Species-specific effects of sarcoplasmic extracts on lipid oxidation in vitro.

The degree to which lipid and myoglobin (Mb) oxidation processes interact in meat can be species-specific. We investigated the effects of beef and pork sarcoplasmic extracts containing different Mb concentrations on lipid oxidation in a liposome system. Sarcoplasm was extracted from beef and pork longissimus dorsi and psoas major muscles. Beef sarcoplasm was diluted with 0.1 M phosphate buffer to obtain a Mb concentration equivalent to that in pork sarcoplasm. Conversely, equine heart Mb was added to pork sarcoplasm to match the myoglobin concentration of beef sarcoplasm. This resulted in beef and pork sarcoplasms, each with 2 different Mb concentrations for the longissimus (0.02 mM and 0.07 mM) and psoas (0.05 and 0.12 mM). Sarcoplasm (or phosphate buffer control) was incorporated within a phosphatidylcholine liposome preparation and incubated at 25 degrees C. Thiobarbituric acid reactive substances (TBARS) were measured at 0, 30, 60, 90, and 120 min of incubation. Regardless of species, greater Mb concentration within the sarcoplasm increased lipid oxidation (P < 0.05). Across muscles, pork sarcoplasm had lower TBARS values than beef sarcoplasm (P < 0.05). Our results suggest that pork sarcoplasm has a lesser effect on lipid oxidation than beef sarcoplasm for a common Mb concentration. However, increased myoglobin concentration within sarcoplasm promotes lipid oxidation regardless of species.

Interaction of fish myoglobin and myofibrillar proteins.

Interactions between fish myoglobin (Mb) and myofibrillar proteins were investigated in a Mb-natural actomyosin (NAM) model at 4 degrees C. Increases in metmyoglobin (MetMb) formation and the relative content of bound Mb were observed, as were decreases in whiteness and Ca(2+)-ATPase activity (P < 0.05). During the first 6 h of incubation, Mb bound preferably to myosin at domains other than the head portion, as evidenced by measurable ATPase activity. The potential binding of Mb to myosin heads occurred after 24-h incubation as evidenced by the marked decrease in Ca(2+)-ATPase activity of the NAM-Mb mixture when compared to that of NAM alone (P < 0.05). The interaction between fish Mb and myofibrillar proteins was more pronounced with increased storage time; formation of high-molecular-weight aggregates (> 206 kDa) also increased with time. Electrophoretic study revealed that disulfide bonds were not involved in Mb-NAM interactions.

Effect of ionic strength and temperature on interaction between fish myoglobin and myofibrillar proteins.

The effects of ionic strength (0, 0.3, and 0.6 M KCl) and temperature (4 and 25 degrees C) on the interaction between fish myoglobin and myofibrillar proteins were investigated in a model system. Increases in the relative content of bound myoglobin and metmyoglobin formation in myoglobin-natural actomyosin (NAM) mixtures with concurrent decreases in whiteness and Ca(2+)-ATPase activity were observed with increasing ionic strength (P < 0.05). The relative content of bound myoglobin and the oxidation of oxymyoglobin were generally greater at 25 degrees C than at 4 degrees C (P < 0.05). Binding of myoglobin to NAM resulted in decreased whiteness (P < 0.05). Ca(2+)-ATPase was not affected by temperature (P > 0.05). SDS-PAGE patterns of protein samples suggested that myoglobin-NAM interactions did not involve disulfide bonds. The formation of high-molecular-weight aggregates (>206 kDa) was observed and was more pronounced at higher ionic strength and higher temperature.

Influence of carbon monoxide in package atmospheres containing oxygen on colour, reducing activity, and oxygen consumption of five bovine muscles.

Steaks from five bovine muscles [psoas major (PM), longissimus lumborum (LL), deep semimembranosus (DSM), superficial semimembranosus (SSM), and semitendinosus (ST)] were packaged in atmospheres containing 20% or 80% oxygen, with and without 0.4% carbon monoxide. Steaks were evaluated on d 0, 4, and 7 of retail display for instrumental (CIE L(∗), a(∗), and b(∗)) and visual colour, total- and metmyoglobin-reducing activity, and oxygen consumption rate. Combining carbon monoxide with either oxygen level had no effect (P>0.05) on any measured attribute. Using higher oxygen levels increased colour stability and reduced variability (P<0.05) among muscles for all measured attributes. In general, colour stability and reducing activity for the muscles were LL>ST>SSM>PM>DSM. Including 0.4% carbon monoxide with 20% or 80% oxygen may not have impacted colour, due to preferential formation of oxymyoglobin, rather than carboxymyoglobin, at these oxygen levels.

