Meat Science 95 (2013) Contents lists available at SciVerse ScienceDirect. Meat Science. journal homepage:

Transcription

1 Meat Science 95 (2013) Contents lists available at SciVerse ScienceDirect Meat Science journal homepage: Review Control of fresh meat quality through manipulation of muscle fiber characteristics S.T. Joo a,,1, G.D. Kim b,1, Y.H. Hwang a, Y.C. Ryu b a Department of Animal Science, Institute of Agriculture & Life Science, Gyeongsang National University, Jinju, Gyeongnam , South Korea b Division of Biotechnology, College of Applied Life Sciences, Jeju National University, 102 Jejudaehakro, Jeju , South Korea article info abstract Article history: Received 8 February 2013 Received in revised form 14 April 2013 Accepted 15 April 2013 Keywords: Meat quality traits Muscle fiber characteristics Muscle fiber types Variations of fresh meat quality exist because the quality traits are affected by various intrinsic and extrinsic factors. Because the meat quality is basically dependent on muscle fiber characteristics, numerous studies have reported the relationship between quality traits and fiber characteristics. Despite intensive research, the relationship is yet to be fully established, however, the present knowledge suggests several potential ways to manipulate muscle fiber characteristics to improve meat quality. The present paper reviews the definition of fresh meat quality, meat quality traits and variations of meat quality. Also, this review presents recent knowledge underlying the relationship between fresh meat quality traits and muscle fiber characteristics. Finally, the present work proposes several potential factors including breed, genotype, sex, hormone, growth performance, diet, muscle location, exercise and ambient temperature that can be used to manipulate muscle fiber characteristics and subsequently meat quality in animals The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-SA license. 1. Introduction Meat quality has always been important to the consumer, and it is an especially critical issue for the meat industry in the 21st century. As consumer demand for high quality meat is increasing in most countries, the meat industry should consistently produce and supply quality meat that is tasty, safe and healthy for the consumer to ensure continued consumption of meat products. In order to produce high quality meat, it is necessary to understand the characteristics of meat quality traits and factors to control them. Fresh meat quality is difficult to define because it is a complex concept determined by consumer preferences. Because fresh meat is animal tissue that is suitable for use as food, the quality characteristics are influenced by various factors such as muscle structure, chemical composition, chemical environment, interaction of chemical constituents, postmortem (p.m.) changes in muscle tissues, stress and pre-slaughter effects, product handling, processing and storage, microbiological numbers and populations, etc. In particular, fresh meat quality is directly related to muscle fiber characteristics because skeletal muscles mainly consist of muscle fibers. The muscle fibers are characterized by their morphological traits, and contractile and metabolic properties (Lee, Joo, & Ryu, 2010). Morphology traits such as total number of fibers (TNF) and cross-sectional area of fibers (CSAF) are major determinant factors of muscle mass as well as meat quality. Also, contractile and metabolic properties of muscle are differentiated by muscle fiber types, and thus fresh meat quality is strongly related to fiber type composition (FTC) in muscle. In general, there are four different muscle fiber types in adult skeletal muscle, which are slow-oxidative or type I, fast oxido-glycolytic or type IIA, and fast glycolytic IIX and IIB (Schiaffino & Reggiani, 1996). All of these fiber types are observed in most muscles of meat animals, and their relative composition in the different muscles can determine the predominance of muscle's metabolic properties (Ozawa et al., 2000; Ryu & Kim, 2005). Consequently, p.m. muscle metabolism which is a crucial factor to determine fresh meat quality is affected by TNF, CSAF and FTC (Kim, Jeong, et al., 2013; Ryu, Lee, Lee, & Kim, 2006). These muscle fiber characteristics vary by various factors including breed (Ryu et al., 2008), selection (Larzul et al., 1999), gender (Ozawa et al., 2000), hormone (Rehfeldt, Fiedler, & Stickland, 2004), growth performance (Gondret, Lefaucheur, Juin, Louveau, & Lebret, 2006; Kim, Kim, et al., 2013), diet (Jeong et al., 2012) and muscle location (Beermann et al., 1990; Hwang, Kim, Jeong, Hur, & Joo, 2010). Therefore, understanding the relationship between muscle fiber characteristics and meat quality traits will improve the production of quality meat, and manipulation of muscle fiber characteristics would have profound impacts on the profitability of the meat industry. The present paper reviews the scientific literature in meat quality traits, muscle fiber characteristics and potential factors to manipulate muscle fiber characteristics. 2. Fresh meat quality Corresponding author. Tel.: ; fax: address: stjoo@gnu.ac.kr (S.T. Joo). 1 These authors contributed equally to this work. The term fresh meat quality is very ambiguous because its definition varies depending on the background of consumers in different The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-SA license.

