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ANIMAL PRODUCTS |
Department of Animal Science, Interdepartmental Nutrition Program, North Carolina State University, Raleigh 27695
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Key Words: age • growth rate • magnesium • pork • quality • water
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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reported that increased frequency of feed withdrawal associated with increased age at marketing within a pen had a negative effect on several pork quality characteristics. However, the results did not indicate whether the effect on pork quality was caused by frequency of feed withdrawal or age of the pigs when marketed because the study was not designed to specifically address that question.
Short-term supplemental dietary Mg has been reported to decrease water loss and improve color of pork (D’Souza et al., 1998, 2000), although results have not been consistent (reviewed by van Heugten and Frederick, 2004). Furthermore, dietary Mg potentially decreases lipid oxidation of stored pork (Apple et al., 2001).
Most of the recent nutritional approaches to improve pork quality have focused on supplementation through feed delivery. This practice may be difficult to implement for brief supplementation periods (2 d) and is further complicated by multiple marketings within pens. Developing an approach of supplementing Mg through the water to improve pork quality would simplify delivery by ensuring proper timing of supplementation (Frederick et al., 2004). Therefore, the objectives of this study were to determine if the age of pigs at slaughter at the same market weight (and, therefore, rate of growth) affects pork quality and if Mg supplementation through drinking water could affect pork quality in pigs of different ages.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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All animal procedures were approved by the Institutional Animal Care and Use Committee of North Carolina State University. A total of 32 crossbred pigs (109 ± 0.6 kg of BW) were used to determine the effect of age of pigs and Mg supplementation through drinking water on pork quality. The goal of the experiment was to select 2 groups of pigs of similar market weight, but the groups were intended to differ by approximately 30 d in age. To accomplish the objective, 2 initial groups of 50 pigs, approximately 28 kg of BW, were selected from 2 groups of sows that had farrowed 30 d apart. The initial groups of pigs were fed the same grower and finishing diets during the appropriate weight ranges, and diets met or exceeded nutrient requirements for each phase of growth (NRC, 1998). Specifically, pigs from each group were moved from a nursery to a finishing barn at 60 d of age, penned by BW (4 or 5 pigs/pen), and fed the grower diet on an ad libitum basis until the average pen BW was 68 kg. Pigs were then fed the finisher diet on an ad libitum basis from 68 kg of BW until they were marketed.
Sixteen average pigs were selected from each of the 2 initial groups of 50, such that the average initial weight and the weight distribution were similar for each of the groups. Thus, 16 pigs were selected from the older initial group, representing slow-growing pigs that reached market weight (110 kg of BW) at 180 d of age, and 16 pigs were selected from the younger initial group (fast growing pigs), which reached market weight (108 kg of BW) at 153 d of age. Halothane and Napole mutations in these 32 pigs were identified retrospectively at slaughter by DNA tests (GeneSeek Inc., Lincoln, NE) and were performed on LM samples. All pigs tested negative for the Halothane mutation. However, 8 pigs were carriers for the Napole gene and 3 pigs were homozygous for the Napole mutation. The distribution of the Napole mutation was 4 carriers and 2 homozygous animals in the younger, fast growing pig group without Mg supplementation; 1 carrier and 1 homozygous animal in the younger, fast-growing pig group with Mg supplementation; 2 carriers in the older, slow-growing pig group without Mg supplementation; and 1 carrier in the older, slow-growing pig group with Mg supplementation.
The 32 pigs selected for this study were placed into individual pens (2.03 x 0.74 m) and provided with free access to water via a nipple waterer. Pigs were fed 2.7 kg of feed per day (Table 1; containing 0.13% Mg) for a 7-d adjustment period. After the adjustment period, pigs were blocked by sex and BW and randomly allotted within block to water supplemented with 900 mg of Mg/L as Mg sulfate heptahydrate (9.8% Mg, 12.9% S; Giles Chemical Corp., Waynesville, NC) of drinking water for either 0 or 2 d before slaughter. Plastic water containers (23 L capacity) were filled daily with 15 L of water containing appropriate Mg concentrations. Water containers were suspended from the ceiling and gravimetrically (approximately 600 mL/min) emptied into a galvanized pipe leading to a nipple waterer. Daily water disappearance volumes were determined by weight.
