Genetics of Pork Quality
Allan P. Schinckel
Department of Animal Sciences, Purdue University
There has been a shift in consumer attitudes towards their purchases of meat products.
Greater emphasis is being placed on palatability characteristics such as tenderness, juiciness, and
flavor versus issues such as cost, food safety or leanness. The food industry is becoming more
consumer driven which is driving segmentation of the market to specific products. For example,
upper scale restaurants and hotels demand consistent pork products with above average quality in
terms of tenderness and flavor. The export market has above average requirements in terms of
color and marbling. To be able to produce pork products to meet specific market demands, we
must identify and select specific genetic populations. It is also important that the quality of our
U.S. fresh pork be competitive with alternative meat products. Otherwise, the pork consumption
share of the domestic market will decrease.
Pork Quality
Pork quality is a general term including a combinations of traits that provide for a
wholesome product, attractive in appearance and is appetizing, nutritious, and palatable to eat
(Kauffman et al., 1990). To be consistently evaluated and interpreted, the best measures of pork
quality are quantitative and repeatable (day to day, lab to lab, etc.). The seven sets of pork
quality measurements are: (1) color; (2) firmness; (3) water holding capacity (drip loss); (4)
ultimate pH; (5) marbling - intramuscular fat; (6) tenderness - shear force; and (7) palatability
traits such as taste panel flavor or juiciness.
1
Genetic Variation and Relationships
The heritability of many of these pork quality measurements have been estimated.
Intramuscular fat, like measures of carcass composition are highly heritable (.40 to .50, Table 1).
Ultimate pH, Hunter L scores and tenderness are moderately heritable. In each case, subjective
visual scores (color-marbling) have a slightly lower heritability than the objective quantitative
measurement (Hunter L, Japanese color score).
The measures of pork quality are genetically correlated (Table 2). Ultimate (45h) pH is
highly correlated with drip loss, color score and L*. Thus, lower pHu is associated with light
colored loin muscle. Also, pHu is negatively correlated with percent lean (-.37) and positively
correlated with backfat depth (.21). It is important to realize that the proportion of the total
genetic variation in one trait can be explained by the variation in another as the R2 coefficient of
variation. Based on this relationship, 40-60% of the genetic variation in drip loss and color can
be accounted for by 24h pH. The relationships between percent lean and pork quality are only
weak to moderate in magnitude. Thus, selection for improved pork quality and percent lean is
possible.
Impact of Lean European Lines
In the spring and fall of 1995, two large swine lean growth trials were conducted at Purdue
University. Participants had the choice of providing pigs in sets of either 24 or 32 per genotype
and sex. In the spring of 1995, the terminal cross pigs of four new genetic sources were
evaluated with four U.S. terminal crosses (2-Hampshire-Duroc by Yorkshire-Landrace, 1-Duroc
by Yorkshire-Landrace and 1-Hampshire by Yorkshire-Landrace). The European pigs had been
selected for lean efficiency and contained a higher percent Large-White. The European terminal
cross pigs had lower feed intake (13.7%) and had lower carcass fat gain (28.7%; Table 3). The
2
lean European lines required 21.2% less feed per unit fat-free lean gain from 60-250 lbs live
weight. Assuming grow-finish feed costs represent 48% of total costs and other costs were the
same, the lean European lines had about 10% lower cost per lb than that of the 1995 U.S.
terminal cross. However, the lean European lines had lower color scores, firmness scores and
marbling scores (Table 4). During the mid 1990's U.S. purebred producers did three things to
genetically reduce backfat thickness and increase carcass lean percentage. First, real-time
ultrasound use increased which was more accurate than previous A-mode ultrasound. Second,
selection pressure was placed on pigs that was reflected by substantial STAGES genetic trends
for reduced backfat thickness and increased pounds of lean. Third, lean sires from Europe,
Sweden, Britain, and Denmark were incorporated in the U.S. purebred populations.
In 1998, a genotype by environment interaction trial was completed. The European
terminal cross was the top European Hampshire Duroc line mated to one of the leanest European
maternal lines based on previous Purdue and NPPC trials. The U. S. Duroc × YorkshireLandrace pigs were produced by sampling numerous Duroc sires from four sources mated to
average U.S. purebred derived Yorkshire-Landrace females. The trial included 288 pigs reared
in two health environments. The differences between the European and U.S. pigs have been
substantially reduced (Tables 5, 6, and 7) for all performance, carcass measurements, and
measures of pork quality. Selection for leanness and incorporation of lean European lines into
the U.S. purebred populations has produced pigs more similar to the European terminal crosses.