2 S.T. Joo et al. / Meat Science 95 (2013) regions of the world. Accordingly, first of all, meat quality should be defined by most consumer preferences. Consumer preferences are related directly to the human senses such as appearance, smell, taste and mouthfeel. Also, fresh meat quality can be defined by scientific factors including composition, nutrients, colorants, water-holding capacity (WHC), tenderness, functionality, flavors, spoilage, contamination, etc. The quality of fresh meat indicates its usefulness to the consumer and its acceptability for cooking. The important quality traits for fresh meat are color, WHC, texture and amount of fat (intramuscular fat/ intermuscular fat/subcutaneous fat), while the important traits for eating quality of cooked meat are tenderness, flavor and juiciness. In general, consumers rate color as the most important quality trait for fresh meat, while tenderness is rated as the most important palatability trait for cooked meat followed by flavor and juiciness (Glitsh, 2000). However, this can vary among consumers depending upon past experiences and cultural background. Therefore, the order of importance of meat quality traits can vary by country (Warner, Greenwood, Pethick, & Ferguson, 2010). The appearance of meat is determined by meat color, packaged meat color, amount and distribution of fat, fat color, amount of drip on the surface of the meat, purge in the tray, and texture of the meat (Becker, 2000). These appearance quality traits (AQT) strongly influence the consumer's decision to select good quality meat at the point of purchase. However, the consumer determines the actual meat quality at the point of consumption with eating quality traits (EQT) such as tenderness, flavor, juiciness and succulence (Acebron & Dopico, 2000). Additionally, consumers assess meat quality by reliance quality traits (RQT) such as safety, nutrition, animal welfare, ethics, price, product presentation, origin, and brand of meat products (Troy & Kerry, 2010). Therefore, it is appropriate to define the term fresh meat quality by consumer preferences that are determined by RQT as well as AQT and EQT of meat (Joo & Kim, 2011). 3. Meat quality traits Quality traits of fresh meat are categorized based on major intrinsic and extrinsic factors. Generally, intrinsic factors are the physiological characteristics of meat such as AQT and EQT, whereas extrinsic factors are the RQT of meat products (Joo & Kim, 2011). All these traits contribute to the consumer's expectation of high quality meat. Consumers determine quality meat as one with desirable color, firm texture, less drip, high marbling, and moderate visible fat and fresh meat odor, while discoloration, soft texture, large amount of drip, less marbling, excessive visible fat and abnormal meat odor are considered as poor quality traits for fresh meat. Also, the consumer expects quality meat that is reliable in relation to safety, nutrition, sustainability and ethics (Troy & Kerry, 2010) Appearance quality traits (AQT) Meat color is the most important AQT because it is the first factor seen by the consumer and is used as an indication of freshness and wholesomeness. Basically, meat color is dependent on species, age and muscle type, and the color differences are due to the different content of myoglobin (Mb) in muscle. The higher Mb content in type I muscle fiber is due to Mb's function of storing and delivering oxygen in the muscle. The Mb content in muscle is affected by factors such as exercise and diet of the animal as well as genetic and environmental factors. Many factors contribute to the discoloration of meat during processing, storage and display. The predominant determinant of meat color stability is the rate of OxyMb oxidation (Faustman, Sun, Mancini, & Suman, 2010), and the rate of discoloration in meat is muscle-specific. Rapid discoloration occurs in muscles that contain greater relative proportions of type I muscle fibers because of higher oxygen consumption rate (Jeong et al., 2009). Two other important AQT for fresh meat are the amount of drip on the surface of meat and purge in the tray. Drip and purge loss depend on the WHC of meat, and WHC is closely related to the color of meat due both to its role in the loss of Mb and reflectance at the surface of the meat (Joo, Kauffman, Kim, & Kim, 1995). Additionally, WHC influences other physical properties including texture and firmness of raw meat, and eating properties of cooked meat. Drip loss originates from the spaces between muscle fiber bundles and the perimysial network, and the spaces between muscle fibers and the endomysial network (Offer & Cousins, 1992). These spaces appear during rigor development when muscle converts to meat. It is well known that excessive drip exudation and soft texture result from the combination of rapid ph decline, and high temperature in p.m. muscle (Joo, Kauffman, Kim, & Park, 1999; Warner, Kauffman, & Greaser, 1997). This is an especially prevalent problem for pork which contains greater relative proportions of type II muscle fibers compared to beef or lamb. Meat texture is directly related to the size of muscle fiber and the amount of connective tissue, and is partially affected by the quantity of intramuscular fat (IMF). Relatively large muscle bundles are responsible for the coarse, undesirable texture on the transversely cut surface of meat. The diversity of muscle is attributed to the heterogeneous characteristics of the individual muscle fibers and the mosaic composition (Taber, 1998). Muscle fiber diameter varies with species, chronological age, state of nutrition of the animal, genetic background and composition of muscle fiber types (Karlsson et al., 1993). The coarseness of the meat surface is increased with thickened connective-tissue strands as well as increased size of muscle bundles. The connective tissue content of meat varies with species, chronological age, state of nutrition of the animal and muscle fiber characteristics (Klont, Brocks, & Eikelenboom, 1998). Meat firmness is also influenced by the status and quantity of the subcutaneous fat surrounding muscles and IMF. Because IMF deposits mainly in the