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On d 3 (0800) of the experimental period, all pigs were loaded and transported 110 km (1 h 50 min) to a commercial abattoir. Pigs were unloaded by abattoir personal. After 2 h 30 min of lairage, pigs were moved by block, 15 m to the stunning area. Pigs were electrically stunned, and blood was collected during exsanguination for plasma Mg. Hot carcass weights were collected before refrigeration to determine dressing percent. The temperature and pH of the loin was measured between the 10th and 11th rib at 45 min postmortem using an Argus Sentron (Gig Harbor, WA) pH meter.
Fabrication and Storage
After 20 h of chilling at 2° C, the entire right side loin and ham were removed and transported 60 km (45 min) at 4° C to a commercial meat fabrication facility for further processing. A total of 3 chops (2.54 cm thick) of the LM were obtained beginning at the seventh and eighth rib interface and extending caudal. The first LM chop was placed in a plastic bag and stored at – 20° C for DM and tissue Mg determination. The second chop caudal to the first was placed on an absorbent pad (Cryovac Sealed Air Corp., Saddle Brook, NY) within a Styrofoam tray (Cryovac Sealed Air Corp.), wrapped with an oxygen-permeable film (Cryovac Sealed Air Corp.), and stored at 4° C in the presence of fluorescent lighting to simulate retail display for 4 d. At the end of the 4-d display storage period the chops were analyzed for extent of oxidation measured by thiobarbituric acid reactive substances (TBARS). The third and final chop was taken immediately caudal to the second chop and stored in a similar environment for 8 d of displayed storage. The remaining caudal portion of the LM was split into equal sections, vacuum packaged in B2651T Cryovac bags with a Multivac machine (Cryovac, Duncan, SC), and stored at 4° C in the absence of light for 25 or 50 d. The SM muscle was removed from the ham. Three SM chops (2.54 cm) were obtained and processed similarly to the LM chops, with the exception that SM was not vacuum-packaged for storage.
Pork Quality Measurements
Fluid loss in the LM and SM was evaluated by 2 separate methods. Surface exudate was determined by a method developed by Kauffman et al. (1986). Briefly, a preweighed filter paper, 4.5 cm diam. (#589, Schleicher and Schuell Inc., Keene, NH), was placed on the surface of each muscle for 2 s, 20 min after the initial cut. The filter paper was reweighed to determine weight gain of the filter paper associated with the extent of accumulation of surface exudate.
Display fluid loss was determined on chops designated for 8 d of display storage. The chops from each muscle were removed from the tray on d 2, 4, 6, and 8, placed on a paper towel for 5 s, and reweighed to determine display fluid loss, expressed as a percentage of initial chop weight. Purge loss was identified as the amount of fluid lost from the LM muscle after 25 or 50 d of vacuum-packaged storage and was reported as the weight loss during storage as a percentage of initial muscle weight.
Color of the LM and SM was objectively evaluated by Minolta lightness (L*), redness (a*), and yellowness (b*) measurements using a Minolta Chroma Meter (CR-200, Ramsey, NJ) using D65 illuminant and calibrated with a standard white plate. Minolta values were reported as the average color values collected at 4 positions, in a diamond pattern, on the surface of each chop. The initial measurement of color was performed 45 min after the initial cut. Additionally, color was determined every 2 d for 8 d of display storage. Color of vacuum-packaged LM was determined on an interior chop after a 45 min bloom period for the 25- and 50-d vacuum-packaged storage periods.
Chemical Analyses
Plasma and muscle Mg concentration was determined in duplicate by atomic absorption spectrophotometry, as described previously (Frederick et al., 2004). Each muscle was ground and passed twice through a 5-mm screen (Oster Food Grinder, Sunbeam Corp. Ltd., Mississauga, ON, Canada) before digestion using nitric acid, followed by treatment with hydrogen peroxide.
Longissimus and SM chops that had been displayed for 4 or 8 d, and the LM loin sections vacuum-packaged for 25 or 50 d, were used to determine oxidation by TBARS analysis as described by Witte et al. (1970). Tetraethoxypropane (Sigma Chemical, St. Louis, MO) was used as a standard at concentrations of 2, 4, 8, 10, 20, 40, and 80 x 10– 7 M. Reported TBARS values reflect a correction for percent recovery, which was determined in concurrent duplicate meat samples to which 1 mL of 80 x 10– 7 M tetraethoxypropane was added (Frederick et al., 2004). Recoveries ranged from 92 to 102%.
