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Research Direction generate
Branch de la recherche
Technical Bulletin 1 988-1 1 E
a technical review
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Die 5 lis Ml
Tf ■ 6 1989 >
STATIul :hes CT
LEThbrioge rA. P
CANADA AC fURC
a technical review
S.D.M. JONES, A.C. MURRAY, A.P. SATHER,
L.E. JEREMIAH, and G.G. GREER
Technical Bulletin 1988-1 IE
Lacombe Technical Bulletin No. 2
Copies of this publication are available from
Research Branch, Agriculture Canada
Bag Service 5000
Produced by Research Program Service
©Minister of Supply and Services Canada 1988
Cat. No.: A54-8/1 988-1 IE
Egalement disponible en frangais sous le titre
La qualhe du pore: revue technique
The dots on the map represent Agriculture
Canada research establishments.
1. Identification of PSE/DFD Pork 3
A. Definition of pork muscle quality
B. Subjective quality standards for pork
C. Objective measures of pork muscle quality
D. Prospects for the industrial indentif ication of PSE/DFD pork
2. Processing considerations regarding PSE/DFD pork 9
A. Processing problems and meat quality
B. Muscle quality and processing yield
3. PSE/DFD pork and retail case life 13
4. Palatability and PSE/DFD pork 15
A. Pork quality and palatability
B. Texture profiles
C. Flavour profiles
5. The genetics of porcine stress susceptibility 20
A. Stress susceptibility in man and other animals
B. The genetics of stress susceptibility in the pig
C. Relationship of genetic stress susceptibility with reproductive traits
D. Relationship of genetic stress susceptibility with performance traits
E. Relationship of genetic stress susceptibility with carcass yield and
F. Distribution in pig populations
G. Effects of halothane testing
H. Selection Strategies
6. The detection of and testing for stress susceptibility 31
A. Genetic markers
B. Physiological markers
7. Ante-mortem influences on pork quality 35
A. Swine deaths
B. Carcass bruising
C. Carcass weight loss
8. Post-mortem influences on pork quality 42
A. PSE pork
B. DFD pork
D. Side of shackling
F. Rate of carcass cooling
G. Electrical stimulation
9. Executive Summary 48
10. Acknowledgements 49
11. Bibliography 50
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Canadian pork production has shown a substantial increase during the last
decade and in 1986 approximately 15 million market pigs will have been
produced in Canada. Approximately 30% of the pork produced in Canada is
exported mainly to the USA and Japan and this trade has an annual value in
excess of $800 million. In all the major pork producing countries of the
world there have been reports which indicate that pork quality appears to have
declined. The major problem is the apparent increased frequency of pale, soft
exudative pork (PSE) and to a lesser degree dark, firm and dry (DFD) pork. In
1981, a Work Planning Meeting on PSE and DFD pork organized by Agriculture
Canada was held in Ottawa on April 27-28. One of the non-research
recommendations resulting from this meeting was the preparation of a technical
literature review on PSE/DFD pork. At that time the amount of Canadian
information was extremely limited and during the last 5 years a considerable
amount of applied work has been completed. In 1986, the Lacombe Research
Station at the request of the Industry/Government Committee on PSE/DFD pork
agreed to complete a technical review.
Chapter 1 examines the identification of PSE/DFD pork. National subjective
pork quality standards have been developed in Canada and their use and
relationship with objective measurements of muscle quality is explained. New
methods which have the potential to measure PSE/DFD at different times
post-mortem are also reviewed. Chapter 2 considers the processing problems
which arise when pork from different quality groups is processed. The yield
figures presented can be used by industry to estimate the financial
implications of processing PSE and DFD pork. The retail case life of PSE/DFD
pork is reviewed in Chapter 3 and its palatability in Chapter A. The
remainder of the bulletin reviews the current knowledge on the main factors
responsible for causing increased frequences of poor quality pork. Chapter 5
deals with the genetics of stress susceptibility which is an area of intensive
research particularly in Europe. The reader may find some of the information
difficult to understand but further simplification would lead to losses in
content. The methods that have been used to identify stress susceptible pigs
other than halothane testing are covered in Chapter 6 and special reference is
made to the identification of the stress susceptibility gene (halothane) in
carrier pigs. Chapter 7 deals with the factors between the farm gate and the
abattoir prior to slaughter which have been shown to influence pork quality.
Post-mortem factors which have a bearing on muscle quality are covered in
This bulletin was not intended as a complete documentation of all the
scientific reports in the literature. We have attempted to highlight the most
important studies and the references should be consulted for further
information, since their interpretation in most cases is our own and we would
be the first to acknowledge that the events leading to aberrant pork quality
are far from understood.
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In Canada the frequency of PSE and DFD pork is variable but for both conditions
is estimated to range between 10-30% (Murray 1986) depending on the season of
the year, and to cost the industry in excess of $20 million per annum. The
objective of this review was to highlight the problems of aberrant muscle
quality and to consider the main causal factors leading to a high frequency of
Head, Red Meats and Beef Production
Lacombe Research Station
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1. IDENTIFICATION OF PSE/DFD PORK
A.C. Murray and S.D.M. Jones
A. Definition of pork muscle quality
The characteristics of fresh uncooked pork muscle which are considered to
be major determinants of quality include: color, textural appearance and
water holding capacity.
Quality may have different meanings to different people. Color, textural
appearance and water holding capacity (drip) are obviously of importance
to the consumer. Color and textural appearance are of importance for the
export market. Water holding capacity is of importance to the processor
because of its great influence on the curing yield. Water holding
capacity is also of importance to the pork producer and the processor
because of its relationship to carcass weight losses. Color is of
importance to the retailer, not only as it relates to consumer acceptance,
but also because of the relationship of color to retail shelf life.
Textural appearance is highly related to water holding capacity. Therefore
measures of color and textural appearance and/or water holding capacity
are the minimum requirements to characterize lean pork quality.
B. Subjective quality standards for pork
The simplest method for the characterization of lean pork quality is that
of subjective evaluation. The first system for the subjective scoring of
pork lean quality was developed at the University of Wisconsin in 1963.
It made use of a 5 point scoring system, with scores ranging from 1 -
extremely Pale, Soft, Exudative (PSE) to 5 - extremely Dark, Firm, Dry
(DFD) with 3 being normal pork. Agriculture Canada Pork Quality Standards
released in 1984 have improved considerably upon the Wisconsin system.
The Agriculture Canada bulletin is illustrated in color and describes a
two part subjective scoring system, one part for color and one part for
structure, as indicated below:
Agriculture Canada Pork Quality Standardst
1 - Extremely Pale
2 - Pale
3 - Normal
4 - Dark
5 - Extremely Dark
1 - Extremely Soft, Exudative
2 - Soft, Exudative
3 - Normal
4 - Firm, Dry
5 - Extremely Firm, Dry
t Agriculture Canada 1984a,
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Soft, exudative pork is not always pale. Conversely pale pork is not
always soft and exudative. These standards allow color to vary
independently from softness and exudativeness and have been particularly
useful in identifying a commonly occurring quality type with essentially
normal color but soft and exudative structure. Quality of this type has
also been identified by other researchers (Monin and Sellier, 1985).
C. Objective measures of pork muscle quality.
Pork lean muscle quality has also been measured objectively or
instrumentally. Such methods/devices have been most commonly used within
about 1 hour or at about 24 hour after slaughter to estimate or predict
the ultimate quality. A number of these are listed below:
1. Color/Reflectance. Two basic configurations of reflectance meters
have been developed. One measures at the meat surface, the other at
the interior of the muscle. Reflectance meters have been shown to be
of value for the measure of muscle color at times equal to and
exceeding 24 hr post slaughter. Table 1.1 shows the relationship
between subjective Agriculture Canada pork quality scores and the
readings of one surface-measuring reflectance meter (Minolta Chroma
Meter II) for the longissimus dorsi muscle.
Table 1.1. The relationship of subjective pork quality to
Minolta reflectance meter L, a and b values (CLE.)
for the longissimus dorsi muscle.
Score t Mean SD Mean SD Mean SD
t Color/Structure Score 1-4,
Agriculture Canada Pork Quality Standards.
Murray and Nemeth 1986
Although deep muscle reflectance meters (Fat-O-Meater , Fiber Optic
Probe [MRI, Bristol], Destron and Hennessy) show some promise for the
prediction of ultimate quality from measurements within one hour after
slaughter, these instruments have not yet been shown to be accurate
enough for prediction of final color from measurements soon after
slaughter (Jones et al , 1984; Somers et al. 1985). Fortin and Raymond
(1987) recently concluded that electronic grading probes were
unsatisfactory for detecting PSE/DFD pork at any stage post-mortem.
- 5 -
Because of the potential utility of such meters, their development and
testing should continue. However, Swatland (1986a) found that
internal reflectance of ham muscles measured with a portable fibre
optic spectrophotometer (Colormet, Instrumar Ltd, St. John's,
Newfoundland) at 24 h post-mortem could distinguish between normal and
2. pH. Table 1.2 indicates the expected pH values for various quality
types of loin eye (longissimus dorsi muscle) at two times
post-slaughter. The pH at 1 h has some potential to distinguish PSE
pork from the other quality types while the pH at 24 h has the
potential to distinguish DFD from other quality types. Because pH is
difficult to measure in a reproducible fashion, and at 1 hour is
markedly influenced by amount of available energy in the muscle, this
method is not always a good predictor of ultimate muscle quality.
Additionally, Table 1.2 shows there is considerable overlap in pH at I
h for the different pork muscle quality groups. However, this method
has been adopted in one country (Switzerland) for the estimation of
pork quality on the slaughter floor and is a factor in settlement of
Table 1.2. The relationship between time post-slaughter and pH
for longissimus dorsi muscles of different quality.
Color/Structure Score 1-4, Agriculture Canada Pork Quality
Murray and Nemeth 1986
3. Drip/water holding capacity. PSE pork has a lower and DFD has a
higher water holding capacity than does pork of normal quality.
Essentially, all techniques for indirectly determining water holding
capacity have been reviewed by Kauffman et al. (1986). These methods
included, among others, measurement of: 1. The amount of drip from a
standard sized muscle sample, 2. The weight of liquid expressed during
centr if ugation, 3. The amount of swelling of a ground meat sample, 4.
The amount of liquid expressed onto filter paper by pressure causing
devices, 5. The amount of moisture absorbed by a piece of filter paper
from a meat surface. All methods proved useful, although methods 1,
3, and 5 proved to be most precise in distinguishing all quality
types. Table 1.3 presents an example of data from two commonly used
methods for estimating water holding capacity. These procedures are
used at times equal to or greater than 24 hr post-slaughter.
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Table 1.3. The relationship between pork quality and measures of
water holding capacity for the longissimus dorsi
t Color/Structure Scores 1-4,
Agriculture Canada Pork Quality Standards.