Effect of antioxidants on stabilization of meat products fortified with n-3 fatty acids.

The effects of an n-3 oil emulsion, with and without added antioxidants, on lipid oxidation in n-3 polyunsaturated fatty acid (PUFA)-fortified meat products were studied. An emulsion of n-3 PUFAs was prepared (25% algal oil, 2.5% whey protein isolates, 10mM sodium citrate, 0.2% potassium sorbate, 500ppm of 70% mixed tocopherols, 100µM EDTA, pH 3, pasteurized at 75°C for 30min) and incorporated into fresh ground turkey, and fresh pork sausage (20% fat) to achieve a concentration of 500mg n-3 PUFA/110g meat. An antioxidant combination containing rosemary (0.2% w/w; radical quencher), citrate (0.5% w/w; sequestrant) and erythorbate (1g/kg product; reductant) was prepared and incorporated into ground turkey patties (5cm dia, 1.5cm thick) or fresh pork sausages (5cm dia, 1.5cm thick). Meat products were stored at 4°C or -18°C and analyzed for color (L*, a*, b* values), lipid oxidation (TBARS and lipid hydroperoxides) and n-3 PUFA profile. a* Values of refrigerated ground turkey patties decreased with storage, and an antioxidant combination effect was observed after 4 days (P<0.05). For fresh pork sausages at 4°C, control+antioxidant (CON+ANTI), and n-3+antioxidant (n-3+ANTI) groups showed greater a* values than controls (CON) indicating that the antioxidant combination stabilized meat color. TBARS and lipid hydroperoxides of both n-3 PUFA-enhanced meat products increased with storage (P<0.05); there were no significant changes in TBARS or lipid hydroperoxides for treatments containing the antioxidant combination (P<0.05). The actual level of n-3 PUFA incorporation in both meat products was greater than 87%; n-3 PUFA concentrations did not change within any treatment during storage (P>0.05). These results provide support for including antioxidant protection in n-3 PUFA fortified meat products.

Metmyoglobin reducing activity.

Meat colour is a major factor that influences the purchase decision by consumers. Many intrinsic and extrinsic factors contribute to the final colour of meat. The role of the MetMb reducing system in maintaining meat colour has been controversial and a considerable amount of work has been published since [Giddings, G. G., 1974. Reduction of ferrimyoglobin in meat. CRC Critical Reviews in Food Technology 5, 143-173] classic review. Historically, the activity of MetMb reductase was classified under different names, for example, diaphorases, aerobic and anaerobic reducing systems, cytochrome b(5) MetMb reductase, and NADH dependent metmyoglobin reducing enzyme system. Several techniques have been proposed to measure the enzyme activity including reflectance spectrophotometry and absorbance spectrophotometry. However, the variations in the reductase systems and techniques used to measure them have yielded inconsistent results from different investigators. This review seeks to characterize the current understanding of metmyoglobin reduction in meat especially with reference to recent developments in this area. Because many systems (different enzymatic systems and non-enzymatic systems) have been reported to reduce MetMb, the term MetMb reductase is not appropriate to be used to reflect "the MetMb reducing activity" in meat. The need for a standardized approach for measuring MetMb reduction is discussed in order for future research to ensure a greater understanding of this important reaction.

Effect of erythorbate, storage and high-oxygen packaging on premature browning in ground beef.

Premature browning (PMB) was investigated in ground beef patties with (0.04%, w/w) and without erythorbate. In Experiment 1, patties were stored at 4 °C for 48 h; at -18 °C for 21 days; or at -18 °C for 21 days, thawed at 4 °C for 24 h; and cooked. Bulk ground beef was stored at -18 °C for 24 days, thawed for 24 h at 4 °C, and patties prepared and cooked immediately. In Experiment 2, fresh patties were overwrapped with oxygen-permeable film or packaged in 80% O(2)/20% N(2) (MAP), and stored for 48 h at 4 °C, or at -18 °C for 21 days, and cooked. Total reducing activity and color (L*, a* and b* values) were measured immediately prior to cooking. Patties were cooked to internal temperatures of 60, 66, 71 and 77 °C and internal cooked color was measured. Total reducing activity was higher for the erythorbate treatment than controls for all storage conditions (P<0.05). a* Values of cooked patties were higher for erythorbate than control treatments under all storage and packaging conditions at 60 and 66 °C (P<0.05). The presence of erythorbate in ground beef patties appeared to maintain red color at cooked internal temperatures of 60 and 66 °C. Frozen bulk storage appeared to increase the susceptibility of ground beef to PMB when compared to fresh and frozen patties. Patties cooked directly from frozen state appeared less susceptible to PMB than frozen-thawed and bulk storage. Ground beef appeared predisposed to PMB when stored in high-oxygen MAP at 4 °C for 48 h.