Statistical Analyses
Data were analyzed by split-plot design with age as the main plot and Mg supplementation as the subplot using the GLM procedure of SAS (SAS Inst. Inc., Cary, NC). Pigs were blocked by weight within sex, and pig was the experimental unit. The age ( block interaction was used as the error term to test age effects. Data that were measured over time were analyzed as a repeated measures analysis using the Mixed procedure of SAS. Linear, quadratic, and cubic (where appropriate) orthogonal contrasts were conducted to evaluate the effects of display time. The presence of the Napole gene was used as a covariate (0 = normal, 1 = heterozygous, 2 = homozygous) in the statistical analysis.
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RESULTS AND DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Pigs differed in age (P < 0.001) by 27 ± 0.3 d (153 vs. 180 d of age, respectively; Table 2). However, live BW did not differ on the day of slaughter (P = 0.13). Therefore, younger pigs grew 96 ± 5 g of BW/d faster than older pigs (P < 0.001). Dressing percent was not affected by age (P = 0.99). Therefore, our goal of obtaining pigs that were raised under the same conditions and differed by age at a similar final BW was accomplished.
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). Water disappearance of older pigs was lower than that of younger pigs (P = 0.05), but water disappearance relative to feed intake did not differ (P = 0.93). No differences in growth performance or water disappearance were observed due to Mg supplementation.
Magnesium concentration of water used to dilute the stock solution was 3 mg/L. The final Mg concentrations of drinking water was 3 and 903 mg/L for control pigs and pigs supplemented with 900 mg of Mg/L, respectively. Therefore, the maximum mean intake of Mg from drinking water were 0.03 and 8.7 g of Mg/d for pigs receiving water without and with Mg supplementation, respectively. Water disappearance averaged 4.2 L per kg of feed consumed and included water wastage, which was not measured in this study. The actual water requirement of pigs has been estimated to be 2 L of water per 1 kg of feed (Cumby, 1986). Assuming this water to feed ratio, mean Mg intake from drinking water would have been approximately 0.01 and 4.4 g of Mg/d for nonsupplemented and supplemented, respectively. Thus, Mg intake was close to the 3.2 g of Mg/d intake that improved pork quality in previous studies (D’Souza et al., 1998, 1999, 2000).
Plasma Mg tended to increase by 10.6% (P = 0.06) with Mg supplementation (Table 2). However, Mg concentrations in either the LM (P = 0.29) or SM (P = 0.27) were not affected by Mg supplementation. Magnesium concentrations in the current study were within the commonly observed range of 17 to 25 ppm of Mg for plasma and approximately 1,000 ppm of Mg for muscle (Apple et al., 2001; Frederick et al., 2004, 2006). Schaefer et al. (1993) reported that plasma Mg increased 34% when pigs were supplemented with 25.2 mg of Mg per day (calculated from supplementing 20 g/d of Mg aspartate product, containing 1.3% Mg aspartate, which contained 9.7% elemental Mg, as a top dress for 5 d). However, Mg concentration of skeletal muscle, the liver, and heart tissue did not change with Mg supplementation in that study. We have previously reported a linear increase in plasma Mg concentration when 0, 300, 600, or 900 ppm of Mg was supplemented for 2 d in water for finishing pigs, without affecting Mg concentration in muscle (Frederick et al., 2004).
Pork Quality Measurements
In LM, Mg supplementation did not alter muscle pH at 24 h; however, muscle pH was lower for older pigs than younger pigs when supplemented with Mg (interaction, P = 0.04; Table 3). Magnesium supplementation lowered pH of the SM at 24 h postmortem in older pigs but did not alter pH in younger pigs (interaction, P = 0.04). There is no clear explanation for these interactions, however, differences were relatively small and may not represent biologically relevant effects. The pH of the LM at 45 min postmortem was greater (P = 0.04) for older, slower growing pigs than younger, faster growing pigs indicating a slower rate of pH decline during early postmortem processes (Table 3). However, the extent of pH decline in the LM (P = 0.48) or SM (P = 0.62) was not affected by age because the pH at 24 h postmortem was not different between age groups. These data are in agreement with results reported by Lonergan et al. (2001), who observed Duroc pigs not selected for rate of lean gain had greater pH of the LM at 15, 30, and 45 min but not 24 h postmortem than Duroc pigs selected for a greater rate of lean gain. However, in the same study a muscle similar to SM, the semitendinosus, had a greater pH at both 15 min and 24 h postmortem when rate of lean gain of Duroc pigs was not selected for. Huff-Lonergan et al. (1997) found similar results in the semitendinosus from the previous 2 generations of selection of Duroc pigs in the same line of pigs used in the study by Lonergan et al. (2001). Oksbjerg et al. (2000) reported no difference in pH of the LM at 45 min or 24 h postmortem when slow-growing pigs were compared with fast-growing Landrace pigs. Furthermore, Goerl et al. (1995) reported no difference in pH when a fast-growing Hampshire line was compared with a 14-breed composition line selected for reproductive traits.