Murray and Nemeth 1986
Murray et al. 1987
4. Protein Solubility. The denaturation of muscle proteins is at the
heart of the PSE pork quality problem. Two commonly used methods
(Table 1.4) which indirectly assess the processing value of meat are
based on protein solubility: % transmission (Hart 1962) and
salt-soluble protein (Barton-Gade 1981). The % transmission method
measures the turbidity of muscle sarcoplasmic proteins at a standard
pH (Table 1.4). The salt-soluble protein method measures the extent
of solubility of muscle proteins in a standard salt solution. Both
are very acceptable indicators of pork muscle quality and are used at
times equal to or exceeding 24 hr post-slaughter.
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Table 1.4. The relationship between pork quality and measures of
protein solubility transmission for the longissimus
Color/Structure % Transmission Protein Solubility
Score t (g/100 g wet wt)
Mean SD Mean SD
t Color/Structure Scores 1-4,
Agriculture Canada Pork. Quality Standards.
Murray and Nemeth 1986
5. Electrical Properties of Muscle. The electrical capacitance or
ability of a muscle to store electricity declines post-slaughter, the
rate of decline being faster in muscle destined to become PSE than in
normal meat. This phenomenon has been utilized for the prediction of
pork quality (Swatland, 1982). Although the capacitance of the
adductor has been shown to be a better predictor of ultimate quality
than is the pH at 1.5 hr post mortem, it has yet to be proven accurate
enough for predictive purposes. Other meters based on changes in the
dielectric properties of muscle (MS Tester (Testron), QM Meter) have
been developed in Europe, but have not been completely evaluated for
use in Canada.
6. ATP Breakdown. This method is based on the change in the optical
properties of ATP (adenosine triphosphate) as it is broken down to
provide energy in the muscle post-slaughter (Honikel and Fischer
19 77). Although the method was designed to be used in combination
with pH measurement at 1 hour after slaughter, it is technically very
cumbersome since it requires the removal, homogenization and filtering
of a meat sample, followed by measurement of absorbance on a
spectrophotometer. Therefore it is unlikely to find a use in industry.
7. Onset of Rigor. When the muscle ATP concentration reaches a certain
low level after slaughter, rigor mortis ensues. Muscle destined to
become PSE is much more active immediately after slaughter, and, as a
result, it goes into rigor sooner than normal muscle. For example, in
stress susceptible pigs rigor can be attained on the slaughter floor,
whereas in normal pigs the onset of rigor is usually between 4 and 6
hours post-mortem. The stiffness/flexibility and orientation of the
front limb (Davis et al. 1978) have been used as indicators of rigor.
Attempts to use this phenomenon to predict ultimate pork quality have
not been entirely successful, primarily because of lack of a practical
- 8 -
on-carcass method to obtain a meaningful monitoring of the rigor
process. This limitation may be overcome by extension of methodology
such as that described by Swatland (1987). However, the rate of onset
of rigor has not yet been shown to be intimately related with ultimate
8. Nuclear Magnetic Resonance (NMR). The nuclear magnetic resonance
technique allows a more direct measure of the water binding status of
muscle tissue. NMR parameters have been found to be correlated with a
number of meat quality measures (Renou et al , 1985). This application
of NMR measurements to the prediction of pork quality is very new and
further evaluation is required.
D. Prospects for the industrial identification of PSE/DFD pork.
Instruments with the capability of recording deep muscle reflectance/light
scatter (Fibre optic probe - Bristol, Colormet - Newfoundland) appear to
have good potential for the measurement of pork quality in intact
carcasses 24 hours post-mortem. These instruments could be used in
coolers by industry to sort carcasses into different quality groups. The
industrial prediction of pork muscle quality by measurements made on the
warm carcass soon after slaughter is not possible with current
methodology. Some of the technical problems have been discussed by
Swatland (1986b). Further development of existing technology (fibre optic
spectrophotometry, electrical properties of muscle) is recommended as a
high research priority. However, at least 95% accuracy would be needed
for the early detection of PSE meat for such a measurement to be used in
the classification or grading of carcasses.
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2. PROCESSING CONSIDERATIONS REGARDING PSE/DFD PORK QUALITY
Many factors influence the transformation of muscle to meat and thereby, the
functional properties of muscle proteins. Scheper (1971) observed that PSE
pork cuts had limited moisture absorption, low water binding capacity and
excessive weight loss (shrinkage), while DFD cuts possessed elevated moisture
absorption and normal water binding capacity and weight loss (shrinkage). A
detailed description of the properties of these conditions and the factors
contributing to their formation has been provided by Briskey (1964).
A. Processing Problems and Meat Quality
Many processing problems have been shown to be associated with aberrations
(PSE or DFD) in meat quality including: fat separation (breakdown of
stable muscle protein/fat emulsions); water separation in processed foods
made from meat; variable cured color intensity and stability; and textural
problems, such as raushiness in finely comminuted products. It is widely
recognized that low pH meat in comminuted sausages results in inferior
quality and the use of extremely high pH meat produces sausage emulsions
with only low viscosity.
Water binding properties of muscle are a function of protein solubility,
and pH and are also influenced by microbiological growth, salt
concentration and temperature. Reductions in water binding capacity lead
to excessive juice separation in canned hams and similar products.
Therefore, DFD muscle generally is considered superior to normal muscle
for use in sausage products due to its superior binding and emulsifying
properties, while PSE muscle generally is not considered desirable as a
component in comminuted meat products because of its inferior binding and
In general, PSE muscle results in processed meat products that are paler
than normal, while DFD muscle results in processed meat products that are
darker than normal. In some cases, there is a two-toning effect in
certain cuts, (e.g. hams) where certain muscles or portions of muscle
became PSE while other muscles or portions of muscles remain normal or
became DFD. These off-colors have been observed to be aesthetically less
attractive to the consumer, particularly when they exist within the same
cut (Kramlich et al. 1975).
B. Muscle Quality and Processing Yields
Although PSE primals have been reported to have only 3 to 5% higher
smokehouse shrinks than normal primals, Kauffman et al. (1978) found that
PSE hams lost substantially more weight during transit and processing than
either normal or DFD hams, and estimated that this excess shrinkage may
amount to as much as 1,000,000 kg per year in the United States. Other
workers have also observed that PSE muscle produced lower yields (Cassens
et al. 1975) and that DFD muscle produced higher yields (Kauffman et al.
1964, 1978) after curing and smoking. Cassens et al. (1975) reported that
PSE muscle consistently had 2 to 8% higher gelatinous cookout when
- 10 -
processed into canned hams, and Leest et al . (1971) observed similar
gelatinous cook out from PSE muscle processed into luncheon meats. It was
also shown that such gelatinous cookout increased dramatically when
processing temperatures exceeded 77°C.
Such results have promoted the following conclusions:
1. That the primary importance of high pH or DFD meat to the industry is
less shrinkage during processing, thereby, resulting in higher yields.
2. That PSE hams represented high potential economic losses to the
processor through excessive shrinkage and lower quality products.
3. That PSE muscle was less suitable for certain processed products than
Recent research conducted by Lacombe Research Station staff in cooperation
with industry personnel evaluated differences in shrinkage and yield among
pork muscle quality (Agriculture Canada 1984a) groups when processed fresh
and following frozen storage and thawing under commercial conditions
(Jeremiah and Wilson 1986). This research demonstrated that the use of
frozen and thawed cuts for processing substantially reduced total
processing yields (1.7 to 5.2% in hams, 4.4 to 14.3% in backs, 1.3 to 2.0%
in picnics, and 2.2 to 2.7% in bellies) when compared with fresh cuts,
depending upon the inherent muscle quality; and that differences in
inherent muscle quality substantially affected the total processing yields
of various fresh (up to 7.7% in hams, 10.7% in backs, 4.8% in picnics, and
0.5% in bellies) and frozen and thawed (up to 13.5% in hams, 20.5% in
backs, 4.6% in picnics, and 0.9% in bellies) cuts (Figure 2.1.). It is
also of interest that freezing and thawing resulted in greater processing
yields from normal and DFD hams. Therefore, it is clear that both the
inherent muscle quality of pork cuts and the decision to process frozen
and thawed cuts can exert substantial influences on the profitability of
pork processing operations.
The research findings compiled to date show that aberrations in meat
quality are associated with factors that produce inferior processed meat
products, and may also have detrimental effects on the profitability of
meat processing operations through reductions in yield.
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FROZEN & THAWED
— i 1 1 1 1
DFD NORMAL PSE EPSE
— i 1 1 1 1 —
DFD NORMAL NCSE PSE EPSE
Figure 2.1. The relationship between total processing yields resulting from
curing and smoking operations and inherent muscle quality in (a)
hams, (b) backs, (c) picnics, and (d) bellies
DFD = Dark, firm, dry
N = Normal
NCSE = Normal color, soft, exudative
PSE = Pale, soft, exudative
EPSE = Extremely pale, soft, exudative
— i 1 1 —
DFD NORMAL PSE
I . , , j
DFD NORMAL PSE
- 13 -
3. PSE/DFD PORK AND RETAIL CASE LIFE
It is conceivable that the biochemical, physical and structural differences
associated with pork of different muscle quality may exert a pronounced effect
upon the quantity and quality of the bacterial flora. In view of the
potential effect on storage life, export to distant markets and the retail
case life of fresh pork, any differences in spoilage rates attributable to
variation in muscle quality become important considerations.
Although most researchers would concur that muscle of darker quality is more
susceptible to spoilage than normal muscle, existing data is limited and
contradictory (Newton and Gill, 1980). In this regard, some workers have
proposed that qualitative and quantitative differences in bacterial
populations developing on DFD, PSE and normal muscle can be attributed, solely
to pH. Thus, the higher pH inherent in DFD muscle was considered to be more
conducive to bacterial proliferation. This speculation, however, has been
contested by others who reported the growth of meat spoilage bacteria to be
unaffected by pH within the range of 5.5 to 7.0. These latter investigators
contend that muscle of darker quality has very low sugar reserves.
Consequently, contaminating bacteria are forced to use amino acids as growth
substrates with the generation of malodorous volatiles. It would then follow
that these offensive off-odors should not arise in PSE or meat of normal
muscle quality until the abundant supply of glucose is depleted.
Since the physiology of bacterial growth and spoilage in muscle of different
quality is far from being resolved, studies at Lacombe were designed to
provide more comprehensive data. The objectives of the research were to
assess differences in the growth of the aerobic spoilage flora on pork of
different muscle quality as classified by the Canadian Pork Quality Standards.
Results of a preliminary study are presented in Figure 3.1. The data compare
the growth of cold tolerant spoilage bacteria on loin chops of quality groups
1 to 5 for up to 4 days of simulated retail display. These results extend the
observations of previous researchers (Rey et al. 1976) by demonstrating that
as one proceeds sequentially from quality groups 1 through 5 there is a
corresponding increase in the levels of contaminating bacteria. That is,
during display, muscle of the darker quality groups sustained significantly
greater populations of bacteria. This increase can be attributed to a
decrease in the length of the lag phase prior to the onset of bacterial growth.
