Pig meat quality has become an important issue over the past decade,
and interest in improving attributes such as color, water-holding
capacity, palatability, and consistency has increased dramatically.
A variety of approaches have evolved to improve the quality of
pig meat. These approaches include manipulating factors such
as genetics, management, nutrition, animal handling, slaughter
procedures, and postmortem handling of the carcasses. The focus
of this discussion will involve a genetic component of meat quality.
Many factors influence the rate and extent of pH decline in postmortem
muscle. Historically, the major contributors to reduced water
holding capacity and pale muscle color have been the combination
of a rapid pH decline and elevated muscle temperature which combine
to produce the Pale, Soft, Exudative (PSE) condition. Monin and
Sellier (1985) were the first to suggest another cause for reduced
water holding capacity and pale muscle color from studies involving
the Hampshire breed. They identified a condition in animals that
had normal rates of muscle pH decline postmortem; however, the
ultimate pH was relatively low and the meat was of low quality.
This so-called "acid meat" condition has been attributed
to high muscle glycolytic potential (GP), a measure of the capacity
for postmortem glycolysis. High GP in Hampshires is associated
with abnormally high levels of muscle glycogen which are converted
into lactate after slaughter, resulting in a lower ultimate pH.
Naveau (1986) suggested that high GP levels were associated with
a single dominant gene known as the Rendement Napole (RN) gene,
and subsequent studies have suggested the presence of RN gene
in pig populations containing Hampshire ancestry in Sweden (Lundstrom
et al., 1996), France (Fernandez and Monin, 1994), and the United
Stated (Sutton, 1997; Miller, 1998).
Glycolytic Potential Determination
The major source of muscle carbohydrate is glycogen. Glycogen
is a polymer of glucose units linked with á1-4
and á1-6 linkages formed around a "foundation"
protein, P-glycogen (Brooks et al., 1996). When energy is required,
glycogen is hydrolyzed to glucose by the action of the enzyme
glycogen phopshorylase, which cleaves á1-4 bonds. De-branching
enzyme is required to cleave the á1-6 bonds. Collectively,
the hydrolyzation of glycogen is referred to as glycogenolysis.
The glucose products that are produced by glycogenolysis are
metabolized via glycolysis. As a result of glucose metabolism,
a net yield of ATP occurs supplying the body with an energy source.
The end product of this metabolism is lactic acid. In living
tissue, lactic acid is transported to the liver and reconverted
to pyruvic acid which is used as an energy source. However, in
postmortem muscle, lactic acid accumulates and accounts for the
Monin and Sellier (1985) suggested
the use of glycolytic potential as an index of the muscle's capacity
for postmortem glycolysis. Glycolytic potential is an estimate
of the levels of all compounds in the muscle that are involved
in the transformation of glycogen to lactic acid using the following
Assays for the combination
of glycogen, glucose, and glucose-6-phosphate combined and for
lactate are carried out using standard procedures. Glycolytic
potential is an expressed in ìmole lactate equivalent per
gram of fresh tissue.
Identification and Classification
of RN Genotypes
At present, the RN gene cannot be identified directly, because
there is no DNA-based test, and, consequently, an animal's genotype
is predicted from its GP relative to the frequency distribution
of GP values for the population. The frequency distribution for
populations within which the dominant allele of the RN gene (RN-)
is segregating is biomodal (Naveau, 1986; LeRoy et al., 1990;
Fernandez et al., 1992). A typical distribution for GP found
in a US population is illustrated in Figure 1 (Miller, 1998).
Animals in the upper part of the bimodal distribution with the
higher glycolytic potential are classified as homozygous dominant
(RN-RN-) and/or heterozygous (RN-rn+).
It is not possible to distinguish between the homozygous dominant
and the heterozygote, which is one of the weaknesses of this classification
procedure. The animals located in the lower part of the bimodal
distribution are classified as homozygous normal (rn+rn+).
The threshold values for glycolytic potential used to distinguish
between the upper and lower parts of the frequency distribution
vary with population studied and sampling method used. A summary
of threshold values reported in the literature is presented in
|Enfalt et al., 1997|
|Lundstrom & Enfalt, 1997|
|Lundstrom et al., 1996|
|Estrade et al., 1993|
|Fernandez et al., 1992|
|aìmol/g. bMedian. NA = not available|
The determination of GP for genotype prediction requires a muscle
sample. The muscle sample can be taken either from the live animal
or postmortem. Sampling live animals requires biopsy equipment
which has been developed to ensure adequate sample retrieval with
limited injury and discomfort to the animal. Initial techniques
for obtaining skeletal muscle biopsies for meat quality evaluation
were reported by Schmidt et al. (1971) and Sybesma et al. (1972).
