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The effect of pH on shelf-life of pork during aging and simulated retail display
Meat Science 82 (2009) 86–93
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The effect of pH on shelf-life of pork during aging and simulated retail display S.F. Holmer a, R.O. McKeith b, D.D. Boler a, A.C. Dilger a, J.M. Eggert c, D.B. Petry d, F.K. McKeith a, K.L. Jones b, J. Killefer a,* a
Department of Animal Science, University of Illinois at Urbana-Champaign, 205 Meat Science Lab, 1503 S. Maryland Drive, Urbana, IL 61801, USA Department of Animal Science, Food and Nutrition, Southern Illinois University, 1205 Lincoln Drive, MC 4417, Carbondale, IL 62901, USA c Newsham Choice Genetics, 5058 Grand Ridge Drive, Suite 200, West Des Moines, IA 50265, USA d Triumph Foods LLC, 5302 Stockyards Expressway, Saint Joseph, MO 64504, USA b
a r t i c l e
i n f o
Article history: Received 3 August 2008 Received in revised form 13 December 2008 Accepted 15 December 2008
Keywords: Aerobic plate count Color Pork pH Shelf-life
a b s t r a c t The pork industry uses pH to differentiate product of varying quality; thus, the effect of pH on shelf-life is important as time during transport is extended. The objective was to develop regression equations to predict shelf-life over a range of ultimate pH (5.42–6.26). Shelf-life was evaluated after vacuum aging pork loin sections 0, 7, 14, 21, or 28 d and during 3 d of simulated retail display (4.5 °C) for pork loin chops. Correlation coefficients indicated a strong relationship between pH and quality measurements. Regression analysis with Aging Day and pH was able to explain 87% of the variation in aerobic plate counts for pork. After 28 d of vacuum aging, loin sections from the upper end of the pH distribution had about a 3 log(1000X) greater aerobic plate count than did the lower end pH product. An increase in pH resulted in pork with lower L*, a*, b* and R630 R580 values and as Aging Day increased, instrumental measurements of color increased slightly. Although higher pH is associated with improved pork quality, higher pH and longer aging periods will result in increased microbial proliferation and decreased shelf-life. Thus, an intermediate pH may provide the most desirable combination of quality and shelf-life when extensive aging is used. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Measurement of pH is used by the pork industry to differentiate product of varying quality. Loin pH is significantly correlated to attributes such as color and water-holding capacity (Huff-Lonergan et al., 2002), which are important characteristics when consumers make purchasing decisions (Brewer & McKeith, 1999). Over a wide range of pH (4.86–7.15), Bidner et al. (2004) developed regression equations to describe the relationship of ultimate pH to various quality measurements. Ultimate pH explained 79% of the variation in color, 57% of variation in drip loss, and 77% of the variation in purge loss (Bidner et al., 2004). Generally, higher pH pork will have superior quality compared to lower pH pork; yet, pH can have a dramatic effect on shelf-life from both a microbiological and color stability perspective. As Price and Schweigert (1987) reported, meat with a pH greater than 5.8 may be more conducive to spoilage and result in decreased shelf-life. Furthermore, Newton and Gill (1981) reviewed the effect of dark, firm, and dry (DFD) meat on microbiological characteristics, and found that the lower glucose content and high
* Corresponding author. Tel.: +1 217 333 8482; fax: +1 217 244 5142. E-mail address: [email protected] (J. Killefer). 0309-1740/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.meatsci.2008.12.008
pH of DFD meat resulted in increased spoilage. Numerous other studies (Fox, Wolfram, Kemp, & Langlois, 1980; Greer & Murray, 1988; Knox, van Laack, & Davidson, 2008) have also examined the effect of pH on shelf-life. These authors have concluded that higher pH products have greater microbial counts and a decrease in shelf-life, which is observed when packaged either under vacuum (Rousset & Renerre, 1991) or overwrapped with an oxygen permeable film (Rey, Kraft, Topel, Parrish, & Hotchkiss, 1976). In addition to changes in microbial proliferation over aging, physical changes that affect color can also occur. These color changes occur based on the form of myoglobin (deoxymyoglobin, oxymyoglobin, and metmyoglobin) and are both time and pH dependent (Ledward, 1992). Generally an increase in pH will result in darker (L or L*) product (Bidner et al., 2004; Huff-Lonergan et al., 2002), but, within a given pH range, L* will remain constant over display (Andrews et al., 2007). A decrease in a* value (becoming less red) will occur over display time, but will decrease to a greater extent in PSE muscle (Zhu & Brewer, 1998). In addition, lower pH product will visually have a shorter shelf-life (Greer & Murray, 1988) compared to higher pH product, which may be due to less enzymatic reduction and faster rate of myoglobin oxidation, which is favored at lower pH (Ledward, 1984). As most of the literature has categorized meat based on discrete pH levels (e.g., PSE or DFD; high or low; <5.4, 5.4–5.6, etc.), it would
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2. Materials and methods
at 2.0 ± 0.27 °C. After the respective Aging Day in vacuum bags, loin sections were removed and the down facing cut surface was swabbed for microbial counts. From each section, a cut was made to expose two fresh surfaces. One fresh surface was overwrapped with PVC and displayed, while the opposing fresh cut surface was swabbed as an initial microbial count for the display chop. Thus, a chop was used for display following each Aging Day.
2.1. Product selection and quality measurements
2.3. Evaluation of PVC overwrapped product in simulated retail display
Over two consecutive weeks, 39 boneless pork loins (19 and 20 loins/week, respectively) were selected from a population of 306 loins based on ultimate (approximately 30 h postmortem) pH measured on Longissimus muscle at the 10th rib (MPI pH-Meter, Meat Probes Inc., Topeka, KS, USA). Starting at pH of 5.40 and increasing every 0.05 units, 1–2 loins/week were randomly selected within each pH increment, so that there was a continuous range in pH (Fig. 1). Because there were only a few loins at the upper end of the distribution, all loins with a pH of 5.85–6.30 were selected. In addition to pH at the 10th rib, visual color (NPPC, 1999), marbling (NPPC, 1999), and firmness (NPPC, 1991) were evaluated by trained, university personnel. CIE L*, a*, and b* values (CIE, 1978) were measured with a Minolta Chromameter CR-300 (Minolta Camera Co., Osaka, Japan) with D65 illuminant, 0° observer and calibrated against a standard white tile.
