 = small discoloration, 4 = modest discoloration, 5 = moderate discoloration, and 6 = extensive discoloration.

Lipid oxidation

There was a packaging × muscle type interaction for lipid oxidation (Fig. 2). Dark-cutting muscles showed less (P < 0.001) lipid oxidation in PVC and HiOx-MAP packaging when compared with normal-pH steaks. HiOx-MAP packaging had greater lipid oxidation for both muscle types than PVC and CO-MAP. However, there were no differences (P = 0.25) in lipid oxidation between normal-pH and dark-cutting when packaged in CO-MAP.

Figure 2.

Least squares means of muscle type × packaging interaction for lipid oxidation. Standard error bars are indicated at each time point. ^a–eLeast square means with different letters are different (P < 0.05).

Discussion

Previous research from our laboratory has indicated that blooming properties of dark-cutting steaks were improved with extended aging (English et al., 2016a). Greater myoglobin oxygenation, in part, can be due to decreased mitochondrial activity. Cornforth and Egbert (1985) validated the role of mitochondria in bloom by the addition of rotenone (complex I electron-transport chain inhibitor) to increase redness of pre-rigor meat. In the current study, we combined 2 different post-harvest processing techniques (aging and modified atmosphere packaging) to enhance the appearance of dark-cutting beef. Greater oxygen content in HiOx-MAP (80% oxygen) can partially satisfy mitochondrial oxygen demand, leading to oxygenation of deoxymyoglobin. In addition, aging time can increase muscle proteolysis (Koohmaraie, 1994) and lower water holding capacity. Hence, more water will be available on the surface of the meat to reflect light. A higher oxygen partial pressure within the package can saturate both myoglobin and water, further enhancing oxymyoglobin formation. Earlier studies also demonstrated a bright-red color when normal-pH steaks packaged in HiOx-MAP compared with PVC (Jayasingh et al., 2001; Jakobsen and Bertelsen, 2002; John et al., 2005). Dark-cutting steaks in HiOx-MAP had increased redness than normal-pH HiOx-MAP on d 6. Less surface discoloration in dark-cutting steaks can be attributed to the protective effect of pH on oxymyoglobin and lipid oxidation. In-vitro experiments utilizing purified myoglobin also demonstrated greater stability of oxymyoglobin at pH 7.4 than at pH 5.6 (Suman et al., 2006). Previous research also reported lower TBARS values in dark-cutting beef than normal-pH steaks (Sawyer et al., 2009). Increased aging time coupled with high oxygen levels in normal-pH conditions can promote lipid oxidation and decrease metmyoglobin reducing activity, leading to lower color stability in normal-pH steaks packaged in HiOx-MAP (English et al., 2016b).

Improved color stability of normal-pH steaks in CO-MAP had been reported by various researchers (Jayasingh et al., 2001; Krause et al., 2003; Hunt et al., 2004; John et al., 2005; English et al., 2016b). However, limited studies have determined carbon monoxide’s effects in high-pH meat conditions. In the current study, dark-cutting steaks achieved the greatest bright-red color in CO-MAP by the end of the display. Carbon monoxide has a higher affinity for myoglobin than oxygen and can form a strong bond with heme. Moreover, carbon monoxide is a known mitochondrial inhibitor (Lanier et al., 1978; Cooper and Brown, 2008). Further, anaerobic condition within CO-MAP can limit lipid oxidation. Hence, we speculate that a greater affinity of myoglobin for carbon monoxide, less oxidative stress due to anaerobic condition, and its effects on mitochondrial oxygen consumption may have improved the surface color of dark-cutting beef. In support, Hamling and Calkins (2008) reported CO-MAP improved appearance of ammonium hydroxide-enhanced (high-pH) beef.