The techniques involved the use of "koffler tongs"
to remove samples from the live animal; however, local anaesthesia
was required. Schoberlein (1976) and Lahucky et al. (1980) developed
an improved biopsy technique that reduced injury and eliminated
the need for local anaesthesia. Several studies have utilized
live-animal biopsies to evaluate correlations between muscle metabolites
and pig meat quality (Hennebach et al., 1980; Henneback et al.,
1982; Lahucky et al., 1982; Kovac et al., 1985; Lahucky, 1987).
Talment et al. (1989) were the first to use biopsy samples to
determine GP of muscle. The most widely used approach to obtain
muscle samples for glycolytic potential determination is using
spring-loaded biopsy equipment. The assumption is that this approach
can provide a sample of resting muscle (Fernandez et al., 1992).
However, the time required to obtain the sample and freeze it
in liquid nitrogen may result in degradation of muscle glycogen.
Fernandez et al. (1992) reported that the lactate content of
biopsy samples taken from the live animal using spring-loaded
biopsy equipment was 5 ± 3 umol g-1, which indicates
that limited glycogenolysis occurred during sampling and storage.
Several authors have used longissimus samples taken postmortem
for glycolytic potential determination (Estrade et al., 1993;
Lundstrom et al., 1996; Enfalt et al., 1997; Sutton, 1997). This
method avoids the problems and issues associated with live-animal
sampling and with potential carcass depreciation. However, sampling
postmortem will not produce resting muscle metabolite level.
Pre-slaughter handling is critical in determining muscle metabolite
levels and will influence the accuracy of the assay results.
Several environmental factors are likely to affect muscle glycogen
levels at slaughter including handling procedures at the farm
and during loading, conditions during transport, and handling
procedures in the slaughter plant prior to slaughter. It is important
when sampling a population that these environmental factors are
consistent across all pigs. All of these factors should be closely
monitored and controlled if accurate results are to be obtained;
otherwise, phenotype misclassifications are likely to occur, particularly
in the case of animals with high GP.
Lundstrom and Enfalt (1997) proposed a quicker method to predict
the RN genotype based on a simple and rapid method to measure
glucose levels. Meat juice from slightly thawed longissimus samples
was collected and analyzed using a device for checking blood glucose
levels in diabetic patients (Glucometer Elite, Bayer Diagnostics).
This blood glucose analyzer determines free glucose concentration
within the sample very rapidly, with results being available within
60 seconds. Comparison of RN genotype classification based on
either GP or this rapid method showed that only 4 of 53 samples
were "misclassified" by the glucose test on meat juice
compared to classification based on GP values determined on a
live-animal biopsy (Lundstrom and Enfalt, 1997). These results
suggest that this could be a very rapid, inexpensive method for
phenotyping of animals within a population in which the RN gene
The Occurrence and Frequency of
the Rendement Napole Gene
Several authors have used the bimodal distribution of GP as evidence
of the presence of the dominant allele (RN-) within
populations, and to date this distribution has only been reported
in purebred and crossbred Hampshire populations or in composite
lines with Hampshire inclusion (Monin and Sellier, 1985; Naveau,
1986; Fernandez et al., 1992; Estrade et al., 1993; Sutton, 1997;
Miller, 1998). However, relatively few studies have investigated
muscle GP within other breeds. Talment et al. (1989) and Enfalt
et al. (1994) sampled Hampshire and Yorkshire populations and
showed that GP in Yorkshires was normally distributed. In comparison
with Yorkshires, Hampshires have been shown to have increased
GP, lower ultimate pH, reduced water-holding capacity and Napole
Yield (a laboratory test of curing and processing yields), greater
cooking loss, and higher muscle reflectance scores indicating
paler meat (Monin and Sellier, 1986; Enfalt et al., 1994).
A summary of the literature relating to GP levels in populations
within which the RN gene is segregating is presented in Table
1. Comparisons between Hampshires and Yorkshires (Enfalt et al.,
1994), Hampshires, Large Whites and Pietrains (Monin and Sellier,
1985), and Hampshires, Yorkshires and Swedish Landraces (Essen-Gustavsson
and Fjelkner-Modig, 1992) all reported higher glycolytic potential
in the Hampshire populations with values being approximately 70-80
umol/g greater than in the other breeds. Miller (1998) reported
a difference in glycolytic potential of 92.5 umol/g between American
Hampshires (homozygous RN-RN- and heterozygous
RN-rn+) and American Yorkshires. Yorkshires
also had a higher muscle ultimate pH (+0.13) and muscle protein
content (+1.5%), and lower drip loss (-1.3%) and cooking loss
(-2.8%) than the Hampshires.