After display for approximately 15 h, initial (1 d) evaluations were made, with subsequent evaluations at 24 (2 d) and 48 h (3 d) later (Display Days: 1, 2, or 3). All evaluations were made through the PVC film. Visual evaluations were made by a fivemember panel experienced in meat color evaluations, but visual color was evaluated only on the first day of simulated retail display. Visual discoloration (i.e., metmyoglobin formation) was evaluated each day using a 10 cm line scale (reference marks at 0%, 25%, 50%, and 100%), where every 1 cm equaled 10% discoloration on the face of the chop. Each day, L*, a*, and b* were measured with a Hunter Lab Miniscan XE Plus (Model 45/0-L; Hunter Associates Laboratory Inc., Reston, VA, USA) using illuminant D65 and a 10° observer. The spectrocolorimeter was calibrated daily against the black and white tiles covered with PVC film. The difference in reflectance at 630 and 580 nm (R630 R580) was used as an indicator of color stability (Strange, Benedict, Gugger, Metzger, & Swift, 1974). Lastly, microbial swabs were collected on the pork surface after the last evaluation (3 d).
benefit the pork industry to understand the relationship of a continuous pH range and shelf-life. Therefore, the objective was to predict shelf-life characteristics over a continuous range in pH for pork loin sections following aging in vacuum bags and pork loin chops under simulated retail display.
2.2. Loin fabrication, processing, and packaging From each loin, a knife (not sterilized between each sample) was used to cut a fresh surface and an initial swab from the exposed surface was performed to enumerate microbial numbers. This swab served as the 0 d sample for both vacuum packaged loin sections and the oxygen permeable polyvinyl chloride shrink film (PVC) overwrapped chops. Next, a 2.5 cm chop was cut and packaged in a white foam tray (Pactiv, Lake Forest, IL, USA) with a absorbent pad (Dri-LocÒ Pad; Sealed Air Corp., Duncan, SC, USA) and overwrapped in PVC film (O2 transmission rate (OTR) = 1400 cm3/m2/24 h; Bush Brothers, Urbana, IL, USA). This chop was stored at 4.5 ± 1.33 °C under constant lighting (900– 1100 Lux) to simulate retail display. The remainder of the loin was cut into four sections, vacuum packaged (OTR = 102.3 cm3/ m2/24 h at 23 °C, 65% RH; moisture vapor transmission rate = 7.9 g/m2/24 h at 37.7 °C, 90% RH; Bush Brothers, Urbana, IL, USA) and randomly assigned to Aging Days of 7, 14, 21, or 28 d
2.4. Enumeration of microbial contamination Microbial swabs were performed with a sterile 15 cm cottontipped applicator (FisherbrandÒ; Fisher Scientific, Pittsburgh, PA, USA) and a 5 5 cm template. After each swab, the applicator tip was broken off in a 15 ml conical tube (FisherbrandÒ; Fisher Scientific, Pittsburgh, PA, USA) containing 10 ml of sterile 0.1% peptone (Difco Laboratories, Detroit, MI, USA). After vortexing for 10 s, samples were serially diluted and plated on PetrifilmTM (3M, St. Paul, MN, USA) in duplicate to enumerate bacteria colonies (aerobic plate counts, APC). After incubation of films for 48 h at 37 °C, films containing 25–250 colonies were counted and are reported as log colony forming units per square centimeter (log CFU/cm2). 2.5. Statistical analysis
6
5
Count
4
3
2
1
0 5.4
5.5
5.6
5.7
5.8
5.9
6.0
6.1
6.2
pH Fig. 1. pH distribution of loins selected for this experiment.
6.3
All data were analyzed with SAS (Version 9.1; SAS Inst., Inc., Cary, NC, USA). The means procedure was used to calculate the mean, standard deviation, minimum and maximum values for the measured parameters, whereas Pearson correlation coefficients between pH and pork quality measurements were calculated with the correlation procedure. Regression analysis was performed with the general linear models procedure. Initial model evaluation for APC following vacuum aging, initial visual color, and DAPC over simulated retail display included the linear, quadratic, and linear interaction for the independent variables of pH and Aging Day (0, 7, 14, 21, and 28 d). For the instrumental measurements of L*, a*, b*, and R630 R580, the initial model included the linear, quadratic, and all linear two-way interactions of pH, Aging Day, and simulated retail Display Day (1, 2, and 3 d). Only significant (P < 0.05) terms were included in the model. If the quadratic term was significant, both the linear and quadratic terms were included in the regression model. Models were evaluated for outliers and consistency of the error variances. For regression analysis of visual color score, data was transformed with 1/visual color score2 due to increasing error variance.
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Table 1 Summary statistics for the selected loins. Parmeter
Mean
SDa
Minimum
Maximum
Correlation with pH
pH Colorb Marblingc Firmnessd L* a* b*
5.76 3.6 3.1 3.1 46.4 7.4 3.7
0.227 0.96 1.19 0.86 2.96 1.66 1.25
5.42 1.0 1.0 1.0 41.5 5.0 1.5
6.26 5.0 6.0 4.0 53.8 11.4 7.2
NA 0.63* 0.64* 0.63* 0.63* 0.25 0.32*
a b c d *
Standard deviation. 1 = lightest to 6 = darkest (NPPC, 1999). Estimated % lipid (NPPC, 1999). 1 = softest to 5 = firmest (NPPC, 1991). P < 0.05.
3. Results and discussion 3.1. Summary statistics and simple correlations Mean pH for the population was 5.76 and ranged from 5.42 to 6.26 (Table 1). Wright et al. (2005) characterized pork in the US retail market and reported that loin chops had an average pH of 5.64, with a range of 5.10–6.36. While the pH range in the current study was not as low as those reported by Wright et al. (2005), this study would represent a majority of pork currently produced in the industry. Bidner et al. (2004) evaluated pork which ranged in pH from 4.86 to 7.15, but indicated the majority (57.4%) of loins had an ultimate pH between 5.4 and 6.1. Difference in pH between studies could have been the result of numerous factors, such as genetics or other management practices, but in the case of Bidner et al. (2004), the higher pH was achieved with an injection of epinephrine prior to slaughter. Pearson correlation coefficients (r) are also presented in Table 1. Loin pH was significantly correlated with color (r = 0.63), marbling (r = 0.64), firmness (r = 0.63), L* (r = 0.63), and b* (r = 0.32). These results are similar to Boler et al. (in press), who reported significant correlations between pH and color, firmness, marbling,
and L*. Significant correlations between pH and color, marbling, and firmness were also reported by Huff-Lonergan et al. (2002). Correlations to quality measurements presented in this study are higher than both preceding studies and may result from the selection method used. Loins for the current study were selected to represent a distribution across pH, whereas the previous reports (Boler et al., in press; Huff-Lonergan et al., 2002) were not selected for a continuous distribution in pH and would have more loins centered around the mean pH for the population. Nonetheless, these results reinforce pH as a useful tool for the pork industry in differentiating product of varying quality. 3.2. Aerobic plate counts Data for APC of 0 d samples was excluded from analysis, as most of these samples had APC below detectable limits. As Aging Day increased from 7 to 28 d, APC increased (Fig. 2), which is not surprising because increased aging time in a vacuum bag will result in increased bacterial proliferation (Blixt & Borch, 2002; Hodges, Cahill, & Ockerman, 1974; Knox et al., 2008). Furthermore, as pH increased, APC increased, and these results are similar to those of Rousset and Renerre (1991) that indicated bacterial counts were 10- to 100-fold greater on high pH (=6.20) meat than normal pH (=5.55) meat at various aging durations. In this study, there was a significant interaction effect for pH and Aging Day on APC (Fig. 2). As Aging Day increased, pH has a more pronounced effect on APC, with higher pH product having increased APC. At 7 d of aging, APC is relatively unaffected by pH, but, after 28 d of aging, pork with the highest pH had about a 3 log(1000X) greater APC density than pork with the lowest pH. The following regression equation was established based on pH and Aging Day and the significant interaction between the two parameters: predicted APC (log CFU/cm2) = 105.0832 (0.9071 Aging Day) + (0.0014 Aging Day2) + (37.5674 pH) (3.3300 pH2) + (0.1775 [Aging Day pH]); R2 = 0.87. Similar to these results, Knox et al. (2008) performed regression analysis and reported an increasing linear relationship between pH
Fig. 2. Predicted aerobic plate counts (APC; log CFU/cm2) from vacuum stored pork loin sections. Predicted APC = Day2) + (37.5674 pH) (3.3300 pH2) + (0.1775 [Aging Day pH]); R2 = 0.87.
