CHANGES I N P O R C I N E MUSCLE

Transcription

1 276. S P E C I F I C B I O P H Y S I C A L F E A T U R E S OF POSTMORTEM CHANGES I N P O R C I N E MUSCLE JOHN D. SINK The r a t e and extent of t h e postmortem changes t h a t occur i n t h e transformation of muscle t o meat r e f l e c t many underlying, complex physicochemica1 i n t e r a c t ions/ t en sion s The se b iolog ical/ph y si o l o g i c a 1 a l t e r a t ions influence meat q u a l i t y and thus contribute t o t h e acceptance of meat as a food.. One of t h e major problems t h a t has confronted meat s c i e n t i s t s and technologists i s t h e v a r i a t i o n i n raw materials. Although various antemortem and postmortem f a c t o r s have been studied, more d e f i n i t i v e information i s needed. U n t i l we f u l l y understand these basic b i o l o g i c a l phenomena, t h e e f f e c t i v e control of meat q u a l i t y w i l l indeed be hard t o achieve. The most important of t h e postmortem changes t h a t occur i n muscle t i s s u e a r e those associated with t h e process of r i g o r mortis. The usual review papers (BateSmith, 1948; Whitaker, 1959; Bendall, 1960a; Lawrie, 1962), considering these postmortem changes and t h e i r subsequent e f f e c t on t h e various muscle properties, have concerned themselves primarily with describing t h e nature of t h e underlying biochemical changes. This paper w i l l attempt t o present s p e c i f i c biophysical f e a t u r e s of t h e various postmortem changes t h a t occur i n muscle t i s s u e and more s p e c i f i c a l l y, those t h a t occur i n porcine muscle t i s s u e. The PostMortem Biophysical Changes The general aspects of postmortem biophysical changes i n muscle t i s s u e a r e outlined i n Figure 1. A s a consequence of c i r c u l a t o r y f a i l u r e o r exsanguination, t h e thermodynamic equilibrium, c h a r a c t e r i s t i c of t h e a c t i v e i n vivo s t a t e of muscle, i s destroyed. With t h e deprivat i o n of oxygen, t h e oxidationreduction p o t e n t i a l drops and t h e aerobic production of energy f o r t h e various metabolic processes u l t i m a t e l y ceases. A s a r e s u l t of t h i s i n t e r r u p t i o n i n t h e metabolic i n t e g r i t y of muscle, t h e a b i l i t y t o maintain: (1)a d i f f e r e n t i a l l y permeable and polarized membrane, (2) an isothermal environment, (3) an osmotic and vapor pressure equilibrium, and (4) t h e appropriate and necessary ion concentrations, i s r a p i d l y l o s t. 'National Science Foundation Postdoctoral Fellow, a t The University of Wisconsin.

2 277. Figure 1. PostModem Biophysical Changes in Muscle Exsanguination Thermodynamic equilibrium de stroyed OxidationReduction potential drops Thermal equilibrium, equilibrium 4;/ Viscoelasticity changes destroyed, < \I E x t ensibility/ekcitabilit y decreases. /,