In light of the implications for storage quality, subsequent studies are in
progress to collect more extensive data on bacterial growth, odor case life
and subjective and objective measures of the deterioration in acceptable
muscle color. Only when these data become available can relevant conclusions
be made concerning the effects of pork muscle quality on spoilage potential.
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1 2 3
DAYS ON DISPLAY
Figure 1. Effect of pork muscle quality on the growth of bacteria on
chops during retail display. Data represents the mean of 6
determinations for each quality group. Canadian Pork
Standard Quality groups 1(B), 2(D), 3(A), 4(A) and
5( • ) are compared.
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4. PALATABILITY AND PSE/DFD PORK
In general, research results within the literature are inconclusive and
contradictory regarding the influence that inherent muscle quality exerts on
palatability. Therefore, as a result Cassens et al. (1975), concluded that it
was difficult to draw general conclusions regarding relative palatability
differences among different levels of muscle quality; and Krol (1971)
expressed a need for additional research to establish relationships of muscle
quality with organoleptic attributes and consumer acceptance.
A. Pork quality and palatability
Recent research at Lacombe (Figure 4.1) has found that a range of pork
products (ham steaks, loin chops, bacon slices, shoulder roasts and
sausage patties) prepared from PSE, normal and DFD pork were all within
the acceptable range for palatability (Jeremiah, 1986). However,
important differences were identified in some products. For example, PSE
hams were less juicy and had less desirable flavor than normal and DFD
hams and PSE loin chops had less acceptable flavor and overall
palatability than normal loin chops. In addition, meaningful differences
in cooking losses were also observed in PSE loin chops and bacon compared
to their normal counterparts. These findings are in agreement with other
published studies (Kauffman et al. 1964; Merkel 1971; Cassens et al. 1975)
Although, it should be noted conflicting reports exist in the literature
and some authors have found no differences in cooking losses that could be
attributed to muscle quality (Topel et al. 1976; Jeremiah 1984), the
studies conducted to date suggest that pork quality exerts a relatively
important effect on cooking losses, but has a relatively minor influence
B. Texture profiles
In recent years, sensory evaluation of food products has become more
sophisticated. With the use of trained and specialized panels, profiles
which contain a large number of variables to fully describe the components
of meat texture and flavour have been developed at Lacombe. Evaluation of
the texture profiles of pork loins with different levels of inherent
muscle quality, indicated that pork with normal, as opposed to aberrent
muscle quality, has a firmer, more elastic and cohesive texture, which is
stringier, more fibrous and harder to compress, thereby, resulting in a
slower rate of breakdown to particles which tend to be fibrous, grainy and
mealy. Such findings aid in explaining that PSE loins were more tender
than their normal counterparts (Fox et al. 1980; Kemp et al. 1976), but
fail to support other reports that:
1. There was a lack of difference in the tenderness of loins from
different quality groups (Merkel 1971; Searcy et al. 1969; Jeremiah
2. PSE loins were the most tender and DFD loins were the least tender
(Judge et al. 1960; Deethardt and Tuma 1971)
- 16 -
3. PSE loins were less tender than their normal counterparts when
evaluated by a taste panel (Buchter and Zeuthen 1971) and consumers
(Topel et al. 1976).
4. PSE loins were the least tender and DFD loins were the most tender
(Huffman and Adams 1972).
The PSE condition appears to result in a drier texture with less moisture
and fat released during mastication and a greater amount of moisture being
absorbed from the mouth. These findings aid in explaining previous
1. PSE loins were the least juicy (Bennett et al. 1973).
2. PSE loins were less juicy than normal loins (Buchter and Zeuthen
1971, Merkel 1971, Fox et al. 1980, Kemp et al. 1976).
3. PSE loins were less juicy than DFD loins (Kauffman et al . 1964;
DFD muscle has a juicier texture than normal with greater amounts of fat
and moisture being released into the mouth and a softer texture which was
less cohesive, fiberous, and stringy; and easier to chew. Such findings
may aid in explaining why some consumer surveys have shown preferences for
DFD and discrimination against PSE pork.
C. Flavour Profiles
Evaluation of the flavour profiles recently formulated at Lacombe revealed
that cooked PSE meat was associated with a predominance of sour character
notes which detracted from the flavor amplitude of samples possessing PSE
properties and resulted in extremely PSE samples receiving low flavor
ratings. This predominance of sour character notes in samples with PSE
properties may well be associated with the rapid postmortem glycolysis and
more extensive build-up of lactic acid. Further evaluation revealed that
DFD samples were associated with a predominance of porky, sweet, and fatty
character notes, which may explain why they received the highest flavor
amplitude ratings and aid in explaining why consumers in a previous study
(Jeremiah 1985) showed a preference for DFD chops and discriminated
against PSE chops. However, as the DFD condition became extreme more
character notes contributing to off-flavors were detected.
Therefore, evaluation of the research conducted to date, relating inherent
muscle quality differences to the cooking and palatability traits of pork
1. In general, all pork cuts are well within the acceptable range in
palatability, but that meaningful differences in cooking losses from
bacon slices and loin chops occur among muscle quality groups
2. The PSE condition is associated with a predominance of sour character
notes, which reduces the flavor amplitude and the desirability of the
flavor and overall palatability of pork cuts
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3. The PSE condition also is associated with a drier texture that reduces
the texture amplitude and contributes to lower juiciness and overall
A. The DFD condition is associated with a predominance of porky, sweet
and fatty character notes which enhance the flavor amplitude and the
desirability of the flavor and overall palatability of pork cuts
5. The DFD condition is also associated with a juicier and softer
texture, which is less cohesive, fiberous, and stringy and is easier
to chew, thereby, enhancing juiciness, tenderness, and overall
6. As the DFD condition becomes extreme, the texture becomes excessively
soft, crumbly and mushy which reduces the texture amplitude, while
enhancing tenderness ratings and reducing the desirability of the
7. As the DFD condition becomes extreme a predominance of character
notes, contributing to off-flavors, is also noted, thereby, reducing
the flavor amplitude and the desirability of the flavor and overall
palatability of pork.
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PSE NORMAL DFD
I 7 ' 5
PSE NORMAL DFD
Figure 4.1. The relationship between palatability and cooking
properties and inherent muscle quality:
a) flavor desirability, b) juiciness, c) tenderness,
d) overall palatability, and e) percent cooking
PSE = Pale, soft, exudative
N = Normal
DFD = Dark, firm, dry
- 19 -
— i 1 1 —
PSE NORMAL DFD
— i 1 1 —
PSE NORMAL DFD
PSE NORMAL DFD
- 20 -
5. THE GENETICS OF PORCINE STRESS SUSCEPTIBILITY
A. Sather and S.D.M. Jones
It has been recognized for many years that a small number of pigs die
suddenly, often during transportation, but also on the farm as a result of
normal, but stressful experiences (e.g. fighting, exercise, high ambient
temperature). The cause of this sudden death was collectively named the
Porcine Stress Syndrome (PSS). About 20 years ago certain pigs were found to
exhibit an uncontrolled elevation in body temperature when exposed to the
anesthetic halothane or by a muscle relaxant such as succinylcholine. This
reaction is generally referred to as Malignant Hyperthermia (MH). In addition
to the deviation of body temperature other clinical signs included muscle
rigidity in the rear limbs, increased respiration rate, increased heart rate
and cyanosis or blotchiness of the skin. Further work revealed that MH was an
inherited condition and could be transmitted to successive generations. Pigs
carrying MH are generally termed stress susceptible. In this bulletin stress
susceptibility and halothane reactivity are considered to be synonynous.
There may be other forms of stress susceptibility which are not related to the
There is a close association between MH and PSS. Autopsies conducted on a
large proportion of pigs diagnosed as PSS have shown similar biochemical
changes in their muscle tissue as those pigs exhibiting MH. It seems clear
that quite a high proportion of deaths from PSS can be directly attributed to
MH. However, it should be recognized that while stress susceptible or
halothane positive pigs (characterized by the MH reaction) are the most likely
animals to succumb to PSS, normal pigs (no MH reaction) can also exhibit PSS
depending of the amount and duration of stressful events.
MH is closely related to aberrant muscle quality or pale, soft and exudative
(PSE) pork in pigs. The stress susceptible line of pigs developed at Lacombe
is 100% MH and about 85% of the pork from these pigs is classified as PSE. On
the other hand the Lacombe breed under Research Station conditions is 0% MH
and with minimal pre-slaughter stress has less than 10% PSE. Therefore, the
frequency of PSE pork found in abattoirs is dependent on both genetic and
environmental factors and their interaction. The following chapter will
review the genetics of stress susceptibility, whereas environmental factors
will be covered in Chapters 7 and 8.
A. Stress susceptibility in man and other animals
Malignant hyperthermia was first identified in man by Denborough and
Lovell (1960) when several members of a family had adverse reactions to
halothane. MH has been reported in many other species, including the dog
(Bagshaw et al. 1978), the cat (De Jong et al. 1974), the horse (Klein
1975) the fallow deer (Pertz and Sunberg 1978) and in poultry (Korczyn et
B. The genetics of stress susceptibility in the pig
1. History. The association of the halothane anesthetic with death in
the pig was first reported by Hall et al. (1966) after the
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administration of a halothane/ suxamethonium anesthetic to 3 pigs, and
then by Harrison et al . (1968) who reported MH in pigs given halothane
alone. Research into the inheritance of the condition was initiated
by the Dutch researchers, Eikelenboom and Minkema (1974) who proposed
a single recessive gene model (Table 5.1) with reduced penetrance
(i.e. the proportion of animals that genetically should react to
halothane which actually produce a positive test) and variable
expressivity (i.e. variation in the degree of reaction to halothane).
The model proposes 3 genotypes (nn, Nn and NN) which relate to 3
phenotypes (halothane positive, carrier, halothane negative). This
model, but with high to complete penetrance has generally been
accepted today (Smith and Bampton 1977, Eikelenboom et al. 1977,
Andresen 1979, Webb 1981, Sather and Murray, 1986) as being most
useful in detecting MH in pigs. Ollivier et al . (1975) however,
proposed a model of incomplete penetrance. The Eikelenboom model has
demonstrated the greatest potential for field testing and has resulted
in the use of halothane testing of breeding stock in Canadian swine
improvement programs. However, because the inheritance of stress
susceptibility is generally considered to be a recessive trait,
considerable effort continues to be expended to develop techniques
that identify the heterozygous (i.e. carrier) pig which produces a
negative halothane test (Table 5.1).