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). D’Souza et al. (1998) supplemented pigs with 3.2 g of Mg/d from Mg aspartate for 5 d and reported a greater initial and ultimate pH in longissimus and biceps femoris muscle of pigs slaughtered using either minimal or negative handling techniques. Similarly, Swigert et al. (2004) observed a greater ultimate pH in pigs supplemented with 3.5 g of Mg/d from MgSO4. Apple et al. (2005) observed greater 0 and 45 min pH values with supplementation of 0.2% Mg from Mg mica to pigs subjected to short-duration transportation stress. However, other studies (D’Souza et al., 1999, 2000; Hamilton et al., 2002, 2003) have not observed changes in either initial or ultimate pH with the supplementation of similar levels of Mg from a variety of sources.
Exudate of the LM measured 24 h postmortem was not affected (P = 0.22) by age of pigs at slaughter (Table 3). However, initial exudate of the SM was reduced (P = 0.05) by 18% in older, slow-growing pigs compared with younger, fast-growing pigs. Display fluid loss (Table 4) of the LM and SM increased linearly as display time increased (P < 0.001) but was not affected on any days measured (P > 0.10) by age of the pig. Kauffman et al. (1986) demonstrated a strong, positive correlation between surface exudate determined with filter paper and fluid loss during storage. However, in the current study correlations between surface exudate and fluid loss after 8 d of display storage of the LM and SM were consistent across muscles but low, r2 = 0.41 and r2 = 0.37, respectively. Nevertheless, the fluid loss during display has more practical significance than surface exudate. The CV associated with display fluid loss was greater than surface exudates measurements (13 vs. 10%, respectively). Therefore, the measurement of surface exudates enabled us to detect a smaller difference than the display fluid loss method.
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; Huff-Lonergan et al., 1997). Furthermore, drip loss was greater in the LM, SM, and semitendinosus from Duroc pigs with a faster rate of lean gain, which was most noticeable at storage periods of more than 2 d (Lonergan et al., 2001). Conversely, faster growing Landrace pigs did not have greater drip loss from the LM than slower growing Landrace pigs (Oksbjerg et al., 2000), which is in agreement with results from the current study.
Magnesium supplementation had no effects on initial surface exudate (Table 3) or display fluid loss from either LM or SM (Table 4). These results are in agreement with Apple et al. (2000, 2002, 2005), who reported no effects on drip loss when pigs were supplemented throughout the finishing period with either 0.1 or 0.2% of Mg from Mg mica. Similarly, no effects on drip loss have been observed when supplementing pigs with Mg from Mg aspartate (Caine et al., 2000), MgSO4 (Frederick et al., 2004; Swigert et al., 2004), or Mg acetate (Geesink et al., 2004). In contrast, earlier reports (D’Souza et al., 1998, 1999, 2000; Hamilton et al., 2002) demonstrated a reduction in drip loss when Mg was supplemented as Mg aspartate, MgSO4, or MgCl2.
Lightness (Minolta L*) of the LM increased linearly with time (P < 0.001), but was not affected by pig age (P > 0.05, Table 5). Redness of the LM increased quadratically (P < 0.001) as display time increased, but no effects of age were noted. Yellowness of the LM increased linearly with time (P < 0.001), and LM chops appeared to be less yellow (P < 0.05) in older, slower growing pigs than younger, faster growing pigs on d 4 and 8 of display storage. The SM from older pigs was darker initially and after 2, 6, and 8 d of display storage (P 
0.02) than younger pigs (Table 6). This effect appeared to be less evident when older pigs were supplemented with Mg (P < 0.05). Redness and yellowness of the SM were not affected (P > 0.10) by age of pig.
