Pork Quality: Beyond the Stress Gene

D.E. Gerrard

Department of Animal Sciences

Purdue University

West Lafayette, Indiana 47907

INTRODUCTION

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 muscle growth.

BACKGROUND

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 lines.

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 IIb fibers.

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.

CONCLUSIONS

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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