Figure 1. Simplified diagram of glycogen conversion in the muscle.
R. S. Emnett,
S. J. Moeller1,
K. M. Irvin,
and D. L. Meeker
The Ohio State University Department of Animal Sciences
1For more information, contact at: The Ohio State University,
Animal Science Building, 2029 Fyffe Road,
Columbus, OH 43210; 614-688-3686; Fax: 614-292-2929; e-mail: moeller.29@osu.edu.
Improvement of meat quality is one of the primary concerns of the pork industry. Many genetic and environmental factors contribute to the quality of fresh and processed meat products. The dominant Rendement Napole gene (RN) has been found to have both positive and negative effects on pork quality. Currently, the best method for classification of animals as RN positive (RN-RN-, RN-rn+) or RN negative (rn+rn+) is the glycolytic potential test (GP). High glycolytic potential indicates that the animal is a carrier of the RN gene. This study investigates the effect of GP on pork quality traits for a population of 576 post-mortem longissimus dorsi samples from the 1998 National Barrow Show Progeny Test. Animals were classified as high glycolytic potential (n = 26) or low glycolytic potential (n = 550), based on a GP threshold of 160 µmoles lactate equivalents per gram for the population bimodal distribution. Objective muscle-quality traits measured included loin pH (pH), water-holding capacity (WHC) measured as the weight (mg) of exudate absorbed on a filter paper, Instron tenderness (INS), percent cooking loss (CL), and Minolta color (MIN). Sensory scores evaluated included tenderness (TEN) and juiciness (JUC). Residual correlations between GP and pH, INS, WHC, CL, and MIN were -0.55, 0.15, 0.20, 0.29, and 0.29, respectively. High GP pigs had significantly (P < 0.01) lower pH (5.42 vs. 5.57) and WHC (0.055 vs. 0.037), greater CL (22.0 vs. 19.2) and paler MIN color (25.24 vs. 23.02) than low GP pigs. No statistical differences were found between low and high GP pigs for INS, JUC, or TEN. Breed was a significant source of variation for all traits evaluated. Berkshire and Chester White breeds exhibited significantly (P < 0.001) lower GP values than Hampshire or Hampshire crossbred samples. The results of this study agree with previous research indicating that high GP values are associated with lower pH, poorer WHC, higher CL, and paler color. The differences in GP across breeds warrant future studies to determine the relationship of GP with muscle quality and sensory traits.
In order to strengthen consumer acceptance, meat-quality improvement has recently become one of the top priorities of the pork industry. Many different environmental and genetic factors can influence the quality of fresh or processed pork products. Most of the past genetic research relating to pork quality has focused on the Halothane or "stress" gene, which is associated with pale, soft, and exudative (PSE) pork. Recently, the Rendement Napole (RN), or "acid meat" gene, has been shown in European and U.S. research to have both negative and positive effects on pork quality.
Carriers of the dominant RN gene have been shown to exhibit paler color, reduced pH and water-holding capacity, as well as increased drip and cooking losses (Monin and Sellier, 1985; Enfalt et al ., 1994; LeRoy et al ., 1996; Lundstrom et al ., 1996; Sutton, 1997; Enfalt et al ., 1997). Positive effects of increased tenderness, juiciness, growth, and carcass advantages have also been reported for Napole carriers (Enfalt et al ., 1994; LeRoy et al ., 1996; Sutton, 1997).
While PSE pork is attributed to a combination of higher temperature and a rapid rate of pH decline in muscle, the effects of the RN gene are the result of lower ultimate muscle pH. U.S. researchers, Sayre et al . (1963), were the first to report that Hampshire pigs had lower ultimate pH and paler color. However, these differences were not further investigated until Monin and Sellier (1985) reported that this lower ultimate pH was the result of high "glycolytic potential." Glycogen is the major source of energy in the muscle. When needed, glycogen is broken down through the glycolytic pathway, of which lactic acid is the end result in post-mortem muscle tissue (Figure 1).
Figure 1. Simplified diagram of glycogen conversion in the muscle.
The RN gene is mainly associated with lines of Hampshire breeding, with reports of the gene frequency being as high as 0.627 in U.S. Hampshire populations (Miller, 1998). Given this high frequency within a popular terminal sire breed, there is great potential for increased economic losses from the undesirable pork-quality effects of the RN gene. Few studies on pigs with diverse genetic backgrounds have been completed for U.S. populations, which leaves open the possibility of the presence of RN in other breeds.