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. In contrast, Huff-Lonergan et al. (1997) reported lower lightness (Hunter L), but not redness or yellowness (Hunter a or b), of the LM from pigs selected for high rates of lean gain and efficiency of gain. Lonergan et al. (2001) reported no difference in Hunter L, a, or b when the fifth generation of the same Duroc line was compared with previous generations. However, Goerl et al. (1995) reported greater Hunter L, a, and b when a fast-growing Hampshire line was compared with a slow-growing 14 breed composite line. In that study the incidence of the Napole gene was not identified and may have been greater in the Hampshire line than the composite line. A possible explanation for the observed differences, primarily in color, could be related to muscle fiber type composition, which is known to correlate with growth rate and muscle pH, glycolytic potential, and lightness (Dietl et al., 1993; Larzul et al., 1997; Oksbjerg et al., 2000). However, differences in fiber types between age groups were not evaluated in the current study.
Color of the LM or SM after display storage was not affected by Mg supplementation, with the exception of greater redness of LM chops displayed for 6 d when Mg was supplemented (P = 0.05). D’Souza et al. (1998) reported a decrease in paleness of LM, but not the biceps femoris, when pigs were supplemented with 3.2 g of Mg/d from Mg aspartate for 5 d. In a subsequent study, they confirmed the reduction in paleness with Mg supplementation and noted a greater response in pigs fed Mg aspartate compared with MgSO4 (D’Souza et al., 2000). Geesink et al. (2004) observed reduced paleness and increased redness in LM of pigs supplemented with 1.19 g of Mg from Mg acetate for 5 d when pigs had not been held in lairage before slaughter but reported no effects in pigs held in lairage for 2 h. Decreased paleness and yellowness of muscle were noted by Hamilton et al. (2003) when supplementing pigs with Mg proteinate or MgSO4 for 1 or 5 d but not for 2 d. Others studies reported no effects of Mg supplementation on the color of pork (D’Souza et al., 1999; Caine et al., 2000; Frederick et al., 2004).
Purge loss and color of the LM stored for 25 or 50 d in vacuum-packaged bags were not affected (P > 0.10) by the age of pigs or Mg supplementation (data not shown). Frederick et al. (2004) reported that Mg supplementation through the water at 900 ppm for 2 or 4 d, but not 6 d, reduced the lightness of chops from vacuum-packaged LM sections stored for 25 d compared with control pigs, but this effect was not observed after 50 d of vacuum-packaged storage. In a subsequent study (Frederick et al., 2006), lightness of the LM was greater after 25 and 50 d of vacuum-packed storage when Mg was supplemented for 2 d at 300 and 600 mg/L compared with 0 or 900 mg/L. Apple et al. (2001) found that supplementation of 0.1% Mg from Mg mica reduced redness and yellowness of boneless loins, regardless of storage duration, but supplementation of 0.2% Mg mica did not result in differences from nonsupplemented pigs.
Oxidation of the LM and SM increased linearly (P < 0.001) with display storage time (Table 7). Younger pigs had greater oxidation of the LM after 8 d of display storage (P < 0.01) than older pigs. Within the younger pigs, Mg supplementation reduced oxidation of the LM (P < 0.05). For SM, oxidation was greater in younger pigs than older pigs when no supplemental Mg was provided (P < 0.05). Oxidation of the SM or LM during vacuum-packaged storage was not affected (P > 0.10) by age or Mg supplementation (data not shown). Our previous results indicated that 900 ppm of Mg supplemented via the water for 2 d reduced the extent of oxidation of the LM after 4 d of display storage and that increasing the duration of supplementation from 2 to 8 d incrementally increased oxidation to levels similar to that of the control treatment (Frederick et al., 2004). Subsequent results (Frederick et al., 2006) demonstrated that the extent of oxidation of the LM increased linearly when the level of Mg supplementation during a 2-d period was increased from 0 to 900 mg/L. An inconsistent effect was observed by Apple et al. (2001), who reported that 0.1% of Mg from Mg mica in the starter, grower, and finisher diets increased oxidation after 28 d of vacuum-packaged storage compared with pigs fed 0.2% Mg from Mg mica, but results were opposite after 56 d of vacuum-packaged storage.
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Footnotes |
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2 Current address: Christensen Family Farms, Sleepy Eye, MN 56085.
3 Corresponding author: Eric_vanHeugten{at}ncsu.edu
Received for publication July 23, 2005. Accepted for publication January 18, 2006.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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