perimysium between muscle bundles, meat firmness is partially influenced by the IMF firmness which is affected by composition of fatty acids and temperature. It is known that IMF produces effects on flavor, juiciness, tenderness and visual characteristics of meat with increased marbling in meat, although there has been extensive debate about the contribution of IMF to the tenderness of meat. The quantity of IMF is affected by many factors including animal breed, slaughter weight (Park et al., 2002), feeding strategy (Du, Yin, & Zhu, 2010), and growth rate (Smith et al., 2009). In animals, adipogenesis occurs the earliest in the visceral fat deposit, closely followed by subcutaneous and intermuscular deposits, and adipogenesis in intramuscular fat occurs last (Hausman et al., 2009). This adipogenesis can be affected by genetic, nutritional and environmental factors that are the key signaling pathways regulating adipogenesis in skeletal muscle (Du & Dodson, 2011). Although there are variations among species, IMF tends to increase with advancing age when the major stages of muscle growth have been completed. IMF deposition is highly heritable and is positively correlated with general body fatness in the animal. Moreover, IMF is positively correlated with percentage of red muscle fiber, but negatively correlated with white muscle fiber in muscle (Hwang et al., 2010) Eating quality traits (EQT) Tenderness is the most important EQT because it strongly influences consumer's perceptions of acceptability. Meat tenderness is mainly affected by the amount and solubility of connective tissue, the composition and contractile state of muscle fibers, and the extent of proteolysis in rigor muscle. Also, IMF content indirectly affects meat tenderness. Tenderness is more important for red meat such as beef and lamb because of a high composition of red muscle fibers and connective tissue compared to pork or chicken. The content of connective tissue is related to muscle fiber characteristics because

3 830 S.T. Joo et al. / Meat Science 95 (2013) muscle fibers occupy 75 90% of the muscle volume, and the morphology of the muscle fiber is a major determinant factor of mass (Lee et al., 2010). The heterogeneity of muscle fiber characteristics in different muscles is known to influence tenderness (Maltin, Balcerzak, Tilley, & Delday, 2003). However, the relationship between muscle fiber characteristics and meat tenderness is still controversial. Muscles with diverse muscle fiber characteristics have different patterns of p.m. change during the conversion of muscle to meat. If type II fibers are predominant in muscle, p.m. glycolysis is rapid, resulting in an accelerated ph decline in the muscle. In addition throughout the p.m. period, sarcomere lengths in muscle vary because each muscle fiber goes into rigor at different times. Consequently meat tenderness varies with the rate of glycolysis, the rigor onset post-slaughter and the extent of glycolysis, which are all related to muscle temperature as well as muscle fiber characteristics (Ali et al., 2008). Flavor is also important for the eating quality of meat because people expect certain attributes such as savoriness. Because meats consist mainly of the lean portion and the fat portion, the meat flavor is primarily dependent on the pool of flavor precursors in these two tissues. Meat flavor is affected by species, sex, age, stress level, amount of fat, and diet of animal. Beef, pork, lamb, and poultry have distinctive flavor characteristics due to the variation of the flavor precursors generally in the fat between and within species. The effect of animal gender on meat flavor is highly related to testosterone and skatole that are produced in intact males and females, respectively. Boar taint in pork from intact males is an unpleasant urine-like and sweaty odor that is related to the presence of androstenone (5α-androst-16-en-3-one) and skatole (3-methylindole) (Grindflek et al., 2011). Androstenone is a metabolite of testosterone, and skatole is the major contributor to pastoral-flavor (Teixeira, Batista, Delfa, & Cadavez, 2005). Testosterone increases muscle growth and decreases intramuscular lipid deposition. In general, intact males deposit less fat throughout the body and within muscle, and are more susceptible to long-term pre-slaughter stress than females or castrated males. Increasing serum-like bloody aromatics and metallic flavor are due to increased levels of Mb in the meat of older animals. Juiciness is positively related with the WHC of meat and the IMF content in meat. The IMF content directly affects juiciness as well as flavor (Hocquette et al., 2010), and the human perception of juiciness is increased as the IMF content in meat increases (Jeremiah, Gibson, Aalhus, & Dugan, 2003). Moreover, the feel of juiciness in the oral cavity is generally sustained when meat has a large amount of IMF. In general, juiciness is a more important sensory trait for pork because consumers of pork place a higher rating on juiciness than flavor or tenderness (Aaslyng et al., 2007), while consumers of beef rate tenderness as the most important palatability trait (Cho et al., 2010). A lack of juiciness is a major quality issue in pork, and pork muscle that lacks marbling exhibits a lack of juiciness. IMF content affects juiciness by enhancing the WHC of meat, by lubricating the muscle fibers during cooking, by increasing the tenderness of meat, and thus the apparent sensation of juiciness, or by stimulating salivary flow during mastication (Luchak et al., 1998). It is well known that meat with a high IMF content has improved juiciness after relatively long-heating in a moist environment, whereas meat of lower IMF content is not deteriorated by severe short-heating under dry cooking conditions Reliance quality traits (RQT) Safety is always more important than AQT and EQT, and the microbial level in meat is the most important RQT for fresh meat. The categories of meat safety also include physical and chemical residues, food additives and animal identification of meat products. In general, consumers evaluate meat safety by visual and odor evaluations which are the