Genetic by environmental interactions were significant for measures of growth rate, feed intake
tenderness and juiciness.
3
Breed Differences
Research has found substantial genetic variation amongst different genetic populations for pork
quality. Berkshires, Chester Whites and Durocs have better pork quality than Hampshires,
Landrace and Yorkshires (Table 8). Increasing the percentage of the three higher pork quality
breeds into terminal crosses will improve pork quality.
Major Genes
Two major genes have been identified and characterized that impact pork quality. The
effects of the porcine stress gene (PSS) on carcass composition and quality are well documented
(Tables 9 and 10). More than a decade ago, PSS was described as acting as a complete recessive
possibly because the results of halophane tests (positive stiff as a board or no response) and
measures of muscle damage (CPK levels). However, data in the 1990's demonstrated that carrier
pigs had lower pork quality and a substantially higher probability of PSE pork. Based on these
results the vast majority of seedstock producers have estimated the stress gene through DNA
testing.
The other major gene is called the Napole gene. The Napole gene is currently considered
to act as a completely dominate gene. In the past, pigs carrying one or two copies of the Napole
gene were identified by having substantially highly glycolytic potential (GP) where (GP) = 2
(glycogen + glucose + glucose-6-phosphate + lactate). In the fall of 2000, the DNA test for the
Napole gene was made commercially available. The DNA test will allow better research into the
effects of the Napole gene. The Napole gene in Hampshires results in lower ultimate pH,
increased drip loss and cooking loss (Tables 11 and 12). However, the shear force was lower for
the Napole pigs. It is interesting to note that the Napole gene changes in 24h Ph vary from trial
to trial.
4
Issues to be Dealt With
The field of genetics makes a number of underlying assumptions. Sometimes these
assumptions are not totally correct. Within line selection for pork quality assumes that the
improvement realized in the purebreds will be realized in the crossbreds. Also, it assumes that
the genetic improvement realized under one set of conditions (processing methods, nutrition,
post-slaughter handling, rate of chill) will be realized under different conditions.
One trial examining these concepts was recently completed at the University of
Tennesses (Van Laack et al., 2001). In their study, Berkshire, Duroc, and Hampshire sires were
mated to Yorkshire-Landrace F1 females. Half of the pigs were within 2h of transport and the
other half withheld from feed 14h prior to transport. The means for the sire line and feed
treatments are shown in Table 13. Within the Duroc breed, a significant negative correlation (.42) existed between intramuscular fat percentage and shear force. No significant relationship
existed between shear force and intramuscular fat in the Berkshire or Hampshire sired pigs. In
Hampshires there was a positive correlation between ultimate pH and shear force and negative
relationships in the Duroc and Berkshire sired pigs. The 14h fast increased the ultimate pH of
the Duroc and Berkshire sired pigs; but, had no impact on the Hampshire sired Napole carrier
pigs. A significant sire line by storage time (2, 7 or 14d) interaction was found for shear force
(Table 14). This data shows that "general" relationships amongst the pork quality measurement
may vary amongst different genetic populations (pure lines or crosses). The differences in
quality between genetic populations may vary somewhat depending on the handling, storage and
processing of the pork products.
Further research needs to be conducted to evaluate the consistency of the genetic and
phenotypic correlations when estimated within line and in crosses. For example, if purebred
5
Durocs are a breed likely to be selected as a high quality terminal sire, then a trial evaluating the
genetic and phenotypic parameters of the pork quality and carcass composition variables should
be completed within the purebred Durocs and when the sires are used to produce crossbreds.
Then, the Duroc breeder must ask how do the Duroc sired pigs need to be improved to better
meet the specifications of the identified value added market. The traits measured and selected
for must move the genetics of the Durocs such that the crossbreds achieve the specifications set
by the purchaser of their commercial customers' pigs.
Summary
As pork quality specifications are established, commercial producers will have to decide
which market will they are producing pigs for. If it is for a high quality value, added market, the
pigs will have to contain a higher percentage of lines superior in pork quality. Seedstock
producers will have to determine which market or markets their breeding stock is going to fit.