3 278. Further, actin and myosin, normally prevented from associating by relaxing factor and electrostatic forces, now begin to unite, as the muscle contracts, to form the more insoluble, gellike actomyosin complex. Thus, all these changes result in a decrease in the extensibility and excitability of muscle tissue which eventually lead to the characteristic rigor mortis condition. Of these many complex biophysical phenomena, four will be discussed in this paper, namely, extensibility, excitability, contractility and viscoelasticity. Extensibility Changes. Although the study of the extensible properties of matter can be traced to the early work of Galileo Galilei in i636, it remained for Robert Hooke, some 40 years later, to relate the amount of deformation in a material to the deforming force applied to it (Westergaard, 1952). It was not until the twentieth century, however, that serious efforts were initiated relating this classical law to the changes in muscle tissue. Living skeletal muscle differs from most other materials in that it can recover completely and rapidly from stretch deformations up to approximately 150 percent of its rest or noncontracted length. However, when it dies and passes into rigor mortis, its extensibility to an applied load falls about 3040 times, and it is no longer able to recover completely from deformations of more than 3 percent of its rest length. Of the various physical changes which occur in muscle after death, the most easily measured is this loss of extensibility. This is because only one parameter is involved, the stretch deformation of the fibers (Bendall, 19 60b ). Various manual, mechanical and electrical devices have been developed to measure the time course of these extensibility changes (BateWth & Bendall, 1949; DeFremery & Pool, 1960; Briskey, Sayre & Cassens, 1962). An excised muscle strip (1 cm2 x 68 cm), which is loaded and unloaded (50gm weights) at specific intervals (2min), is used in these "rigorometers." The readout is a printed record of extensibility change with time postmortem. Extensibility changes have been taken as the main criteria in delineating and defining the several "phases" in the process of rigor mortis (BateSmith & Bendall, 1949; Briskey, Sayre & Cassens, 1962): a delay phase, virtually no change in extensibility b onset phase, continuous reduction in extensibility c completion phase, complete loss of extensibility In the following table (Table l), it can be observed that there is a wide variation in the response of muscle as it passes into rigor mortis, influenced by such biological factors as muscle and breed, and possibly body size and sex.

4 279 I Table 1. PostMortem Changes i n t h e E k t e n s i b i l i t y of Porcine Muscle Source of variation Delay phase duration h s e t phase duration Total time t o completion min min min Breed (1. d o r s i ) Yorkshire Poland China Hampshire Chester White Body S i z e (1. d o r s i ) 25 kilograms 50 kilograms 90 kilograms 120 kilograms kscle Longissimus d o r s i Light semitendinosus Dark semitendinosus sex (1.d o r s i ) Barrows Gilts E x c i t a b i l i t y Changes. The study of t h e e x c i t a b i l i t y or i r r i t a b i l i t y of muscle t i s s u e has had a r a t h e r long and i n t e r e s t i n g h i s t o r y since Luigi Galvani performed h i s c l a s s i c experiments almost two c e n t u r i e s ago (Loeb, 1905; Opatowski, 1951; Galambos, 1962). The e x c i t a b i l i t y of l i v i n g systems depends, t o a l a r g e extent, on t h e i r metabolic i n t e g r i t y. When this i n t e g r i t y i s i n t e r r u p t e d ( i. e., death), t h e r e i s a l o s s of e x c i t a b i l i t y concomitant with t h e depletion of energy s t o r e s (Ungar, 1963). Each muscle f i b e r maintains, i n t h e r e s t i n g s t a t e, a p o t e n t i a l (ca.90 m v ) negative t o t h e outside. Excitation a l t e r s t h i s d. c. p o t e n t i a l from 90 t o +40 mv (Buchthal, 1957). Repolarization occurs a s a r e s u l t of oxidative enzymatic and thermodynamic processes (Nachmansohn & Wilson, 1955). The r e s t i n g muscle f i b e r i s excited under t h e influence of a stimulus (Pay, Goodall & SzentGyorgyi, 1953). I n most laboratory investigat i o n s, e l e c t r i c a l stimulation has been used t o study these e x c i t a b l e propert i e s of muscle. The two parameters of an e l e c t r i c a l stimulus usually evaluated a r e t h e e x c i t a b i l i t y threshold voltage and t h e l o s s i n muscle response t o continuous stimulation a t a constant voltage.