most rapid indications of meat spoilage, although they are unreliable indicators of safety. The importance of meat as a carrier of bacterial pathogens is considerable in terms of public health. Therefore, strict and stringent safety requirements in the processing of meat have been developed and implemented in many countries. The Hazard Analysis Critical Control Point (HACCP) system provides the basis for the meat safety management system within the meat chain (Troy & Kerry, 2010). There is no doubt that quality meat is one with high nutritional value, and meat is one of the most nutritional foods. However, recently, the concept of nutrition has changed as the nutrition of food has reached an all-time high. In the past, quality meat was more closely related to the sensory perceptions, freshness, and safety aspects of meat products, whereas more recently it is associated with nutrition, well-being and functionality in relation to human health. Consequently, consumers may consider the high content of fat and cholesterol in meat as undesirable and unhealthy, although meat is nutritious because it is a rich source of protein, essential amino acids, minerals and vitamins. Meat composition can be manipulated to alter the nutritional profile in most cases. Dietary supplementation is the key factor which can most easily be manipulated and has one of the most profound effects on meat composition. Furthermore, the effect of diet on nutritional profile is more profound in meat derived from monogastric animals. These kinds of meats are categorized as functional foods which are defined as foods with nutritional profiles that exceed conventional products (Decker & Park, 2010; Hur, Park, & Joo, 2007). In recent years there has been a considerable increase in consumer concern with regard to how meat is produced. Concern about animal welfare has greatly increased around the world, and there has been an enormous development of the organic rearing of animals. Consumers demand that animals are reared, transported and slaughtered under humane conditions. Also, consumers want to be confident that the meat they purchase is derived from ethically robust production systems. Consequently, farmers, veterinarians, packers and scientists need to become more knowledgeable on how to assess and audit animal welfare at the farm and slaughter plant (Grandin, 2010). It should be emphasized that the importance of traceability has increased in relation to RQT. Regulatory agencies in many countries have insisted on the implementation and application of traceability systems (Troy & Kerry, 2010). 4. Variation of meat quality traits Attempts to identify common standards of fresh meat quality have been done for international trade across a number of countries. However consumers still have difficulty in accurately predicting quality by perception at the point of purchase (Glitsh, 2000). The eating quality and the assessment of beef from the same animals in eight countries in the EU have been investigated, and the results showed that consumer's preferences differ among countries (Dransfield et al., 1984). Irish and English panelists preferred flavor more than tenderness and juiciness, but Italian panelists tended to value tenderness more highly than flavor. Pethick, Warner, and Banks (2006) reported that consumers of lamb in Australia usually place the greatest weight on flavor/odor, followed by tenderness and finally juiciness. This is in contrast to consumers of beef who generally rate tenderness as the most important palatability trait (Moon, Yang, Park, & Joo, 2006). According to Warner et al. (2010), flavor has increased in importance for beef consumers as tenderness variation in meat has been reduced. It is well known that inherent differences in eating quality between muscles exist. Jeremiah et al. (2003) found that 30 anatomically defined bovine muscles differed significantly in EQT. It should be emphasized that meat is composed of numerous tissues such as adipose, epithelial, connective and nervous tissues, although the major component is muscle. Consequently, regardless of muscle quality, variation of meat quality could be influenced by these intrinsic factors. For example, some muscles contain relatively large quantities

4 S.T. Joo et al. / Meat Science 95 (2013) of connective tissue, which are associated with meat toughness. If meat contains either an unexpected excess or low amount of fat, it would be considered a low quality meat. It is obvious that fat content, connective tissue and muscle fiber characteristics have a significant influence on meat quality. Also, storage conditions and temperature are very important because meat quality can deteriorate via spoilage due to adverse storage conditions or temperature abuse. Moreover, AQT and EQT are significantly influenced by the method of cooking and preparation of meat cuts. 5. Relationship between meat quality traits and muscle fiber characteristics It is commonly reported that many aspects of meat quality are related with muscle fiber characteristics that are represented by TNF, CSAF and FTC in muscle. In particular, these muscle fiber characteristics are closely related with muscle ph that is commonly considered as an indicator of pork quality. Especially, FTC in muscle is related to the rate of p.m. ph decline. Increasing the proportion of fast-twitch glycolytic fibers in porcine longissimus muscle has been shown to increase the rate and extent of p.m. ph decline (Choi, Ryu, & Kim, 2007; Kim, Jeong, et al., 2013; Ryu & Kim, 2006). Moreover, oxidative fibers are susceptible to cold temperature and thus rapid temperature decline p.m. could increase muscle shortening especially if the muscles contain a high amount of oxidative fibers (Lonergan, Zhang, & Lonergan, 2010). Muscle fiber characteristics influence AQT including meat color, WHC, texture and marbling in meat. The Mb content and the rate of Mb oxidation are muscle-specific, and increasing the proportion of red muscle fibers is known to increase the redness and Mb content of meat (Kim et al., 2010). It is well documented that increasing the proportion of type I fibers decreases color stability with a