The relationship between muscle fiber characteristics, postmortem metabolic rate, and meat quality of pig longissimus dorsi muscle

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1 Meat Science 71 (2005) MEAT SCIENCE The relationship between muscle fiber characteristics, postmortem metabolic rate, and meat quality of pig longissimus dorsi muscle Y.C. Ryu, B.C. Kim * Division of Food Science, College of Life and Environmental Sciences, Korea University, 5-1 Anam-dong, Sungbuk-gu, Seoul , Republic of Korea Received 22 December 2004; received in revised form 7 April 2005; accepted 7 April 2005 Abstract The aim of this study was to investigate the histochemical parameters of muscle fibers, and to estimate the correlation of muscle fiber characteristic to postmortem metabolic rate and meat quality traits in pigs. A total of 231 crossbred pigs were evaluated. Samples of the longissimus dorsi muscle were taken to evaluate the histochemical characteristics, postmortem metabolic rate and meat Fiber type composition was mainly related to postmortem metabolic rate and meat quality traits among various muscle fiber characteristics. The percentage of type IIb fiber was negatively related to ph 45 min (r = 0.33) and positively to R-value (r = 0.32). Drip loss was negatively related to fiber area percentages of type I and IIa (r = 0.25 and 0.26, respectively) and positively related to type IIb percentage (r = 0.39). A similar tendency was found between lightness and fiber area percentage. In conclusion, increasing the percentage of type IIb fiber is related to increasing the postmortem metabolic rate, and is related to the deterioration of meat Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Muscle fiber; Postmortem metabolic rate; Pork quality 1. Introduction Mechanisms controlling pork quality development are often associated with altered postmortem muscle metabolism. Specifically, changes in the extent or rate of glycolysis can create unfavorable muscle ph. A high rate of ph decline and a low ultimate ph result in muscle protein denaturation and diminished quality parameters (Hammelman et al., 2003; Henckel, Karlsson, Oksbjerg, & Petersen, 2000). One of the main factors determining muscle biochemical pathways is fiber type composition: skeletal muscle is composed of different types of fibers, which are the results of coordinated expression of distinct sets of structural proteins and metabolic enzymes (Chang et al., 2003; Schiaffino & Reggiani, 1996). Fiber types are often * Corresponding author. Tel.: ; fax: address: bckim@korea.ac.kr (B.C. Kim). defined by the isoforms of myosin heavy chain (MHC) that are present. According to the major MHC isoforms found in adult mammalian skeletal muscles, four different single MHC isoforms have been identified in ATPase based fiber types: MHC-1 in type I, MHC-2a in type IIa, MHC-2b in type IIb and MHC-2d(x) in type IId(x) (Pette & Staron, 2000). The type I and IIb fibers, also known as slow-oxidative and fast-glycolytic, respectively, represent two extreme metabolic profiles. The type IIa and IIx fibers are intermediate fast oxidative-glycolytic fibers (Klont, Brocks, & Eikelenboom, 1998). Because muscle fibers contain different myosin heavy chains, which are responsible for their different ATPase activities (Picard et al., 1999), it is possible that fiber type composition may be associated with postmortem changes in the conversion of muscle to meat and subsequently meat quality (Karlsson, Klont, & Fernandez, 1999). Therefore, the variation in fiber type characteristics can explain part of the /$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi: /j.meatsci

2 352 Y.C. Ryu, B.C. Kim / Meat Science 71 (2005) variation in some meat quality traits (Essen-Gustavsson, Karlsson, Lundstrom, & Enfalt, 1994). It is well known that muscle fiber number, size, and fiber type composition are closely related to each other (Ryu, Rhee, & Kim, 2004), and some recent studies found correlations between muscle fiber characteristics and meat quality traits in beef (Maltin et al., 1998; Ozawa et al., 2000) and in pigs (Eggert, Depreux, Schinckel, Grant, & Gerrard, 2002; Karlsson et al., 1999). For the practical application of this knowledge to improve and control meat quality, more information of the effects of the fiber type characteristics on postmortem metabolic rate is necessary. Therefore, the aim of this study was to investigate the morphological and histochemical