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J. Anim. Sci. 2006. 84:894-901
© 2006 American Society of Animal Science

ANIMAL PRODUCTS

Comparison of histochemical characteristics in various pork groups categorized by postmortem metabolic rate and pork quality1

Y. C. Ryu and B. C. Kim2

Division of Food Science, College of Life and Environmental Sciences, Korea University, Seoul 136713, Korea


   Abstract
Top
 Abstract
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 IMPLICATIONS
 LITERATURE CITED
 
The purpose of the present study was to investigate the variations in histochemical characteristics of muscle samples segregated according to metabolic rates (MR) and pork quality attributes. A total of 231 crossbred Duroc x (Yorkshire x Landrace) pigs was evaluated. Samples of the LM were taken to evaluate histochemical characteristics, postmortem MR, and meat quality. Samples were classified into fast, normal, and slow MR groups based on muscle pH at 45 min and R-value. Drip loss and lightness (L*) were used to assign samples to 1 of 4 quality classes. Pale, soft, and exudative pork belonging in the fast group had the greatest (P < 0.05) percentage of type IIb fibers, and RSE (reddish-pink, soft, and exudative) pork belonging in the fast group had a similar tendency. Additionally, RFN (reddish-pink, firm, and nonexudative) pork belonging in the normal group showed a lower (P < 0.05) percentage of type IIb fibers than PSE or RSE, regardless of MR, and DFD pork had the lowest (P < 0.05) percentage of type IIb fibers. In general, the fast-glycolyzing PSE pork with the lowest pH at both 45 min and 24 h had greater percentages of type IIb fibers than the fast-glycolyzing RFN pork. There were more fiber-type composition differences between quality classes in pork undergoing a fast rate of metabolism compared with pork undergoing a normal rate of metabolism. It can be concluded that muscle histochemical characteristics are associated with early postmortem MR, the extent of glycolysis, and, thereby, pork quality; however, these effects are limited to the pigs exhibiting a fast glycolytic rate.

Key Words: metabolic rate • muscle fiber • pork quality


   INTRODUCTION
Top
 Abstract
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 IMPLICATIONS
 LITERATURE CITED
 
Mechanisms controlling pork quality are often associated with altered postmortem muscle metabolism. Specifically, changes in the extent or rate of glycolysis can create unfavorable muscle pH. A high rate of pH decline and a low ultimate pH result in muscle protein denaturation and diminish quality parameters (Henckel et al., 2000Go; Hammelman et al., 2003Go). One of the main factors determining muscle biochemical pathways is fiber-type composition; skeletal muscle is composed of different types of fibers, which are the results of coordinated expression of distinct sets of structural proteins and metabolic enzymes (Schiaffino and Reggiani, 1996Go; Chang et al., 2003Go).

Using classical histochemical techniques (myosin ATPase staining), 3 types of muscle fibers exist in adult porcine muscle. The type I and IIb fibers, also known as slow-oxidative and fast-glycolytic, respectively, represent 2 extreme metabolic profiles. The type IIa fibers are intermediate between type I and IIb fibers with respect to energy metabolism (Klont et al., 1998Go). Because muscle fibers contain different myosin-heavy chains, which are responsible for their different ATPase activity (Picard et al., 1999Go), it is possible that fiber-type composition may be associated with postmortem changes in the conversion of muscle to meat and subsequently meat quality (Karlsson et al., 1999Go; Brocks et al., 2000Go). Therefore, the variation in fiber-type characteristics can explain part of the variation in some meat quality traits (Essen-Gustavsson et al., 1994Go).

It is well known that muscle fiber number, size, and fiber-type composition are closely related to each other (Ryu et al., 2004Go). Some recent studies found correlations between muscle fiber characteristics and meat quality traits in beef (Ozawa et al., 2000Go) and in pigs (Karlsson et al., 1999Go; Eggert et al., 2002Go; Ryu and Kim, 2005Go). For the practical application of this knowledge to improve and control meat quality, more information on the effects of the fiber-type characteristics on postmortem metabolic rate (MR) is necessary. Therefore, the purpose of the present study was to investigate the variations in histochemical characteristics of muscle samples segregated according to MR and pork quality attributes.