Table 5.1. Genotype and associated phenotypes of the Halothane
Gene assuming a single recessive Mendelian gene
model with complete penetrance
nn Halothane positive
Nn Halothane negative (carrier)
NN Halothane negative (normal)
2. Single vs Multiple genes. When the halothane test (Eikelenboom and
Minkema, 1974) is defined as a simple (halothane only), short term
exposure (up to but not exceeding 5 minutes) to a high concentration
of halothane anesthetic (4-5%), a trait that follows single gene
Mendelian inheritance is observed. With longer test periods, of up to
20 minutes, this condition appears "...to be a complex dominant with a
modified single dominant gene or two dominant genes in concert to
produce a graded series of phenotypes..." (Williams et al. 1977). The
same authors also recommended a succinylcholine test in addition to
the halothane test for a pig to be considered normal, but then they
reported difficulty with pigs producing successive negative tests. The
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work of Hall et al . (1972) also suggested that when the standard
Eikelenboom halothane test is augmented with succinylcholine the
inheritance of the gene appears to be under the control of a dominant
gene. Britt et al . (1977) using both the caffeine contracture test on
muscle biopsies with a halothane challenge of up to 100 minutes
suggested that the inheritance of this trait was under the control of
more than one gene. Thus, only when attempts are made to make the gene
appear dominant or to identify the carrier pig does the inheritance
fail to follow classical Mendelian patterns.
3. Dominant vs Recessive gene model. Webb et al. (1986) administered a
standard 3 minute halothane challenge to 6-7 weeks old pigs followed
by an injection of succinylcholine. However, while this test did
produce differences among the genotypes , it was not sufficiently
precise to identify heterozygotes (carriers). This testing procedure
also requires a high degree of sophistication to administer making it
unsuitable for field testing of pigs. While it is well established
that the pharmacological aspects of PSS are recessive, meat quality
aspects may be additive (Jensen and Barton-Gade 1985, Murray and
Sather, 1986). That is the meat quality traits (PSE) of the carrier
are intermediate to stress susceptible and normal pigs.
4. Penetrance. The standard test (3-5 min , 4-5% halothane) has generally
produced a highly penetrant trait. Eikelenboom and Minkema (1974)
first reported the gene to have reduced penetrance but later
(Eikelenboom et al , 1977) found the trait to be fully penetrant,
assuming a single recessive gene. Sather and Murray (1986), concur
with this model and reported the gene to be essentially fully
penetrant in their populations. Ollivier et al. (1975) suggested
incomplete penetrance. It should also be noted that false positive
reactions to halothane are essentially unknown. Variation in
penetrance reported by these workers may be a function of the
populations (eg breeds) tested.
Carden and Webb (1984) reported that the penetrance of the halothane
positive reaction increased with age. Penetrance was low (<0.75) for
pigs less that 35 days of age and high (>0.85) for pigs greater than
56 days of age. Since expression of the halothane positive genotype is
dependent upon age, this factor must be considered when testing pigs.
In the Lacombe halothane testing program, pigs are routinely tested
between 49 and 56 days of age.
5. Variable Expressivity. The separate expression of two distinct
phenotypes, halothane positive and halothane negative, is not
absolute. Webb (1980) described a third, infrequent phenotype,
"doubtful", which fits neither of the two major classifications.
Sather and Murray (1986), as well as other researchers, have reported
variation in the degree of reaction. Pigs with an nn genotype can
produce a range of reactions including those so mild to be called
doubtful to those pigs that succumb to halothane challenge. No
researchers have been able to provide a completely satisfactory
explanation of the variation in terms of degree of reaction or time
of onset except to state that modifier genes exist. It is also
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probable that the physiological state of the pig prior to testing may
also influence the degree of intensity of reaction.
6. Pleiotrophy. Genetic stress susceptibility has also been associated
with a wide range of other traits that are directly connected with the
strict pharmacological definition of MH. Most notably halothane
positive pigs are more prone to stress than halothane negative pigs
and they have reduced meat quality (PSE) (e.g. color, drip loss, water
holding capacity). In addition, the condition is often associated with
a heavy muscled conformation, reduced carcass fat, increased carcass
lean, changes in growth rate and food conversion, reduced female
reproductive potential including both reduced litter size and losses
in life time sow productivity as well as changes in semen
characteristics. Thus, the(se) gene(s) has a wide range of effects
that go beyond the original concerns of reduced meat quality and
sudden death of the PSS/PSE syndrome complex.
C Relationship of genetic stress susceptibility with reproductive traits.
Genetic stress susceptibility has a significant effect upon reproductive
performance of the sow in terms of litter size birth and at weaning. Webb
and Jordan (1978) reported a reduction of 1.6 and 1.1 pigs at birth and at
weaning from halothane positive sows. Webb et al. (1982) reported a 24%
reduction in conception rate of halothane positive females. Carden et al.
(1985) found 1.07 and 1.56 fewer pigs, born and weaned in their stress
susceptible line (SS) when compared to their to stress resistant line
(SR). Litters of carrier pigs, based on the assumption of a single
recessive gene model, were produced by mating SR to SS females. The
resulting litters were 1.27 and 1.88 pigs smaller when compared to similar
litters produced from SR dams. This may suggest that the halothane gene
can also increase perinatal and postnatal mortality of apparently stress
resistant pigs. Lampo et al. (1985) reported that there were no
differences in prolificacy among halothane negative and positive gilts,
but second litter halothane negative sows had 0.4 more piglets born alive
than did similar halothane positive sows. This difference increased to a
full pig in subsequent litters. Willeke (1986) also reported not only
smaller litters at birth of the halothane positive sow of nearly 0.2 pig,
but a decrease in litter size weaned of over 0.5 pigs. Halothane positive
sows also produced 1.1 few litters that did their halothane negative
Schlenker et al. (1984) reported reduced ejaculate volume, forward
motility, normal spermatozoa and total sperm per ejaculate from halothane
positive compared to halothane negative boars. 72.0% of the halothane
negative boars produced semen of satisfactory quality compared to only
64.5% for halothane positive boars.
D. Relationship of the genetic stress susceptibility with performance traits.
It is generally accepted that genetic stress susceptibility depresses
growth rate (Carlson et al. 1980; De Wilde 1984), but this effect has not
always been reported. While Webb and Jordan (1978) reported a
nonsignificant reduction of 15 g per day in growth rate, Vogeli et al.
- 24 -
(1983) found significant reductions in growth rate. Hanset et al . (1982)
demonstrated that while both males and females had a depressed growth rate
associated with a positive halothane test, the differences were
significant only for females. Webb (1980) summarizing 12 studies, reported
a range in the difference between halothane positive and halothane
negative pigs from -47 to 28 g per day, with a mean of -2 g per day. Daily
food consumption and food conversion ratios are closely associated with
growth rate. Webb and Jordan (1978) found no differences in average
backfat for halothane positive and negative animals. De Wilde (1984)
however, reported less backfat on halothane positive pigs, while Hanset et
al. (1982) could only demonstrate a significant difference in female pigs
on the shoulder and back, but both sexes of halothane positive pigs had
less backfat on the loin. Jones et al . (1987a) suggested that the
distribution of subcutaneous fat in carcasses from halothane positive pigs
may differ from that of halothane negative pigs. This in turn has
important implications for the carcass grading system as these pigs may
have greater backfat at different sites along the loin but a 4-5% increase
in carcass lean yield.
E. Relationship of the genetic stress susceptibility with carcass yield and
The association of halothane sensitivity with increased lean tissue
content has been well established (Carlson et al . 1980, Webb and Jordan
1978, De Wilde 1984, Vogeli et al. 1983). Based on 14 studies, Webb et
al . (1982) reported a decrease of 1 mm in average carcass backfat. He
also reported that the meat color from halothane positive pigs was "paler"
and meat quality was "worse" than that observed for halothane negative
pigs. Webb and Jordan (1978) reported an incidence of 31.7% PSE meat from
carcasses of halothane positive pigs, but only 10.2% from halothane
negative pigs. Murray and Sather (1986), found their halothane positive
line of pigs produced a frequency in excess of 80% PSE meat, while the
same laboratory reported less than 10% PSE from a 100% halothane negative
Lacombe line. The halothane gene however may not necessarily produce
similar effects on meat quality in all breeds. Barton-Gade and Olsen
(1987) reported a lower incidence of PSE pork in halothane positive Danish
Large White pigs than they observed in halothane positive Danish Landrace
pigs. They also reported that these Large White pigs had more dark, firm
and dry pork than did the Landrace pigs. These results suggest the
possibility that either different halothane positive alleles or modifying
genes exist among different breeds.
F. Distribution of pig populations
Webb et al. (1982) reviewed the distribution of the genetic stress
susceptibility in British and other pigs. He reported a very low incidence
in Large White or Yorkshire pigs at close to 0% and up to 100% in the
Dutch Pietrain (Table 5.2).
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Table 5.2. Incidences of halothane positive pigs (HP) in breeds
from different countries
updated from Webb et al , 1982
- 26 -
Since halothane testing was not being carried out in Canada at that time,
no Canadian data was reported in his study. In 1983, the overall
incidence in Canada was 1.82%, averaged over the 3 White breeds
(Yorkshire, Landrace and Lacorabe) reporting reactor pigs. As in other
world populations, the incidence is higher in the Canadian Landrace than
in the Yorkshire (Table 5.3). The incidence of halothane positive pigs in
Canada is one of the lowest found in the world. It is interesting to note
that in the Lacorabe breed halothane genes have been identified, while the
Lacombe Research Station with an extensive halothane testing program has
yet to detect a halothane positive Lacombe pig (Sather and Murray 1986).
The colored Canadian pigs have reported no halothane positive reactions
since halothane testing began in Canada, although only relatively small
numbers have been tested.
Table 5.3. Number of herds station testing boars in the
Canadian Swine ROP Program and percentage those herds
reporting at least 1 halothane positive boar
Agriculture Canada 1986 tNS = Nova Scotia, PQ = Quebec,
ON = Ontario, MN = Manitoba, SA = Saskatchewan, AB = Alberta
- 27 -
Although Table 5.3 tends to present a somewhat biased picture on the
frequency of halothane positive animals since herds are identified rather
than individuals, it can be seen that genetic stress susceptibility is
well established in all Canadian white breeds throughout all regions
across Canada. The initial incidence of the halothane gene was greatest in
the Landrace and somewhat less in the Yorkshire. However, it appears that
the Landrace breeders have been successful in reducing the number of herds
reporting the gene compared to that of the Yorkshire breed. There are too
few Lacombe herds to make a consistent estimate of the frequency of
halothane positive reporting herds. However, these results indicate that
the while the gene frequency is low compared to that in other countries,
it is nevertheless well distributed across the Canadian national herd.
Clearly, because of low gene frequency of the halothane gene and uniform
distribution of halothane sensitive pigs across Canada, halothane testing
alone will not be very effective tool in reducing the frequency of the
halothane gene. However, if the halothane test is used in conjunction
with other genetic tools that can assist in the identification of the
heterozygous pig (Nn) , such as genetic markers (e.g. the Pgd , Phi loci),
it may be possible to identify carrier pigs, and in that way more
effectively reduce the incidence of animals carrying stress susceptibility.