Limited gene frequency estimates are available; however, the frequency
of the dominant allele is high in Hampshire populations studied.
Lundstrom, cited by Fernandez and Monin (1994), reported a frequency
for the dominant allele (RN-) of .5 within a Hampshire
population in Sweden. Enfalt et al. (1994) estimated the frequency
at .72 from a population of Swedish Hampshires, and Miller (1998)
reported a frequency of .63 in a population of purebred Hamphires
in the US.
The Effects of the Rendement Napole
Poorer meat quality in Hampshires compared to other breeds has
been described by a number of authors (Sayre et al., 1963; Monin
and Sellier, 1985). These earlier findings have been confirmed
more recently by the National Pork Producers Council "Terminal
Line Program Results" (1995) which showed that Hampshires
had the lowest ultimate pH when compared to a range of other sire
lines and breeds commonly used in the US industry. Several authors
have subsequently characterized the poorer meat quality effects
in Hampshires and attributed them to the RN gene. A summary of
published results on the effects of the RN gene on fresh meat
quality traits is presented in Table 2. Longissimus ultimate
pH has consistently been shown to be lower for homozygous dominant
(RN-RN-) and heterozygotes (RN-rn+)
with differences in ultimate pH ranging from -.12 (Lundstrom et
al., 1996) to -.22 (LeRoy et al., 1996). Only one study has compared
homozygous dominant (RN-RN-) and recessive
(rn+rn+) animals and found a difference
in ultimate pH of -.20 (LeRoy et al., 1996). Lundstrom et al.
(1996) analyzed the relationship between GP and ultimate pH of
the longissimus and reported a phenotypic correlation coefficient
of .86. Miller (1998) reported a phenotypic correlation of .50
between GP and longissimus pH in a population of hybrid commercial
animals with Hampshire inclusion.
|Table 2. Literature summary of breed differences in and the effects of the RN gene on meat quality traits|
|Enfalt et al., 1997|
|Lundstrom et al., 1996|
|LeRoy et al., 1996|
|Enfalt et al., 1994|
|Monin and Sellier, 1985|
|*, **, ***, P<.05, P<.0l, P<.00l, respectively.|
|a Ultimate pH of the longissimus.b Water-holding capacity, %.c Fiber optic probe. d Hunter L* value.e EEL reflectance.|
Sayre et al. (1963) and Monin and Sellier (1985) observed higher
reflectance indicating paler meat for Hampshires compared to Yorkshires.
In addition, a number of studies have shown higher muscle reflectance
and fiber optic probe measurements for homozygous dominant (RN-RN-)
and heterozygous (RN-rn+) animals compared
to those homozygous recessive individuals (rn+rn+;
Table 2). Miller (1998) reported higher Hunter L* values, indicating
paler muscle color, for pigs with glycolytic potentials of greater
than 220 ìmol/g compared
to animals with values below 180 ìmol/g.
The Napole Yield technique
was developed by Naveau (1986) as a laboratory procedure to estimate
the yield of Paris style of processing hams. The technique has
been used as a measure of water holding capacity and processing
yields. In several studies, Napole Yields were lowered for RN-
carrier animals compared to normal genotypes and also for purebred
Hampshires compared to Yorkshires (Table 2). These differences
appear to be relatively consistent across studies (Table 2), and
Napole Yield has been used to classify animals for RN genotype.
A summary of the literature relating to the effects of the RN
gene on eating quality traits is presented in Table 3. Although
fresh meat quality is impaired by the RN gene, some studies have
observed advantages in eating quality for animals with the RN-
allele. A number of studies
|Table 3. Literature summary of breed differences and the effects of the RN gene on eating quality traits|
|Lundstrom et al., 1996|
|LeRoy et al., 1996|
|Enfalt et al., 1994|
|Monin and Sellier, 1985|
|*, **, ***, P<.05, P<.01, P<.001, respectively.|
|a0 or tender, juicy, no off-flavor, non acidic - 10 or tough, dry, off flavor, acidic.|
|b0 tough, dry, off flavor, acidic - 15 tender, juicy no off flavor.|
have observed reductions in Warner-Brazler shear force for RN-
heterozygotes compared to homozygous recessive animals, indicating
more tender meat for this genotype (Table 3; Lundstrom et al.,
1996; Sutton, 1997; Miller, 1998). In addition, improvements
in taste panel tenderness scores (Table 3) for animals carrying
at least one copy of the dominant allele compared to homozygous
recessive individuals were observed in the studies of Miller (1998)
and LeRoy et al. (1996) but not in the study of Lundstrom et al.