possible shift to a brownish MetMb color (Renerre, 1990). The composition of fast-twitch glycolytic (IIB) fibers in pork muscle is related to higher lightness and lower WHC (Kim, Jeong, et al., 2013). Hypertrophy of fast-twitch oxido-glycolytic fibers (IIA) is more specifically detrimental to WHC (Larzul et al., 1997; Maltin et al., 1998). The size of muscle fibers affects muscle growth potential and the size of the fiber bundle, resulting in the visible coarseness of transverse sections of meats (Kim, Kim, et al., 2013; Rehfeldt & Kuhn, 2006; Ryu & Kim, 2005). The content of connective tissue including IMF also varies with muscle fiber characteristics (Klont et al., 1998). There is a strong positive genetic correlation between CSAF and IMF content in porcine longissimus muscle (Larzul et al., 1997). Kim, Jeong, et al. (2013), and Kim, Kim, et al. (2013) also reported that the proportion and size of type IIB fibers are positively related with IMF content in porcine longissimus muscle. In beef muscle, IMF is positively correlated with the percentage of red muscle fiber, but negatively correlated with white muscle fiber (Hwang et al., 2010). It is commonly stated that red oxidative muscles contain more IMF than white glycolytic muscle. However, Lefaucheur (2010) reported no universal relationship between IMF and FTC, and suggested that both characteristics are rather independent and can be manipulated separately. EQT are also closely associated with muscle fiber characteristics. The heterogeneity of muscle fiber type in different muscles is known to influence meat tenderness (Maltin et al., 2003). Slow-twitch muscles have been reported to contain more collagen, which plays an important role in binding muscle fibers and decreasing tenderness of meat (Kovanen, Suominen, & Heikkinen, 1984). However, the relationship between FTC and meat tenderness is still controversial, and no clear relationship between collagen content and FTC has been reported in livestock species (Lefaucheur, 2010). Muscles with a larger fiber size, especially type IIB fiber, exhibit tougher meat than muscles of smaller fiber size in cattle (Renand, Picard, Touraille, Berge, & Lepetit, 2001) and in pig (Karlsson et al., 1993). Conversely, Berri et al. (2007) reported that CSAF showed a positive relationship with tenderness in broiler breast muscle. FTC of muscle is related to p.m. proteolytic degradation as well as glycolysis and the rate of ph decline. Fast-twitch IIB fibers are known to be highly glycolytic fibers and their metabolism contributes to a fast metabolic rate early p.m. period (Ryu & Kim, 2005, 2006). If fast-twitch glycolytic fibers are predominant in muscle, rapid glycolysis is induced, resulting in a rapid ph decline in the muscle (Choe et al., 2008). Thus, the composition of type IIB fibers are negatively related to muscle ph but positively related to R-value (adenine/inosine ratio), allowing for the determination of ATP depletion in the early p.m. period (Ryu et al., 2008). Contrarily, increasing the proportion of type I fibers in muscle decreases the rate and extent of p.m. ph decline (Choi et al., 2007). These differences in p.m. muscle properties are due to different FTCs and influence meat tenderness. Moreover, type II fast fibers are more susceptible to early p.m. proteolytic degradation than type I slow fibers (Xiong et al., 2007). Hwang et al. (2010) also reported improvement of tenderness via increasing the percentage of type I fibers and decreasing the percentage of type IIB in cattle muscle. The influence of muscle fiber characteristics on p.m. aging is another important aspect of meat quality. The increase of fast-twitch glycolytic fibers has beneficial effects on p.m. aging and tenderness in cattle (Seideman, Crouse, & Cross, 1986). In p.m. aging of meat, the rate of aging is faster in fast-twitch muscles than in slow-twitch oxidative muscles (Totland, Kryvi, & Slinde, 1988). The calpain/calpastatin ratio is higher in fast-twitch glycolytic muscles than in slow-switch oxidative muscles, which could partly explain the faster rate of aging in glycolytic muscles (Ouali & Talmant, 1990). Fast-twitch fibers have a more extensively developed sarcoplasmic reticulum, transverse-tubule system, and thinner Z-band than slow-twitch fibers, and proteins that comprise the Z-band in fast-twitch fibers are more susceptible to early p.m. proteolytic degradation than those in slow-twitch fibers (Xiong, 2004). Meat flavor and juiciness are strongly affected by IMF content in muscle that is positively correlated with proportion of type I fibers in muscle (Maltin et al., 1998). A high proportion of type I fibers is associated with a high level of phospholipids which are important determinant of cooked meat flavor (Hwang et al., 2010). Also, the content of type I fiber is positively related with juiciness (Calkins, Duston, Smith, Carpenter, & Davis, 1981). However, Lefaucheur (2010) suggested that intramuscular adipocytes within a muscle were not related to its FTC, although research has shown that red oxidative muscles contain more total IMF than white glycolytic muscles. Therefore, the relationship among FTC, IMF and EQT still remains a challenge. 6. Potential factors to manipulate muscle fiber characteristics 6.1. Breed and genotype One of the important factors that influence TNF, CSAF and FTC of a given muscle within a species is breed. In general, wild animals contain more oxidative fibers, less glycolytic fibers and smaller fibers compared to domesticated animals (Lefaucheur, 2010). Animal selection for increased growth rate and lean meat content shifts muscle metabolism towards a more white glycolytic and less red oxidative type (Rahelic & Puac, 1981). For example, Meishan pigs have a higher oxidative and a lower glycolytic metabolism with a decrease in TNF and CSAF compared to LW pigs (Bonneau, Mourot, Noblet, Lefaucheur, & Bidanel, 1990), and LW pigs contain more type I fibers than miniature pigs (Stickland & Handel, 1986). Also, there is a positive relationship between high muscularity and a high proportion of myosin heavy chain (MHC) IIb transcript in different pig breeds (Wimmers et al., 2008). The longissimus and gluteus medius muscles from Hampshire pigs have a greater oxidative capacity, a lower glycolytic capacity, and a higher concentration of glycogen than those of