parameters of muscle fibers, and to estimate the correlation of muscle fiber characteristic to postmortem metabolic rate and meat quality traits in pigs. 2. Materials and methods 2.1. Animals A total of 231 crossbred Duroc (Yorkshire Landrace) pigs (149 gilts and 82 castrated male pigs) were evaluated. All pigs tested by DNA probe (Fujii et al., 1991) and normal on ryr1 gene pigs were used in the experiment. Pigs were slaughtered by electrical stunning at ± 1.7 days of age. The abattoir used a traditional scalding-singeing process. After evisceration, the carcasses were weighed (82.7 ± 7.97 kg) and backfat thickness was measured at the 11th and last thoracic vertebrae. The mean of these two measurements was used as backfat thickness (20.6 ± 5.9 mm) Histochemical analysis Within 45 min postmortem, muscle samples for histochemical analysis were taken from the longissimus muscle at the 8th thoracic vertebrae. Samples were cut into cm pieces, promptly frozen in isopentane cooled by liquid nitrogen, and stored at 80 C until subsequent analyses. Serial transverse muscle sections (10 lm) were obtained from each sample with a cryostat (CM1850, Leica, Germany) at 20 C and mounted on glass slides. The myosin adenosine triphosphatase activities were detected after acid (ph 4.7) or alkaline (ph 10.7) preincubation (Lind & Kernell, 1991). All histochemical samples were examined by an image analysis system. The operational system consisted of an optical microscope equipped with a CCD color camera (IK-642K, Toshiba, Japan), and a standard workstation computer which controls all the image analysis system (Image-Pro Plus, Media Cybernetics, L.P., USA). All portions of the sections analyzed were free from tissue disruption and freeze damage. About 600 fibers per sample were evaluated. On the two series of photographs in each tissue taken at the same location on the different specimens, the muscle fibers were divided into types I, IIa, and IIb according to the nomenclature of Brooke and Kaiser (1970). Fiber number percentage was obtained from the ratio of the number of each fiber type to the total number of fiber counted, and fiber area percentage was the ratio of the total cross-sectional area (CSA) of each fiber type to the total measured fiber area. The average CSA, diameter and perimeter of the typeidentified fibers were also measured. Fiber density was calculated from the mean number of fibers per mm 2. For the calculation of the total fiber number, the fiber density per mm 2 was multiplied with the loin-eye area (cm 2 ) determined at the level of the last rib Postmortem metabolic rate Samples from the longissimus muscle at the 8th thoracic vertebrae were taken at 45 min postmortem and immediately frozen in liquid nitrogen to determine postmortem metabolic rate. Early postmortem muscle ph and temperature were determined directly on carcass after the muscle samples were removed. The ph was measured using a spear type electrode (Model 290A, Orion Research Inc., USA) at 45 min (ph 45 min ) and 24 h postmortem (ph 24 h ). Muscle temperature was measured in the center of the muscle using a portable thermometer (Model TES-1300, TES Electrical Electronic Co., Taiwan). To determine the depletion of ATP postmortem, R-value (adenine/inosine ratio) was measured by the procedure of Calkins, Dutson, Smith, and Carpenter (1982). Briefly, samples placed in 6% perchloric acid were homogenized (Ace Homogenizer AM-8, Nissei Co., Japan) at 5000 rpm for 90 s and then centrifuged (Centrikon T-124, Kontron instruments Co., Switzerland) at 3000g for 10 min at 2 C. The absorbance were measured using a calibrated spectrophotometer (Model Du-64, Beckman Co., USA), and adenine/inosine ratio was determined as the ratio of the absorbance readings at 250/260 nm Meat quality traits Following 24 h of chilling, M. longissimus dorsi was taken to evaluate the meat quality traits. For each of the 231 loins, every effort was made to maintain consistency in using the same anatomical location for each procedure. Drip loss was determined by suspending muscle samples standardized for surface area in an inflated plastic bag for 48 h at 2 C (Honikel, 1987). Filter-paper fluid uptake (FFU) was also measured as described by Kauff-