   MATERIALS AND METHODS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 IMPLICATIONS
 LITERATURE CITED
 
Animals
A total of 231 crossbred Duroc x (Yorkshire x Landrace) pigs (149 gilts and 82 castrated male pigs) was evaluated. Pigs were slaughtered during the winter period at 172.7 ± 1.7 d of age. The abattoir used a traditional scalding-singeing process. After electrical stunning, the carcasses were exsanguinated before scalding in 65°C hot water. After evisceration, the carcasses were weighed, and backfat thickness was measured at the 11th and last thoracic vertebrae. The mean of these 2 measurements was used as backfat thickness. The loin eye area was measured at the level of the last rib.

Histochemical Analyses
Within 45 min postmortem, muscle samples for histochemical analysis were taken from the LM at the 8th thoracic vertebrae. Samples were cut into 0.5- x 0.5- x 1.0-cm pieces, promptly frozen in isopentane cooled by liquid nitrogen, and stored at –80°C until subsequent analyses. Serial transverse muscle sections (10 µm) were obtained from each sample with a cryostat (CM1850, Leica, Germany) at –20°C and mounted on glass slides.

Myosin ATP activities (Brooke and Kaiser, 1970Go) were detected after acid (pH 4.7) or alkaline (pH 10.7) preincubation (Lind and Kernell, 1991Go). Unfixed sections were preincubated at room temperature for 5 min in a buffer consisting of 100 mM potassium chloride in 100 mM sodium acetate and adjusted to pH 4.7 with acetic acid (Lind and Kernell, 1991Go). After preincubation, the sections were subjected to the following steps: 1) washing in 4 rinses of distilled water, 2) washing for 30 s in a 20 mM glycine buffer (pH 9.4) containing 20 mM CaCl2, 3) incubation at room temperature for 25 min in a freshly prepared medium (40 mM glycine buffer containing 20 mM CaCl2 and 2.5 mM ATP disodium salt (pH 9.4), 4) washing in three 30-s changes of 1% CaCl2, 5) washing in 2% cobalt chloride for 3 min, 6) washing in 3 changes of distilled water, 7) immersing in 1% yellow ammonium sulfide for 30 s, 8) washing in several changes of distilled water, and 9) mounting in glycerol jelly (20 g of gelatin, 2.4 g of phenol crystals, 60 mL of glycerol, and 70 mL of distilled water).

All histochemical samples were examined by an image analysis system. The operational system consisted of an optical microscope equipped with a charge-coupled device color camera (IK-642K, Toshiba, Japan) and a standard workstation computer that controls the image analysis system (Image-Pro Plus, Media Cybernetics, Silver Springs, MD).

All portions of the sections analyzed were free from tissue disruption and freeze damage. Approximately 600 fibers were evaluated per sample. On the 2 series of photographs in each tissue taken at the same location on the different specimens, the muscle fibers were divided into types I, IIa, and IIb according to the nomenclature of Brooke and Kaiser (1970)Go. Fiber number percentage was obtained from the ratio of the number of each fiber type to the total number of fibers counted, and fiber area percentage was the ratio of the total cross-sectional area (CSA) of each fiber type to the total measured fiber area. The average CSA, diameter, and perimeter of the type-identified fibers were also measured. Fiber density was calculated from the mean number of fibers per mm2. For the calculation of the total fiber number (the estimated number of fibers), the fiber density was multiplied with the loin eye area (cm2) determined at the level of the last rib.

