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I ^ fl Agriculture 


Research Direction generate 
Branch de la recherche 

Technical Bulletin 1 988-1 1 E 

Pork quality: 

a technical review 




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LEThbrioge rA. P 


Pork quality: 

a technical review 

Research Station 
Lacombe, Alberta 

Technical Bulletin 1988-1 IE 
Lacombe Technical Bulletin No. 2 

Research Branch 
Agriculture Canada 

Copies of this publication are available from 


Research Station 

Research Branch, Agriculture Canada 

Bag Service 5000 

Lacombe, Alberta 


Produced by Research Program Service 

©Minister of Supply and Services Canada 1988 
Cat. No.: A54-8/1 988-1 IE 
ISBN: 0-662-16570-5 

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 
meat quality 

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 

D. Marketing 

8. Post-mortem influences on pork quality 42 

A. PSE pork 

B. DFD pork 

C. Stunning 

D. Side of shackling 

E. Scalding 

F. Rate of carcass cooling 

G. Electrical stimulation 

9. Executive Summary 48 

10. Acknowledgements 49 

11. Bibliography 50 

- 1 - 


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 
Chapter 8. 

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. 

- 2 - 

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 
PSE/DFD pork. 

S.D.M. Jones 

Head, Red Meats and Beef Production 

Agriculture Canada 

Lacombe Research Station 

- 3 - 

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 

Subjective Color 

Subjective Structure 

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, 

- 4 - 

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. 

Color /Structure 

























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 
PSE pork. 

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 
carcass value. 

Table 1.2. The relationship between time post-slaughter and pH 
for longissimus dorsi muscles of different quality. 


Time Post-Slaughter 

1 hr 


24 hr 





5.5 - 

- 6.4 



5.5 ■ 

- 6.4 



5.7 - 

- 6.8 



6.4 • 

- 7.0 



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. 

- 6 - 

Table 1.3. The relationship between pork quality and measures of 
water holding capacity for the longissimus dorsi 

Color/ Structure 
Score t 

Expressible Juice 
(Centrifuge Method) 























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. 

- 7 - 

Table 1.4. The relationship between pork quality and measures of 
protein solubility transmission for the longissimus 
dorsi muscle. 

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 
pork quality. 

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. 

- 9 - 


L.E. Jeremiah 

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 
emulsifying properties. 

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 
normal muscle. 

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. 

- 11 - 











— i 1 1 1 1 


100 -, 


— i 1 1 1 1 — 




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 

12 - 



130 n 








— i 1 1 — 


I iu- 





w 100- 




^** **• 





I . , , j 




- 13 - 


G.G. Greer 

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. 

- 14 - 











1 2 3 

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. 

- 15 - 


L.E. Jeremiah 

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 
on palatability. 

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 
reports that: 

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; 
Jeremiah, 1984). 

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 
indicate that: 

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 

- 17 - 

3. The PSE condition also is associated with a drier texture that reduces 
the texture amplitude and contributes to lower juiciness and overall 
palatability ratings 

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 
palatability ratings 

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 
overall palatability 

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. 

- 18 - 




10.0 1 




9.5 H 



</> 8.0 



I 7 ' 5 

-> 7.0 
6.5 H 




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 - 



J2 8.0 

E 7.5 

w 7.0 


— i 1 1 — 






















w 6.5 





— i 1 1 — 






























- 20 - 


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 
halothane gene. 

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 
al. 1980). 

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 

- 21 - 

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 

Genotype Phenotype 

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 " 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 

- 22 - 

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 

- 23 - 

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 
meat quality. 

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). 

- 25 - 

Table 5.2. Incidences of halothane positive pigs (HP) in breeds 
from different countries 








% HP 

Yorkshire/Large White: 




























German (GDR) 







German (GFR) 
















































White Meat: 


Lacombe : 



Hampshire : 






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 

% Halothane 


r of 


Positive Herd 





























































































































































Grand Total 







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 
test stations 




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) 

Gene Frequency 

1983 1984 1985 

Selection intensity 

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 
findings : 

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 
inferior quality. 

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. 

- 31 - 


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 
progeny tests. 

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 
levels . 

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 
is warranted. 

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 - 


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. 

Winter .09 


Spring . 11 


Summer .17 

(June -Aug) 

Fall . 14 










06 .18 

,06 .19 

,06 .27 



.24 .17 

.21 .11 

.32 .16 

.18 .13 

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 
also crucial. 

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 












75 -I 

Davidson era/, (1968) 

Bowland and Standish, (1966) 
(0.20% /h) 
Jones et al, (1985) 

(0.1 6% /h) •* 

Warriss and Down (1985) 





FAST (h) 

- 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 

D. Marketing 

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 
emotional component. 

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 
against frostbite. 

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 
Braathen (1981). 

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. 

Fed or 


Holding Time 

No. of 

% PSE 






































Neilson 1981 

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! . 

Transport Duration 

Less than 1 hour 

Greater than 2 hours 

Resting Pe 


TColor Si 











+ 1.2 






t Study conducted during the summer months. 

T Color Score 1-4, Agriculture Canada Pork Quality Standards, 

Fortin 1986 

- 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. 




ed Restriction 




Line of Pigs 


NN Nn nn 








Murray 1986 

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 
quality reduced. 

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 - 


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. 







5.0 -U 




-i 1 1 1 

6 12 18 24 

A. PSE Pork 

TIME (hr) 

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. 

C. Stunning 

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 
blank cartridge. 

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 
Johansen 1984). 

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 
>300 volts). 

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 

Larsen 1983 

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. 

E. Scalding 

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 
structure scorest. 

Frequency of Scores 

Chilling Treatment 



Lean Color 


611 141 



518 225 
Lean Structure 




599 144 




545 180 


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 
susceptibility gene. 

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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