5 832 S.T. Joo et al. / Meat Science 95 (2013) the Swedish Yorkshire (Essén-Gustavsson & Fjelkner-Modig, 1985). The longissimus dorsi muscle of Berkshire pigs has a larger percentage of type I fiber compared to Landrace and Yorkshire pigs (Ryu et al., 2008). Therefore, it should be emphasized that an increase in carcass lean percentage is positively related to glycolytic and oxidative metabolism in longissimus muscle as well as lean tissue growth rate (Karlsson et al., 1993), although no opposite or significant correlations between lean tissue growth rate and FTC have been reported (Candek-Potokar, Lefaucheur, Zlender, & Bonneau, 1999; Ryu, Rhee, & Kim, 2004). Consequently, there is a strong relationship between FTC and growth performance when comparing extremely different genetic breeds, but this is often controversial within conventional domesticated animals (Lefaucheur, 2010). All these findings imply that FTC in muscle could be manipulated by breeding of animals. On the other hand, the progressive improvement in meat quality can be achieved through selection of animals that have a quality gene. According to Warner, Greenwood, and Ferguson (2011), interaction between genotype and environment contributes to the phenotypic variation in AQT and EQT, and increasing muscle mass is achievable by targeted selection for specific mutations, for example, the myostatin gene. The major genes that induce an increase in carcass lean content are the double-muscling myostatin gene in cattle, the Callipyge gene in sheep and Ryr1, RN (Rendement Napole) or IGF-II genes in pig. The effects of these genes on TNF, CSAF and FTC are well documented, and only the mutated myostatin gene is associated with an additional increase in TNF (Lefaucheur, 2010). The improvement of tenderness in double-muscled cattle is due to a lower total and higher soluble collagen content (Ngapo et al., 2002). Increased muscle content of Callipyge sheep is related to increased proportion of white muscle fibers (Koohmaraie, Shackelford, Wheeler, Lonergan, & Doumit, 1995). The specific increased CSAF of fast muscle fiber in Callipyge sheep reduces protein degradation, including a lower p.m. proteolysis, maturation and tenderization of meat (Lorenzen et al., 2000). Increasing muscle content of Ryr1 pigs is due to increased CSAF with similar or reduced TNF (Depreux, Grant, & Gerrard, 2002). The Ryr1 gene pigs show decreased IIA mrna and increased MHC IIX mrna in muscle, while the RN mutation decreases MHC IIB mrna and increases IIA and IIX mrna expression (Park, Gunawan, Scheffler, Grant, & Gerrard, 2009). The Ryr1 gene induces a punctual mutation in the SR Ca 2+ channel, leading to an abnormally high release of Ca 2+ in the sarcoplasm and rapid ph decline early p.m., resulting in PSE pork (Lefaucheur, 2010). All these changes in FTC due to mutated genes in pigs are strongly related to poor pork quality. Therefore, efforts to eliminate these genes from pig herds are necessary to improve the AQT and EQT of pork Sex and hormone There are sex differences relative to muscle fiber characteristics, although Staun (1963), Rowe and Goldspink (1969) and Miller, Garwood, and Judge (1975) did not find any sex differences in TNF, in CSAF and in FTC, respectively. The smaller type IIA and type IIB fibers in the longissimus dorsi muscles from boars indicate that TNF is higher in male pigs compared to female pigs (Petersen, Henckel, Oksbjerg, & Sørensen, 1998). The CSAF of fiber types I, IIA and IIB is smaller in the longissimus dorsi muscle of entire male pigs compared to gilts (Karlsson et al., 1993). Essentially, females exhibit larger fibers with no difference in fiber type percentages and relative areas (Larzul et al., 1999; Miller et al., 1975; Solomon, Campbell, & Steele, 1990). Some hormones have a profound influence on the muscle fiber characteristics of specific muscles. Differences in TNF and CSAF are primarily controlled by sex hormones. Differences in TNF between males and females can arise by hormonal action if differences in androgen hormones are sufficiently high during periods of prenatal fiber formation (Rehfeldt et al., 2004). Yoshioka, Boivin, Bolduc, and St-Amand (2007) reported that testosterone treatment in later postnatal periods can stimulate muscle hypertrophy in a direct or indirect manner by satellite cell proliferation and muscle protein synthesis, without increasing TNF. Hormonal differences, especially testosterone, may also contribute to the gender differences in specific fiber type sizes that ultimately affect the relative concentrations of MHC isoforms (Staron et al., 2000). Muscle fiber phenotypes are dramatically changed by thyroid hormones. Hypothyroidism causes fast-to-slow transitions, while hyperthyroidism induces transitions in the reverse direction (Fitts, Winder, Brooke, Kaiser, & Holloszy, 1980; Ianuzzo, Patel, Chen, O'Brien, & Williams, 1977). Many studies show that low levels of thyroid hormones cause fast-to-slow shifts in MHC isoform expression: MHC IIB MHC IIX(D) MHC IIA MHC I/slow, whereas high levels of thyroid hormones induce slow-to-fast shifts in MHC isoform expression: MHC I/slow MHC IIA MHC IIX(D) MHC IIB (Caiozzo, Herrick, & Baldwin, 1992; Fitzsimons, Herrick, & Baldwin, 1990; Izumo, Nadal-Ginard, & Mahdavi, 1986). In addition, low thyroid hormone levels inhibit or delay the appearance of adult fast-twitch muscle fibers, whereas high levels of thyroid hormone accelerates the transition from developmental fibers to adult fast-twitch fibers (Adams, McCue, Zeng, & Baldwin, 1999). According to Ryu, Choi, Ko, and Kim (2007), serum insulin-like growth factor-i (IGF-I) concentrations are negatively related to the composition of type I fibers, whereas serum epidermal growth factor is positively correlated to the composition of type I fiber. The proportion of type IIB fibers has a positive correlation with IGF-I expression (Owens, Campbell, Francis, & Quinn, 1994). In β-agonist-fed lambs, muscle hypertrophy is associated