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ANIMAL PRODUCTS |
Department of Animal Science and Interdepartmental Nutrition Program, North Carolina State University, Raleigh 27695-7621
Abstract
Thirty-two pigs were used to determine the timing effect of magnesium (Mg) supplementation given through drinking water on pork quality. Pigs (16 barrows and 16 gilts) were individually penned, provided 2.7 kg of feed (0.12% Mg) daily (as-fed basis), and allowed free access to water via a nipple waterer for the duration of the study. After 5 d of adjustment, pigs (120 ± 0.8 kg BW) were allotted randomly by weight and sex to 900 mg/L of supplemental Mg from magnesium sulfate heptahydrate in drinking water for –6, –4, –2, or 0 d relative to slaughter. The LM and semimembranosus (SM) muscles were removed 24 h postmortem. Retail display storage was simulated for 8 d, and the LM was vacuum-packaged for 25 or 50 d at 4°C. Magnesium did not affect the pH of the LM at either 45 min (P = 0.15) or 24 h postmortem (P = 0.23). However, the pH of the SM at 24 h postmortem tended to be greater (P = 0.08) for pigs consuming Mg for 2 d than for those not supplemented. Fluid loss after 8 d of storage was less (P < 0.05) in the LM of pigs supplemented with Mg for 6 d than in those without supplementation. Furthermore, fluid loss from the SM of pigs provided supplemental Mg for 2 d, but not for 4 or 6 d, was lower (P < 0.05) on each day of retail display than the SM of unsupplemented pigs. Minolta L*, a*, and b* color measurements of the LM during display storage were not (P > 0.10) affected by Mg supplementation. However, Mg supplementation for 2 or 4 d decreased paleness (lower L* value) after 25 d (P < 0.05), but not 50 d (P > 0.10) of vacuum-packaged storage. Magnesium addition for 2 d decreased the extent of oxidation (thiobarbituric acid-reactive substances) of the LM after 4 d of display storage compared with 0 d of Mg (P < 0.05). Oxidation of the SM during 8 d of display storage increased linearly (P < 0.05) as duration of supplementation increased from 2 to 6 d but did not differ (P = 0.22) from 0 d of Mg supplementation. Although the response to Mg supplementation was variable, supplementation for 2 d before slaughter was considered most efficacious because of the following: decreased fluid loss from the SM, and lower lipid oxidation formation in the LM during retail storage; a darker, more desirable LM color after 25 d of vacuum-packaged storage; and cost reductions compared with longer durations.
Key Words: Magnesium • Pigs • Pork • Quality • Water
Introduction
Magnesium is an important divalent cation involved with over 300 enzymes essential for metabolism, including protein and energy metabolism. Furthermore, Mg has been shown to decrease the acute stress response resulting from handling before slaughter (Kietzmann and Jablonski, 1985
), to control intracellular calcium (Laver et al., 1997), and to delay the initiation of glycolysis by maintaining high-energy phosphates postmortem (Moesgaard et al., 1993). For these reasons, Mg has been evaluated as a nutritional means of improving pork quality (D’Souza et al, 1998; Apple et al., 2001; Hamilton et al., 2002).
Indeed, short-term supplemental Mg has been reported to improve pork water-holding capacity (D’Souza et al., 1998, 1999, 2000) and color (D’Souza et al., 1998, 2000), and it may affect lipid oxidation of stored pork (Apple et al., 2001). The positive effects of Mg supplementation on pork quality do not seem to depend on Mg source (D’Souza et al., 1999); however, the response may, or may not, depend on timing of supplementation (D’Souza et al., 2000; Hamilton et al., 2002). Conversely, others have not demonstrated a positive, or consistent, effect of Mg on pork quality (van Laack, 2000; Hamilton et al., 2002). Although many studies have concentrated on supplemental dietary Mg in feed, supplementation through drinking water is a novel, practical approach that has not been reported previously. Therefore, the objective of this study was to determine the optimal duration of Mg supplementation through drinking water to improve pork quality.