G. Effects of halothane testing
The frequency of genetic stress susceptibility in 1983 varied from 0.069
for Yorkshire to 0.238 for Lacombes with a national average gene frequency
over all breeds of 0.135 (Table 5.4). If this is accepted as an
appropriate gene frequency for the national herd, and assuming that the
populations are in genetic equilibrium, then the calculations shown in
Table 5.5 consider the changes in gene frequency that can be expected
through different selection intensities.
Table 5.4. Frequency of halothane positive (H+) pigs and estimated gene
frequency (Q) in pigs tested for the halothane reaction in
pigs H + t %H +
pigs H %H H
Yorkshire 1851 9 0.49 0.069 1887 22 1.17 0.108 1604 11 0.68 0.083
Landrace 1268 43 3.39 0.184 1334 25 1.87 0.137 1122 9 0.80 0.090
Lacombe 124 7 5.65 0.238 142 0.00 0.0 146 4 2.74 0.166
Total 3243 59 1.82 0.135 3363 47 1.40 0.118 2872 24 0.84 0.091
t H+ - halothane positive, T Q = gene frequencies.
- 28 -
Table 5.5. Expected changes in gene frequency (Q) with different
levels of selection intensity(s)
1983 1984 1985
1.0 0.135 0.119 0.106
0.5 0.135 0.127 0.120
0.1 0.135 0.133 0.132
0.05 0.135 0.134 0.133
Observed 0.135 0.118 0.091
The selection intensity (s) is the proportion of halothane positive pigs
culled. If all pigs (boars and gilts) were tested and only negative pigs
used for breeding, then s=l. Since only boars are presently tested s can
have a maximum value of only 0.5. It has been estimated that only 10% of
the boars used by seed stock producers are station tested and thus
halothane tested. Therefore, for present Canadian conditions s cannot be
greater than 0.05 per generation. The generation interval was assumed to
be one year. In a typical pig herd, this ranges from 2.5 to 3 years.
Thus, the estimated gene frequency should decrease by approximately .001
per generation or 0.0004 per year, a rate that could not be detected under
our present testing conditions and sampling procedures. However, from
Table 5.4 the apparent frequency of halothane positive pigs is decreasing
at a rate greater than that possible under the most ideal conditions of
selection (s=1.0). Thus, it is evident that the assumed equilibrium
conditions have not been met. The most important deviation from
equilibrium probably arises from non-random mating with respect to the
halothane gene. This would result in an underestimation of the frequency
of the halothane gene, since the nn genotype would be found less
frequently than that expected under equilibrium conditions. This breeding
strategy while reducing the number of reactor pigs (nn) in the test
station, may do little to reduce the overall incidence of PSE arising from
carrier pigs (Nn).
To further illustrate this point, the Large White was considered to be
free of the halothane gene in the U.K. (Webb et al , 1982), from survey
testing procedures similar to that found in Canada. When Southwood (1985)
progeny tested the British Large White with reactor pigs, a gene
frequency of 0.11 was detected. Thus, while halothane testing may be an
effective monitoring procedure, unless it is also accompanied with proper
sampling procedures supported by progeny testing or estimated from within
designed experiments, it should not be used for gene frequency estimation.
H. Selection strategies
Reduction (or removal) of genetic stress susceptibility from the Canadian
swine population will be difficult (or nearly impossible). The gene is
- 29 -
well entrenched in all white breeds, in all regions in Canada. However,
the frequency of halothane positive pigs can only account for a small
proportion of the PSE pork seen in Canada. A number of reports (Jensen and
Barton-Gade 1985; Webb et al. 1986, Murray 1987) have suggested that
carrier (Nn) pigs may also have reduced meat quality. If these reports
can be demonstrated to be applicable to Canadian breeds, then this
additive genetic model could account for a substantial proportion of the
meat quality problems seen in Canada. If a gene frequency of 0.135 is
accepted, then, while only 1.8% of pigs would be halothane positive (nn),
23.3% would be carrier pigs (Nn) and at greater risk of producing PSE than
normal pigs (NN). This genetic model then implies that the halothane gene
would be much more important in determining meat quality for Canadian pigs
than previously thought under the assumptions of the recessive gene model,
and be responsible for up to half of the current incidence of PSE meat
found in commercial abbatoirs. There are a number of areas where our
knowledge of the halotnane gene is limited. In Canada, the Lacombe work
on the meat quality of carrier pigs has shown the frequency of PSE meat to
be intermediate to halothane positive and halothane negative pigs. The
halothane positive line at Lacombe was based on crosses between purebred
Pietrain and Lacombe breeding stock. The same results may not have been
obtained with other white breeds such as the Yorkshire or Landrace. Thus,
we would suggest that there is some urgency to evaluate the meat quality
of carrier pigs (halothane positive x normal and their reciprocal crosses)
for the major breeds used in Canada. If the same model was found to be
generally applicable to the breeds commonly used in Canada, efforts would
then be needed to estimate the frequency of carriers in the national herd.
Methodology to accomplish this is discussed in the next chapter. In the
mean time, we would recommend that all imported breeding stock and A.I.
boars be progeny tested for stress susceptibility. This would entail
maintaining halothane positive lines of pigs. However recent work has
suggested that some carriers do react to the halothane test and this could
result in false positives being identified.
Since the frequency of PSE meat has increased in most countries we would
conclude this chapter by presenting several hypothesis to explain these
1. That there is a general relationship of carcass lean meat content with
muscle quality (color, water holding capacity) and that selection for
increased lean growth rate will also increase the incidence of PSE
2. That certain types of lean, heavily muscled pigs produce meat with
3. That the more intensive nature of the industry (large production units
and central abattoirs) has imposed greater degrees of stress on the
modern pig resulting in an increased incidence of PSE meat.
Hypothesis 1., if true, presents very serious problems to the swine
industry, since breeding programs place very high emphasis upon efficient
production of lean tissue. While long term selection experiments for
increased lean growth rate have generally not reported any substantial
decline of meat quality as a correlated genetic response, they have not in
general considered the genetically lean pig (e.g. less than 12 mm fat at
- 30 -
the last-rib at 100 kg live weight). However, the available evidence
(Sather et al , 1981) suggests that certain types of pigs (e.g. heavy
muscled conformation) may be responsible for a substantial proportion,
although not all, of the meat quality problems. Long term genetic
studies, using lines that are free of genetic stress susceptibility
(halothane gene) are still required to verify hypothesis 1.
The development of halothane positive lines of pigs (e.g. Webb 1981,
Jensen and Barton-Gade 1985, Murray and Sather, 1986) has confirmed
hypothesis 2. Halothane positive (nn genotype) and carrier (Nn genotype)
pigs have a higher lean carcass yield than normal pigs (NN genotype) and
under certain breeding programs may be considered superior animals in
terms of carcass yield traits.
However, recent evidence (Jensen and Barton-Gade 1985, Murray and Sather,
1986) now suggests that carrier pigs (Nn) may have intermediate meat
quality relative to either homozygote (i.e. NN vs nn) . These results imply
that even if the frequency of halothane positive pigs are low (say,
approximately 1.8%), the incidence of halothane carrier pigs can be
substantial (23.3%) and contribute significantly to the amount of PSE meat
found in Canadian pork. However, it is becoming increasingly clear that
environment (hypothesis 3) has a vital bearing on the overall incidence of
PSE pork. Canadian pork production has not only expanded but also become
more intensive in the last 25 years. Large abattoirs now routinely
slaughter up to 30,000 pigs a week. There is little doubt that the modern
pig is subjected to a greater degree of stress during marketing (farm gate
to slaughter) than in the past.
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6. THE DETECTION OF AND TESTING FOR STRESS SUSCEPTIBILITY
A. Sather and A.C. Murray
A. Genetic markers.
Halothane testing has proven to be very useful for the indentif ication of
homozygous stress susceptible pigs (genotype nn - Table 6.1) but is
ineffective as a tool to identify the heterozygote (genotype Nn). In
addition, halothane testing at the field level in Canada because of its
expense is usually confined to pigs housed in test stations. For these
reasons, considerable work has been underway in Europe to evaluate genetic
markers which require a blood sample to determine the presence of absence
of certain blood group loci, Archibald and Imlah (1985) pointed out that
the halothane gene was a member of a linkage group consisting of 5 other
linked gene loci:
1. S(A-O) blood group locus which suppresses the expression of the A-0
blood group. The S allele suppresses the expression of the A-0 blood
groups and is recessive to the dominant S allele allowing expression
of the A-0 blood antigens.
2. H blood group locus is a multiallelic system of 6 alleles
( al te rna tat ive form of the same gene) that controls a series of
erythrocyte antigens. The alleles of this system have been usually
classified^ as the H allele (presence of the "a" antigen) and the
compound H "allele" (absence of the "a" antigen).
3. Pig erythrocyte 6-phosphogluconate dehydrogenase (Pgd) locus has two
codominant alleles Pgd and Pgd .
4. Pig blood serum protein postalbumin-2 (Po-2) locus has two codominant
alleles Po2 and Po2 .
5. Pig erythrocyte phosx>hohexose isomerase (Phi) locus has two codominant
alleles Phi and Phi .
Codominant loci, in which both genes at a single gene locus are
simultaneously expressed, are particularly useful since the complete
genotype at that locus can be readily established.
The use of genetic markers as a predictive tool to assign a probability
that a pig is a carrier of stress susceptibility is dependent upon linkage
disequilibrium. That is, certain haplotypes (combinations of specific
alleles at different loci on a single chromosome) have greater probability
of existing than that expected if a population were in equilibrium. While
the mathematics of this topic is beyond the scope of this review several
points can be discussed in general that pertains to the use of genetic
markers as a tool for locating the carrier pig.
1. Linkage disequilibrium means that specific alleles (variants of a
gene) from two gene loci are associated or correlated with each other.
The implication is that a marker gene can only provide a probability
statement as to whether or not a pig is a carrier. The presence or
- 32 -
absence of the marker gene is not proof, but rather a good indication,
that the pig is a carrier. Greater certainty of identifying a carrier
pig can be achieved by using two or more marker loci.
2. Since two or more gene loci are involved, recombination events can
occur between these loci. Thus, the phase of a marker gene can change.
Crossing over between two gene loci leads to a degeneration of linkage
disequilibrium. Thus, linkage disequilibrium will be more stable with
closely linked loci. If there is no linkage between two loci, then all
"linkage disequilibrium" will be lost in one generation of random
3. The linkage disequilibrium that exists between any two loci is
population dependent and subject to genetic drift (i.e. variation is
linkage disequilibrium from one population to the next) and sampling
errors. The parameters and thus the usefulness of a marker must be
determined for each specific population.