(1996). These later authors hypothesized that any tenderness
advantage may be related to the larger amount of sarcoplasm around
the myofibril in RN- animals due to the higher glycogen
content, which may dilute any toughness effect caused by myofibrillar
protein. Enfalt et al. (1994) showed shear force advantages for
purebred Hampshires compared to Yorkshires. Lundstrom et al.
(1996) showed a stronger taste and an increase in acid flavor,
for animals which carry RN- allele. An increase in
acid flavor for heterozygous (RN-rn+) compared
to homozygous recessive animals was also reported by Sutton (1997),
suggesting a potential negative effect on palatability for this
Interestingly, another effect of the gene is to increase cooking
loss, which could negatively impact eating quality (Table 3).
Several studies have reported higher losses during cooking from
RN- animals compared to homozygous recessive pigs in
the range 0.09 to 3.5 percentage units. As discussed above, this
may be the result of a combination of reduced water binding characteristics
resulting from the low ultimate pH and the breakdown of excess
glycogen present in muscle during the cooking process, resulting
in moisture release, and an increase in cooking loss for those
animals which have high residual muscle glycogen.
There have been a limited number of studies that have examined
the effects of the RN gene on growth and carcass characteristics
(Table 4). LeRoy et al. (1996) compared the three possible genotypes,
namely heterozygotes, and the two homozygotes and found that homozygous
dominant (RN-RN-) and heterozygous pigs
grew faster than homozygous recessive animals.
|Table 4. Literature summary of breed differences in and the effects of the RN gene on growth and carcass traits|
|Enfalt et al., 1997|
|LeRoy et al., 1996|
Enfalt et al. (1997) also reported an advantage in average daily
gain of 26 g/day for heterozygotes compared to homozygous recessive
animals. These results suggest a small advantage in average daily
gain (ranging from 10 to 50 g per day) for animals with the
RN- allele. Miller (1998) reported numerical
advantages in growth (+36 g/day) and backfat depth (-1.9 mm) for
animals with high glycolytic potential; however, these differences
were not statistically significant.
Along with possible growth advantages, there is evidence of decreased
backfat depth for RN- homozygotes and heterozygotes
compared to rn+ homozygotes; however, the differences
are relatively small (Table 4). LeRoy et al. (1996) reported
decreases in backfat depth of 1.3 and 2.1 mm for heterozygotes
(RN-rn+) and homozygous dominant (RN-RN-)
animals, respectively, compared to homozygous recessive pigs (rn+rn+).
Enfalt et al. (1997) also reported a trend for decreased backfat
(-1.2 mm) and increased carcass lean percentage (+1.0%) for RN-
homozgyotes and heterozygotes compared to rn+ homozygotes.
Miller (1998) reported a small advantage in gain:feed ratio for
pigs with high compared to low glycolytic potential.
The RN gene can be detected within a population based on the glycolytic
potential of the pigs measured either on biopsy samples taken
from the live animal or on postmortem muscle samples. If the
RN gene is segregating within the population, a bimodal frequency
distribution for glycolytic potential is observed. The breakpoint
between the two distributions is used as the threshold value to
determine the predicted genotype of the individual pigs. A number
of studies have investigated the effects of the gene on fresh
meat and eating quality. Decreased ultimate pH and water-holding
capacity and paler muscle color have been reported by several
authors; some studies have indicated tenderness advantages for
animals carrying the dominant allele. Data on the effects of
the gene on on-farm performance and carcass characteristics are
limited; however, a small advantage for RN- animals
in terms of average daily gain, decreased backfat depths, and
increased carcass lean content has been observed.
Brooks, A.B., Fahey, T.D., and White, T.P. 1996.
Exercise Physiology: Human Bioenergetics and Its Applications.
Mayfield Publishing Co., Mountain View, CA.
Enfalt, A.C., Lundstrom, K., Lundkvist, L., Karisson,
A., and Hansson, I. 1994. Technological meat quality and the
frequency of the RN-gene in purebred Swedish Hampshire and Yorkshire
pigs. 40th IcoMST, The Hague, Netherlands.
Enfalt, A.C., Lundstrom, K., Hansson, I., Johansen,
S., and Nystrom, P. 1997. Comparison of non-carrier and heterozygous
and heterozygous carriers of the RN- allele for carcass
composition, muscle distribution and technological meat quality
in Hampshire-sired pigs. Livestock Prod. Sci. 47:221.
Essen-Gustavsson, B., Karlstrom, K., and Lundstrom,
K. 1992. Muscle fiber characteristics and metabolic response
at slaughter in pigs of different halothane genotypes and their
relation to meat quality. Meat Sci. 3l:l.