The selection must be geared towards their purelines producing terminal cross pigs that meet
established pork quality specifications.
Literature Cited
Kauffman, R. G., W. Sybesma and G. Eikelenboom. 1990. In Search of Quality. J. Can. Inst.
Food Sci. Technol. Vol. 23, No. 4/5.
van Laack, R. L. J. M., S. G. Stevens, and K. J. Stalder. 2001. The influence of ultimate pH and
intramuscular fat content on pork tenderness and tenderization. J. Anim. Sci. 79:392-397.
6
Table 1. Heritability of Pork Quality Traits and Performance.
Pork Quality
h2
pH - 1 hr
.19
pH - 24 hr
.19
Hunter L*
.31
a*
.47
b*
.42
Color score-Japan
.22
Drip loss
.10
Intramuscular fat
.44
Marbling score
.24
Eating Quality
Tenderness, kg
.26
Tenderness/sensory
.29
Flavor
.09
Juiciness
.08
Overall acceptability
.25
Eating Quality
Tenderness, kg
.26
Tenderness/sensory
.29
Flavor
.09
Juiciness
.08
Overall acceptability
.25
Copenhafer Literature Review, 2001.
Table 2. Genetic Correlations
pH 24
Drip
Color score-Japan
IMF
Marb Score
% Lean
Drip
-.71
Color score
Japan
-.62
-.41
IMF
-.20
-.21
n/a
Marb
Score
.26
n/a
.25
n/a
%
Lean
-.37
n/a
-.15
-.12
.02
BF
.21
n/a
-.02
n/a
.31
-.77
L*
-.80
n/a
-.99
.02
-.23
-.18
Copenhafer Literature Review, 2001.
7
Table 3. 1995 Spring Lean Growth Trail
Performance Trait
European
US
% Difference
ADG, lbs/d
2.02
2.11
-4.3
ADFI, lbs
4.41
5.34
-13.7
Live Wt. F/G
2.28
2.53
-9.9
Fat Depth, in2
.71
1.18
-39.1
Fat-Free lean gain, lbs/d
.74
.67
+10.5
Carcass Fat gain g/d
.45
.63
-28.7
6.23
7.91
-21.2
Lean Feed Conversion
(lbs Feed/lbs F.F. Lean)
Test Period 60-250 lbs
Mean of three European terminal crosses and four U.S. terminal crosses
with either 24 or 32 pigs per genetic cross and sex.
Table 4. 1995 Spring Lean Growth Trial Pork Quality Traits
Pork Quality
Lean European
U.S.
Color
2.22
2.84
Firmness
1.71
2.90
Marbling
2.00
2.74
Table 5. Genetic by Environmental Interactions for Growth Rate and Feed Intake (GxE 3)
Conv. Weaning-Cont
Flow
SEW Three Site
EUR
D-YL
EUR
D-YL
Sign.
30-51 d
ADG, lb/d
.72
.90
1.09
1.03
G**E**, GxE**
ADFI, lb/d
.99
1.16
1.43
1.51
G**GxS*
51 d - 250lb
ADG, lb/d
1.69
1.81
1.87
1.87
G**E**GxE*T**ExT*
*
ADFI, lb/d
4.60
5.04
5.04
5.22
G**E***GxE*T**ExT
*
Gain/Feed
.367
.361
.373
.359
Days to 250lb 181.3
171.8
163.7
164.8
G*E**GxE**T**S**
8
Table 6. LSMEANS for the Carcass Traits (G x E 3)
Conv. WeaningCont. Flow
SEW Three Site
EUR
D-YL
EUR
D-YL
FD 10 Rib in
.74
.83
.80
.86
2
Loin Eye Area, in
7.30
7.15
7.48
7.38
% Lean
52.1
50.8
51.7
50.6
Color
2.13
2.00
2.26
2.16
Marbling
1.56
1.76
1.59
1.65
Firmness
2.15
2.29
2.20
2.22
Table 7. LS means for Pork Quality Traits (GxE 3)
Conv. WeaningCont. Flow
SEW Three Site
EUR
D-YL
EUR
D-YL
Tenderness
6.79
6.81
7.00
5.65
Juiciness
6.42