105.0832
(0.9071 Aging Day) + (0.0014 Aging
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and APC at 14, 24, and 34 d of aging. However, it would appear that after only 14 d of aging, APC approached 6 log CFU/cm2 (Knox et al., 2008), whereas, in the current study, only a few of the samples at the upper end of the pH range approached this limit after 28 d of aging. Pork which was approaching this 6 log CFU/cm2, are at a level in which products will be considered spoiled (Banwart, 1981). Pork from the lower end of the pH range, with much lower APC, could be stored for 28 d without crossing this threshold level of spoilage. Because higher pH loins are selected for export (directly by measuring pH or indirectly by selecting darker colored product) and will spend prolonged periods of time in a vacuum bag, considerations for shelf-life are especially important. Results show that high pH (>6.0) pork, aged for prolonged periods of time (similar to export product) may be at risk for spoilage after 28 d of aging in a vacuum bag. The predicted DAPC (final APC of chop after display initial APC of chop) is shown in Fig. 3. It is important to evaluate how the APC changed from the initial to the final value, as the 3 d evaluations may have been high due to initial contamination as the chops were cut. The regression equation illustrates (predicted DAPC = 147.6438 + (0.2467 Aging Day) (0.0054 Aging Day2) + (48.1093 pH) (3.9279 pH2); R2 = 0.52), that both Aging Day and pH affected the change in APC between the first and last days of display. As Aging Day increased, there was an increase in the rate in which the APC changed over display, and this was also the case for pH; yet, as pH increased, there was a greater change in APC compared to Aging Day. Greer and Murray (1988) reported that under simulated retail display, pork loin chops classified as PSE (pH 5.40) had lower bacterial counts than DFD (pH 6.41) chops. For all three groups (PSE, normal (pH 5.58), DFD) bacterial counts increased as time in display increased; therefore, as pork is stored for longer time periods, the risk of spoilage during retail display increases across all pH levels. Both Fox et al. (1980) and Rey et al. (1976) observed that PSE (lower pH) pork had lower microbial counts during retail display than that of higher pH product.
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3.3. Color evaluation Visual color scores (Fig. 4) were evaluated 15 h (1 d) after fabrication following 0, 7, 14, 21, or 28 d of vacuum packaged aging and the predicted transformed visual color score (1/color score2) = 3.0530 (0.0014 Aging Day) + (0.00004 Aging Day2) (0.9543 pH) + (0.0764 pH2); R2 = 0.44. Even though Aging Day and Aging Day2 were significant variables in the regression equation, pH, and not Aging Day, contributed the most to the prediction of visual pork color scores. Similar to the current data, other researchers (Boler et al., in press; Huff-Lonergan et al., 2002) have also reported the importance that pH plays on color scores. Discoloration was not analyzed because, after 28 d in a vacuum bag and 3 d of display, the majority (34 of 39) of samples were evaluated as <10% discolored. Instrumental measurements for color (L*, a*, b*, and R630 R580) were evaluated during the 3 d of simulated retail display. For the dependent variable L* (Fig. 5), Display Day (1, 2, or 3 d) was not significant nor did Display Day have a significant interaction with either pH or Aging Day (predicted L* = 616.0093 + (0.0405 Aging Day) (187.2003 pH) + (15.3807 pH2); R2 = 0.35). This indicates that during 3 d simulated retail display, there were minimal changes in L* values over the 3 d display period, regardless of aging time. Lindahl, Karlsson, Lundström, and Andersen (2006) indicated that aging time did affect L* value when pork chops were wrapped with an oxygen permeable film and displayed. However, other research has reported no change in L* value after 7 d on display (Zhu & Brewer, 1998) or after removal from vacuum bags after 28 d of postmortem aging (Zhu, Bidner, & Brewer, 2001). The predicted a* value (26.1004 + (0.0433 Aging Day) (0.0014 Aging Day2) (3.2192 pH) + (0.5159 Display Day) (0.0237 [Aging Day Display Day]); R2 = 0.30), predicted b* value (100.3634 + (0.0829 Aging Day) (26.9048 pH) + (2.0366 Day) (0.3907 Display Day2) pH2) + (1.8886 Display 2 (0.0138 [Aging Day Display Day]); R = 0.52), and predicted (11.4417 pH) + R630 R580 (86.1003 (0.3560 Aging Day)
Fig. 3. Predicted change in aerobic plate counts (DAPC; log CFU/cm2) of pork chops under simulated retail display from 1 to 3 d. Predicted DAPC = Aging Day) (0.0054 Aging Day2) + (48.1093 pH) (3.9279 pH2); R2 = 0.52.
147.6438 + (0.2467
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Fig. 4. Predicted transformed visual color score (1/visual color score2). Predicted transformed color score = 3.05 Day2) (0.9543 pH) + (0.0765 pH2); R2 = 0.44.
(0.0843 [Aging Day pH]) (12.7216 Display Day) (0.0215 [Aging Day Display Day]) + (2.1555 [pH Display Day]); R2 = 0.42) results are presented in Figs. 6–8, respectively. As the three prediction equations had all three parameters (pH, Aging Day, and Display Day) in the prediction equation, Display Day (1, 2, or 3 d) was held constant in each equation so that the response due to Aging Day and pH could be visualized.