Estrade, M., Vignon, X., and Monin, G. 1993. Effect
of the RN- gene on ultrastructure and protein fractions
in pig muscle. Meat Sci. 35:313.
Fernandez, X., Tornberg, E., Naveau, J., Talmant,
A., and Monin, G. 1992. Bimodal distribution of the muscle glycolytic
potential in French and Swedish populations of Hampshire crossbred
pigs. J. Sci. Food Agric. 59:307.
Fernandez, X., and Monin, G. 1994. A major gene
affecting pork quality: the RN gene. Meat Focus, 332.
Hennebach, H., Albrecht, V., and von Lengerken, G.
1980. Investigation for prediction of meat quality by use of
shot biopsy in live pigs. Archiv. Tierzucht, Berlin. 23:183.
Hennebach, H., and von Lengerken, G. 1982. Investigation
for prediction of meat quality by use of shot biopsy in live pig.
Archiv Tierzucht, Berlin. 25:131.
Kovac, L., Sidor, V., Mlynek, J., Vavrisinova, K.,
and Lahucky, R. 1985. Evaluation of pork meat quality by use
of biopsy method in relation to other premortal diagnostic methods
and possibility of its practical use. 4th Int. Symposium. Rational
Prod. and meat quality under the Large Scale Prod.Conditions,
Nitra, Czechoslovakia, September 24-26th.
Lahucky, R., Rajtar, V., Kovac, L., and Sidor, V.
1980. Investigation of biochemical values in muscle tissue by
biopsy assays in pigs. Scientific Symposium on Early Performance
Information in Pigs, Leipzig, GDR, November 5, p. 79.
Lahucky, R. 1987. Recent findings using the muscle
shot biopsy to evaluate meat quality in pigs. Pig News and
LeRoy, P., Naveau, J., Elsen, J.M., and Sellier,
P. 1990. Evidence for a new major gene influencing meat quality
in pigs. Genet. Res. Camb. 55:33.
LeRoy, P., Monin, G., Elsen, J.M., Cartiez, J.C.,
Talmant, A., Lebret, B., LeFaucheur, L., Mourot, J, Juin, H.,
and Sellier, P. 1996. Effect of the RN genotype on growth and
carcass traits in pigs. 47th EAAP, Lillehammer, Norway.
Lundstrom, K., Anderson, A., and Hansson, A. 1996.
Effect of the RN gene on technological and sensory meat quality
in crossbred pigs with Hampshire as terminal sire. Meat Sci.
Lundstrom, K., and Enfalt, A.C. 1997. Rapid prediction
of RN phenotypic in pigs by means of meat juice. Meat Sci.
Miller, K.D. 1998. The detection and characterization
of pigs with differing glycolytic potential levels within United
States Swine populations. Ph.D. Thesis, University of Illinois,
Monin, G., and Sellier, P. 1985. Pork of low technological
quality with a normal rate of muscle pH fall in the immediate
postmortem period: The case of the Hampshire breed. Meat Sci.
Naveau, J. 1986. Contribution à l'étude
du determinisme genetique de la qualité de la viande porcine.
Heritabilité du rendement technologique Napole. J.
Rech. Porcine en France 18:265.
NPPC. 1995. Genetic Evaluation: Terminal Line Program
Results. National Pork Production Council, Des Moines, IA.
Sayre, R.N., Briskey, E.J., and Hoekstra, W.G. 1963.
Comparison of muscle characteristics and postmortem glycolysis
in three breeds of swine. J. Anim. Sci. 22:1012.
Schmidt, G.R., Zuidam, L., and Sybesma, W. 1971.
Biopsy technique and analysis for predicting pork quality. J.
Anim. Sci. 34:25.
Shoberlein, L. 1976. The shot biopsy - a new method
of sampling the muscle tissue. J. Anim. Sci. 32:457.
Sutton, D.S. 1997. The meat quality and processing
characteristics of RN carrier and non-carrier pigs. Ph.D. Thesis,
University of Illinois, Urbana, IL.
Sybesma, W., Minkema, D., van der Wal, P.G., and
Walstra, P. 1972. Muscle biopsy analysis and meat quality.
23rd Ann. Mtg. European Assoc. Anim. Prod., Verona, Italy, October
Talment, A., Fernandez, X., Sellier, P., and Monin,
G. 1989. Glycolytic potential in longissimus dorsi muscle of
Large White pigs as measured after in vivo sampling. Proc. 35th
ICoMST, Copenhagen, Denmark, pp 1129.