with a selective increase in CSAF of glycolytic fibers, whereas effects on FTC are unclear (Beermann et al., 1987). In pigs, treatment with β-agonist increases the frequency of type IIB fibers, mainly at the expense of type IIA fibers, resulting in lower activities of oxidative enzymes (Oksbjerg, Henckel, & Rolph, 1994) Growth performance and diet The muscle fiber characteristics are significant for growth performance. In most farm animals, myogenesis is a biphasic phenomenon involving the successive differentiation of a primary and secondary generation of myotubes during the fetal period. The number of secondary generation fibers varies by fetus weight or gestation period of animals. In general, TNF is considered to be established by 90 dg of fetus (i.e. after 80% of gestation) in pig and 180 dg of fetus (i.e. after 66% of gestation) in cattle (Picard, Lefaucheur, Berri, & Duclos, 2002; Wigmore & Stickland, 1983). The fetal stage of animals is characterized by the differentiation of fibers strongly expressing the slow MHC isoform derived from primary fibers, and of fibers strongly expressing the neonatal MHC isoform derived from both primary and secondary fibers (Condon, Silberstein, Blau, & Thompson, 1990; Lyons, Ontell, Cox, Sassoon, & Buckingham, 1990). CSAF remains constant during gestation, but FTC shifts dramatically to a greater extent in future glycolytic than oxidative fibers after birth (Lefaucheur, 2010). At birth, muscle is composed of oxidative fibers and mrnas of the fast MHC isoform can be detected a few days after birth (Moody, Enser, Wood, Restall, & Lister, 1978). The proportion of oxidative fibers decreases while the proportion of glycolytic fibers increases during growth (Lefaucheur & Vigneron, 1986; Solomon et al., 1990). TNF, CSAF and FTC are factors that affect muscle growth performance and meat quality (Rehfeldt & Kuhn, 2006). The size of various fiber types in longissimus dorsi muscle is increased at different rates, while the CSAF of all fibers is increased during growth (Hegarty & Allen, 1978). The diameter of type II fibers is increased faster than that of type I (Oksbjerg et al., 1994). Carter et al. (2010) reported that fast MHC expression increased during aging in the rat soleus muscle due to a marked increase in fibers that co-express both fast

6 S.T. Joo et al. / Meat Science 95 (2013) and slow MHC isoforms. Recently, Kim, Kim, et al. (2013) reported a decrease in type I and IIA fibers and an increase in type IIB fibers with increasing pig carcass weight. In the carcass weight range of 70 kg to 109 kg, the diameter of type IIB is positively correlated with loin-eye area and carcass weight, and an excessively high weight of carcass has an influence on pale and exudative properties in pork because of muscle fiber characteristics, especially increased type IIB fibers (Kim, Kim, et al., 2013). TNF and CSAF are negatively correlated with each other, and muscle mass is closely related to CSAF at a constant TNF (Kim, Jeong, et al., 2013; Rehfeldt, Fiedler, Dietl, & Ender, 2000). However, when low birth weight pigs are compared to normal weight pigs, CSAF shows a negative correlation with lean meat content in carcass, and low birth weight pigs have a low TNF, larger CSAF, higher IMF content in muscle and a decreased lean meat content in carcasses compared to those of normal birth weight pigs (Gondret et al., 2006; Rehfeldt & Kuhn, 2006). These differences imply that low birth weight pigs induce a higher physiological maturity, and muscle fiber hypertrophy is achieved earlier because of a lower TNF (Lefaucheur, 2010). Therefore, a high TNF combined with normal CSAF is suggested as a way to increase muscle growth potential without decreasing oxidative capacity which could be induced by excessive muscle fiber hypertrophy (Lefaucheur, 2010). On the other hand, the adequate level of energy and the balance of nutrients play an important role in determining growth rate and feed efficiency in the fetal as well as in the postnatal period. During the fetal period, malnutrition specifically decreases the number of secondary fibers, leading to a permanent decrease in postnatal muscle growth potential (Hegarty & Allen, 1978; Wigmore & Stickland, 1983). In contrast, over-nutrition of the sow between 25 and 50 days of gestation can increase the total number of fibers in the developing pigs (Rehfeldt, Fiedler, Weikard, Kanitz, & Ender, 1993). In lactating sows, a selective decrease in CSAF of glycolytic fibers, and an increase in relative CSAF of type I fibers are found in longissimus muscle (Lefaucheur, 1990). In general, muscle fiber hypertrophy is impaired by restricting feed intake regardless of the pig's growth stage, whereas the effect of restricted feeding on FTC appears to vary depending on the growth stage. According to Harrison, Rowlerson, and Dauncey (1996), feed restriction at an early stage (between 3 and 7 weeks of age) does not change FTC in longissimus muscle, but does lead to a dramatic increase in the proportion of type I fibers in the red rhomboideus muscle and lower CSAF of all fibers. Lefaucheur (1990) also reported that feed restriction between 7 and 100 kg of body weight did not change fiber type percentages in pig longissimus and tibialis cranialis muscles, but increased the CSAF. The FTC in longissimus muscle is not changed due to feed restriction in post-weaning and growing-finishing pigs (Bee et al., 2007). However, restricted feeding induces a lower proportion of type IIB fibers and more type IIA fibers in porcine longissimus muscle during growth (Solomon & Lynch, 1988). These changes in muscle fiber characteristics due to restricted feeding would be related to deterioration of meat quality. Candek-Potokar et al. (1999) reported that a 30% restriction in feed intake during the growing-finishing period caused an increase in drip loss and lightness with a decrease in IMF content in porcine longissimus dorsi muscle. In lambs, feed restriction induced atrophy of muscle fibers and increased the percentage of type I fibers at the expense of type IIB fibers (Solomon, Caperna, Mroz, & Steele, 1994). The energetic density of the diet (2.79 vs 1.87 Mcal