While genetic markers may be of value in determining whether or not a pig
is a carrier, considerable ground work must be done prior to the general
application and adoption of such techniques. The effectiveness of the use
of genetic markers are dependent upon two genetic parameters that can vary
from population to population:
1. gene frequency of both the marker and the primary gene, and
2. linkage disequilibrium between the marker and the primary gene.
If genetic techniques are to be used to improve meat quality in Canada,
then the first priority should be to make stress resistance pigs available
at A.I. centers. This could be done with a progeny test to a known line of
halothane positive sows, and halothane testing the resulting progeny.
After blood typing procedures have been adapted for use as genetic
markers, these should be used to replace the costly and time consuming
B. Physiological Markers
1. Blood enzymes. A number of blood enzymes have been tested for their
ability to predict stress susceptibility as indicated by a positive
halothane test (Sybesma and Eikelenboom 1978). For example, levels of
serum lactate hydrogenase (LDH) and glutamate oxalate transferase
(GOT) and plasma aldolase have been found to be higher in
halothane-positive (H ) pigs than in halothane-negative (H ) pigs
although enzyme levels were not highly correlated highly with
halothane reactivity. Since these enzymes are found in most tissues
of the body, many varied causes can contribute to increase their blood
Creatine phosphokinase (CPK) has shown promise as an indicator of
stress susceptibility (Mitchell and Heffron 1982). This enzyme is
concentrated in muscle and brain tissues. Serum CPK usually emanates
from the muscle and is a useful indicator of muscle deterioration
through disease or injury. Although serum levels vary greatly due to
- 33 -
factors not related to stress susceptibility such as other muscle
disorders, diurnal variation, muscle activity such as exercise, and
age, they are consistently higher for H than for H pigs. CPK
measurements may have some potential if all confounding sources of
variation can be understood and controlled.
Erythrocyte osmotic fragility. Red blood cells from H pigs _have a
different resistance to hemolysis by salts than do those from H pigs.
The measurement of erythrocyte osmotic fragility is a simple test
which has shown limited potential to detect heterozygous carriers of
the halothane gene (Harrison and Verburn 1973). However the technique
appears to show breed differences which are not related to stress
susceptibility. It requires further examination and refinement to
maximize differences between halothane genotypes and to minimize
Mitochondrial calcium efflux. The rate of efflux of calcium from
muscle mitochondria has been found to be higher in H pigs than in H
pigs (Cheah and Cheah 1979). This finding is unlikely to be
incorporated into a diagnostic test for stress susceptibility since it
would require the use of a biopsy technique, and a sophisticated
procedure for the isolation of mitochondria.
Blood platelet morphology and membrane bound calcium. The electron
microscopic examination of blood platelets has found to be useful to
distinguish between H and H pigs (Basrur et al. 1983). The area of
the platelet open canalicular system is greater for the H than for
the H pigs. However, this approach is far too costly and and time
consuming to offer potential as a routine test for stress
Blood hormone levels. The stress hormones (Cortisol, catecholamines,
thyroid hormones) are in general higher in H than in H pigs,
although the levels are far too variable to be of any predictive value.
Heat production. Observations show that H pigs, but not H pigs,
exhibit a considerable increase in muscle temperature as measured by a
rectal thermometer during the administration of the anesthetic,
halothane. The fact that this temperature increase is quite erratic,
and by the time it is great enough to measure the recovery of the pig
is unlikely, makes single muscle temperature measurements an unlikely
candidate as a predictive tool. The evaluation of infrared
thermography techniques for the detection of temperature changes in
stressed and unstressed pigs differing with respect to their reaction
to halothane is currently in progress.
Microscopic examination of muscle tissue. Microscopic examination of
muscle tissue using both light and electron microscopes can detect
structural anomalies related to stress susceptibility, but these
techniques have been found to detect only a small percentage of H
- 34 -
8. Muscle contracture test. The usual test to predict susceptibility to
malignant hyperthermia (MH) in humans is the in vitro contracture test
in which strips of muscle are exposed to drugs such as halothane,
caffeine and succinylcholine which may cause contraction. This test
has shown a great degree of variability in pigs and it is very
demanding technically, but because of the fact that it appears to
identify a greater number of carriers than even the halothane test,
further research into its use as a predictor of stress susceptibility
9. Muscle metabolism. The changes in glycolytic rate and pH, which occur
in muscle post-slaughter, also occur in a muscle sample removed from a
live pig* Increase in muscle glucose-6-phosphate (G-6-P) and
halothane-induced decrease in adenosine triphosphate (ATP) have been
shown to be related to stress susceptibility (Sybesma and Eikelenboom
1978). These methods are not only deficient in accuracy but are
somewhat technically demanding to be of general use.
Considerable research efforts are being expanded particularly in Europe on
tests that will accurately identify pigs carrying genetic stress
susceptibility. The most promising research area concerns the use of
genetic markers such as blood groups, although there is no work underway
in this field in Canada.
- 35 -
7. ANTE-MORTEM INFLUENCES ON PORK QUALITY
A.C. Murray and S.D.M. Jones
The treatment of pigs during the period up to 48 hours prior to slaughter has
a major influence on the economic losses due to transit deaths, loss in
carcass yield (shrink and/or bruising) and inferior lean meat quality. To a
great degree these losses relate to the ability of a pig to cope with
stressors. Certain pigs, including those which react in a unique way to the
anesthetic, halothane , are particularly susceptible to stressors.
A. Swine Deaths
Within Canada death losses during transit over the last few years has
averaged about 1.2 pigs/1000 pigs transported. On a Canada wide basis the
economic loss to the industry through transportation deaths Is estimated
to be over $2 million per annum. Transportation deaths based on pigs
found dead on arrival at abattoirs are shown in Table 8.1 by region and
season. In most regions, transportation deaths tend to be more frequent in
the summer months when high temperature and humidity levels prevail,
leading to increased stress. There are large regional differences with
the Western Provinces (Saskatchewan and Alberta) recording the highest
death losses due to transportation in most seasons. At the present time
the reasons why transport deaths are almost three times higher in Alberta
and Saskatchewan compared to other regions of Canada are not clear. Some
of these regional differences may be accounted for deaths in assembly
yards before the pigs reach the abattoir.
Table 7.1. Market pigs found dead (% of total slaughter in region) on
arrival at the abattoir by region and season (1984-1985).
Atlantic Quebec Ontario Manitoba Sask. Alberta B.C.
Spring . 11
Fall . 14
Overall deaths 1984-1985 = 0.117 based on 13,138,237 pigs slaughtered.
Information supplied by Meat Hygiene Division of Agriculture Canada.
- 36 -
Stress susceptible pigs are more prone to transit death. Immediately prior
to death these pigs usually exhibit at least some of the symptoms of
classical "porcine stress syndrome" (PSS). The symptoms include: labored
breathing, blotchiness of the skin, hyperthermia and rigidity of the limbs.
B. Carcass Bruising
The extent of the losses through bruising has been reviewed by Warriss
(1986) for Britain. In the UK between 1969 and 1975 about 2.6 pigs per
thousand were condemned either partially or wholly because of bruising.
Of particular concern was the skin damage caused mostly through fighting,
as these carcasses were often unsuitable for the production of rind-on
bacon. This fighting problem may be somewhat more prevalent in the UK
because of the extensive slaughter of young boars for meat production. A
brochure by Grandin, describing the situation for the U.S.A., placed
annual losses to the livestock industry due to bruising at $46 million.
Approximately two thirds of all bruises in pigs were found to occur in the
very valuable ham area. Both hams were often ruined in the case of a
spreader injury, which might result from slippery flooring. Grandin
suggested that a substantial proportion of all bruises was due to careless
or abusive handling. The movement of pigs more at their own pace with
canvass slappers (which are not frozen) or boards instead of canes, clubs,
electric prods, kicking, etc will greatly reduce bruising. The design of
facilities to permit good footing and to avoid steeply inclined ramps is
C. Carcass Weight Loss
If pigs do not eat for extended periods of time they will inevitably lose
weight. Figure 7.1 shows the extent to which duration of feed restriction
will affect the live pig weight (Warriss, 1986). Live weight losses occur
at the rate of approximately 0.2% per hour.
Figure 7.1 The effect of length of feed restriction on live pig
Davidson era/, (1968)
Bowland and Standish, (1966)
Jones et al, (1985)
(0.1 6% /h) •*
Warriss and Down (1985)
- 37 -
Carcass weight losses probably begin between 9 and 18 hours after the last
feeding and occur at the rate of about 0.1% per hour thereafter (Warriss,
1986). Although transit distance and time have been used to explain
weight losses, probably the major factor influencing losses is the
duration of feed and water restriction. Of course stress situations, such
as handling, mixing, transit and elevated temperatures would be expected
to exacerbate any effects of feed and water restriction. The feeding of
sucrose solutions to pigs immediately prior to slaughter has been shown to
increase carcass yields by up to 3% (Fernandes et al, 1979). However, the
concomitant decrease in meat quality, as evidenced by a decreased in
muscle pH at 45 minutes post-slaughter, may offset any potential weight
Many of the PSE/DFD-relat ed losses have been attributed to treatment
during the period immediately prior to slaughter, and are thought to be
due to exposure of the pig to stress situations which it has not
previously encountered to any great degree. The following stressors are
among the most common and may be considered to have both a physical and
1. Interaction among pigs. Pigs usually exist in a very well defined
social structure and they are very possessive of their territory. The
mixing of unfamiliar pigs usually results in a great deal of fighting
during the social regrouping process. This situation is one of
extreme stress. In addition, fighting causes losses due to bruising
and it can have an effect on the quality of certain muscles (Warriss
and Brown, 1985; Warriss, 1986). The effects of mixing can be
minimized by using adequate partitioning during transit and proper pen
design at the packing plant. Since pigs prefer to stand at the
perimeter of the pen along the fences, long narrow pens are preferred
at assembly points and in lairage areas of packing plants.
2. CI imate/Micro-environment . Pigs are particularly sensitive to both
high temperatures and extreme fluctuations in temperature. The
incidence of PSE pork and the incidence of transit deaths are known to
be at their highest during the hottest periods of the year. Since
pigs do not have a great ability to compensate for temperature
changes, every effort must be made to assure proper temperature and
humidity control. Because of the extreme cold temperatures in most
parts of Canada during the winter months, pigs must also be protected
Control of temperature during the transportation of pigs to market is
critical. This is accomplished by assuring that trucks have adequate
levels of ventilation. Overcrowding should be avoided. Loading
densities ranging between 0.34 m and 0.41 m per hog, depending on
the weather, have been recommended (Agriculture Canada 1984b). Very
little work has been conducted in Canada relating to stocking density
in a truck, its interaction with environmental conditions and the
effects on pork quality. Recent work in Holland (Lambooy et al. 1985)
examined long transportation periods (44 hrs) in conjunction with 3
- 38 -
stocking densities on a triple deck truck (0.66, 0.44 and 0.33
m /pig). Pigs stocked at 0.66 m /pig sat or lay down quietly within
the first two hours of the journey. About 1/3 of the compartment was
not occupied. The pigs stocked at 0.44 m /pig sat down or lay quietly,
about 15-30 minutes later than the lowest stocking density. The area
of the compartment was almost completely occupied. In the compartment
with the highest density (0.33 m /pig), not all animals could lie down
at the same time and as a result the pigs were continually changing
their positions. The authors concluded that the highest stocking
density should be 0.44 m /pig (for 100 kg pigs) for reasons of animal
welfare and meat quality. Prolonged rest stops and prolonged waiting
at time of unloading should be avoided. Transportation during the
hottest part of the day in the summer months should also be avoided.