5 280. Using t h e electromyographic apparatus described by Forrest e t a l. (1965), a preliminary study was conducted at Wisconsin recently t o examine t h e nature of postmrtem changes i n t h e response of muscle t o e l e c t r i c a l stimulation. From t h e d a t a presented i n Table 2, t h e loss of e x c i t a b i l i t y i s evident f r o m t h e increase i n e x c i t a b i l i t y threshold voltage necessary t o stimulate t h e muscle s t r i p i n i t i a l l y, and t h e decrease i n response t o t h e r e F e t i t i v e stimulation at a constant voltage (50v) w i t h time Fostmortem. Table 2. Source of variation Ohr postmortem Hampshire Chester White Postlrlortem Changes i n t h e E x c i t a b i l i t y of Porcine Muscle Excitability t h r e sholda Response t o 50volt stimulationb Maximum force T o t a l response durat ionc amount V g sec cm t min postmortem Hampshire Chester White min postmortem Hampshire Chester White l h r postmortem Hampshire Chester White t a 2 dhinimum v o l t a g e required to produce a c o n t r a c t l l e response. % e p e t l t l v e s t i m u l a t i o n a t a frequency o f 2 c y c l e s / s e c o n d and a s t i m u l u s duration o r 0.1 millisecond. 'Tntal time the e x c i s e d muscle s t r i p (I f o r c e o f >10 grams. cm 2 x 6 cm) was a b l e to produce a c o n t r a c t i l e * T o t a l area o r the s t i m u l a t l n n response p a t t e r n rrom F = l n l t i a l to F = grams. C o n t r a c t i l i t y Changes. The usual description shows s k e l e t a l muscle t o be composed of bundles of p a r a l l e l f i b e r s, t h e f i b e r s i n t u r n a r e made up of bundles of p a r a l l e l myofibrils, and t h e myofibrils a r e composed of a hexagonal a r r a y of myofilaments containing molecular chains of t h e c o n t r a c t i l e proteins, a c t i n (A) and myosin (M) (Huxley, 1958). The c r o s s s t r i a t i o n s, c h r a c t e r i s t i c of s k e l e t a l muscle, a r i s e from a repeating v a r i a t i o n i n t h e p r o t e i n density along t h e myofibril. However, these do not represent permanent s t r u c t u r e s but r a t h e r physiological states.

6 281. Under a phase contrast microscape (Figure 2 ), t h e r e i s a regular a l t e r n a t i o n of dense (Abands) and l i g h t e r (Ibands). The c e n t r a l region of t h e Aband i s less dense than t h e rest of t h e band, and i s known as t h e Hband. I n t h e center of t h i s band i s a denser Mline. The Iband i s b i s e c t e d by a narrow, dense l i n e c a l l e d t h e Zline. From one Zline t o t h e next i s usually taken as t h e repeating u n i t of myofibrillar s t r u c t u r e and i s designated a sarcomere. I n most v e r t e b r a t e s t h i s length i s approximately 3 A. Since earliest times, t h e contraction of s t r i a t e d muscle has been r e l a t e d t o changes i n these c r o s s s t r i a t i o n s (Buchthal & Knappeis, 1943; Rorvath, 1952; Gilev, 1962) as w e l l as t o changes i n t h e periodic i t y or sarcomere length (Hodge, 1955; Perry, 1956; Huxley, 1957). Even t h e r e l a t i o n s h i p between s t r i a t i o n p a t t e r n and sarcomere length has been investigated (Garamvolgyi, Kerner & CserSchultz, 1964). FIGURE Z I' 2. STRl ATlON P A T T E R N OF M SARCOrnRF: THE NONCONTRACTED Z 'I MYOFl B R I L.

7 2 82 Table 3 summarizes the results of an experiment in which we studied the striation pattern as a function of sarcomere length in three porcine muscles. It can be observed that each sarcomere type has limiting sarcomere lengths. These results with porcine muscles are in good agreement with those reported to occur posttcortem in the muscles of other vertebrates. Table 3. Distribution of Sarcomere Types in PostMortem Porcine Muscle Wscle Sarcomere type* / length range Trapezius thoracis Rectus femoris Longissimus dorsi SS/ > RL/Z , SC/ , NC/l HC/l , EC/( 1.39, la.o $ $ o Mean sarcomere length , 1.94/y *Note: SS = slightly stretched; RL = relaxed or noncontracted; SC slightly contracted; NC = normally contracted; HC = highly contracted; EC = extremely contracted.