3 Y.C. Ryu, B.C. Kim / Meat Science 71 (2005) man, Eikelenboom, Van der Wal, Merkus, and Zaar (1986). The color of the meat was measured at 45 min and 24 h postmortem with a Minolta chromameter (CR- 300, Minolta Camera Co., Japan) after exposing the surface to the air for 30 min at 2 C. The average of triplicate measurements was recorded and the results were expressed as C.I.E. (Commission International de lõeclairage) L*, a*, and b*. Warner Bratzler shear force (WBS) was determined using an Instron Universal Testing Machine (Model 1011, Instron Corp., USA) equipped with a Warner Bratzler shearing device. Six cores (1.27 cm diameter) were removed from each steak parallel to the longitudinal orientation of muscle fibers. Samples were sheared perpendicular to the long axis of the core Statistical analysis Descriptive statistics were performed using the MEANS procedure of the SAS PC software (SAS, 2001) to calculate mean values and standard deviations for all variables. Pearson correlation coefficients were evaluated to describe the relationship between postmortem metabolic rate, meat quality traits and muscle fiber characteristics using partial correlation coefficients (CORR procedure of SAS, 2001). 3. Results and discussion 3.1. Meat quality traits and postmortem metabolic rate The mean values, standard deviations and overall ranges for early postmortem parameters and meat quality traits measured are given in Table 1. Broad ranges in all variables were observed at 45 min and 24 h postmortem (PM). The variations in postmortem metabolic rate Table 1 Early postmortem parameters and meat quality traits of the longissimus dorsi muscle in pigs Mean SD Min Max ph 45 min R-value Lightness (L*) Redness (a*) Yellowness (b*) h Postmortem ph 24 h Drip loss (%) FFU (mg) Lightness (L*) Redness (a*) Yellowness (b*) WBS (N) Filter-paper fluid uptake. 2 Warner Bratzler shear force. and meat quality traits of finishing pigs used in the present study were similar to those of other literature (Allison, Bates, Booren, Johnson, & Doumit, 2003; Gentry, McGlone, Blanton, & Miller, 2002; Larzul et al., 1997). These results indicate that there are many differences in postmortem metabolic rate and meat Differences in lightness at 45 min PM probably resulted from variation in the early postmortem metabolic rate and reflect the variation in early postmortem meat The correlation coefficients between early postmortem parameters and meat quality traits are presented in Table 2. Different measures of water-holding capacity (drip loss and FFU) were highly related to each other (r = 0.68). L* value at 24 h PM also correlated highly with 45 min PM measures of L* value (r = 0.56). For drip loss, ultimate meat color traits of L* (r = 0.58) and b* (r = 0.44) showed positive correlation, while shear force (r = 0.29) and ph 24 h (r = 0.50) were neg- Table 2 Correlation coefficients (r) within/between metabolic rate measurement and meat quality trait of longissimus dorsi muscle in pigs ph 45 min R-value L* 45 min a* 45 min b* 45 min ph 24 h Drip loss FFU L* 24 h a* 24 h b* 24 h R-value 0.73 z 1 L* 45 min 0.66 z 0.65 z 2 a* 45 min b* 45 min 0.20 y 0.15 x 0.44 z 0.67 z ph 24 h 0.32 z 0.24 z 0.18 y 0.15 x 0.05 Drip loss 0.40 z 0.41 z 0.42 z y 0.50 z FFU z 0.43 z 0.43 z z 0.68 z 1 L* 24 h 0.51 z 0.41 z 0.56 z y 0.56 z 0.58 z 0.62 z 2 a* 24 h x x b* 24 h 0.31 z 0.30 z 0.31 z z 0.44 z 0.43 z 0.62 z 0.63 z WBS x 0.28 y 0.26 x 0.29 y y z 1 3 Lightness (L*), redness (a*) and yellowness (b*) measured at 45 min and 24 h postmortem. Levels of significance: x P < 0.05; y P < 0.01; z P <

4 354 Y.C. Ryu, B.C. Kim / Meat Science 71 (2005) atively correlated. L* value at 24 h PM also correlated inversely with shear force (r = 0.33) and ph 24 h (r = 0.56). These results indicate that pork with a lower ultimate ph becomes exudative and lighter, as suggested by Allison et al. (2003). Early postmortem parameters correlated moderately with meat quality traits. Both drip loss and L* value correlated positively with R-value (r = 0.41 and 0.41, respectively) and were inversely related to ph (r = 0.40 and 0.51, respectively) measured 45 min PM. With increasing lightness and R-value measured 45 min PM, the drip loss was higher, the color was paler, and the ultimate ph was lower. Muscle ph 45 min reflects the rate of postmortem metabolism and influences the degree of protein denaturation (Sellier & Monin, 1994). Offer and Knight (1988) reported that the rate of ph decline and ultimate ph are the two main determinants of WHC and meat color. Brewer, Zhu, Bidner, Meisinger, and McKeith (2001) also demonstrated that as ph approaches the isoelectric points of muscle