6.22
6.94
5.33
Shear force
5.47
5.64
5.03
5.67
Cooking Loss, %
32.8
28.7
31.2
29.3
IMF, g
1.57
1.94
1.95
2.22
Ph24h
Drop loss 24h
Hunter L*
5.55
3.62
58.1
5.64
1.43
57.2
Table 8. Breed Differences - NBS®
Growth
ADG
BF10
lb/day
inches
Berkshire
1.70
1.18
Chester
1.64
1.24
White
Duroc
1.78
1.02
Hampshire
1.64
.91
Landrace
1.72
1.02
Yorkshire
1.69
1.00
5.43
3.74
57.9
5.53
2.39
57.2
Carcass
LMA Minolta
sq in
reflect
5.39
22.9
5.59
23.3
5.76
6.49
5.97
5.89
24.6
22.5
27.6
24.7
Sign
G**,GxT*,S*E*
E,S***, GxExS**
G**,S**
E*, S**
G*,S**
S*,ExS*
Sign
G*,GxE**, GxT*
G***, GxE**, GxT*
G**, GxExT*
G***, ExS*
G**,E**,S**
G**E**
G*** GxS1
E, GxS*
Meat Quality
24h pH
IMF
pH
%
5.84
3.24
5.82
2.93
5.72
5.54
5.61
5.68
3.97
2.35
2.31
2.24
9
Table 9. Means for Stress Carriers versus Non-Carriers
Nn
NN
Sign
LS Means
ADG
2.15
2.13
NS
Gain:Feed
.36
.33
.01
Dressing Percent
75.3
74.4
.001
10th Rib Fat Depth
.94
1.02
NS
2
LEA, in
6.64
6.43
NS
Ham Lean
15.21
14.55
.01
Pork Quality Means
45 min pH
6.4
6.6
.001
24h pH
5.6
5.7
.01
Color
2.2
2.7
.001
Firmness
2.2
2.9
.001
Minolta L*
45.7
42.0
.001
a*
9.5
9.2
NS
b*
7.3
5.8
.001
Drip Loss
5.2
3.4
.001
Cooking Loss
27.1
25.6
NS
Lipid Content
2.0
2.3
NS
144 stress carrier and normal
242, 275, and 309 lbs
Leach et al., 2000.
University of IL
Table 10. NBS-Stress Gene Data
Trait
ADG
LENGTH
BF10
LMA
MINOLTA
Phu
IMF
Nn-NN
-0.2 lb/day
0 inches
-.05 inches
.37 sq in
2.2
-.01
-.34%
Quality effect
of one n gene
None
None
Better
Better
Poorer
None
Poorer
Table 11. Effects of Napole in Hampshire Sired Pigs.
Glycolytic Potential
RN (198)
rn rn (32)
Biopsy
238.5a
145.7b
a
Post Mortem
224.7
165.5b
Phu
5.42a
5.52b
Drip Loss
5.87
4.32
Cooking Loss
25.5a
21.2b
Shear Force, kg
2.85
2.99
(Miller et al., 1998. University of Illinois)
10
Table 12. U. S. Research on the Napole Gene
Napole
Napole
Sign
Carrier
Free
N
56
55
Dressing %
73.8
73.6
Backfat
.93
.89
Ph 24th
5.26
5.50
.001
Color
1.90
2.24
.05
Firmness
2.26
2.00
Marbling
1.54
1.78
Minolta L*
50.3
54.5
.001
Minolta a*
8.71
8.75
Minolta b*
9.13
Drip Loss
7.02
WB-shear, kg
1.54
(Hamilton et al., 2000. Univ. of Illinois)
7.28
4.67
1.78
.001
.001
.05
Table 13. Effect of Sire Breed on Ultimate pH.
Sire
N
Ultimate pH
Berkshire
60
5.61
Duroc
54
5.63
Hampshire
82
5.51
RN__
30
5.41
rn rn
32
5.60
Feed
Unfed
Fed
88
83
5.62
5.55
Van Laack, Stevens, Stalder, 2001
Table 14. Warner-Bratzler shear force (kg) of pork longissimus muscle
as affected by sire genetic type and storage time (LS means)
Storage time, d
Sire genetic type
2
7
14
Berkshire
4.95av
4.28wx
3.57yz
Duroc
5.18v
4.19wx
3.58yz
w
x
Hampshire
4.53
4.01
3.39yz
v
wx
Non-RN carrier
4.87
4.14
3.56yz
RN carrier
4.24w
3.95xy
3.27z
a
Standard error is 0.2.
Van Laack et al., 2001
11