8 283. I n another experiment (Sink e t al., 1965), we demonstrated t h a t t h e sarcomere shortening i s q u i t e severe when t h e delay phase of r i g o r mortis i s of short duration. However, when t h e delay phase of r i g o r mortis i s of long duration, t h e sarcomere shortening t h a t occurs i s much less. Consequently, it can be s t a t e d t h a t t h e amount of sarcomere shortening o r contraction, coincident with t h e r i g o r mortis onset, i s highly dependent upon t h e time course of r i g o r mortis (r = 0.9; P <.Ol). V i s c o e l a s t i c i t y Changes. The physical change of state from t h e p l a s t i c condition of f r e s h muscle t o t h e hard condition of muscle i n a s t a t e of r i g o r mortis has long been noted. When t h e thermodynamic processes of muscle a r e disturbed following exsanguination, concomitant changes i n t h e v i s c o e l a s t i c p r o p e r t i e s begin t o occur (Pryor, 1952). Since muscle t i s s u e i s a c o l l o i d a l system (protein t water), it undergoes a phase change from a r a t h e r l i q u i d condition, known a s a sol, t o a s o l i d or semisolid state, known as a g e l, during t h e development of r i g o r mortis (Ferry, 1948). Although various models have been proposed t o explain these v i s c o e l a s t i c p r o p e r t i e s of muscle (Will.de, 1954; Pringle, 1960), none appear t o be q u i t e s a t i s f a c t o r y. This s o l + g e l transformation plays an important r u l e i n postmortem muscle physiology. The assumption t h a t t h e b a s i c physical process of muscular contraction may c o n s i s t of a s o l + g e l transformation has been advanced (Polissar, 1952). Actually, t h i s transformation occurs a s a r e s u l t of t h e crossbond formation between a c t i n and myosin t o form actomyosin. All of t h e v i s c o e l a s t i c changes i n muscle are not s o l e l y due t o t h i s actomyosin formation, but a r e due p a r t l y t o t h e formation of cross l i n k s and adhesions between other p a r t i c l e s which permit l i t t l e r e l a t i v e m t i o n. The binding of a c t i n and myosin r e s u l t s i n p r o p e r t i e s which a r e q u i t e d i f f e r e n t from t h e p r o p e r t i e s of f r e e A and f r e e M (Hasselbach, 1952). This molecular crosslinking increases t h e i n t e r n a l v i s c o s i t y of muscle and decreases i t s e x t e n s i b i l i t y and e x c i t a b i l i t y. If a c t i n and myosin a r e l e f t f o r a s u f f i c i e n t time i n t h i s associated condition, secondary h i s t e r e t i c changes take place, i n which links t h a t were r e v e r s i b l e become i r r e v e r s i b l e (SzentGyorgyi, 1953). Although other methods (Catton, 1957) a r e a v a i l a b l e f o r determining t h e postmortem v i s c o e l a s t i c p r o p e r t i e s of muscle, t h e l o s s i n s o l u b i l i t y of t h e myofibrillar proteins, o r t h e DeutickeKamp e f f e c t, i s w e l l established and can be used as an indication of t h e amount of crossl i n k i n g t h a t has occurred. The following table (Table 4) presents some comparative data on t h e postmortem s o l u b i l i t y changes i n t h e myofibrillar and sarcoplasmic p r o t e i n s p r o t e i n s i n two porcine muscles. A g r e a t e r reduction I n t h e s o l u b i l i t y of t h e myofibrillar p r o t e i n s i s observed.