The effects of antioxidant combinations on color and lipid oxidation in n-3 oil fortified ground beef patties.

This study was carried out to determine an effective combination of chelators, reductants and free radical scavengers for enhancing color stability and minimizing lipid oxidation in muscle foods fortified with n-3 fatty acids. Chelators (sodium tripolyphosphate, STPP; sodium citrate, CIT), reductants (sodium erythorbate, ERY) and radical scavengers (butylhydroxyanisole, BHA; mixed tocopherols from two different sources, 30 or 95TOC; rosemary extract, ROSE) were incorporated in various combinations into ground beef (15% fat) with or without n-3 oil fortification (n=8). Individually, STPP and CIT had no significant effect on a* values except day 4, but showed higher a* values when combined with ERY (STPP+ERY and CIT+ERY) (P<0.05). CIT had lower hue angle values than STPP on days 4 and 6, but CIT+ERY showed more discoloration than STPP+ERY (P<0.05). CIT+ERY showed less lipid oxidation than CIT alone (P<0.05), whereas there was no difference between STPP and STPP+ERY. CIT+ERY+ROSE demonstrated higher a* values than CIT+ERY+95TOC on days 4 and 6 (P<0.05); there was no difference between ROSE and 95TOC groups when n-3 oil was incorporated into ground beef patties (P>0.05). The combination of ROSE and ERY appeared to be effective in slowing the decline of a* values. All antioxidant combinations were effective at delaying lipid oxidation when compared to CON or n-3. A combination of CIT, ERY and ROSE was most effective for stabilizing color and delaying lipid oxidation.

Effect of muscle source on premature browning in ground beef.

Premature browning (PMB) describes cooked beef that may appear done before reaching 71 °C. Ground beef from paired Longissimus lumborum (LL) and Psoas major (PM) muscles was formed into patties. Patties were cooked immediately, and after 48 and 96 h storage at 4 °C. Total reducing activity (TRA) and external color were measured immediately prior to cooking. Patties were cooked to internal end point temperatures of 60, 66, 71 or 77 °C and internal cooked color (L(*), a(*) and b(*) values) was measured. Raw PM patties had greater L(*) values and lesser a(*) values than those from LL (P<0.05). For LL and PM, raw a(*) and b(*) values decreased with storage from 0 h to 96 h (P<0.05). At 0 and 48 h storage, cooked patties prepared from PM had greater a(*) values than those prepared from LL at all internal endpoint temperatures (P<0.05). Internal cooked a(*) values of patties from PM decreased with storage of raw patties, whereas it was stable for LL patties (P<0.05).

Induction of redox instability of bovine myoglobin by adduction with 4-hydroxy-2-nonenal.

The redox stability of myoglobin (Mb) is compromised by many factors, including lipid oxidation and its products. 4-Hydroxy-2-nonenal (HNE) is an alpha,beta-unsaturated aldehyde derived from the oxidation of omega-6 polyunsaturated fatty acids and is highly reactive and cytotoxic. Our objective was to study potential binding of HNE to Mb and determine how it affects redox stability. OxyMb (0.15 mM) was incubated with HNE (1 mM) at 4, 25, and 37 degrees C at pH 7.4 or 5.6. Samples were analyzed for MetMb formation and by Western blot analyses, LC-MS, LC-MS-MS, circular dichroism (CD), and differential scanning calorimetry (DSC). MetMb formation increased with increasing temperature and was greater at pH 5.6 than at pH 7.4 (P < 0.05). At 37 degrees C, HNE accelerated oxidation at pH 7.4 but not at pH 5.6 (P < 0.05). At both 25 and 4 degrees C, HNE accelerated oxidation at pH 7.4 and 5.6 (P < 0.05). LC-MS revealed the covalent binding of HNE to Mb at both pH values via Michael addition, while Western blot analysis indicated that HNE was bound to histidine (HIS) residues. LC-MS-MS identified six histidine residues of Mb that were readily adducted by HNE, including the proximal (HIS 93) and distal (HIS 64) histidine associated with the heme group. Secondary structure differences between control Mb and Mb incubated with HNE were not detected by CD. However, DSC revealed a decreased T(m) for Mb reacted with HNE at pH 7.4, indicating Mb tertiary structure was altered in a manner consistent with destabilization. These results suggest that HNE accelerates bovine skeletal muscle OxyMb oxidation in vitro by covalent modification at histidine residues.