Materials and Methods
Animals and Treatments
All animal procedures were approved by the Institutional Animal Care and Use Committee of North Carolina State University. Sixteen barrows and 16 gilts weighing 120 ± 0.8 kg from the North Carolina State University Swine Education Unit were placed into 2.03- x 0.74-m individual pens and provided free access to water via a nipple waterer. Pigs were fed (as-fed basis) 2.7 kg of a diet containing 0.12% Mg from feedstuffs per day (Table 1) during a 5-d adjustment and 6-d treatment period. After the adjustment period, pigs were allotted by weight within sex to water supplemented with 900 mg/L of elemental Mg from magnesium sulfate heptahydrate (9.8% Mg; Giles Chemical Corp., Waynesville, NC) for 6, 4, 2, and 0 d before slaughter. Plastic water containers (23-L capacity) were filled daily with 15 L of water containing the appropriate Mg concentration. These containers were suspended from the ceiling and gravimetrically emptied into a galvanized pipe leading to a nipple waterer regulated to dispense 600 mL of water/min. Daily water disappearance volumes were determined by weight loss of the water containers assuming that 1 kg was equivalent to 1 L of water. Feed was removed 12 h before transport; however, pigs had free access to experimental water treatments until transport to the abattoir.
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On d 7 (at 0800), all pigs were loaded and transported 110 km (1.75 h) to a commercial abattoir. Pigs were unloaded by abattoir personnel. After 45 min of lairage, pigs were moved, by replicate, 15 m to the stunning area. Pigs were electrically stunned, and blood was collected during exsanguination for determination of plasma Mg concentration. Hot carcass weights were recorded before chilling to calculate dressing percent. The temperature and pH of the LM were measured between the 10th and 11th ribs at 45 min and 24 h postmortem using an Argus Sentron (Gig Harbor, WA) pH meter.
After a 20-h chilling period at 2°C, entire, bone-in loins and hams from the right sides were removed and transported 60 km (45 min) at 4°C to a commercial meat-fabrication facility for further processing. The semimembranosus (SM) was removed from the ham, and four 2.54-cm-thick chops were cut. Additionally, a total of four 2.54-cm-thick chops from the LM were obtained posterior to the seventh-and-eighth-rib interface. The first SM and LM chops were placed in a plastic bag and stored at –20°C for DM and tissue Mg determination. The second chop (located posterior to the first chop) was used for drip loss determination on the same day of collection. The third LM and SM chops, posterior to the second, were placed on an absorbent pad (Cryovac Sealed Air Corp., Saddle Brook, NY) within a Styrofoam tray (Cryovac Sealed Air Corp.), and wrapped with a polyvinyl chloride film (Cryovac Sealed Air Corp.). Packages were then stored in a random fashion on two tables in a walk-in refrigerator at 4°C, approximately 1.5 m below two shop lights (1.22 m in length) containing two fluorescent lights each (40 W; F40CW-EX; General Electric Co., Cleveland, OH) to simulate retail display for 4 d. At the end of the 4-d display storage period, chops were analyzed for extent of oxidation. The final chops were stored for 8 d in a display environment similar to that used for the third chop. The remaining posterior portion of the LM was divided into equal sections, weighed, vacuum-packaged in B2651T Cryovac bags with a Multivac machine (Cryovac, Ducan, SC), and stored at 4°C in the absence of light for 25 or 50 d.
Pork Quality Measurements
Fluid loss of the LM and SM was evaluated by two separate methods. Drip loss was determined by a method developed by Honikel et al. (1986). Briefly, a 70-g core sample of each muscle was manually removed and suspended by a fish hook (barb removed) in a plastic, covered container, and stored at 4°C for 48 h. Drip loss was reported as the weight loss of the sample after 48 h of storage divided by the initial weight of the muscle before storage and multiplied by 100. Display fluid loss was determined on chops designated for 8 d of display storage. The muscle was 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. Display fluid loss was reported as the weight loss of the displayed chop as a percentage of the initial weight of the chop before storage. Each chop was returned to its original tray, rewrapped, and returned to display storage for subsequent measurements.
Purge loss was identified as the amount of fluid lost from the LM (450 g) defined in the previous section after 25 or 50 d of vacuum-packaged storage. After the appropriate storage period, each muscle section was removed from the package, blotted with a paper towel, and reweighed. Purge loss was reported as the weight loss during storage divided by the initial muscle weight and multiplied by 100.
Color of the LM and SM was objectively evaluated by L*, a*, and b* measurements using a Minolta CR-200 Chroma Meter (Minolta Corp., Ramsey, New Jersey) calibrated with a standard white plate using D65 illuminant and an 8-mm-diameter viewing area. Values were reported as the average color values collected at four positions in a diamond pattern on the surface of each chop. Color of the LM and SM was conducted on chops designated for 8 d of display storage. The initial measurement of color was performed after 45 min of bloom at 4°C. Additionally, color was determined every 2 to 8 d of display storage. The color of vacuum-packaged LM was determined on an interior chop after a 45-min bloom period at 4°C. Chops displayed for 4 or 8 d and the LM chops from vacuum-packaged loin sections for 25 or 50 d were vacuum-packaged in Cryovac bags (B2651T) and stored at –20°C until oxidation was determined by thiobarbituric acid-reactive substances (TBARS).