Postmortem MR
Within 45 min postmortem, samples from the LM at the 8th thoracic vertebrae were taken and immediately frozen in liquid nitrogen to determine postmortem MR. Early postmortem muscle pH and temperature were determined directly on the carcass at the 7th-8th thoracic vertebrae after the muscle samples were removed. The pH was measured using a spear-type electrode (Model 290A, Orion Research, Inc., Boston, MA) at 45 min (pH45min) and 24 h postmortem (pH24h). Muscle temperature was measured in the center of the muscle using a portable thermometer (Model TES-1300, TES Electrical Electronic Co., Taiwan).

To determine the postmortem energy metabolism, the R-value, which is the ratio of inosine to adenosine nucleotides, was measured by the procedure of Calkins et al. (1982)Go. Approximately 2 g of each sample was placed in 6% perchloric acid and homogenized (Ace Homogenizer AM-8, Nissei Co., Japan) at 5,000 rpm for 90 s and then centrifuged (Centrikon T-124, Kontron Instruments Co., Switzerland) at 3,000 x g for 10 min at 2°C. Absorbances were measured using a calibrated spectrophotometer (Model Du-64, Beckman Co., Fullerton, CA), and R248, R250, and R258 were defined as the ratios of A248/A260, A250/A260, and A258/A250, respectively.

Samples were classified based on muscle pH45min and R-value into 1 of 3 MR groups as described subsequently (Honikel and Fischer, 1977Go):


Formula


Formula


Formula

Meat Quality Traits
Following 24 h of chilling, LM was taken to evaluate the meat quality traits. For each of the 231 loins, every effort was made to maintain consistency in using the same anatomical location for each procedure.

Drip loss was determined by suspending muscle samples standardized for surface area in an inflated plastic bag for 48 h at 2°C (Honikel, 1987Go). Filter paper fluid uptake was also measured as described by Kauffman et al. (1986)Go.

The color of the meat was measured at the 7th-8th thoracic vertebrae at 45 min and at the 8th-9th thoracic vertebrae at 24 h postmortem with a chromameter (CR-300, Minolta Camera Co., Japan) after exposing the surface to the air for 30 min at 2°C. The average of triplicate measurements was recorded, and the results were expressed as C.I.E. (Commission International de l’Eclairage) L*, a*, and b*. The magnitude of the total color difference was represented by a single number, {Delta}E:


Formula

where L* represents lightness, a* represents redness-greenness, and b* represents yellowness-blueness. This formula provides numeric data that represent the differences in color perceived between 45 min and 24 h postmortem measurements.

Drip loss and lightness (L*) measured at 24 h postmortem were used to assign samples to 1 of 4 quality classes (QC) as described subsequently (Joo et al., 1999Go):


Formula


Formula


Formula


Formula

Statistical Analysis
A General Linear Model (SAS Inst., Inc., Cary, NC) was used to evaluate significant differences (P < 0.05) among the postmortem MR groups and QC. The model included the effects of MR, QC, and MR x QC. When significant differences (P < 0.05) were detected, the mean values were separated by the probability difference (PDIFF) option at a predetermined probability rate of 5%. The results were presented as least squares means for the groups together with the standard errors of these least squares means.


   RESULTS AND DISCUSSION
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
IMPLICATIONS
 LITERATURE CITED
 
Postmortem MR and Meat Quality Traits
There were significant differences in early postmortem MR (Figure 1Go) and meat quality traits (Figure 2Go) among pork groups categorized by MR or QC. The results were consistent with the well-established glycolytic rate, water-holding capacity (WHC), and meat color in pork (Joo et al., 1995Go; Candek-Potokar et al., 1998Go). The PSE pork showed a low pH45min value and a high R-value. Conversely, in DFD pork, a high R-value was found to be combined with a high pH45min value. In normal prerigor muscle, with its high content of adenosine nucleotides, the R-value is expected to be low. In PSE and DFD meat, however, high R-values should be found because of high inosine and hypoxanthine concentrations.