(0.0014 Aging Day) + (0.00004 Aging
The a* and b* values decreased (became less red and less yellow) as pH increased, and the decrease was observed across all 3 d of simulated retail display. Zhu and Brewer (1998) documented no difference in a* values for normal or DFD pork; yet, both normal and DFD pork were higher than that of PSE pork. As Aging Day increased from 0 to 28 d, there was a slight increase for a* and b* values, indicating that vacuum packaged pork became redder and more yellow over time. Lindahl et al. (2006) found that both a*
Fig. 5. Predicted L* value. Predicted L* = 616.0093 + (0.0405 Aging Day)
(187.2003 pH) + (15.3807 pH2); R2 = 0.35.
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Fig. 6. Predicted a* value where each response surface represents 1, 2, or 3 d evaluation. Predicted a* = 26.1004 + (0.0433 Aging Day) Day2) (3.2192 pH) + (0.5159 Display Day) (0.0237 [Aging Day Display Day]); R2 = 0.30.
and b* values were higher in pork aged 8 d when compared to pork aged only 1 d. Lindahl et al. (2006) also reported that there was an increase for a* during the first day of display. The current results (Fig. 6) illustrates an increase in a* (becomes more red) over the 3 d of display when loins were stored up to 21 d, after which a* val-
Fig. Day)
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(0.0014 Aging
ues decreased with increasing Display Day when loins were aged out to 28 d. Zhu and Brewer (1998) reported that b* values increased from 0 to 1 d of display, with a decrease in b* values in subsequent days of display. Similarly in this study, chops from unaged (0 d) loins had lower b* values on 1 d than on 2 or 3 d of simulated
7. Predicted b* value where each response surface represents 1, (26.9048 pH) + (2.0366 pH2) + (1.8886 Display Day) (0.3907 Display Day2)
2, or 3d evaluation. Predicted b* = 100.3634 + (0.0829 Aging (0.0138 [Aging Day Display Day]); R2 = 0.52.
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Fig. 8. Predicted R630 R580 where each response surface represents 1, 2, or 3 d evaluation. Predicted R630 R580 = 86.1003 (0.3560 Aging (11.4417 pH) + (0.0843 [Aging Day pH]) (12.7216 Display Day) (0.0215 [Aging Day Display Day]) + (2.1555 [pH Display Day]); R2 = 0.42.
retail display. Throughout the aging period and across the pH range, Display Day 2 had higher b* values than both day 1 and 3. Rousset and Renerre (1991) observed that beef with a high pH, previously held in a vacuum bag, had lower R630 R580 values than beef with normal pH. Coincidentally, R630 R580 values decreased (less surface oxymyoglobin) as pH increased in pork loins (Fig. 8). Even though the decrease in R630 R580 values was observed on all 3 d of display, 1 d R630 R580 values of low pH chops were higher than 2 and 3 d. Yet, at 0 d of aging and when pH was greater than 5.80, R630 R580 values were higher after 3 d of display than both proceeding days. Likewise, after 28 d of aging those samples having a pH greater than 6.18 had higher R630 R580 values at Display Day 3 than both proceeding days. Zhu and Brewer (1998) reported that pork classified as ether PSE, normal, and DFD had similar R630 R580 values until 2–3 d of aging time, but PSE pork will have a greater decline in R630 R580 values than the other conditions. Because none of the pork chops in the current study exhibited the classic PSE condition, this may explain the difference in results between the current experiment and those of Zhu and Brewer (1998). 4. Conclusions From our results, it is apparent that pH can have a dramatic affect on the shelf-life of pork after longer aging times typically associated with postmortem aging, distribution, and/or exportation. We developed regression equations that can be used to predict shelf-life characteristics based on the ultimate pH of the Longissimus, duration of aging, and length of retail display. Results demonstrated that increasing aging time of high pH pork will increase the amount of microbial proliferation. Similar results will be observed when high pH product undergoes retail display. Color stability is affected more by loin pH than by duration of aging (up to 28 d)
Day) -
or display (up to 3 d). Pork loins of higher pH will have more microbial growth during aging, chops with more rapid bacterial proliferation during display, and a shorter shelf-life. Pork products with an intermediate pH may provide the best balance between quality and shelf-life when extensive aging is desired or necessary. Acknowledgement The authors would like to thank Newsham Choice Genetics for partial funding and support of this project. References Andrews, B. S., Hutchison, S., Unruh, J. A., Hunt, M. C., Boyer, J. E., & Johnson, R. C. (2007). Influence of pH at 24 h postmortem on quality characteristics of pork loins aged 45 days postmortem. Journal of Muscle Foods, 18(4), 401–419. Banwart, G. J. (1981). Basic Food Microbiology. Westport, Connecticut: The AVI Publishing Company, Inc. Bidner, B. S., Ellis, M., Brewer, M. S., Campion, D., Wilson, E. R., & McKeith, F. K. (2004). Effect of ultimate pH on the quality characteristics of pork. Journal of Muscle Foods, 15(2), 139–154. Blixt, Y., & Borch, E. (2002). Comparison of shelf life of vacuum-packed pork and beef. Meat Science, 60(4), 371–378. Boler, D. D., Dilger, A. C., Bidner, B. S., Carr, S. N., Eggert, J. M., Day, J. W., et al. (in press). Ultimate pH explains variation in pork quality traits. Journal of Muscle Foods. Brewer, M. S., & McKeith, F. K. (1999). Consumer-rated quality characteristics as related to purchase intent of fresh pork. Journal of Food Science, 64(1), 171–174. CIE (Commission Internationale de l’eclairage) (1978). Recommendations on uniform color spaces—color equations, psychometric color terms. Supp. No. 2 to CIE Publ. No. 15 (E-1.3.L) 1971 (9TC-1-3), CIE, Paris, France. Fox, J. D., Wolfram, S. A., Kemp, J. D., & Langlois, B. E. (1980). Physical, chemical, sensory, and microbiological properties and shelf life of PSE and normal pork chops. Journal of Food Science, 45(4), 787–790. Greer, G. G., & Murray, A. C. (1988). Effects of pork muscle quality on bacterial growth and retail case life. Meat Science, 24(1), 61–71. Hodges, J. H., Cahill, V. R., & Ockerman, H. W. (1974). Effect of vacuum packaging on weight loss, microbial growth and palatability of fresh beef wholesale cuts. Journal of Food Science, 39(1), 143–146.