Dark-cutting beef has more myoglobin concentration than normal-pH beef (McKeith et al., 2016; English et al., 2016a). With a greater myoglobin concentration, one can speculate a bright-red color for dark-cutting steaks in CO-MAP than normal-pH steaks in CO-MAP. On the contrary, on d 6 of display, dark-cutting steaks in CO-MAP had lower a* and chroma values and greater muscle darkening than normal-pH steaks, which suggests the effect of carbon monoxide on high-pH meat is pH-dependent. This can be, in part, due to impaired diffusion of carbon monoxide deep into meat due to muscle swelling. Further, research utilizing electrochemical techniques noted that affinity of oxygen to myoglobin was greater at pH 7.4 than 5.6 (Nerimetla et al., 2017). However, limited information is currently available on the pH-dependent effects of carbon monoxide binding to myoglobin.

Conclusion

Surface color of dark-cutting beef color that had been aged for 21 d was improved with the use of HiOx- and CO-MAP. HiOx-MAP dark-cutting steaks were similar in color to normal-pH steaks on d 1 of display. However, dark-cutting steaks in CO-MAP resulted in a more stable red color than HiOx-MAP. Lipid oxidation was greater in HiOx-MAP dark-cutting steaks than in CO-MAP. An elevated muscle pH enhances bacterial growth and can affect flavor profile. Hence, characterizing the effects of modified atmosphere packaging on microbial growth and taste attributes are critical to maximizing the benefits of improved surface color in dark-cutting beef.

Notes

This research was supported by the Oklahoma Center for the Advancement of Science and Technology, Oklahoma Applied Research Support (AR 15-017), project funded in 2015. [^{^}]

Literature Cited

American Meat Science Association. 2012. Meat color measurement guidelines. Am. Meat Sci. Assoc., Chicago, IL.

American Meat Science Association. 1991. Guidelines for Meat Color Evaluation. Am. Meat Sci. Assoc., Chicago, IL.

Ashmore, C. R.Doerr, L.Foster, G.Carroll, F.. 1971. Respiration of mitochondria isolated from dark-cutting beef. J. Anim. Sci. 33:574–577. doi:10.2527/jas1971.333574xhttp://dx.doi.org/10.2527/jas1971.333574x

Ashmore, C. R.Doerr, L.Parker, W.. 1972. Respiration of mitochondria isolated from dark-cutting beef: Postmortem changes. J. Anim. Sci. 34:46–48. doi:10.2527/jas1972.34146xhttp://dx.doi.org/10.2527/jas1972.34146x

Apple, J. K.Sawyer, J. T.Muellenet, J. F.Yancey, J. W. S.Wharton, M. D.. 2011. Lactic acid enhancement can improve the fresh and cooked color of dark-cutting beef. J. Anim. Sci. 89(12):4207–4220. doi:10.2527/jas.2011-4147http://dx.doi.org/10.2527/jas.2011-4147

Cooper, C. E.Brown, G. C.. 2008. The inhibition of mitochondrial cytochrome oxidase by the gases carbon monoxide, nitric oxide, hydrogen cyanide and hydrogen sulfide: Chemical mechanism and physiological significance. J. Bioenerg. Biomembr. 40:533–539. doi:10.1007/s10863-008-9166-6http://dx.doi.org/10.1007/s10863-008-9166-6

Cornforth, D. P.Hunt, M.. 2008. Low-oxygen packaging of fresh meat with carbon monoxide; meat quality, microbiology, and safety. AMSA White Paper Series. 2:1–10.

Cornforth, D. P.Egbert, W. R.. 1985. Effect of rotenone and pH on the color of pre-rigor muscle. J. Food Sci. 50:34–35. doi:10.1111/j.1365-2621.1985.tb13271.x

Egbert, W. R.Cornforth, D. P.. 1986. Factors influencing color of dark cutting beef muscle. J. Food Sci. 51:57–59. doi:10.1111/j.1365-2621.1986.tb10835.xhttp://dx.doi.org/10.1111/j.1365-2621.1986.tb10835.x

English, A. R.Wills, K. M.Harsh, B. N.Mafi, G. G.VanOverbeke, D. L.Ramanathan, R.. 2016a. Effects of aging on the fundamental color chemistry of dark-cutting beef. J. Anim. Sci. 94:4040–4048. doi:10.2527/jas.2016-0561

English, A. R.Mafi, G. G.VanOverbeke, D. L.Ramanathan, R.. 2016b. Effects of extended aging and modified atmospheric packaging on beef top loin steak color. J. Anim. Sci. 94:1727–1737. doi:10.2527/jas.2015-0149