Figure 1
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  		Figure 1. Muscle pH and R-values measured 45 min postmortem in pork groups categorized by quality class. Results are expressed as means ± SE. Significance (P < 0.05) is indicated by differing letters. PSE: drip loss > 6.0%, L* > 50; RSE (reddish pink, soft, and exudative): drip loss > 6.0%, L* ≤ 50; RFN (reddish pink, firm, and nonexudative): drip loss ≤ 6.0%, L* ≤ 50; and DFD: drip loss < 2.0%, L* < 43. pH45min = pH measured at 45 min; R248 = ratio of A248/A260.

 

Figure 2
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  		Figure 2. Drip loss, filter paper fluid uptake (FFU), and lightness of LM in pork groups categorized by metabolic rate (A, B) or quality class (C, D). Results are expressed as means ± SE. Significance (P < 0.05) is indicated by differing letters. PSE: drip loss > 6.0%, L* > 50; RSE (reddish pink, soft, and exudative): drip loss > 6.0%, L* ≤ 50; RFN (reddish pink, firm, and nonexudative): drip loss ≤ 6.0%, L* ≤ 50; and DFD: drip loss < 2.0%, L* < 43. PM = postmortem

 
Muscle pH45min reflects the rate of postmortem metabolism and influences the degree of protein denaturation (Sellier and Monin, 1994Go). Ryu et al. (2005)Go reported that initial levels of metabolites related closely to muscle pH, and fast-glycolyzing pigs exhibited severe protein denaturation during the early postmortem period. Offer and Knight (1988)Go reported that the rate of pH decline and ultimate pH are the 2 main determinants of WHC and meat color. Brewer et al. (2001)Go also demonstrated that as pH approaches the isoelectric points of muscle proteins, free water increases and scatters more light. Thus, decreasing pH45min is related to increasing drip loss and lightness, which is deteriorative to meat quality.

The fast-glycolyzing group produced 56.3% PSE, 16.7% RSE, and 27.0% RFN pork. The normal group produced 73.1% RFN, 20.2% RSE, and 6.7% PSE pork, and the slow group produced only DFD pork. Therefore, we analyzed all data on the basis of the simultaneous consideration of MR group and QC.

The WHC and L* values were different depending on both MR group and QC (Table 1Go). Fast-glycolyzing PSE pork showed the greatest lightness measured at 24 h postmortem. To evaluate the color difference between 45 min and 24 h postmortem, the color change value was calculated. The fast group had a lower color change value than the normal group (4.72 vs. 6.79; P < 0.001). In the fast group, the color change value was similar among QC, except for PSE, which tended to be greater than the other classes. In the normal group, PSE pork had a greater color change value than RSE or RFN. Additionally, RSE and RFN pork in the normal group showed more color change compared with those in the fast group.


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  		Table 1. Meat quality traits and postmortem (PM) glycolytic rate in various pork groups categorized by metabolic rate (MR) and quality class (QC)1
 
The pH45min and R-values were different depending on the MR group; the pH24h was varied in QC. The PSE pork showed the lowest pH24h value regardless of MR. In the fast-glycolyzing group, the pH differences between pH45min and pH24h were similar among QC. In the normal group, PSE and RSE pork showed a lower pH24h and greater pH differences compared with RFN pork.

The reclassification of traditionally normal pork into RFN and RSE by Kauffman et al. (1992)Go, who reported only 15% of meat to be ideal (RFN) and >50% to be RSE with an unacceptably high drip loss, was of great interest to meat scientists working on pig meat quality. In this study, meat samples were classified into PSE (16.9%), RSE (19.1%), RFN (61.9%), and DFD (2.1%) pork. For PSE pork, 69.2% was produced by the accelerated rate of postmortem metabolism, and 30.8% was produced in the normal-glycolyzing group. Most of the RSE pork (81.8%) was produced in the normal-glycolyzing group.