9 284. Table 4. PostMortem Changes i n t h e Protein S o l u b i l i t y of Porcine Muscle Protein nitrogen s o l u b i l i t y a Myofibrillar Muscle 0HR l$ Sarcoplasmic 24HR 0HR 24KR k % $ Longissimus d o r s i Semitendinosus Dark portion Light portion aexpressed as percent of t o t a l tissue nitrogen. The Biophysical Changes and Meat Q u a l i t y In t h e course of t h i s paper, a number of s p e c i f i c biophysical f e a t u r e s of postmortem changes i n procine muscle have been presented. The l i m i t a t i o n of t i m e has made it impossible t o discuss but a small p a r t of t h e biophysics involved i n t h e transformation of muscle t o meat (Ernst, 1963). Finally, w e might ask how these postmortem biophysical changes a r e r e l a t e d t o meat q u a l i t y. Any abnormalities i n t h e usual postmortem biophysical changes t h a t occur a r e associated with t h e development of t h e pale, s o f t, exudative condition i n porcine muscle, r e c e n t l y described i n an extensive review by Briskey (1964). The extent of t h e v i s c o e l a s t i c changes appears t o play an important p a r t i n t h e fatemulsifying p r o p e r t i e s of porcine muscle (Trautman, 1964). The t i m e course of r i g o r mortis has been shown t o subsequently e f f e c t t h e cooking c h a r a c t e r i s t i c s of muscle (Sayre, =ernat & Briskey, 1964). Recent s t u d i e s a t New Zealand (Locker, 1960) and Wisconsin (Herring, Cassens & Briskey, 1965) have investigated t h e r e l a t i o n s h i p between sarcomere length and tenderness. It has a l s o been demonstrated that t h e r a t e of l o s s of s o l u b i l i t y i n t h e myofibrillar p r o t e i n f r a c t i o n p a r a l l e l s t h e rate of development of toughness (Connell, 1960). Finally, w e a r e beginning t o examine tenderness a t t h e c e l l u l a r and molecular l e v e l. While t h e exact nature of meat q u a l i t y i s yet unknown, an understanding of t h e postmortem biophysical changes t h a t occur i n muscle can perhaps l e a d us t o more p r e c i s e l y control i t s development. If an antemortem o r e a r l y postmortem prediction of t h e ultimate meat q u a l i t y can be made by biophysical or other methods (Forrest e t a l., 1965), appropriate control measures could be developed (Briskey, 1963; Hallund & Bendall, 1965) t h a t would eventually lead t o desired degrees of tenderness, t e x t u r e, j u i c i n e s s, f l a v o r and color.

10 285. While the postmortem biophysics of muscle discussed in this paper may not necessarily reflect the actual carcass conditions, they represent our knowledge today and offer us a starting point for much needed further investigations. BateSmith, E. C The physiology and chemistry of rigor mortis, with special reference to the aging of beef. Adv. Food Res. &:138. BateSmith, E. C. and J. R. Bendall Factors determining the time course of rigor mortis. 2. Physiol. (London) 110:4765. Bay, Z., M. C. Goodall and A. SzentGyorgyi The transmission of excitation from the membrane to actomyosin. Bull. Math. Biophysics 15 : Bendall, J. R. 1960a. Postmortem changes in muscle, in Structure and Function of Mzscle, Vol. lll, (G. H. Bourne, ea.) Academic Press, New York, p Bendall, J. R. 1960b. The stretch deformation of skeletal muscle, in Flow Properties of Blood and other Biological Systems (A. L. Copely & G. Stainsbz eds.), Pergamon Press, New York, p Briskey, E. J Influence of ante and postmortem handling practices on properties of muscle which are related to tenderness, Roc. Meat Tenderness Sm., Campbell Soup Co., p Briskey, E. J., Etiological status and associated studies of pale, soft, exudative porcine musculature. Adv. Food Sci. 13: Briskey, E. J., R. N. Sayre and R. G. Cassens Development and application of an apparatus for continuous measurement of muscle extensibility and elasticity before and during rigor mortis. J. Food Sei. 27: Buchthal, F & Introduction to Electromyography, Gyldendal, Denmark, p Buchthal, F. and G. G. Knappeis Propagation of contraction in the isolated striated muscle fibre. Acta Physiol. Scand. 5 : Catton, W. T Physical Methods in Physiology, Philosophical Library, New York, p Connell, J. J Mechanical properties of fish and fish products, in Flow Properties of Blood and other Biological Systems (A. L. Copely & G. Stainsbz, Pergamon Press, New York, p