S.F. Holmer et al. / Meat Science 82 (2009) 86–93 Huff-Lonergan, E., Baas, T. J., Malek, M., Dekkers, J. C., Prusa, K., & Rothschild, M. F. (2002). Correlations among selected pork quality traits. Journal of Animal Science, 80(3), 617–627. Knox, B. L., van Laack, R. L. J. M., & Davidson, P. M. (2008). Relationships between ultimate pH and microbial, chemical, and physical characteristics of vacuumpackaged pork loins. Journal of Food Science, 73(3), M104–M110. Ledward, D. A. (1984). Haemoproteins in mean and meat products. In B. J. F. Hudson (Ed.). Developments in food proteins (Vol. 3, pp. 33–68). New York, New York: Elsevier Applied Science Publishers Ltd. Ledward, D. A. (1992). Color of raw and cooked meat. In D. A. Ledward, D. E. Johnston, & M. K. Knight (Eds.), The chemistry of muscle-based foods (pp. 128–144). Cambridge: The Royal Society of Chemistry. Lindahl, G., Karlsson, A. H., Lundström, K., & Andersen, H. J. (2006). Significance of aging time on degree of blooming and colour stability of pork loin from different crossbreeds. Meat Science, 72(4), 603–612. Newton, K. G., & Gill, C. O. (1981). The microbiology of DFD fresh meats: A review. Meat Science, 5(3), 223–232. NPPC (1991). Procedures to evaluate market hogs (3rd ed.). Des Moines, IA: National Pork Producers Council. NPPC (1999). Pork quality standards. Des Moines, IA: National Pork Producers Council.
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Price, J. F., & Schweigert, B. S. (1987). The science of meat and meat products (3rd ed.). Westport, Connecticut: Food and Nutrition Press, Inc. Rey, C. R., Kraft, A. A., Topel, D. G., Parrish, F. C., & Hotchkiss, D. K. (1976). Microbiology of pale, dark and normal pork. Journal of Food Science, 41(1), 111–116. Rousset, S., & Renerre, M. (1991). Effect of CO2 or vacuum packaging on normal and high pH meat shelf-life. International Journal of Food Science and Technology, 26(6), 641–652. Strange, E. D., Benedict, R. C., Gugger, R. E., Metzger, V. G., & Swift, C. E. (1974). Simplified methodology for measuring meat color. Journal of Food Science, 39(5), 988–992. Wright, L. I., Scanga, J. A., Belk, K. E., Engle, T. E., Tatum, J. D., Person, R. C., et al. (2005). Benchmarking value in the pork supply chain: Characterization of US pork in the retail marketplace. Meat Science, 71(3), 451–463. Zhu, L. G., Bidner, B., & Brewer, M. S. (2001). Postmortem pH, muscle, and refrigerated aging effects on ability of vacuum-packaged pork to bloom. Journal of Food Science, 66(8), 1230–1235. Zhu, L. G., & Brewer, M. S. (1998). Discoloration of fresh pork as related to muscle and display conditions. Journal of Food Science, 63(5), 763–767.
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The effect of pH on shelf-life of pork during aging and simulated retail display S.F. Holmer a, R.O. McKeith b, D.D. Boler a, A.C. Dilger a, J.M. Eggert c, D.B. Petry d, F.K. McKeith a, K.L. Jones b, J. Killefer a,* a
Department of Animal Science, University of Illinois at Urbana-Champaign, 205 Meat Science Lab, 1503 S. Maryland Drive, Urbana, IL 61801, USA Department of Animal Science, Food and Nutrition, Southern Illinois University, 1205 Lincoln Drive, MC 4417, Carbondale, IL 62901, USA c Newsham Choice Genetics, 5058 Grand Ridge Drive, Suite 200, West Des Moines, IA 50265, USA d Triumph Foods LLC, 5302 Stockyards Expressway, Saint Joseph, MO 64504, USA b
a r t i c l e
i n f o
Article history: Received 3 August 2008 Received in revised form 13 December 2008 Accepted 15 December 2008
Keywords: Aerobic plate count Color Pork pH Shelf-life
a b s t r a c t The pork industry uses pH to differentiate product of varying quality; thus, the effect of pH on shelf-life is important as time during transport is extended. The objective was to develop regression equations to predict shelf-life over a range of ultimate pH (5.42–6.26). Shelf-life was evaluated after vacuum aging pork loin sections 0, 7, 14, 21, or 28 d and during 3 d of simulated retail display (4.5 °C) for pork loin chops. Correlation coefficients indicated a strong relationship between pH and quality measurements. Regression analysis with Aging Day and pH was able to explain 87% of the variation in aerobic plate counts for pork. After 28 d of vacuum aging, loin sections from the upper end of the pH distribution had about a 3 log(1000X) greater aerobic plate count than did the lower end pH product. An increase in pH resulted in pork with lower L*, a*, b* and R630 R580 values and as Aging Day increased, instrumental measurements of color increased slightly. Although higher pH is associated with improved pork quality, higher pH and longer aging periods will result in increased microbial proliferation and decreased shelf-life. Thus, an intermediate pH may provide the most desirable combination of quality and shelf-life when extensive aging is used. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Measurement of pH is used by the pork industry to differentiate product of varying quality. Loin pH is significantly correlated to attributes such as color and water-holding capacity (Huff-Lonergan et al., 2002), which are important characteristics when consumers make purchasing decisions (Brewer & McKeith, 1999). Over a wide range of pH (4.86–7.15), Bidner et al. (2004) developed regression equations to describe the relationship of ultimate pH to various quality measurements. Ultimate pH explained 79% of the variation in color, 57% of variation in drip loss, and 77% of the variation in purge loss (Bidner et al., 2004). Generally, higher pH pork will have superior quality compared to lower pH pork; yet, pH can have a dramatic effect on shelf-life from both a microbiological and color stability perspective. As Price and Schweigert (1987) reported, meat with a pH greater than 5.8 may be more conducive to spoilage and result in decreased shelf-life. Furthermore, Newton and Gill (1981) reviewed the effect of dark, firm, and dry (DFD) meat on microbiological characteristics, and found that the lower glucose content and high
* Corresponding author. Tel.: +1 217 333 8482; fax: +1 217 244 5142. E-mail address: [email protected] (J. Killefer). 0309-1740/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.meatsci.2008.12.008
pH of DFD meat resulted in increased spoilage. Numerous other studies (Fox, Wolfram, Kemp, & Langlois, 1980; Greer & Murray, 1988; Knox, van Laack, & Davidson, 2008) have also examined the effect of pH on shelf-life. These authors have concluded that higher pH products have greater microbial counts and a decrease in shelf-life, which is observed when packaged either under vacuum (Rousset & Renerre, 1991) or overwrapped with an oxygen permeable film (Rey, Kraft, Topel, Parrish, & Hotchkiss, 1976). In addition to changes in microbial proliferation over aging, physical changes that affect color can also occur. These color changes occur based on the form of myoglobin (deoxymyoglobin, oxymyoglobin, and metmyoglobin) and are both time and pH dependent (Ledward, 1992). Generally an increase in pH will result in darker (L or L*) product (Bidner et al., 2004; Huff-Lonergan et al., 2002), but, within a given pH range, L* will remain constant over display (Andrews et al., 2007). A decrease in a* value (becoming less red) will occur over display time, but will decrease to a greater extent in PSE muscle (Zhu & Brewer, 1998). In addition, lower pH product will visually have a shorter shelf-life (Greer & Murray, 1988) compared to higher pH product, which may be due to less enzymatic reduction and faster rate of myoglobin oxidation, which is favored at lower pH (Ledward, 1984). As most of the literature has categorized meat based on discrete pH levels (e.g., PSE or DFD; high or low; <5.4, 5.4–5.6, etc.), it would
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2. Materials and methods
at 2.0 ± 0.27 °C. After the respective Aging Day in vacuum bags, loin sections were removed and the down facing cut surface was swabbed for microbial counts. From each section, a cut was made to expose two fresh surfaces. One fresh surface was overwrapped with PVC and displayed, while the opposing fresh cut surface was swabbed as an initial microbial count for the display chop. Thus, a chop was used for display following each Aging Day.