Hamling, A. E.Calkins, C. R.. 2008. Enhancement of beef chuck and loin muscles with ammonium hydroxide and salt. J. Anim. Sci. 86:967–971. doi:10.2527/jas.2007-0346http://dx.doi.org/10.2527/jas.2007-0346

Hendrick, H. B.Boillot, J. B.Brady, D. E.Naumann, H. D.. 1959. Etiology of dark-cutting beef. Res. Bull. (Sun Chiwawitthaya Thang Thale Phuket) p. 717 (Sun Chiwawitthaya Thang Thale Phuket). University MO Agric. Exp. Stn., Columbia.

Hunt, M.C.Mancini, R.A.Hachmeister, K.A.Kropf, D.H.Merriman, M.DelDuca, G.. 2004. Carbon monoxide in modified atmosphere packaging affects color, shelf life, and microorganisms of beef steaks and ground beef. J. Food Sci. 69:FCT45–FCT52. doi:10.1111/j.1365-2621.2004.tb17854.x

Jakobsen, M.Bertelsen, G.. 2002. The use of CO2 in packaging of fresh red meats and its effect on chemical quality changes in the meat: A review. J. Muscle Foods 13:143–168. doi:10.1111/j.1745-4573.2002.tb00326.xhttp://dx.doi.org/10.1111/j.1745-4573.2002.tb00326.x

Jayasingh, P.Cornforth, D. P.Carpenter, C. E.Whittier, D.. 2001. Evaluation of carbon monoxide (CO) treatment in modified atmosphere packaging or vacuum packaging to increase color stability of fresh beef. Meat Sci. 59:317–324. doi:10.1016/S0309-1740(01)00086-9http://dx.doi.org/10.1016/S0309-1740(01)00086-9

John, L.Cornforth, D.Carpenter, C. E.Sorheim, O.Pettee, B. C.Whittier, D. R.. 2005. Color and thiobarbituric acid values of cooked top sirloin steaks packaged in modified atmospheres of 80% oxygen, or 0.4% carbon monoxide, or vacuum. Meat Sci. 69:441–449. doi:10.1016/j.meatsci.2004.08.013http://dx.doi.org/10.1016/j.meatsci.2004.08.013

Koohmaraie, M. 1994. Muscle proteinases and meat aging. Meat Sci. 36:93–104. doi:10.1016/0309-1740(94)90036-1http://dx.doi.org/10.1016/0309-1740(94)90036-1

Krause, T. R.Sebranek, J. G.Rust, R. E.Honeyman, M. S.. 2003. Use of carbon monoxide packaging for improving the shelf life of pork. J. Food Sci. 68:2596–2603. doi:10.1111/j.1365-2621.2003.tb07067.xhttp://dx.doi.org/10.1111/j.1365-2621.2003.tb07067.x

Lanier, T. C.Carpenter, J. A.Toledo, R. T.Reagan, J. O.. 1978. Metmyoglobin reduction in beef systems as affected by aerobic, anaerobic, and carbon monoxide-containing environments. J. Food Sci. 43:1788–1792. doi:10.1111/j.1365-2621.1978.tb07415.xhttp://dx.doi.org/10.1111/j.1365-2621.1978.tb07415.x

Lawrie, R. A. 1958. Physiological stress in relation to dark-cutting beef. J. Food Agric. 9:721–727. doi:10.1002/jsfa.2740091106http://dx.doi.org/10.1002/jsfa.2740091106

Lenaz, G.Bovina, C.D’Aurelio, M.Fato, R.Formiggini, G.Genova, M. L.Giuliano, G.Pich, M. M.Paolucci, U.Castelli, G. P.Ventura, B.. 2002. Role of mitochondria in oxidative stress and aging. Ann. N. Y. Acad. Sci. 959:199–213. doi:10.1111/j.1749-6632.2002.tb02094.xhttp://dx.doi.org/10.1111/j.1749-6632.2002.tb02094.x