Chemical Analyses
Plasma and muscle Mg concentrations were determined by atomic absorption spectrophotometry. Briefly, each muscle was ground and passed through a 5-mm screen twice (Oster Food Grinder; Sunbeam Corp., Canada, Ltd., Mississauga, Ontario). Two grams of sample was dried at 103°C overnight, and the dried sample was quantitatively transferred to a polypropylene tube (Corning, Acton, MA). Ten milliliters of nitric acid (Fisher Scientific; Fair Lawn, NJ) was added to the tube and allowed to predigest overnight. The next day, samples were placed in a microwave oven (MARS 5; CEM, Matthews, NC), ramped for 10 min to 110°C, maintained at 110°C by thermowell, and cooled for 20 min before adding 2 mL of hydrogen peroxide (Sigma, St. Louis, MO) to terminate digestion. Tubes were brought up to 25 mL with deionized water. Fifty microliters of digested sample was combined with 5 mL of lanthanum chloride (0.5%, wt/vol) and read by atomic absorption.
Oxidation in muscle was determined by TBARS as described by Witte et al. (1970). Briefly, samples were removed from the –20°C freezer and allowed to thaw overnight at 6°C. Each muscle was ground and passed through a 5-mm screen twice (Oster Food Grinder; Sunbeam Corp.). Four grams of ground muscle was homogenized, in duplicate, with 16 mL of ice-cold phosphate buffer (pH = 7.0) prepared to contain 50 mM Na2HPO4, 50 mM NaH2PO4, 0.1% EDTA, and 0.1% propyl gallate in a stainless steel cup for 20 s using an Omni-Mixer (Model 17105; Sorvall Corp., Newtown, CT). Then, 4 mL of 30% trichloroacetic acid was added, and samples were homogenized for an additional 10 s. The homogenate was filtered (P8; Fisher Scientific, Fair Lawn, NJ), and 2 mL of clear filtrate and 2 mL of 2-thiobarbituric acid (0.02 M) were transferred to 16- x 125-mm screw-cap glass test tubes, vortexed, and heated in a 100°C water bath for 30 min. Test tubes were allowed to cool in an ice-cold water bath for at least 15 min and vortexed before measuring absorbance on a spectrophotometer at 533 nm (model DU 640; Beckman-Coulter Inc., Fullerton, CA). Absorbance of samples was compared to tetraethoxypropane standard concentrations of 2, 4, 8, 10, 20, 40, and 80 x 10–7 M. Reported TBARS values were corrected for percent recovery, which was determined in concurrent duplicate meat samples to which 1 mL of 80 x 10–7 M tetraethoxypropane was added before homogenation. Recovery ranged from 92 to 102%.
Statistical Analyses
Data were analyzed as a randomized complete block design using the GLM procedure of SAS (SAS Inst. Inc., Cary, NC). Pigs within individual pens were considered the experimental unit and were blocked by body weight within sex. Differences between treatments were determined using LSD comparisons. In addition, orthogonal linear, quadratic, and cubic contrast comparisons were conducted where appropriate. Least squares means were reported and differences were considered significant at P < 0.05, whereas tendencies were reported at P < 0.10.
Results and Discussion
Feed and Water Disappearance
Feed intake limiting was implemented to minimize the variation of dietary Mg intake. No feed refusal was observed during the adjustment or treatment periods. Therefore, feed intake was the same across treatments and Mg intake from feed was a constant 3.24 g of elemental Mg per day. Thus, any difference of Mg intake was a result of Mg intake provided through the drinking water.
Magnesium concentration of water used to dilute the stock solution was 3 mg/L. Therefore, the total Mg concentration of water was 3 and 903 mg/L for pigs receiving water without and with Mg supplementation, respectively. Assuming that water disappearance equaled water consumption, the maximum Mg intake from drinking water was 0.03, 10.47, 8.53, and 7.10 g of elemental Mg/d for 0, 2, 4, and 6 d of supplementation, respectively. Water requirements have been estimated to be 2 L of water per kilogram of feed (Cumby, 1986). Assuming this water-to-feed ratio, Mg intake from drinking water would have been approximately 4 to 5 g/d; however, the exact Mg intake from water is not known because we were not able to determine water wastage.