proteins, free water increases and scatters more light. Thus, decreasing ph 45 min is related to increasing drip loss and lightness, which is deteriorative to meat These results indicate that R-value and ph 45 min, which reflect the early postmortem metabolic rate, are reliable parameters to predict ultimate pork Moreover, lightness measured 45 min PM could be used as evidence of the variation in the early postmortem metabolic rate and meat Fig. 1. Serial sections of longissimus thoracis muscle, stained for matpase activity after preincubation at ph 4.7 (a) and ph 10.7 (b). Magnification of 100 was used (bar = 50 lm). Abbreviations: I, type I fiber (slow-twitch, oxidative); IIa, type IIa fiber (fast-twitch, oxido-glycolytic); IIb, type IIb fiber (fast-twitch, glycolytic).

5 Y.C. Ryu, B.C. Kim / Meat Science 71 (2005) Muscle fiber size To evaluate the histochemical characteristics of pig longissimus dorsi muscle, this study used myosin ATPase reactivity (Fig. 1). The relationships between muscle fiber size and meat quality traits were generally limited and only a few significant correlations were found (Table 3). For fiber size parameters, there was a negative correlation with FFU (r = 0.16 to 0.20) and WBS (r = 0.22 to 0.25). However, drip loss and lightness were not associated with muscle fiber size parameters, as suggested by Candek-Potokar, Lefaucheur, Zlender, and Bonneau (1999). Also, no clear relationship between CSA and muscle ph 45 min was observed in agreement with Larzul et al. (1997) Muscle fiber quantity In order to evaluate the fiber quantity, the density of fiber per mm 2 and total number of fibers were determined. The overall relationships between meat quality traits and muscle fiber number are presented in Table 4. Drip loss correlated positively with the density (r = 0.16) and total number (r = 0.25) of type IIb fiber. Higher numbers of type IIb fiber was associated with increasing drip loss. On the other hand, the number of type I and IIa fibers had an inverse relationship with drip loss. Higher numbers of type I and IIa fibers would be expected to result in increased WHC. Consistent with these observations, L* correlated inversely with the number of type I and IIa fibers. In addition, the number of muscle fibers was consistently correlated positively with WBS, and an inverse relation between muscle fiber size and WBS was observed (Table 3). These observations probably resulted from the close inverse relationship between muscle fiber number and size, in accordance with other literature (Larzul et al., 1997). The fiber density was strongly affected by muscle fiber size and negatively related to CSA (Ryu et al., 2004). Maltin et al. (1997) also found that the fiber diameter of type IIb fiber significantly contributed to the variation in meat tenderness. Early postmortem muscle ph was inversely related to the number of type IIb fiber. In contrast, the number of type I fiber was positively related to ph 45 min. A rather strong correlation was observed between R-value and the number of type IIb fiber. Ultimate muscle ph correlated inversely with the number of type IIb fiber, but was related to type IIa fiber. These results indicate that pigs with higher fiber numbers of type I and IIa fiber have lower drip loss, lower L* value, and higher muscle ph at 45 min PM. Higher WHC observed with increased numbers of type I fiber resulted from a direct relationship between postmortem metabolic rate and drip loss. Indeed, ph 45 min and R-value were closely related with drip loss (Table 2). It is generally accepted that low muscle fiber number correlates with fibers that exhibit greater hypertrophy, and these large muscle fibers seem to be associated with poor meat quality (Gentry et al., 2002). In the present study, however, postmortem metabolic rate and meat quality traits were much more related to numerical abundance than muscle fiber size Fiber type composition To understand the effect of fiber type composition, this study analyzed the relationships between histochemical characteristics and meat quality traits (Table 5). Table 3 Correlation coefficients (r) between muscle fiber size and meat quality traits of the longissimus dorsi muscle in pigs Cross-sectional areas Diameters Mean I 1 IIa 2 IIb 3 Mean I IIa IIb ph 45 min R-value L* a* x b* h Postmortem ph 24 h Drip loss FFU z 0.17 x x 0.16 x 0.08 L* a* b* WBS x x x Type I, IIa, and IIb fibers. Level of significance: x P < 0.05.