The problem of overheating has been addressed through the spraying of
pigs in the lairage area. The spraying of pigs prior to stunning
caused a decrease in the temperature of the longissimus dorsi muscle
35 minutes after slaughter and a reduction in the incidence of PSE
meat (Smulders et al. 1983).
In addition to control of temperature, control of lighting is
important. Hogs may experience bright sunlight for the first time
during the loading for transport to market. Movement from low light
intensity into bright sunlight, perhaps in combination with change in
temperature from 18°C to -20°C, is undoubtedly stressful, and this
tends to increase the liklihood for physical abuse during loading and
unloading. Uniform lighting in the abattoir holding area must also be
3. Handling. Physical abuse not only dramatically increases the
incidence of bruising, but is also a contributor to the overall stress
level of the hog (Grandin). Rough handling including kicking and
beating should be avoided. Canvass slappers, which are not frozen,
should be used instead of canes and clubs. The use of electric prods
is discouraged. Van der Waal (1970) after measuring epinephrine levels
concluded that pigs may be considerably stressed by electric prods.
Lewis et al. (1961) found that stress through the application of 18
electric shocks per hour for a period of 5 or more hours caused the
depletion of muscle glycogen reserves and ultimately resulted in DFD
meat. On the other hand much briefer treatments of this type would be
expected to result in PSE meat. Driving boards can decrease the
degree of stress during the moving of pigs, and where possible hogs
should be permitted to move at their own pace.
4. Handling facilities and facility design. Lack of proper design of
handling and holding facilities is a major contributor to bruising and
to stress levels which ultimately result in PSE pork. Elimination of
slippery walkways, steep ramps, sharp corners, sharp protruding
surfaces, noise and vibrations, etc at all points from farm to
slaughter will undoubtedly result in improved pork quality. Extensive
reviews on this subject have been completed by Grandin (1980) and
5. Water and feed restriction. Numerous research reports describe the
effects of feed and water restriction on pork quality. These are
typified by the data of Neilsen (1981) which are presented in Table
- 39 -
7.2. Increasing the duration of feed restriction and resting time
prior to slaughter resulted in a decrease in the incidence of PSE meat
and an increase in the incidence of DFD meat.
Table 7.2. Frequency of PSE and DFD pork in relation to feeding
and holding periods.
Table 7.3. Scores (%) for pork muscle color (differences from
base-line) in pigs transported and rested for
different periods of time prior to slaughter! .
Less than 1 hour
Greater than 2 hours
t Study conducted during the summer months.
T Color Score 1-4, Agriculture Canada Pork Quality Standards,
- 40 -
6. Transportation and resting period. European work with pigs has
indicated that short transportation periods combined with no resting
period at the abattoir results in an increased incidence of PSE pork.
A recent study conducted in Quebec (Fortin, 1986) has confirmed these
findings under Canadian conditions. The frequency of pork colour
scores for transport duration of less than 1 hour and hours resting
were set at (baseline value) and all other frequencies were reported
against the base-line values (Table 7.3). During the summer, pigs
were either transported for less than 1 hour or greater than 2 hours
and slaughtered on arrival at the packing plant or rested for 3 or 6
hours . The baseline values where PSE incidence would be expected to
be highest are shown as in Table 7.3 and the other treatments are
measured against these values in terms of percentage differences.
Short term transportation with no rest prior to slaughter produced a
much higher incidence of color scores of 2 (pale) than pigs
transported for over 2 hours. Short transportation would be expected
to be more stressful than longer transportation, since there is a very
short period of time between loading and unloading. Also, pigs are
often mixed prior to transportation with the result that social
regrouping will continue on the truck. A longer resting period was
associated with improved pork color scores. For example, 6 hour
resting decreased the frequency of pale loins by about 30%. The
incidence of dark pork (color score 4) did not show a major increase
with an increased resting period. Therefore, short resting periods (3
hour or more), particularly following a short truck journey, would
appear to offer considerable benefit in reducing the frequency of PSE
pork. These short rest periods are not likely to have a major
influence on carcass weight shrinkage, but do require the plant to
invest in modern animal handling facilities, and to have deliveries of
pigs extremely well coordinated.
7. Interaction of environment with genetic susceptibility to stress.
Certain pigs, including those that give a positive halothane test, are
particularly susceptible to stressors. As such, little or no stress
is required to trigger the PSE condition. An example of the effect of
stress susceptibility (as determined through halothane testing) and
length of feed restriction on the final lean meat quality (Murray,
1986) is shown in Table 7.4.
Table 7.4. The effect of duration of feed restriction on the
incidence of soft exudative (SE) structuret in the
longissimus dorsi muscle of pigs from three lines.
Line of Pigs
NN Nn nn
t Structure Score, Agriculture Canada Pork Quality Standards.
\ Line of pigs: NN = Lacombe , Nn = Lacombe x Halothane positive,
nn = halothane positive.
- 41 -
For this trial feed restriction was carried out in the barn in which the
pigs were raised with no mixing, but with water available, and slaughter
occurred within one hour of arrival at the Lacombe abattoir. Table 8.4
shows a major effect of line on pork quality. At hour off feed the
incidence of pork with soft exudative structure varied from 0% for the
Lacombe breed (probably non-carriers of the halothane gene) to 87% for the
nn line (all of which were halothane-positive) , while the cross line was
intermediate. At 48 hour off feed both the nn line and the cross line
showed considerable improvements in quality, the cross line giving greater
response to the 48 hour feed restriction than the nn line. Had this
experiment been carried out in a commercial packing plant , the increased
stress of commercial marketing would probably have resulted in even more
obvious interactions between line and time off feed.
Environmental conditions have been shown to be a major contributing factor
in the development of PSE/DFD pork. Although some sound general
principles have been established e.g. handling, facility design, resting
periods prior to slaughter, etc., the area is still poorly researched in
Canada. For example, the average transportation distances to processing
plants and times spent in transportation in the marketing process have not
been defined. A major effort is required to collect this information
before cause and effect can be established, and the incidence of poor meat
Research findings have resulted in recommendations concerning the handling
of pigs at the farm, during transit, and at the abattoir. Information,
written in a non technical style, is available from Canadian sources
(Thompson, 1980; Canadian Pork Council, Canadian Meat Council 1980). The
most comprehensive treatment of the subject is found in the "Recommended
code of practice for care and handling of pigs" (Agriculture Canada 1984b).
- 42 -
8. POST-MORTEM INFLUENCES ON PORK QUALITY
A.C. Murray and S.D.M. Jones
Muscle of the living pig has a pH of near 7.2, that is, it is neither acidic
nor basic. The blood, in addition to supplying the muscle nutrient
requirements, serves to remove metabolic end products, such as lactic acid,
which are produced as a result of muscle activity.
Biochemical reactions within the muscle continue for considerable time after
slaughter, making use of the energy supplies stored mostly in the form of the
carbohydrate, glycogen. The rate of reaction is controlled to a large extent
by the levels of free calcium. However since blood circulation has ceased at
death, lactic acid accumulates. Rigor mortis sets in within about the first 6
hours post-slaughter. The accumulation of lactic acid is evidenced by a
decrease in the pH value to approximately 5.5 within about 12 hours after
slaughter (Figure 8.1). This process constitutes the normal conversion of
muscle to meat.
Figure 8.1. The effect of length of time after slaughter on the
pH of PSE, normal and DFD meat.
-i 1 1 1
6 12 18 24
A. PSE Pork
The timing of the above events is somewhat different In muscles destined
to become PSE. These muscles are well endowed with energy (glycogen), and
because of an apparent inability to control levels of calcium, are
biochemically very active immediately after slaughter, so that lactic acid
is produced much more rapidly than normal and the pH may fall to below 5.6
- 43 -
within one hour post-slaughter while the carcass is still at or near body
temperature (Figure 8.1). This combination of low pH and warm temperature
causes structural changes in the muscle proteins which markedly decrease
their ability to bind water while also influencing the texture and color
of the meat surface. Thus pale, soft, exudative pork results.
Muscles such as the longissimus dorsi (loin eye), the biceps femoris
(outside ham) and the gluteus medius (ham face) are particularly
susceptible to the PSE condition.
B. DFD Pork
Muscles use energy for the pH decline which is required for normal
conversion to meat. Muscles which have been depleted of their energy
supply (glycogen) through long term stress (eg. long exposure to extreme
cold with mixing and holding without feed) are incapable of the producing
much lactic acid and thus remain at a high pH (> 6) (Figure 8.1). Because
of the higher pH, the water holding capacity rcuains high and the texture
is dark, sticky or dry and much firmer than nornal.
A number of practices occurring at or immediately after slaughter can have
an influence on the rate of pH decline and thus the incidence of PSE pork.
Three common methods have been used for the stunning of hogs - captive
bolt, electrical and carbon dioxide stunning. These have been described
and compared in detail (Eikelenboom, 1983). During captive bolt stunning,
a bolt is driven either into or against a pig's head by air pressure or a
Electrical stunning is performed by passing an electric current through
the brain so as to render the pig unconscious. The current is applied at
60-70 volts (low voltage) for up to 20 sec (5-7 seconds is effective -
Gregory 1987) or at 240-1000 volts (high voltage) for 1-5 sec. This is
usually accomplished through electrodes placed either 1. on both sides of
the head midway between the eyes and the ears, 2. on both sides of the
head immediately behind the ears or 3. between the forehead and the nape
of the neck or back (front or head to back stunning).
Carbon dioxide stunning is accomplished by exposing pigs to approximately
70% carbon dioxide in a gas chamber. This technique appears to be more
stressful to the pig, since there is a period of 20-30 sec between
entering the gas and unconsciousness (Hoenderken, 1983), whereas the other
two stunning methods produce instant unconsciousness.
Choice of stunning technique can have a major impact on meat quality.
Captive bolt stunning causes a higher incidence of PSE meat than either of
the other methods (Yang et al. 1983). Agriculture Canada research data
(Murray, 1987) indicates that captive bolt stunning may cause four fold
higher incidence of PSE pork than does electrical stunning (Table 8.1).