While the search for a DNA marker continues (Mariani et al ., 1996; Milan et al ., 1996), the best method of classifying animals as either RN positive (RN-RN- or RN-rn+) or RN negative (rn+ rn+), is the use of the glycolytic potential (GP) test (Monin and Sellier, 1985). Individuals that carry the dominant RN gene exhibit higher glycolytic potential. High or low GP classifications are based on a bimodal distribution threshold value unique to each population (Naveau, 1986).
The first phase of The Ohio State University Napole gene research involves the classification of a diverse population by GP testing, followed by statistical analysis of all correlated production, carcass, meat quality, and sensory traits. Correlations between GP and pork-quality traits of interest, as well as a study of progeny groups and pedigree information, will be completed. Differences between breeds for quality traits will also be reported.
Five hundred seventy-six frozen loin-muscle samples from animals previously characterized for growth, carcass, and meat quality characteristics in the 1998 National Barrow Show Progeny Test were obtained. Berkshire (n = 184), Chester White (n = 99), Duroc (n = 77), Hampshire (n = 22), Landrace (n = 55), Poland China (n = 24), Spot (n = 15), Yorkshire (n = 69), Hampshire crossbred (n = 16), Poland x Duroc F1 crossbred (n = 8), and Tamworth x Hampshire crossbred (n = 7) were represented in the test. All animals were subjected to similar on-test management, carcass evaluation, and sensory analysis.
Glycolytic potential of the loin muscle is the estimated sum of the compounds that have the potential to be transformed into lactic acid in the post-mortem muscle. GP (µmole/g) is calculated as 2([glycogen] + [glucose-6-phosphate] + [glucose]) + [lactate]. Loin samples were subjected to a preparation procedure developed by Dalrymple and Hamm (1973), which allows for the simultaneous extraction of metabolites of interest from the muscle. The concentration of glycogen, glucose, and glucose-6-phosphate was determined by a procedure developed by Keppler and Decker (1972), while the amount of lactate present in the muscle was determined according to the procedures of Bergmeyer (1974). Animals were classified as high glycolytic potential (n = 26) or low glycolytic potential (n = 550) based on a GP threshold of 160 µmoles lactate equivalents per gram for the population bimodal distribution.
Muscle quality and sensory traits were analyzed in a mixed model analysis of SAS (1988) with fixed effects of day off test, breed, and GP status and a random sire (breed) effect. Partial correlation coefficients were calculated among quality traits using the MANOVA statement in SAS (1988).
Residual correlations (P < 0.01) between GP and pH, INS, WHC, CL, and MIN were -0.55, 0.15, 0.20, 0.29, and 0.29, respectively (Table 1). As expected, pH had the strongest relationship and was negatively correlated with GP. This is in agreement with Miller (1998) who also found a negative correlation coefficient of -0.49 for GP and longissimus pH. These results show that as glycolytic potential increases, so does tenderness and cooking loss, while pH and water-holding capacity decrease.
High GP pigs had significantly (P < 0.01) lower pH (5.42 vs. 5.57) and WHC (0.055 vs. 0.037), greater CL (22.0 vs. 19.2) and paler MIN color (25.24 vs. 23.02) than low GP pigs (Table 2). These results are also in agreement with previous research (Lundstrom et al ., 1996; Sutton, 1997; Enfalt et al ., 1997) showing that the high GP samples lose more water than low GP samples. No statistical differences were found in this study between low and high GP pigs for INS, JUC, or TEN, indicating no advantage in tenderness or juiciness for the high glycolytic potential group as was reported by both Miller (1998) and Lundstrom et al . (1996).
Breed was a significant source of variation for all traits evaluated (Table 3). Berkshire (Berk) and Chester White (Chester) breeds exhibited significantly (P < 0.001) lower GP values than Hampshire (Hamp) or Hampshire crossbreed (Hamp-X) samples. However, Hamp GP was not different (P > 0.05) from Hamp-X, Yorkshire (York), Landrace, Spot, or Tam/Hamp-X. Berk and Chester loin pH were also significantly higher than both the Hamp and Hamp-X breeds. Berks exhibited the most desirable WHC and CL when compared with the other breeds represented. Hamp and Hamp-X were only different in WHC from Berk and Chester. Landrace had the least desirable water-holding capacity but were not significantly different than Poland, York, Hamp-X, Spot, Poland-X, or Tam/Ham-X. Landrace also had the highest cooking losses but were only different from Hamp, Chester, Poland-X, Berk, and York. Hamp and Hamp-X were scored the most tender but were not significantly different from Berk, York, Landrace, or Tam/Hamp-X.