Porcine oxymyoglobin and lipid oxidation in vitro.

The present study was carried out to investigate the effect of dietary α-tocopherol supplementation on porcine oxymyoglobin (OxyMb) and lipid oxidation (TBARS) in model lipid systems, and to determine the influence of 4-hydroxynonenal (4-HNE), a secondary product of lipid oxidation, on porcine OxyMb stability. Porcine metmyoglobin (MetMb) formation was greater in the presence of 4-HNE than the control (P<0.05). Western blot analyses confirmed the covalent modification of myoglobin (Mb) histidine residues by 4-HNE. When combined with microsomes, both equine and porcine OxyMb oxidation increased with time of incubation, and was greater at 37 than at 25 °C (P<0.05). Lower TBARS values were observed in microsomes prepared from vitamin E-supplemented than control pork livers (P<0.05). α-Tocopherol concentration did not affect OxyMb oxidation in microsomes at 25 or 37 °C (P>0.05). These results differ from those observed with beef muscle microsomes where both OxyMb and lipid oxidation were delayed with elevated α-tocopherol levels. These results suggest that the factors affecting Mb and lipid oxidation interactions may be species-dependent.

Effect of erythorbic acid on cooked color in ground beef.

Consumers often use the color of cooked ground beef as an indicator of doneness. For safety reasons, it is recommended that the center of ground beef products be heated to 71°C. In some instances beef may appear done before reaching 71°C, a condition termed premature browning (PMB). Ground beef (15% fat), with added erythorbic acid (ERY) at 0.04 and 0.06% was formed into patties, wrapped in oxygen permeable film, and stored in the dark at 4°C. Patties were stored for either 10 h or 58 h and then cooked to internal end point temperatures of 60, 66, 71 or 77°C. Internal cooked color L(∗), a(∗) and b(∗) values were measured. For beef patties stored 10 h, there was no effect of ERY on internal cooked color. After 58 h storage, ground beef with 0.04 and 0.06% ERY had higher a(∗) values than controls at 60°C (P<0.05). Beef with 0.04% ERY cooked to an internal temperature of 66°C had higher a(∗) values than 0.06% ERY and controls (P<0.05). There was no effect of ERY on color of beef patties cooked to 71 or 77°C. The presence of 0.04% ERY in ground beef patties stored 58 h appeared to maintain red color at internal temperatures of 60 and 66°C.

Aldehyde reactivity with 2-thiobarbituric acid and TBARS in freeze-dried beef during accelerated storage.

When lipid oxidation is evaluated in freeze-dried beef, a yellow 450-nm-absorbing pigment develops during the 2-thiobarbituric acid (TBA) assay. TBA analysis and high performance liquid chromatography (HPLC) were applied to measure oxidative changes in salted freeze-dried beef patties (15% fat) initially during storage at 49°C. The TBA pink pigment (λ(max)=532 nm) was most pronounced in unstored salted freeze-dried beef, and yellow pigment (λ(max)=450 nm) predominated in stored samples. An in vitro study of TBA reactivity of different aldehydes, known to be secondary lipid oxidation products, showed that alkanals and alk-2-enals favored TBARS(450) formation, while alka-2,4-dienals favored TBARS(532). Values of TBARS(450) from 95°C TBA incubation were lower than those from 25°C incubation (P<0.05), indicating that the yellow chromophore from the aldehyde-TBA complex was less thermally stable than the pink pigment. 5-Hydroxymethyl-2-furfural, an aldehyde produced from Maillard reaction, also produced strong TBARS(450). Propional, butanal and 5-hydroxymethyl-2-furfural (HMF), were tentatively identified in freeze-dried beef during accelerated storage at 49°C, and have the potential to yield TBARS @450.