Mancini, R. A.Ramanathan, R.. 2014. Effects of postmortem storage time on color and mitochondria in beef. Meat Sci. 98:65–70. doi:10.1016/j.meatsci.2014.04.007http://dx.doi.org/10.1016/j.meatsci.2014.04.007

McKeith, R. O.King, D. A.Grayson, A. L.Shackelford, S. D.Gehring, K. B.Savell, J. W.Wheeler, T. L.. 2016. Mitochondrial abundance and efficiency contribute to lean color of dark cutting beef. Meat Sci. 116:165–173. doi:10.1016/j.meatsci.2016.01.016http://dx.doi.org/10.1016/j.meatsci.2016.01.016

Moore, M. C.Gray, G. D.Hale, D. S.Kerth, C. R.Griffin, D. B.Savell, J. W.Rainest, C. R.Belk, K. E.Woerner, D. R.Tatum, J. D.Igo, J. L.VanOverbeke, D. L.Mafi, G. G.Lawrence, T. E.Delmorell, R. J.Christensenll, L. M.Shakelford, S. D.King, D. A.Wheeler, T. L.Meadows, L. R.O’Connor, M. E.. 2012. National Beef Quality Audit-2011: In-plant survey of targeted carcass characteristics related to quality, quantity, value, and marketing of fed steers and heifers. J. Anim. Sci. 90:5143–5151. doi:10.2527/jas.2012-5550http://dx.doi.org/10.2527/jas.2012-5550

North American Meat Processors Association. 2002. The meat buyers guide. N. Am. Meat Processors Assoc., Reston, VA.

Nerimetla, R.Krishnan, S.Mazumder, S.Mohanty, S.Mafi, G. G.VanOverbeke, D. L.Ramanathan, R.. 2017. Species-specificity in myoglobin oxygenation and reduction potential properties. Meat Muscle Biol. 1:1–7. doi:10.22175/mmb2016.10.0003http://dx.doi.org/10.22175/mmb2016.10.0003

Nishimura, T.Liu, A.Hattori, A.Takashi, K.. 1998. Changes in mechanical strength of intramuscular connective tissue during postmortem aging of beef. J. Anim. Sci. 76:528–532. doi:10.2527/1998.762528xhttp://dx.doi.org/10.2527/1998.762528x

Offer, G.Trinick, J.. 1983. On the mechanism of water holding in meat: The swelling and shrinking of myofibrils. J. Meat Sci. 8(4):245–281. doi:10.1016/0309-1740(83)90013-Xhttp://dx.doi.org/10.1016/0309-1740(83)90013-X

Sawyer, J. T.Apple, J. K.Johnson, Z. B.Baublits, R. T.Yancey, J. W. S.. 2009. Fresh and cooked color of dark-cutting beef can be altered by post-rigor enhancement with lactic acid. Meat Sci. 83:263–270. doi:10.1016/j.meatsci.2009.05.008http://dx.doi.org/10.1016/j.meatsci.2009.05.008

Suman, S. P.Faustman, C.Stamer, S. L.Liebler, D. C.. 2006. Redox instability induced by 4-hydroxy-2-nonenal in porcine and bovine myoglobins at pH 5.6 and 4°C. J. Agric. Food Chem. 54:3402–3408. doi:10.1021/jf052811yhttp://dx.doi.org/10.1021/jf052811y

Tang, J.Faustman, C.Hoagland, T. A.. 2005. Postmortem oxygen consumption of mitochondria and its effects on myoglobin form and stability. J. Agric. Food Chem. 53:1223–1230. doi:10.1021/jf048646ohttp://dx.doi.org/10.1021/jf048646o

Taylor, A. A.MacDougall, D. B.. 1973. Fresh beef packed in mixtures of oxygen and carbon dioxide. J. Food Sci. Technol. 8:453–461. doi:10.1111/j.1365-2621.1973.tb01732.x

Witte, V. C.Krause, G. F.Bailey, M. E.. 1970. A new extraction method for determining 2-thiobarbituric acid values of pork and beef during storage. J. Food Sci. 35:582–585. doi:10.1111/j.1365-2621.1970.tb04815.xhttp://dx.doi.org/10.1111/j.1365-2621.1970.tb04815.x