2.1. Product selection and quality measurements
2.3. Evaluation of PVC overwrapped product in simulated retail display
Over two consecutive weeks, 39 boneless pork loins (19 and 20 loins/week, respectively) were selected from a population of 306 loins based on ultimate (approximately 30 h postmortem) pH measured on Longissimus muscle at the 10th rib (MPI pH-Meter, Meat Probes Inc., Topeka, KS, USA). Starting at pH of 5.40 and increasing every 0.05 units, 1–2 loins/week were randomly selected within each pH increment, so that there was a continuous range in pH (Fig. 1). Because there were only a few loins at the upper end of the distribution, all loins with a pH of 5.85–6.30 were selected. In addition to pH at the 10th rib, visual color (NPPC, 1999), marbling (NPPC, 1999), and firmness (NPPC, 1991) were evaluated by trained, university personnel. CIE L*, a*, and b* values (CIE, 1978) were measured with a Minolta Chromameter CR-300 (Minolta Camera Co., Osaka, Japan) with D65 illuminant, 0° observer and calibrated against a standard white tile.
After display for approximately 15 h, initial (1 d) evaluations were made, with subsequent evaluations at 24 (2 d) and 48 h (3 d) later (Display Days: 1, 2, or 3). All evaluations were made through the PVC film. Visual evaluations were made by a fivemember panel experienced in meat color evaluations, but visual color was evaluated only on the first day of simulated retail display. Visual discoloration (i.e., metmyoglobin formation) was evaluated each day using a 10 cm line scale (reference marks at 0%, 25%, 50%, and 100%), where every 1 cm equaled 10% discoloration on the face of the chop. Each day, L*, a*, and b* were measured with a Hunter Lab Miniscan XE Plus (Model 45/0-L; Hunter Associates Laboratory Inc., Reston, VA, USA) using illuminant D65 and a 10° observer. The spectrocolorimeter was calibrated daily against the black and white tiles covered with PVC film. The difference in reflectance at 630 and 580 nm (R630 R580) was used as an indicator of color stability (Strange, Benedict, Gugger, Metzger, & Swift, 1974). Lastly, microbial swabs were collected on the pork surface after the last evaluation (3 d).
benefit the pork industry to understand the relationship of a continuous pH range and shelf-life. Therefore, the objective was to predict shelf-life characteristics over a continuous range in pH for pork loin sections following aging in vacuum bags and pork loin chops under simulated retail display.
2.2. Loin fabrication, processing, and packaging From each loin, a knife (not sterilized between each sample) was used to cut a fresh surface and an initial swab from the exposed surface was performed to enumerate microbial numbers. This swab served as the 0 d sample for both vacuum packaged loin sections and the oxygen permeable polyvinyl chloride shrink film (PVC) overwrapped chops. Next, a 2.5 cm chop was cut and packaged in a white foam tray (Pactiv, Lake Forest, IL, USA) with a absorbent pad (Dri-LocÒ Pad; Sealed Air Corp., Duncan, SC, USA) and overwrapped in PVC film (O2 transmission rate (OTR) = 1400 cm3/m2/24 h; Bush Brothers, Urbana, IL, USA). This chop was stored at 4.5 ± 1.33 °C under constant lighting (900– 1100 Lux) to simulate retail display. The remainder of the loin was cut into four sections, vacuum packaged (OTR = 102.3 cm3/ m2/24 h at 23 °C, 65% RH; moisture vapor transmission rate = 7.9 g/m2/24 h at 37.7 °C, 90% RH; Bush Brothers, Urbana, IL, USA) and randomly assigned to Aging Days of 7, 14, 21, or 28 d
2.4. Enumeration of microbial contamination Microbial swabs were performed with a sterile 15 cm cottontipped applicator (FisherbrandÒ; Fisher Scientific, Pittsburgh, PA, USA) and a 5 5 cm template. After each swab, the applicator tip was broken off in a 15 ml conical tube (FisherbrandÒ; Fisher Scientific, Pittsburgh, PA, USA) containing 10 ml of sterile 0.1% peptone (Difco Laboratories, Detroit, MI, USA). After vortexing for 10 s, samples were serially diluted and plated on PetrifilmTM (3M, St. Paul, MN, USA) in duplicate to enumerate bacteria colonies (aerobic plate counts, APC). After incubation of films for 48 h at 37 °C, films containing 25–250 colonies were counted and are reported as log colony forming units per square centimeter (log CFU/cm2). 2.5. Statistical analysis
6
5
Count
4
3
2
1
0 5.4
5.5
5.6
5.7
5.8
5.9
6.0
6.1
6.2
pH Fig. 1. pH distribution of loins selected for this experiment.
6.3
All data were analyzed with SAS (Version 9.1; SAS Inst., Inc., Cary, NC, USA). The means procedure was used to calculate the mean, standard deviation, minimum and maximum values for the measured parameters, whereas Pearson correlation coefficients between pH and pork quality measurements were calculated with the correlation procedure. Regression analysis was performed with the general linear models procedure. Initial model evaluation for APC following vacuum aging, initial visual color, and DAPC over simulated retail display included the linear, quadratic, and linear interaction for the independent variables of pH and Aging Day (0, 7, 14, 21, and 28 d). For the instrumental measurements of L*, a*, b*, and R630 R580, the initial model included the linear, quadratic, and all linear two-way interactions of pH, Aging Day, and simulated retail Display Day (1, 2, and 3 d). Only significant (P < 0.05) terms were included in the model. If the quadratic term was significant, both the linear and quadratic terms were included in the regression model. Models were evaluated for outliers and consistency of the error variances. For regression analysis of visual color score, data was transformed with 1/visual color score2 due to increasing error variance.