The RSE pork has high drip loss with minimal muscle protein denaturation. Warner et al. (1997)Go demonstrated that the ultimate pH of RSE pork was 0.1 unit less than that of RFN pork, and van Laack and Kauffman (1999)Go found similar results. In this study, RSE pork had a lower pH24h value than RFN pork, but the difference in pH24h value was only found in the normal group, which had a normal pH decline pattern. Van Laack and Kauffman (1999)Go suggested RSE to be a mild form of PSE, and if the pH decreased further, they indicated that RSE samples would have become PSE pork. In this study, 18.2% of RSE pork was thought to exhibit a mild form of PSE. Our results indicate that PSE pork was mainly produced by the rapid postmortem glycolysis and tended to have an abnormal extent of glycolysis. However, RSE pork was mainly affected by the extent of postmortem glycolysis.

Muscle Fiber Size and Quantity
Carcass weight was similar among all groups and was not dependent on either MR or QC (Table 2Go). The PSE pork belonging in the fast group and the RSE pork in the normal group showed a large loin eye area (P < 0.01) although similar in carcass weight. Fast-glycolyzing PSE pork and normal-glycolyzing RSE pork increased in total number and total number of type IIb fibers (P < 0.05). These groups showed a similar tendency relative to the result of loin eye area. The total number of fibers was affected by both the CSA of muscle fiber and loin eye area (Ryu et al., 2004Go). No interaction between MR and QC was observed through a muscle fiber size measurement. The fast-glycolyzing group had a smaller CSA of type I (P < 0.01) and type IIa (P < 0.05) fiber than the normal or slow groups; mean CSA and type IIb fiber CSA were not affected by the MR group.


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  		Table 2. Carcass traits and muscle fiber characteristics in various pork groups categorized by metabolic rate (MR) and quality class (QC)1
 
Fiber-Type Composition
To evaluate fiber-type composition, the number and area percentages of each fiber type were determined (Table 3Go). The number percentage of type I fiber was different among the MR groups (P < 0.05). Specifically, the fast group (5.99%) had a lower percentage than the normal (8.23%) or slow (10.57%) groups. The percentages of type IIa fibers were affected by QC. The PSE and RSE pork had a lower (P < 0.05) percentage of type IIa fibers than RFN or DFD pork. In particular, PSE pork belonging in the fast group showed the lowest percentage of type IIa fiber. The significant interaction between MR and QC was found in the percentage of type IIb fibers. The fast-glycolyzing PSE pork had the greatest percentage of type IIb fibers (P < 0.05).


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  		Table 3. Fiber-type composition in various pork groups categorized by metabolic rate (MR) and quality class (QC)1
 
A clearer difference in histochemical property was observed in the results of fiber area percentage. The area percentage of type I fiber among the MR groups was different (P < 0.05) in accordance with the result of the number percentage of type I fiber. The fast group (3.95%) had a lower percentage than the normal (6.06%) or slow (7.28%) groups. The PSE pork belonging in the fast group had the lowest area percentage of type IIa fiber and had the greatest area percentage of type IIb fibers.

A few studies have suggested that part of the variation in fiber-type characteristics and metabolic potentials within muscle explain the variation in some meat quality characteristics (Essen-Gustavsson et al., 1994Go; Chang et al., 2003Go). Larzul et al. (1997)Go reported that the percentage of type IIb fibers was negatively related to the pH at 30 min postmortem and was positively related to lightness. In this study, the fast-glycolyzing PSE pork with the lowest pH45min and pH24h values had a greater percentage of type IIb fibers than RFN pork. Fast-glycolyzing RSE pork also had a greater percentage of type IIb fibers than the fast-glycolyzing RFN pork, although there were no differences in pH value.

Comparing the fiber-type composition observed from the different QC in the fast group, the results imply that a low pH value in fast-glycolyzing PSE and RSE pork is due to a high percentage of type IIb fibers. The high proportion of type IIb fibers may be more prone to PSE pork because of its anaerobic nature, greater glycogen content, and lower ultimate pH, as suggested by Solomon et al. (1998)Go. Conversely, a low pH45min value in the fast-glycolyzing RFN pork might be explained by the effect of extrinsic factors (i.e., poor handling) because RFN pork further differentiated by MR did not significantly differ in the percentage of type IIb fibers. In the normal-glycolyzing group, there was no clear difference in the pH45min and R-values among QC. The only difference was a low value of ultimate muscle pH in PSE and RSE pork. These results indicate that PSE and RSE pork had more postmortem glycolysis than RFN pork. However, there was no clear difference in fiber-type composition between the different QC in the normal group. Despite having similar pH declines during the early postmortem period, carcasses in normal group showed variation in meat quality traits.