Effect of dietary α-tocopherol supplementation on color and lipid stability in pork.

Myoglobin and lipid oxidation are major causes of quality deterioration in fresh pork. A process to enhance color and lipid stability would prove valuable to the pork industry given the current trend of centralized packaging and distribution to retail markets. Our objective was to determine the effects of dietary α-tocopherol (α-Toc) supplementation on color and lipid stability in ground pork, and loin chops stored in modified atmosphere packaging (MAP). Yorkshire crossbred pigs (n=20) were randomized into two groups and fed diets containing 48 (CON) or 170 mg α-Toc acetate/kg feed (VIT-E) for 6 weeks before slaughter. Plasma α-Toc concentration was measured weekly. Post-slaughter, Boston butt shoulders were ground, formed into patties with or without 1.5% salt, and stored fresh at 4°C for 0, 2, 4, or 6 days, and frozen at -20°C for 45 or 90 days. Pork loin chops were packaged aerobically and stored at 4°C for 0, 2, 4 or 6 days, or in MAP at 4°C for 7, 10 or 13 days prior to Hunter L*,a*,b* and TBARS analyses. α-Toc concentration of longissimus dorsi, psoas major, biceps femoris, semimembranosus and semitendinosus muscles was determined. Plasma α-Toc was greater (P<0.05) in VIT-E animals compared with CON and α-Toc concentrations were greater (P<0.05) in all VIT-E muscles compared with CON. TBARS values of both fresh and salted patties were less in VIT-E than in CON meat following 6 days at 4°C; VIT-E TBARS of salted patties were less (P<0.05) after 45 days at -20°C compared with CON. α-Toc supplementation did not influence (P>0.05) color of aerobically packaged or MAP chops, or of fresh or salted pork patties. α-Toc supplementation reduced TBARS formation in fresh and salted pork but had no significant impact on color.

Effect of aldehyde lipid oxidation products on myoglobin.

The effects of aldehyde lipid oxidation products on myoglobin (Mb) were investigated at 37 degrees C and pH 7.2. Oxymyoglobin (OxyMb) oxidation increased in the presence of 4-hydroxynonenal (4-HNE) compared to controls (P < 0.05). Preincubation of metmyoglobin (MetMb) with aldehydes rendered the heme protein a poorer substrate for enzymatic MetMb reduction compared to controls, and the effect was inversely proportional to preincubation time; unsaturated aldehydes were more effective than saturated aldehydes (P < 0.05). The order of MetMb reduction as affected by preincubation was control > hexanal > heptanal > octanal > nonanal = decanal = hexenal > heptenal = octenal > nonenal = decenal = 4-HNE (P < 0.05). Preincubation of MetMb with 4-HNE enhanced the subsequent ability of the heme protein to act as a prooxidant in both liposomes and microsomes when compared to controls (P < 0.05); the effect was reduced in microsomes containing elevated concentrations of alpha-tocopherol (P < 0.05). MetMb preincubation with mono-unsaturated aldehydes enhanced the catalytic activity of MetMb to a greater degree than saturated aldehydes (P < 0.05). These results suggest that aldehyde lipid oxidation products can alter Mb stability by increasing OxyMb oxidation, decreasing the ability of MetMb to be enzymatically reduced and enhancing the prooxidant activity of MetMb.

alpha,beta-unsaturated aldehydes accelerate oxymyoglobin oxidation.

This study investigates the potential basis for enhancement of oxymyoglobin (OxyMb) oxidation by lipid oxidation products. Aldehydes known to be formed as secondary lipid oxidation products were combined with OxyMb in aqueous solution at 37 degrees C and pH 7.4. Metmyoglobin (MetMb) formation was greater in the presence of alpha,beta-unsaturated aldehydes than their saturated counterparts of equivalent carbon chain length. Additionally, increasing chain length from hexenal through nonenal resulted in increased MetMb formation (P < 0.05). Electrospray ionization mass spectrometry (ESI-MS) revealed that OxyMb incubated with 4-hydroxynonenal (HNE) at pH 7.4 at 37 degrees C yielded myoglobin molecules adducted with one to three molecules of HNE from 0.5 to 2 h of incubation, respectively. A prooxidant effect of HNE was noted at pH 7.4 but was not apparent at pH 5.6 when compared to the control (P < 0.05). This appeared to be due to rapid OxyMb autoxidation at this pH compared to pH 7.4. ESI-MS demonstrated that adduction of HNE to OxyMb occurred at pH 5.6. This research demonstrates that alpha, beta-unsaturated aldehydes accelerate OxyMb oxidation and appear to do so via covalent attachment.

alpha-tocopherol oxidation in beef and in bovine muscle microsomes.