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Table 1 Summary statistics for the selected loins. Parmeter
Mean
SDa
Minimum
Maximum
Correlation with pH
pH Colorb Marblingc Firmnessd L* a* b*
5.76 3.6 3.1 3.1 46.4 7.4 3.7
0.227 0.96 1.19 0.86 2.96 1.66 1.25
5.42 1.0 1.0 1.0 41.5 5.0 1.5
6.26 5.0 6.0 4.0 53.8 11.4 7.2
NA 0.63* 0.64* 0.63* 0.63* 0.25 0.32*
a b c d *
Standard deviation. 1 = lightest to 6 = darkest (NPPC, 1999). Estimated % lipid (NPPC, 1999). 1 = softest to 5 = firmest (NPPC, 1991). P < 0.05.
3. Results and discussion 3.1. Summary statistics and simple correlations Mean pH for the population was 5.76 and ranged from 5.42 to 6.26 (Table 1). Wright et al. (2005) characterized pork in the US retail market and reported that loin chops had an average pH of 5.64, with a range of 5.10–6.36. While the pH range in the current study was not as low as those reported by Wright et al. (2005), this study would represent a majority of pork currently produced in the industry. Bidner et al. (2004) evaluated pork which ranged in pH from 4.86 to 7.15, but indicated the majority (57.4%) of loins had an ultimate pH between 5.4 and 6.1. Difference in pH between studies could have been the result of numerous factors, such as genetics or other management practices, but in the case of Bidner et al. (2004), the higher pH was achieved with an injection of epinephrine prior to slaughter. Pearson correlation coefficients (r) are also presented in Table 1. Loin pH was significantly correlated with color (r = 0.63), marbling (r = 0.64), firmness (r = 0.63), L* (r = 0.63), and b* (r = 0.32). These results are similar to Boler et al. (in press), who reported significant correlations between pH and color, firmness, marbling,
and L*. Significant correlations between pH and color, marbling, and firmness were also reported by Huff-Lonergan et al. (2002). Correlations to quality measurements presented in this study are higher than both preceding studies and may result from the selection method used. Loins for the current study were selected to represent a distribution across pH, whereas the previous reports (Boler et al., in press; Huff-Lonergan et al., 2002) were not selected for a continuous distribution in pH and would have more loins centered around the mean pH for the population. Nonetheless, these results reinforce pH as a useful tool for the pork industry in differentiating product of varying quality. 3.2. Aerobic plate counts Data for APC of 0 d samples was excluded from analysis, as most of these samples had APC below detectable limits. As Aging Day increased from 7 to 28 d, APC increased (Fig. 2), which is not surprising because increased aging time in a vacuum bag will result in increased bacterial proliferation (Blixt & Borch, 2002; Hodges, Cahill, & Ockerman, 1974; Knox et al., 2008). Furthermore, as pH increased, APC increased, and these results are similar to those of Rousset and Renerre (1991) that indicated bacterial counts were 10- to 100-fold greater on high pH (=6.20) meat than normal pH (=5.55) meat at various aging durations. In this study, there was a significant interaction effect for pH and Aging Day on APC (Fig. 2). As Aging Day increased, pH has a more pronounced effect on APC, with higher pH product having increased APC. At 7 d of aging, APC is relatively unaffected by pH, but, after 28 d of aging, pork with the highest pH had about a 3 log(1000X) greater APC density than pork with the lowest pH. The following regression equation was established based on pH and Aging Day and the significant interaction between the two parameters: predicted APC (log CFU/cm2) = 105.0832 (0.9071 Aging Day) + (0.0014 Aging Day2) + (37.5674 pH) (3.3300 pH2) + (0.1775 [Aging Day pH]); R2 = 0.87. Similar to these results, Knox et al. (2008) performed regression analysis and reported an increasing linear relationship between pH
Fig. 2. Predicted aerobic plate counts (APC; log CFU/cm2) from vacuum stored pork loin sections. Predicted APC = Day2) + (37.5674 pH) (3.3300 pH2) + (0.1775 [Aging Day pH]); R2 = 0.87.
105.0832
(0.9071 Aging Day) + (0.0014 Aging
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and APC at 14, 24, and 34 d of aging. However, it would appear that after only 14 d of aging, APC approached 6 log CFU/cm2 (Knox et al., 2008), whereas, in the current study, only a few of the samples at the upper end of the pH range approached this limit after 28 d of aging. Pork which was approaching this 6 log CFU/cm2, are at a level in which products will be considered spoiled (Banwart, 1981). Pork from the lower end of the pH range, with much lower APC, could be stored for 28 d without crossing this threshold level of spoilage. Because higher pH loins are selected for export (directly by measuring pH or indirectly by selecting darker colored product) and will spend prolonged periods of time in a vacuum bag, considerations for shelf-life are especially important. Results show that high pH (>6.0) pork, aged for prolonged periods of time (similar to export product) may be at risk for spoilage after 28 d of aging in a vacuum bag. The predicted DAPC (final APC of chop after display initial APC of chop) is shown in Fig. 3. It is important to evaluate how the APC changed from the initial to the final value, as the 3 d evaluations may have been high due to initial contamination as the chops were cut. The regression equation illustrates (predicted DAPC = 147.6438 + (0.2467 Aging Day) (0.0054 Aging Day2) + (48.1093 pH) (3.9279 pH2); R2 = 0.52), that both Aging Day and pH affected the change in APC between the first and last days of display. As Aging Day increased, there was an increase in the rate in which the APC changed over display, and this was also the case for pH; yet, as pH increased, there was a greater change in APC compared to Aging Day. Greer and Murray (1988) reported that under simulated retail display, pork loin chops classified as PSE (pH 5.40) had lower bacterial counts than DFD (pH 6.41) chops. For all three groups (PSE, normal (pH 5.58), DFD) bacterial counts increased as time in display increased; therefore, as pork is stored for longer time periods, the risk of spoilage during retail display increases across all pH levels. Both Fox et al. (1980) and Rey et al. (1976) observed that PSE (lower pH) pork had lower microbial counts during retail display than that of higher pH product.