Trends within the breeds for the analysis closely follow those previously reported in past investigations (Goodwin, 1997). However, the limited number of high GP Hampshires in this analysis may lead to results that are not characteristic of the actual breed differences attributed to effects of the RN gene. Another factor to consider is that Hampshires did not contribute all of the 26 individuals classified as high GP, which suggests evidence of the presence of the RN gene in other breeds. Further investigations into these breed differences will be conducted.
| Table 1. Correlations Between Glycolytic Potential and Pork Quality Characteristics.1 | |||||
|---|---|---|---|---|---|
| Glycolytic Potential | pH*** -0.55 |
WHC*** 0.20 |
INS** 0.15 |
CL*** 0.29 |
MIN*** 0.29 |
| *** Significant at P < 0.001, Significant at P < 0.01. 1 pH = Ultimate Loin pH, WHC = Water Holding Capacity (mg of exudate), INS = Instron tenderness, CL = Cooking Loss (%), MIN = Minolta reflectance | |||||
| Table 2. Least Squares Means of High and Low Glycolytic Potential Groups for Pork Quality Characteristics.1 | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Group2 | n | GP*** | pH*** | WHC*** | INS | CL** | MIN** | TEN | JUC |
| High GP | 26 | 177.32 | 5.42 | 0.055 | 4.92 | 22.00 | 25.24 | 6.8 | 5.29 |
| Low GP | 550 | 110.21 | 5.57 | 0.037 | 5.29 | 19.22 | 23.02 | 6.9 | 5.35 |
| *** Significant at P < 0.001, Significant at P < 0.01. 1 GP = Glycolytic Potential (µmole/g), pH = Ultimate Loin pH, WHC = Water Holding Capacity (mg of exudate), INS = Instron tenderness, CL = Cooking Loss (%), MN = Minolta reflectance, TEN = Tenderness Sensory Score (1 = extremely tough), JUC = Juiciness Sensory Score (1 = extremely dry). 2 High GP = greater than 160 µmole/g; Low GP = less than 160 µmole/g. | |||||||||
| Table 3. least Squares Means of Breeds for Pork Quality Characteristics.1 | ||||||
|---|---|---|---|---|---|---|
| Breed | n | GP | pH | WHC | CL | TEN |
| Berkshire | 184 | 131.93 | 5.61 | 0.038 | 18.13 | 7.35 |
| Chester | 99 | 132.09 | 5.60 | 0.039 | 20.12 | 6.35 |
| Duroc | 77 | 141.45 | 5.51 | 0.045 | 21.21 | 6.46 |
| Hampshire | 22 | 150.40 | 5.48 | 0.045 | 20.27 | 7.57 |
| Hamp-X | 16 | 151.11 | 5.46 | 0.045 | 20.57 | 7.99 |
| Landrace | 55 | 151.03 | 5.44 | 0.056 | 22.48 | 6.91 |
| Poland | 24 | 142.59 | 5.49 | 0.051 | 20.86 | 6.20 |
| Poland-X | 8 | 132.99 | 5.53 | 0.049 | 19.12 | 5.71 |
| Spot | 15 | 148.88 | 5.39 | 0.046 | 21.50 | 6.55 |
| Tam/Hamp-X | 7 | 147.51 | 5.51 | 0.042 | 21.43 | 6.99 |
| Yorkshire | 69 | 151.47 | 5.41 | 0.049 | 20.96 | 7.04 |
| 1 GP = Glycolytic Potential (µmole), pH = Ultimate Loin pH, WHC = Water-Holding Capacity (mg of exudate), CL = Cooking Loss (%), TEN = Tenderness Sensory Score (1 = extremely tough). | ||||||
The results of this study agree with previous research indicating that high glycolytic potential values are associated with lower loin ultimate pH, poorer water-holding capacity, higher cooking losses, and paler color. Berkshire and Chester White samples were higher in pH and lower in glycolytic potential than both the Hampshire and Hampshire crossbred samples. However, the differences across breeds for the other quality traits studied warrant future investigations to determine the relationship of GP with muscle quality and sensory traits.
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