6 356 Y.C. Ryu, B.C. Kim / Meat Science 71 (2005) Table 4 Correlation coefficients (r) between muscle fiber number and meat quality traits of the longissimus dorsi muscle in pigs The density of muscle fiber Total number of fibers Sum I 1 IIa 2 IIb 3 Sum I IIa IIb ph 45 min z z R-value y 0.15 x 0.15 x x 0.22 y L* y y a* x x b* x x h postmortem ph 24 h z 0.17 x 0.16 x y 0.24 z Drip loss y 0.24 z 0.16 x 0.16 x 0.17 x 0.17 x 0.25 z FFU x 0.18 y 0.15 x x 0.20 y L* z 0.18 y z 0.17 x 0.11 a* x x b* y x 0.04 WBS y x 0.31 y Type I, IIa, and IIb fibers. Levels of significance: x P < 0.05; y P < 0.01; z P < Table 5 Correlation coefficients (r) between fiber type composition and meat quality traits of the longissimus dorsi muscle in pigs Fiber number percentage Fiber area percentage I 1 IIa 2 IIb 3 I IIa IIb ph 45 min 0.20 y 0.20 y 0.33 z 0.29 z z R-value z 0.32 z 0.22 y 0.23 y 0.31 z L* 0.17 x 0.14 x 0.27 z 0.24 z y a* y 0.14 x y 0.22 y b* 0.16 x y h Postmortem ph 24 h z 0.23 y z 0.33 z Drip loss z 0.36 z 0.25 z 0.26 z 0.39 z FFU z 0.34 z 0.19 y 0.21 y 0.27 z L* 0.18 y 0.23 y 0.34 z 0.29 z 0.15 x 0.33 z a* y b* y 0.19 y y 0.20 y WBS x x Type I, IIa, and IIb fibers. Levels of significance: x P < 0.05, y P < 0.01, z P < Results showed that, among various muscle fiber characteristics, fiber type composition was mainly related to postmortem metabolic rate and meat quality traits. Drip loss was inversely related to fiber area percentages of type I and IIa (r = 0.25 and 0.26, respectively) and positively related to type IIb percentage (r = 0.39). Similar tendencies were found between drip loss and fiber number percentage. Different measurements of water-holding capacity (FFU) also showed the same tendency. Another important quality parameter, lightness, was negatively related to type I and IIa fiber percentages (r = 0.29 and 0.15, respectively) and positively related to the percentage of type IIb fiber (r = 0.33). These results suggested that increasing the percentage of type IIb fiber and decreasing the percentages of type I and IIa fibers is related to increasing the drip loss and lightness, which is deteriorative to meat In general, glycolysis and onset of rigor mortis are faster in white than in red muscles. Type IIa and IIb fibers mainly carry out the glycolytic pathway, and their metabolism contributes to a fast ph decline. These fiber type compositions of porcine muscles are related to the occurrence of the PSE condition (Candek-Potokar et al., 1999). In this study, the percentage of type IIb fiber was negatively related to ph 45 min and positively to R-value. On the contrary, increasing percentages of both type I and IIa fibers was related to decreasing rates of postmortem metabolism. These results were in agreement with a relationship between postmortem metabolic rate and numerical abundance of a given fiber type (Table 4). Ultimate muscle ph correlated negatively with the percentage of type IIb fiber, but was positively related to that of type IIa fiber. This study shows that R-value and ph 45 min, which reflect the early postmortem metabolic rate, are reliable parameters for predicting ultimate pork Lightness measured 45 min postmortem can be used as an evidence of the variation in the early postmortem metabolic rate and meat Increasing the percentage of type IIb fiber was related to increasing the rate of muscle ph decline, drip loss, and lightness. These results imply that accelerated metabolic rate and poor meat quality are partially explained by an increase in percentage of type IIb fiber.

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