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3.3. Color evaluation Visual color scores (Fig. 4) were evaluated 15 h (1 d) after fabrication following 0, 7, 14, 21, or 28 d of vacuum packaged aging and the predicted transformed visual color score (1/color score2) = 3.0530 (0.0014 Aging Day) + (0.00004 Aging Day2) (0.9543 pH) + (0.0764 pH2); R2 = 0.44. Even though Aging Day and Aging Day2 were significant variables in the regression equation, pH, and not Aging Day, contributed the most to the prediction of visual pork color scores. Similar to the current data, other researchers (Boler et al., in press; Huff-Lonergan et al., 2002) have also reported the importance that pH plays on color scores. Discoloration was not analyzed because, after 28 d in a vacuum bag and 3 d of display, the majority (34 of 39) of samples were evaluated as <10% discolored. Instrumental measurements for color (L*, a*, b*, and R630 R580) were evaluated during the 3 d of simulated retail display. For the dependent variable L* (Fig. 5), Display Day (1, 2, or 3 d) was not significant nor did Display Day have a significant interaction with either pH or Aging Day (predicted L* = 616.0093 + (0.0405 Aging Day) (187.2003 pH) + (15.3807 pH2); R2 = 0.35). This indicates that during 3 d simulated retail display, there were minimal changes in L* values over the 3 d display period, regardless of aging time. Lindahl, Karlsson, Lundström, and Andersen (2006) indicated that aging time did affect L* value when pork chops were wrapped with an oxygen permeable film and displayed. However, other research has reported no change in L* value after 7 d on display (Zhu & Brewer, 1998) or after removal from vacuum bags after 28 d of postmortem aging (Zhu, Bidner, & Brewer, 2001). The predicted a* value (26.1004 + (0.0433 Aging Day) (0.0014 Aging Day2) (3.2192 pH) + (0.5159 Display Day) (0.0237 [Aging Day Display Day]); R2 = 0.30), predicted b* value (100.3634 + (0.0829 Aging Day) (26.9048 pH) + (2.0366 Day) (0.3907 Display Day2) pH2) + (1.8886 Display 2 (0.0138 [Aging Day Display Day]); R = 0.52), and predicted (11.4417 pH) + R630 R580 (86.1003 (0.3560 Aging Day)
Fig. 3. Predicted change in aerobic plate counts (DAPC; log CFU/cm2) of pork chops under simulated retail display from 1 to 3 d. Predicted DAPC = Aging Day) (0.0054 Aging Day2) + (48.1093 pH) (3.9279 pH2); R2 = 0.52.
147.6438 + (0.2467
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Fig. 4. Predicted transformed visual color score (1/visual color score2). Predicted transformed color score = 3.05 Day2) (0.9543 pH) + (0.0765 pH2); R2 = 0.44.
(0.0843 [Aging Day pH]) (12.7216 Display Day) (0.0215 [Aging Day Display Day]) + (2.1555 [pH Display Day]); R2 = 0.42) results are presented in Figs. 6–8, respectively. As the three prediction equations had all three parameters (pH, Aging Day, and Display Day) in the prediction equation, Display Day (1, 2, or 3 d) was held constant in each equation so that the response due to Aging Day and pH could be visualized.
(0.0014 Aging Day) + (0.00004 Aging
The a* and b* values decreased (became less red and less yellow) as pH increased, and the decrease was observed across all 3 d of simulated retail display. Zhu and Brewer (1998) documented no difference in a* values for normal or DFD pork; yet, both normal and DFD pork were higher than that of PSE pork. As Aging Day increased from 0 to 28 d, there was a slight increase for a* and b* values, indicating that vacuum packaged pork became redder and more yellow over time. Lindahl et al. (2006) found that both a*
Fig. 5. Predicted L* value. Predicted L* = 616.0093 + (0.0405 Aging Day)
(187.2003 pH) + (15.3807 pH2); R2 = 0.35.
S.F. Holmer et al. / Meat Science 82 (2009) 86–93
Fig. 6. Predicted a* value where each response surface represents 1, 2, or 3 d evaluation. Predicted a* = 26.1004 + (0.0433 Aging Day) Day2) (3.2192 pH) + (0.5159 Display Day) (0.0237 [Aging Day Display Day]); R2 = 0.30.
and b* values were higher in pork aged 8 d when compared to pork aged only 1 d. Lindahl et al. (2006) also reported that there was an increase for a* during the first day of display. The current results (Fig. 6) illustrates an increase in a* (becomes more red) over the 3 d of display when loins were stored up to 21 d, after which a* val-
Fig. Day)
91
(0.0014 Aging
ues decreased with increasing Display Day when loins were aged out to 28 d. Zhu and Brewer (1998) reported that b* values increased from 0 to 1 d of display, with a decrease in b* values in subsequent days of display. Similarly in this study, chops from unaged (0 d) loins had lower b* values on 1 d than on 2 or 3 d of simulated
7. Predicted b* value where each response surface represents 1, (26.9048 pH) + (2.0366 pH2) + (1.8886 Display Day) (0.3907 Display Day2)
2, or 3d evaluation. Predicted b* = 100.3634 + (0.0829 Aging (0.0138 [Aging Day Display Day]); R2 = 0.52.
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Fig. 8. Predicted R630 R580 where each response surface represents 1, 2, or 3 d evaluation. Predicted R630 R580 = 86.1003 (0.3560 Aging (11.4417 pH) + (0.0843 [Aging Day pH]) (12.7216 Display Day) (0.0215 [Aging Day Display Day]) + (2.1555 [pH Display Day]); R2 = 0.42.
retail display. Throughout the aging period and across the pH range, Display Day 2 had higher b* values than both day 1 and 3. Rousset and Renerre (1991) observed that beef with a high pH, previously held in a vacuum bag, had lower R630 R580 values than beef with normal pH. Coincidentally, R630 R580 values decreased (less surface oxymyoglobin) as pH increased in pork loins (Fig. 8). Even though the decrease in R630 R580 values was observed on all 3 d of display, 1 d R630 R580 values of low pH chops were higher than 2 and 3 d. Yet, at 0 d of aging and when pH was greater than 5.80, R630 R580 values were higher after 3 d of display than both proceeding days. Likewise, after 28 d of aging those samples having a pH greater than 6.18 had higher R630 R580 values at Display Day 3 than both proceeding days. Zhu and Brewer (1998) reported that pork classified as ether PSE, normal, and DFD had similar R630 R580 values until 2–3 d of aging time, but PSE pork will have a greater decline in R630 R580 values than the other conditions. Because none of the pork chops in the current study exhibited the classic PSE condition, this may explain the difference in results between the current experiment and those of Zhu and Brewer (1998). 4. Conclusions From our results, it is apparent that pH can have a dramatic affect on the shelf-life of pork after longer aging times typically associated with postmortem aging, distribution, and/or exportation. We developed regression equations that can be used to predict shelf-life characteristics based on the ultimate pH of the Longissimus, duration of aging, and length of retail display. Results demonstrated that increasing aging time of high pH pork will increase the amount of microbial proliferation. Similar results will be observed when high pH product undergoes retail display. Color stability is affected more by loin pH than by duration of aging (up to 28 d)
Day) -
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