11 286. DeFremery, D. and M. F. Pool Biochemistry of chicken muscles as r e l a t e d t o r i g o r mortis and tenderization. Food Res. 25:7387 Ernst, E Biophysics of t h e S t r i a t e d Muscle. Budapest, p Ferry, J. D Protein gels. Akademiai Kiado, E.Protein Chem. 4:l78. Forrest, J. D., M. D. Judge, J. D. Sink, W. G. Hoekstra and E. J. Briskey, Prediction of t h e time course of r i g o r mortis through response of muscle t i s s u e t o e l e c t r i c a l stimulation. J. Food Sci. (submitted). Galambos, R p M s c l e s and Nerves. Doubleday, Garden City, Garamvolgyi, N., J. Kerner and M. CserSchultz. Hung. 24 : Acta Physiol. Gilev, V. P A study of myofibril sarcomere structure during contraction. 2. C e l l Biol. 12: Hallund, 0. and J. R. Bendall The longterm e f f e c t of elect r i c a l stimulation on t h e postmortem f a l l of ph i n t h e muscles of Landrace pigs. 2. Food Sci. =: Hasselbach, W D i e umwandlung von aktomyosinatpase i n Lmy0 sin ATP ase durch aktivatoren und d i e resultierenden aktiviernaturforsch. 7B: ungseffekte. z. Herring, H. K., R. G. Cassens and E. J. Briskey Further studies on bovine muscle tenderness a s influenced by carcass position, J. Food Sci. (submitted.) sarcomere length and f i b e r diameter. Hodge, A. J Studies on the structure of muscle. Biochem. Cytol. 1: s. Biophys. Horvath, B Contraction and c r o s s s t r i a t i o n of muscle. Biochem. Biophys. Acta 8: Huxley, A. F Muscle structure and theories of contraction. Prog. i n Biophysics 1: Huxley, H. E (5) :313. The contraction of muscle. S c i e n t i f i c American L a w r i e, R. A The conversion of muscle t o meat. Food Sci. 1:6882. Rec. Adv. Locker, R. H Degree of muscular contraction as a f a c t o r i n tenderness of beef. Food R e s. 25: Loeb, J On the changes i n t h e nerve and muscle which seem t o underlie t h e electrotonic e f f e c t s on t h e Galvanic current. Univ. Calif. fibl. i n Physiol. 3:915.

12 287. Nachmansohn D., and I. B. Wilson Molecular basis for generation of bioelectric potentials, in Electrochemistry in Biology and Medicine (T. Shedlovsky, ed.), John Wiley & Sons, New Yox, p Opatowski, I On the mathematical theories of excitation. Bull. Math. Biophysics 13:4145 Perry, S. V Relation between chemical and contractile function and structure of skeletal muscle cell. Physiol. Revs. 36:l372. Polissar, M. J Physical chemistry of contractile process in muscle. I. A physiocochemical model of contractile mechanism. her. J. Physiol. 168: Pringle, J. W. S Models of muscle. S~mp. SOC. Exp. Biol. L 14 : Pryor, M. G. M The rheology of muscle, in Deformation and Flow in Biological Systems (A. FreyWyssling, ed.), NorthHolland Publishers, Amsterdam, p Sayre, R. N., B. Kiernat and E. J. Briskey Processing characteristics of procine muscle related to ph and temperature during rigor mortis development and to gross morphology 24hr postmortem. J. Food SCi. 29 : Sink, J. D., R. G. Cassens, W. G. Hoekstra and E. J. Briskey Rigor mortis pattern of skeletal muscle and sarcomere length of the myofibril. Biochem. Biophys. Acta 102: SzentGyorgyi, A Chemical Physiology of Contraction in Body and Heart Muscle. Academic Press, New York, p Trautman, J. C Fatemulsifying properties of prerigor and postrigor pork proteins. Food Tech. 2: Ungar, G Excitation. Charles Thomas, Springfield, p Westergaard, H. M Theory of Elasticity and Plasticity, Harvard Univ. Press, Cambridge, p. 845 Whitaker, J. R Chemical changes associated with aging of meat with emphasis on the proteins. Adv. Food Res. 9:l60. Wilkie, D. R Facts and theories about muscle. Prop,. in Biophysics 4:

13 288. G. R. BEECHER: The t h r e e speakers t h i s morning have presented reviews and observations on t h e r e l a t i o n s h i p of antemortem physiological phenomenon i n t h e porcine animal a s they may be r e l a t e d t o t h e ultimate p r o p e r t i e s of t h e muscle. D r. Zobrisky has indicated t h e r o l e n u t r i t i o n plays i n t h e development and maturation of muscle t i s s u e. He has a l s o presented evidence t h a t t h e enzyme and s u b s t r a t e concentrations i n muscle t i s s u e immediately p r i o r t o exsanguination of t h e animal a r e very important i n determining t h e ultimate postmortem muscle properties. Mr. Forrest has presented very e x c i t i n g observations on t h e r e l a t i o n s h i p of some antemortem physiological measurements t o t h e ultimate p r o p e r t i e s of t h e muscle t i s s u e from t h e same animals. The physiological concepts t h a t John presented a r e wellknown, however they have been applied i n a new manner t o the f i e l d of muscle research. The t h i r d speaker, D r. Sink, has a l s o presented an excellent review of a d i f f i c u l t subject and implicated how physical changes may o r do a f f e c t t h e ultimate properties of muscle. I would l i k e t o expand b r i e f l y on another area t h a t w e f e e l a f f e c t s t h e ultimate p r o p e r t i e s of porcine muscle and c e r t a i n l y accounts f o r a l a r g e portion of t h e musclemuscle v a r i a t i o n observed i n t h e porcine carcass. This area i s t h e red f i b e r content of a muscle. M r. Norman b r i e f l y touched on t h i s a r e a yesterday. Generally, white muscle f i b e r s are characterized by anaerobic o r glycolytic enzyme a c t i v i t y whereas red fibers have aerobic o r Kreb's cycle enzyme a c t i v i t y. Thus, it can be seen t h a t t h e proportion of r e d t o white f i b e r s i n a muscle w i l l p r e d i c t t h e general type of metabolic a c t i v i t y antemortem and d e f i n i t e l y p r e d i c t t h e amount and r a t e of enzyme a c t i v i t y postmortem provided s u f f i c i e n t substrate, i n t h i s case glycogen, i s present. We have observed ranges i n r e d fiber content from a low of 19% i n t h e semitendinosus l i g h t portion t o a high of 47% i n t h e trapezius. The seven porcine muscles studied were c l a s s i f i e d a s e i t h e r red o r white on t h e basis of t h e i r red fiber content. It should be pointed out t h a t those muscles i n t h e porcine carcass which become pale, s o f t, and exudative most frequently (longissimus d o r s i and gluteus medius) a r e white muscles (less than 30% red f i b e r s ) whereas those muscles characterized by a r e s i s t a n m o t h i s condition a r e red muscles ( g r e a t e r than 30% red f i b e r s ). From t h e s e observations, we f e e l t h e r e d f i b e r content of a muscle i s extremely important i n determining t h e p r o p e r t i e s that a muscle will ultimately a t t a i n postmortem and adds another f a c t o r t o t h e many t h a t contribute t o t h e ultimate postmortem p r o p e r t i e s of a muscle. I believe t h e r e i s t i m e f o r a few questions. Although w e a r e running l a t e, I am sure t h a t t h i s a very e x c i t i n g area and we w i l l entert a i n questions from t h e f l o o r. Dr. Webb?

14 289. DR. WEBB: I have a question f o r Mr. Forrest concerning t h e evaluation of these muscles, t h e time a f t e r death they were cut, the temperature of the muscle, t h e l i g h t conditions and t h e general condit i o n a t observation. J. C. FORRESJ!: These carcasses were cut a t 24 hours post mortem. W e did not measure t h e a c t u a l temperature. There was very l i t t l e variation. V. R. CAHILL: As we move i n t o t h i s second phase of the program we are going t o take a look a t those f e a t u r e s of both carcass and e know t h a t over t h e years a great deal of work has been pork product. W done on swine improvement. We a r e out here i n t h i s S t a t e of Kansas where we've heard much about pioneering i n the l a s t f e w days and w e have c a l l e d on a man who has pioneered i n t h e area of pork carcass evaluation and swine improvement. I am r e f e r r i n g t o Wilbur Bmner who i s now pioneering f o r a year i n a new area, t h e Armour Pork Company, and without taking f u r t h e r t i m e, Wilbur, may I t u r n t h i s microphone t o you. #############