The oxidation of alpha-tocopherol (TH) in beef was analyzed using a stable isotope dilution capillary gas chromatography-mass spectrometry assay. TH decreased while alpha-tocopherolquinone (TQ) and 2,3-epoxy-alpha-tocopherolquinone (TQE(2)) increased in ground longissimus lumborum (LL) and psoas major (PM) muscles during storage (P < 0.10). In LL steaks, the relative concentrations of TH decreased and TQ and TQE(2) increased in surface samples; changes were less dramatic in deep samples. Deuterated alpha-tocopherolhydroquinone (THQ) standard was not recovered and endogenous THQ was not detected in meat; THQ was measurable in microsomes isolated from PM and incubated in the presence of 2, 2'-azobis(2-amidopropane)HCl (ABAP) or myoglobin. ABAP-challenged microsomes yielded a tocopherol product profile which favored 5, 6-epoxy-alpha-tocopherolquinone (TQE(1)) and TQE(2), while the use of myoglobin as prooxidant resulted in a higher proportion of TQ and THQ. Results demonstrated that concentrations of TH decreased and TQ and TQE(2) increased in meat during storage and are consistent with the peroxy-radical scavenging function of tocopherol.

Effect of dietary vitamin E supplementation on the colour and lipid stability of fresh, frozen and vacuum-packaged beef.

The effects of dietary vitamin E supplementation on tissue α-tocopherol (α-Toc) levels and on the susceptibility of fresh, frozen and vacuum-packaged beef to lipid oxidation and colour deterioration were investigated. Friesian cattle were fed diets containing 20 (basal, n=5) or 2000 (supplemented, n=5) IU (α-tocopheryl acetate/kg feed/day for approximately 50 days prior to slaughter. α-Toc levels were higher (p<0.05) in muscles from supplemented animals than from those on a basal diet. Significant differences in α-Toc levels were also observed between muscles from different treatment groups, the order of the supplemented group was: M. psoas major (PM)>M. longissimus dorsi (LD)>M. gluteus medius (GM) (p<0.05), and in the basal group the order was: PM>GM>LD (p<0.05). Supplemented fresh, frozen and vacuum packed beef showed greater colour and lipid oxidative stability than meat from the basal group after 7 days retail display at 4°C (p<0.05). Thus, dietary (α-Toc supplementation appeared to retard metmyoglobin and TBARS formation in LD, GM and PM and increased the colour shelf life of these muscles.

Effect of Pseudomonas fluorescens on beef discoloration and oxymyoglobin oxidation in vitro.

The relationship between bacterial growth and oxymyoglobin oxidation in vitro and in meat was studied. In the in vitro study, oxymyoglobin was combined with Pseudomonas fluorescens or sterile nutrient broth (control) in an airtight vessel. P. fluorescens samples showed greater metmyoglobin formation and oxygen consumption than controls. The P. fluorescens population in the reaction vessels was correlated with metmyoglobin formation (r = 0.85, P < 0.05) and oxygen consumption (r = 0.91, P < 0.05). When P. fluorescens and oxymyoglobin were combined in an airtight vessel, reducing the headspace from 13 ml and 9 ml to 3 ml resulted in greater metmyoglobin formation (P < 0.05). In the meat study, beef cores prepared from longissimus lumborum were inoculated with P. fluorescens (10(7) CFU/cm2) or sterile peptone water (control), packaged under 1% O2 (+99% N2), air, or 100% O2 and stored at 4 degrees C. Inoculated beef cores showed higher bacterial loads and metmyoglobin formation than their respective controls during 10 h storage in 1% O2, 3 days in air, and 7 days in 100% O2 (P < 0.05). This finding indicated that P. fluorescens could accelerate beef discoloration. Overall, studies demonstrated that oxygen consumption concomitant with P. fluorescens growth decreased partial oxygen pressure, which accelerated oxymyoglobin oxidation.