Slaughter Data
Plasma Mg tended to increase linearly (P < 0.11) as duration of Mg supplementation increased (Table 2), and was highest (12% increase) when Mg was supplemented for 6 d. Similarly, van Laack (2000) reported a 10% increase in plasma Mg when pigs were supplemented with 2 g of Mg per kilogram of feed as Mg sulfate for 5 d. Furthermore, D’Souza et al. (1999) reported a 10% increase of plasma Mg with dietary supplementation of 3.2 g of Mg/d from Mg sulfate, aspartate, or chloride for 5 d before slaughter. Magnesium supplementation did not significantly affect muscle Mg concentrations in either the LM or SM, which is in agreement with Schaefer et al. (1993) and Apple et al. (2001). Dressing percent was greater (P < 0.05) for pigs receiving Mg supplementation for 6 d compared with 2 d (Table 2).
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, 2000) and Hamilton et al. (2002). However, the pH of the SM from pigs supplemented with Mg for 2 d tended to be greater (P = 0.08) at 24 h postmortem than those not receiving Mg supplementation. Apple et al. (2000) reported that the ultimate pH of the loin was greater when pigs were fed 1.25% Mg mica, although 2.5% did not affect ultimate pH. Even though pH values are often used as an indicator of pork quality, a difference in initial pH or 24-h pH may not be required to alter pork quality (D’Souza et al., 1999, 2000). Fluid Loss
Drip loss, measured after chilling of the carcass (approximately 36 h postmortem), of the LM (P = 0.34) and SM (P = 0.51) was not affected by Mg supplementation (Table 3), which is consistent with other studies using dietary Mg mica, Mg aspartate, or Mg sulfate before slaughter (Apple et al., 2000; Caine et al., 2000; van Laack, 2000). However, drip loss was decreased with Mg supplementation in studies that implemented a negative handling strategy immediately before slaughter (D’Souza et al., 1998, 1999, 2000). Fluid loss of displayed LM chops was not (P > 0.10 ) affected during the first 6 d of storage. However, after 8 d of display storage, fluid loss from the LM chops of pigs supplemented for 6 d was lower (P < 0.05) than chops from pigs that did not receive Mg supplementation. Fluid loss of displayed SM chops was lower (P < 0.05) for pigs consuming supplemental Mg for 2 d before slaughter compared with no Mg supplementation during all storage periods measured, resulting in 36% and 24% reductions after 2 and 8 d of display storage, respectively. A reduction in fluid loss from the SM contradicts the observations by van Laack (2000), who found no effects when pigs were fed 2 g of supplemental Mg per kilogram of feed as Mg sulfate for 5 d.
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), which is in agreement with previous research (Apple et al., 2001; Hamilton et al., 2002). This result may indicate that any effect of Mg on fluid loss is expected to occur during relatively short periods of displayed storage, and the effect is less noticeable during longer periods of vacuum-packaged storage. Color
Magnesium supplementation did not (P > 0.10) affect L*, a*, or b* values of the LM during display storage (Table 4), which is consistent with results reported by D’Souza (1999) and Caine et al. (2000). However, D’Souza et al. (1998, 2000) reported a reduction in the L* of the LM when Mg was supplemented through the feed. Hamilton et al. (2002) also reported a reduction in L* of the LM with 2 d but not 3 or 5 d of Mg supplementation. In the present experiment, Mg supplementation for 2 d decreased (P < 0.05) initial yellowness (b*) of the SM compared with the control, and lowered (P < 0.05) paleness (L* value) after 2 d of display storage compared with 6 d of Mg supplementation. However, this effect was not apparent at any other measurement period during display storage, indicating that there was only a slight and transient effect of Mg supplementation on the color of displayed pork.
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). However, Mg supplementation did not (P = 0.24) affect L* values after 50 d of vacuum-packaged storage. Furthermore, Mg addition did not (P > 0.10) influence redness (a* values) or yellowness (b* values) at 25 or 50 d of vacuum-packaged storage. Apple et al. (2001) found that supplementation of 1.25% Mg mica decreased redness and yellowness of boneless loins, but supplementation of 2.5% Mg mica did not result in differences from unsupplemented pigs. In that same study, neither concentration of Mg mica affected loin paleness. The results from the present experiment indicate that Mg supplementation for 2 or 4 d before slaughter may reduce the paleness of vacuum-packaged loins without negatively affecting redness or yellowness.