- 44 -
Table 8.1. The effect of stunning method on the PSE score of
porcine longissimus dorsi muscle (loin eye).
with PSEt Score
Stun Method of pigs 1/2 3 4/5
Electrical 100 11 85 4
Captive bolt 100 43 55 2
t PSE score 1/2, Extremely pale/soft structure, 3 =
normal, 4/5 dark/firm structure.
The carcass and meat quality defects which can be influenced by stunning
include: bone fractures, blood splash, bruising, inadequate bleeding and
PSE meat (Gregory 1987). Bone fractures can occur in the shoulder blade,
thoracic, and lumbar vertebrae and the pelvis. Shoulder blade fractures
can largely be avoided by the use of a restraining conveyer or by
restricting the duration of current application to a short period (2 sec
when using 320 volt, Brathen and Johansen 1984). Gregory (1987) concluded
that the incidence of bone fractures probably depended on the voltage
used. Blood splash appears as small dots of blood and can be observed in
the muscles of the shoulder, loin and ham. No slaughter method can
completely eliminate blood splash since it has also been recorded in the
muscles of animals subject to ritual slaughter. The evidence in the
literature suggests that high voltage head and back stunning results in
less blood splash than low voltage stunning since head to back stunning
would result in a greater frequency of cardiac arrests. Shorter
electrical stunning times (< = 2 sec) have been shown to give less blood
splash, fewer broken bones and improved pork quality (less PSE) than
longer stunning times (Hoenderken 1978; Van der Waal 1978; Braathen and
Detailed research by Hoenderken (1978, 1983) has lead to the conclusion
that for effective electrical stunning:
1. A current of at least 1.25 amperes must be attained within 1 sec. In
practice this requires a voltage in excess of 240 volts (preferably
2. The electrodes must be placed on the head to assure that the current
passes through the brain.
3. Pigs must be stuck as soon as possible and definitely within 30 sec
after stunning in order to prevent a return to consciousness.
The controversy concerning the relative merits of electrical and carbon
- 45 -
dioxide stunning continues. Prior to 1980, the consensus of research data
indicated that electrical stunning was superior to C0„ for both meat
quality and humaneness reasons (eg. Van der Waal, 1978). As of 1983, C0 9
stunning was not permitted in the Netherlands because of the apparent
additional stress imposed during the period of 20-30 sec between entering
the gas and unconsciousness (Hoenderken, 1983). However, Mosfeldt Laursen
(1983) concluded that electrical stunning with at least 300V and
anesthesia with 70% C02 were both acceptable procedures, and differences
in the humaneness of the procedures was insignificant when the general
background state of stress was considered. Also, low voltage stunning was
considered less humane than the other procedures. In an experiment
involving several thousand pigs (Table 8.2), carbon dioxide stunning with
a Compact Stunner was shown to yield a lower incidence of PSE pork than
electrical stunning at either 300V or 700V (Larsen 1983). In addition,
the former method resulted in less blood splash and fewer bone fractures.
Further comparative testing of these two techniques is required in Canada
to arrive at a recommendation on choice of C0~ versus electrical stunning.
Table 8.2. Comparison of the effect of electrical and carbon
dioxide stunning on quality of the longissimus dorsi
Stun Method % PSE % DFD
Elect. 300V 18.5 5.7
Elect. 700V 15.1 8.3
Carbon dioxide 4.0 6.1
D. Side of Shackling
The effect of shackling side on meat quality has been a concern. Fisher
and Augustini (1981) showed that for the semimembranosus muscle (inside
ham) , the pH at 45 min post-slaughter and a number of other meat quality
measures were dependent upon whether the pig was hung from the left or
right leg during the period between stunning and scalding. The quality was
poorer (more PSE) in the ham of the side that was shackled. This
observation has also been supported by Swatland (1986). Jones and
Murray (1987) however, have shown that the side of shackling had no effect
on the lean meat quality of the longissimus dorsi (loin-eye) muscle.
Lundstrom and Henningson (1986) found that shackling pigs by two legs did
not result in a great enough improvement in the quality of the loin muscle
to be recommended for commercial practices (one to two leg shackling).
The concencus in the literature is the shackled side (usually the left
side) may produce a higher incidence of PSE muscle in the ham, but has
minor influence on the quality of the loin muscle.
The practice of scalding pigs after stunning and sticking to permit
dehairing adds heat to the carcass and would therefore be expected to
- 46 -
exacerbate PSE quality problems. The skinning of carcasses is an obvious
alternative to the scalding process. Skinning can result in faster carcass
chilling and therefore better color and higher quality (Voogd, 1983). In
addition, it has been shown to be more economical than scalding where
markets are available for pig-skin.
F. Rate of carcass cooling
Carcass cooling systems have been developed with an overall objective to
reduce deep muscle temperatures so as to minimize the growth of spoilage
bacteria. Conventional cooling systems for hog carcasses usually employ
air temperatures of close to 1 C and air speed of 0.5 meters/sec. Very
few coolers are designed with sufficient refrigeration capacity to
overcome the initial high heat load. Consequently, air temperature rises,
leading to an extended cooling cycle. Increasing the rate of carcass
cooling will lower the rate of biochemical reactions and pH decline, and
in theory improve meat quality in carcasses susceptible to PSE. Multiple
stage chilling was developed in Denmark and is now finding acceptance in
the Canadian meat Industry. Although these newer systems differ in
specifications, the pork carcass Is chilled for about 1 hr at very low
temperatures (-20° to -40° C) at varying air speeds (1-3 meters/sec)
followed by conventional cooling. Only limited research has been
conducted in Canada on the effects of two stage chilling on pork muscle
quality. The results to date (Table 8.3) show that a blast-chill cycle of
1 hr does result in improved subjective scores for pork quality compared
to conventional chilling (Jones et al. 1987b).
Table 8.3. The effect of chilling treatment on loin color and
Frequency of Scores
Jones et al. 1987b
t Color/Structure Score 1-5, Agriculture Canada Pork Quality
However , it should be noted that there was a very low incidence of PSE
pork (color and structure scores of 1 and 2) encountered in this study for
both chilling treatments. The net effect of blast-chilling was a shift of
subjective scores from 3 to 4, thus producing pork of a darker color and
- 47 -
firmer structure than conventional chilling. Whether the same general
results would apply when a high frequency of PSE pork was encountered is a
matter of speculation. In addition, rapid chilling of pork under certain
conditions may lead to muscle toughening (Dransfield and Lockyer 1985) and
this problem is likely to be of greater importance in leaner pigs.
Clearly more research is still needed under Canadian conditions to
establish optimum chilling rates for pork carcasses.
Spray chilling has been adopted by several pork processing plants in
Canada. Although there has been little research conducted, it appears
that spray chilling for up to 8 hrs can substantially reduce carcass
shrink (2.5% to 0.6%), while having no effect on meat quality (Jones et
al. 1987) Spray chilling has a slight benefit to carcass cooling rate by
reducing deep ham temperature 6 hrs after commencement of chilling by 2-4°
G. Electrical Stimulation
The electrical stimulation of beef carcasses has been shown to increase
the meat tenderness. It also speeds the rate of muscle pH decline and
rigor development, negating any effect of cold toughening which might be
associated with hot boning and/or very rapid chilling. However,
application of the technique to pork carcasses has in some cases adversely
affected pork quality (Crenwelge et al , 1984). This is not surprising
since increasing the rate of pH decline while the muscle is still near
body temperature would be expected to increase the incidence of PSE meat.
Electrical stimulation of pork may however offer some potential if used in
conjunction with very rapid chilling.
- 48 -
1. The current frequency of pale soft and exudative (PSE) pork in Canada is
regarded as an important commercial problem by the Canadian meat industry
and influences the competitive position of Canadian pork in international
2. PSE muscle results increased drip in fresh pork. Losses amount to 4% for
PSE compared to 2% for normal pork. Processed yields for PSE pork are
reduced by about 10% in fresh and 20% in frozen backs compared to normal
3. Limited research shows that the shelf life of PSE pork may be longer than
that of normal and dark pork.
4. Although PSE meat is not rated as high in palatability as normal meat, it
is still generally regarded as acceptable. However, cooking losses are
generally significantly higher in PSE compared to normal and dark pork.
Detailed results obtained by flavor and texture panels show PSE pork has a
sour taste which reduces flavour ratings, and a drier texture that
contributes to lower juiceness.
5. A very high incidence of PSE pork is produced from homozygous stress
susceptible pigs. Stress susceptibility is a genetic disorder and is
probably inherited as a recessive gene. However, after several years of
halothane testing, homozygous stress susceptible pigs only make up 1-2% of
the national swine population. Therefore homozygous stress susceptible
pigs are not a main contributor to the current commercial incidence of PSE
6. Heterozygous pigs which carry the stress susceptibility gene (carriers)
cannot be identified by halothane testing, yet the frequency of carriers
could be up to 24% of the national swine population. Work at Lacombe has
shown that carriers produce meat quality intermediate to stress
susceptible and normal pigs. In simple terras up to 12% of all pigs
slaughtered could produce PSE pork through carrying the stress
7. Transportation, time of last feeding, environmental temperature, stocking
density and the handling of pigs have been shown to be the most important
environmental factors influencing the frequency of PSE pork. Over the
last 20 years the Canadian swine industry has increased in size along with
the development of slaughter facilities which routinely handle up to
30,000 pigs per week. There is little doubt that increased
intensification in the industry combined with larger throughputs at
slaughter plants have resulted in more stress on the market pig. For
example, in one study short transportation distances (< 1 hour) and no
resting prior to slaughter was associated with a 20% increase in PSE pork
compared to pigs rested for 3 hours prior to slaughter. In another
experiment, pigs carrying the stress susceptibility gene had only a 14%
incidence of PSE after 48 hours without feed, but a 60% incidence at
The executive summary highlights the main findings of the technical
review and represent the views of the authors.
- 49 -
hours off feed. Knowledge of marketing conditions across Canada is
limited, but it is clear that environmental factors play an important
causal role in the current frequency of PSE pork. Capital will have to be
expended to improve transportation conditions and animal handling
facilites in assembly yards and abattoirs, to reduce the frequency of PSE
9. The two most important post-mortem factors influencing muscle quality are
stunning and the rate of carcass cooling. High voltage stunning (> 300
volts) has generally been adopted by the Canadian industry. However,
recent work with a compact C0„ stunner has shown a much lower incidence of
PSE meat than that obtained by electrical stunning (4% vs 16.5%).
Blast-chilling of pork carcasses increases the cooling rate compared to
conventional chilling and results in a slightly darker, coloured pork with
a firmer structure. However, it is doubtful if blast-chilling alone can
control the production of PSE meat.
The authors wish to thank Dr. A. Fortin from the Animal Research Centre in
Ottawa and Dr. M.A. Price of the University of Alberta in Edmonton for their
helpful comments during the preparation of this bulletin. Thanks are also
extended to Julie Smith and Irene Thauberger for typing this bulletin.
- 50 -
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