Based on these results, it was concluded that muscle fiber composition is a useful parameter to explain the variation of the early postmortem MR, the extent of glycolysis, and, thereby, meat quality. Accelerated MR and poor meat quality in fast-glycolyzing PSE and RSE pork were explained by an increase in percentage of type IIb fibers. A high proportion of type IIb fibers may be more prone to undesirable pork because of its anaerobic nature.

Bowker et al. (1999)Go suggested that muscle protein differences across genetic lines result in differing susceptibilities to denaturation given similar postmortem muscle pH conditions. Skeletal muscle is composed of different types of fibers, which are the result of coordinated expressions of distinct sets of structural proteins (Schiaffino and Reggiani, 1996Go). Bowker et al. (2004b)Go suggested that myosin isoform composition influences postmortem energy metabolism and/or myosin isoforms differ in their susceptibility to protein denaturation. Fibers expressing fast MHC isoforms, especially 2X and 2B, have greater ATPase activity early postmortem but are more susceptible to inactivation by a rapid pH decline (Bowker et al., 2004aGo). Such differences in structural proteins and their denaturation susceptibility might have led to the variation in meat quality characteristics. Recently, fiber types have been often defined by the isoforms of MHC that are present. According to the major MHC isoforms found in porcine LM, 4 different single MHC isoforms have been identified (Lefaucheur et al., 1998Go). They showed that 18, 31, and 51% of conventional IIb fibers were pure IIx, hybrid IIx/IIb, and pure IIb fibers, respectively. These type IIx fibers account for nearly half of all fibers classified as IIb fibers using traditional methods (Depreux et al., 2000Go) and the aerobic capacity of type IIx fibers intermediate between type IIb and type IIa fibers (Pette and Staron. 2000Go). Conventional IIb fibers used in this study are a heterogeneous population; therefore, further research is needed to divide conventional IIb fibers into pure IIx, hybrid IIx/IIb, and pure IIb fibers to confirm the relationship between histochemical characteristics and meat quality in pigs.


   IMPLICATIONS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 IMPLICATIONS
LITERATURE CITED
 
Fast-glycolyzing, pale, soft, and exudative pork with the lowest muscle pH values had a greater percentage of type IIb fibers than fast-glycolyzing, reddish-pink, firm, and nonexudative pork. In pigs exhibiting a normal glycolytic rate, the only difference was a low value of ultimate muscle pH in pale, soft, and exudative pork and reddish-pink, soft, and exudative pork, and there was no clear difference in fiber-type composition among quality classes. These results imply that accelerated metabolic rate and poor meat quality in fast-glycolyzing, pale, soft, and exudative pork and reddish-pink, soft, and exudative pork are explained by an increase in the percentage of type IIb fibers. It is also proposed that the fast-glycolyzing, reddish-pink, soft, and exudative pork was thought to exhibit a mild form of pale, soft, and exudative, and the greater drip loss observed in the normal-glycolyzing, reddish-pink, soft, and exudative pork was caused mainly by the lower ultimate pH. It can be concluded that muscle histochemical characteristics influence the early postmortem metabolic rate, the extent of glycolysis, and, thereby, meat quality. However, these effects are limited to the fast-glycolyzing pigs.


   Footnotes
 
1 This work was partially supported by a grant from Korea University and Ministry of Education and Human Resources Development in Korea (2005). Back

2 Corresponding author: bckim{at}korea.ac.kr

Received for publication November 29, 2004. Accepted for publication November 28, 2005.


   LITERATURE CITED
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 IMPLICATIONS
 LITERATURE CITED
 


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