The primary goal of the pork industry is to produce the greatest
amount of high quality protein possible for the least amount of
input. Recently, increased selection pressure for superior muscle
growth has created tremendous gains in meat production. As a
result of this aggressive selection strategy, the ratio of major
muscle proteins has been altered and pork quality has suffered.
This has resulted in and may be the cause of pale, soft, exudative
(PSE) pork. Identifying and characterizing which myosin causes
PSE pork will allow for better understanding of reduced pork quality
and provide a basis for designing strategies for improving meat
quality. Therefore, the overall objective of our laboratory
is to understand the underlying molecular mechanisms that cause
lowered pork quality in pigs with the genetic potential for greater
Methods such as maximizing nutrient intake and administration
of exogenous compounds have resulted in large increases in the
amount of muscle produced by an individual pig; however, the bulk
of progress in muscle accretion has been realized through the
use of superior genetics. Unfortunately, many undesirable traits
become closely associated (linked) with economically favorable
traits when single trait selection is implemented over a long
period. In the case of pork production, a higher incidence of
lowered meat quality (PSE) has been the consequence of aggressive
muscle production. Since this generates negative repercussions
in the consumer sector, production of inferior product robs the
industry of potential profits.
Relationships between increased muscle development and pork quality
are poorly understood at the cellular level. Until the details
of this phenomenon are elicited, processes to utilize fully this
type of meat cannot bedeveloped. Therefore, the overall objective
of this laboratory is to understand the underlying molecular mechanisms
that cause lowered pork quality in pigs with the genetic potential
for greater muscle development.
The resurgence in the frequency of PSE (pale, soft, exudative)
pork in the industry is partially due to increased consumer demands
for leaner meats. In addition, production costs and global competition
have driven the industry towards the use of genetic lines with
a high frequency of porcine stress syndrome (PSS). Pig with PSS
produce muscle protein more efficiently than normal pigs. Although
there is not a perfect correlation between PSE meat and PSS, data
clearly demonstrate that genetics with a high frequency of PSS
tend to give rise to more PSE pork than other porcine genetic
Pale, soft and exudative pork represents the major quality problem
in the pork industry (Kauffman et al., 1992). According to the
Pork Chain Quality Audit (1994) and the National Pork Producers
Council (1991), >10% of all pork carcasses generated in the
US contain PSE meat. Development of this well documented condition
is pH-related and has been extensively reviewed (Sebranek and
Judge, 1990). Briefly, after pigs are slaughtered, there is a
conversion of glycogen to lactic acid in the muscle tissue. This
is a natural process that normally takes several hours to occur
and results in an overall pH decline in muscle to about 5.6-5.8.
In dark, firm and dry (DFD) pork, a pH decline does not occur
because there is little energy substrate present at slaughter;
therefore, the resulting muscle pH is unable to decline appreciably
from near about 6.8-7.0. At this this relatively high pH, water
is bound tightly and a dark color develops in the meat (reviewed
by van Laack, 1994). Conversely, in PSE pork, the ultimate pH
is similar to that of normal pork but the rate of pH decline is
accelerated. A lower initial pH and an elevated carcass temperature
immediately after slaughter, results in greater protein denaturation
than that occurring for normal pigs. This results in water loss
and altered light scattering characteristics in PSE muscle (Swatland,
1994). Pork originating from this aberrant situation is normally
undesirable in appearance and possesses altered cooking characteristics
(Hall, 1972). From a consumer standpoint, these altered meat
characteristics are quite unsavory.
Because the combination of a lower pH and higher temperature are
prerequisites for the generation of PSE pork (Penny, 1967), it
is reasonable to expect that by lowering carcass temperature immediately
postmortem would eliminate the occurrence of PSE pork. Application
of such cooling strategies in pork processing has reduced the
magnitude of the problem but does not eliminate it. This whole
process (PSE development) is aggravated by the fact that most
PSS pigs are more muscular (Webb, 1981) and dissipate heat slower
after exsanguination. This complex synergism complicates the
development of a "quick fix" remedy for the industry.
Furthermore, the reason that PSS pigs, as well as most other
highly selected genetics, are capable of depositing larger amounts
of muscle is because of their propensity to develop muscle fibers
with increased glycolytic metabolism (Dildey et al., 1970).
Muscle fiber type is characterized by the relative amount and
type of myosin isoform contained within an individual fiber.
Myosin is one of the most abundant proteins found in muscle and
consists of two heavy chains and 4 light chains. The heavy chain
contains a globular head and helical tail region and has a molecular
weight of approximately 200 kilodaltons (Buckingham et al., 1986).
Currently, there are four major adult isoforms: type I, IIa,
x and b myosin (Mahdavi et al., 1986). Although great homology
exists among the myosin isoform family, subtle differences in
amino acid composition allow different fiber types to function
under a myriad of physiological and biochemical conditions. Postural
muscles, such as the soleus, possess more type I (slow contracting,
oxidative) fibers than type II (fast contracting, glycolytic)
fibers. Conversely, the extensor digitorum longum contains a
predominance of type II fibers.
Animal domestication or selection of animals for superior meat
producing ability involves the propagation of those individuals
possessing greater amounts of type II fibers. From a practical
standpoint of muscle growth and meat animal selection, the type
II muscle fibers represent the bulk of the musculature of pigs
and render a nodal point for the control of muscle/meat production.
It remains intuitive, therefore, that as genetics are selected
for increased muscle protein deposition, geneticists place great
pressure on muscle fiber type, in particular the amount of type
Perhaps the most compelling evidence to support our genetics predisposition
hypothesis is that the PSE condition is difficult to mimic in
a normal pig. Theoretically, raising the ambient temperature
in an early postmortem pig carcass should cause the development
of PSE meat. Likewise, late antemortem stress should cause immediate
mobilization of glycogen and glucose substrates in the muscle
and cause lactic acid build up in the muscles during a period
when evacuation of metabolites is not possible. The fact that
these treatments are difficult to induce in normal pigs suggests
that there is an innate difference among genetic lines that allows
some pigs a greater propensity to develop an abnormal pH decline.
Data collected from our laboratory shows that PSS pigs contain
different amounts of type IIb myosin than controls. Concomitant
with this large increase in IIb myosin abundance is an equal alteration
in type I myosin.
Taken together, the aforementioned data show that an increase
in glycolytic muscle fibers augments the ability of the muscle
to produce an acid environment. However, it is imperative to
know which protein is losing its integrity during the transformation
of muscle to meat and to determine whether processing procedures
can be modified to minimize the losses occurring in PSE meat or
maximizing the processibility of PSE pork. If the type of myosin
is breed (line)-dependent, selection pressure could be applied
to reduce its occurrence in the commercial pig population. Alternatively,
it is reasonable to speculate that the relative amount of each
myosin within a given pig may be used to dictate the type of process
utilized to transform it into a fresh or processed pork product.
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