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Oxidation of the LM after 25 d of vacuum-packaged storage or SM after 4 d of displayed storage was not (P > 0.10) affected by Mg supplementation (Table 6). However, LM chops from pigs receiving Mg supplementation for 2 d had less (P < 0.05) oxidation after 4 d of display storage than pigs supplemented with Mg for 0 or 6 d. The extent of LM oxidation after 4 d, and of the LM and SM after 8 d, of display storage increased (P < 0.05) linearly with duration of Mg supplementation from 2 to 6 d. A similar response was observed in the vacuum-packaged LM after 50 d of storage; however, neither 4 nor 6 d of Mg supplementation affected (P > 0.10) oxidation of either muscle compared with pigs receiving no supplemental Mg at any storage period. Apple et al. (2001) reported greater oxidation of the loin when pigs were fed 1.25% vs. 2.5% Mg mica after 4 wk of vacuum-packaged storage. Over an additional 4 wk, oxidation of loins from pigs provided with 1.25% Mg mica remained constant and the oxidation of loins from pigs provided with 2.5% Mg mica increased to a level higher than that of the 1.25% Mg mica group. Results presented in the current experiment indicate that Mg supplementation can potentially reduce oxidation when supplemented for a brief period of time. However, longer durations of Mg supplementation may increase oxidation relative to brief duration (2 d before slaughter). Thus, briefer periods of Mg supplementation may be more desirable when the length of shelf life is considered.
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Magnesium supplementation through drinking water is a practical, efficient method of providing supplemental magnesium to pigs before slaughter. Results of the present study suggest that magnesium supplementation through drinking water for as little as 2 d before slaughter may improve pork quality; however, results were inconsistent across supplementation durations and muscles.
Footnotes
1 Appreciation is expressed to Clear Run Farms (Warsaw, NC), Parks Family Meats (Harrells, NC), and the North Carolina State Univ. Dept. of Food Sci. for facility use, and O. Phillips, S. Wolford, D. Lee, B. Belstra, and S. Pion for technical support. Funded in part by the North Carolina Pork Council, the North Carolina Agric. Res. Service, and the Institute of Nutrition of the Univ. of North Carolina System. The use of trade names does not imply endorsement by the North Carolina Agric. Res. Service of the products named or criticism of similar ones not mentioned. 
2 Current address: Christensen Family Farms, Sleepy Eye, MN 56085.
3 Correspondence: Box 7621 (phone: 919-513-1116; fax: 919-515-6316; e-mail: eric_vanheugten{at}ncsu.edu).
Received for publication August 1, 2003. Accepted for publication January 21, 2004.
Literature Cited
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Apple, J. K., C. V. Maxwell, B. deRodas, H. B. Watson, and Z. B. Johnson. 2000. Effect of magnesium mica on performance and carcass quality of growing-finishing swine. J. Anim. Sci. 78:2135–2143.
Caine, W. R., A. L. Schaefer, J. L. Aalhus, and M. R. Dugan. 2000. Behaviour, growth performance and pork quality of pigs differing in porcine stress syndrome genotype receiving dietary magnesium aspartate hydrochloride. Can. J. Anim. Sci. 80:175–182.
Cumby, T. R. 1986. Design requirements of liquid feeding systems for pigs: A review. J. Agric. Eng. Res. 34:153–172.
D’Souza, D. N., R. D. Warner, F. R. Dunshea, and B. J. Leury. 1999. Comparison of different dietary magnesium supplements on pork quality. Meat Sci. 51:221–225.
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D’Souza, D. N., R. D. Warner, B. J. Leury, and F. R. Dunshea. 2000. The influence of dietary magnesium supplement type, supplementation dose and duration, on pork quality and the incidence of PSE pork. Aust. J. Agric. Res. 51:185–189.
Hamilton D. N., M. Ellis, M. D. Hemann, F. K. McKeith, K. D. Miller, and K. W. Purser. 2002. The impact of longissimus glycolytic potential and short-term feeding of magnesium sulfate heptahydrate prior to slaughter on carcass characteristics and pork quality. J. Anim. Sci. 80:1586–1592.
Honikel, D. O., C. J. Kim, P. Roncales, and R. Hamm. 1986. Sacromere shortening of pre-rigor muscles and its influence on drip loss. Meat Sci. 16:267–282.
Kietzmann, M., and H. Jablonski. 1985. Blocking of stress in swine with magnesium aspartate hydrochloride. Prakt. Tierz. 661:331–338.
Laver, D. R., V. J. Owen, P. R. Junankar, N. L. Taske, A. F. Dulhunty, and G. D. Lamb. 1997. Reduced inhibitory effect of Mg+ on ryanodine receptor-Ca+ release channels in malignant hyperthermia. Biophys. J. 73:1913–1924.[Medline]
Moesgaard, B., I. Errebo Larsen, B. Quistorff, I. Therkelsen, V. G. Christensen, and P. F. Jorgensen. 1993. Effect of dietary magnesium on postmortem phosphocreatine utilization in skeletal muscle of swine: A non-invasive study using 31P-NMR spectroscopy. Acta Vet. Scand. 34:397–404.[Medline]
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