ME/kg diet) was shown to influence FTC in lambs given ad libitum access to feed and slaughtered at 45 kg of body weight (Solomon & Lynch, 1988). The longissimus muscles from lambs fed the lower-energy diet contained more type I and fewer type IIB fibers (Solomon & Lynch, 1988). Jeong et al. (2012) reported that FTC was not affected by dietary energy level (3.0 vs 3.2 Mcal DE/kg) in porcine longissimus muscle, while type IIB fiber size was greater in the low energy group (3.0 Mcal DE/kg) than in the medium energy group (3.2 Mcal DE/kg). Consequently, control of diet is another potential tool which can be combined with growth performance in different rearing systems to manipulate muscle fiber characteristics in relation to meat quality Muscle location, exercise, and ambient temperature It is well known that muscle fiber characteristics are affected by muscle type, location and function within an animal. Basically, the histochemical characteristics of muscle fiber depend on muscle location and function in animals. In general, a high degree of type IIB fibers is found in porcine longissimus dorsi (Kiessling & Hansson, 1983; Kim, Kim, et al., 2013), gluteus medius (Essén-Gustavsson & Fjelkner-Modig, 1985), biceps femoris (Barton-Gade, 1981), quadriceps femoris (Barton-Gade, 1981), vastus lateralis (Kiessling & Hansson, 1983), and semimembranosus (Barton-Gade, 1981; Monin, Mejenes-Quijano, Talmant, & Sellier, 1987) muscles. Examples of porcine muscles containing a high degree of I and IIA fibers are masseter, trapezius (Monin et al., 1987) andtriceps brachii (Kiessling & Hansson, 1983) muscles. In bovine muscles, longissimus and semimembranosus muscles have a high proportion of IIB fibers (Hwang et al., 2010; Ozawa et al., 2000), whereas a high degree of type I fibers is found in psoas major, biceps brachii and brachialis muscles (Hwang et al., 2010; Kirchofer, Calkins, & Gwartney, 2002). Generally, deep muscles involved in maintaining posture are more oxidative and containmore typeifibers than more superficial muscles involved in rapid movements (Rosser, Norris, & Nemeth, 1992). The proportion of type I fiber in porcine semitendinosus muscle ranges from approximately 4% in the superficial white portion to 45% in the deep red portion (Beermann et al., 1990). In cattle, the increase in fiberdiameterstops later in the light than in the dark area of semitendinosus muscle, coinciding with an increase in the percentage of type IIB fibers in the light area (Dreyer, Naudé, Henning, & Rossouw, 1977). This information regarding muscle fiber characteristics in relation to individual muscles is very useful in producing quality meat cuts in meat markets, as the meat industry in many countries has shown trends towards marketing individual muscles for consumption (Hwang et al., 2010; Jeremiah et al., 2003; Kirchofer et al., 2002). Although muscle fibers are dynamic structures that exhibit high plasticity, fibers undergo type shift following an obligatory pathway: I IIA IIX IIB (Pette & Staron, 2000; Schiaffino & Reggiani, 1996). The FTC in muscle can be changed by physical exercise, depending on the type and duration of the activity. Prolonged endurance induces a IIB IIX IIA I transition in muscles involved in the exercise (Lefaucheur, 2010). This transition of fiber types in muscle has been found in miniature pigs (Mcallister, Reiter, Amann, & Laughlin, 1997), and confirmed in more commercial pigs (Petersen et al., 1998). Recently, understanding the transition mechanisms of fiber types and manipulating FTC in individual muscles via exercise of animals have become important to the meat industry, because of the development of organic rearing of the animal and consumer concern about animal welfare. Moreover, the FTC can be changed through ambient temperature at animal rearing systems. For example, long term cold exposure shows a shift in FTC to a slower type in oxidative muscles involved in posture in LW pigs (Herpin & Lefaucheur, 1992). Contrarily, if pigs are reared in warm environments, both oxidative and glycolytic metabolism decreased (Rinaldo & Le Dividich, 1991). Consequently, the FTC in individual muscles can be manipulated by physical exercise combined with ambient temperature in conventional rearing systems. 7. Conclusion The term fresh meat quality should be defined by consumer's preferences that are determined by AQT and EQT as well as RQT of meat. The AQT include meat color, drip and purge, texture and firmness, and marbling, while EQT consist of tenderness, flavor and

7 834 S.T. Joo et al. / Meat Science 95 (2013) juiciness. The components of RQT are safety, nutritional value, animal welfare and ethics. These quality traits are evaluated by the consumer at the point of purchase, and contribute to the consumer's expectation of high quality meat. However, consumer's preferences are different according to country because their significance is determined by regional preference and by the experiences of the individual consumer. Moreover, variations of meat quality traits exist according to individual muscles, and the inherent differences in eating quality among muscles also exist. Therefore, better control of meat quality requires a thorough understanding of meat quality as a complex global concept submitted to the influence of various meat quality traits. AQT and EQT are dependent on muscle fiber characteristics such as TNF, CSAF and FTC in meat. The rate of p.m. ph decline that influences AQT and EQT is closely related to muscle fiber characteristics, especially FTC in muscle. Numerous studies have reported that a high proportion of fast-twitch glycolytic fibers (type II fibers) in muscle increases the rate and extent of p.m. ph decline, while a high proportion of oxidative fibers (type I fiber) increases muscle shortening and level of phospholipids in muscle influencing tenderness and flavor. Increasing the proportion of type IIB fibers in porcine muscles decreases WHC and tenderness of pork due to increased CSAF, whereas increasing the proportion of type IIB fibers in bovine muscle increases tenderness of beef due to improved p.m. aging. Although these are general findings in the present paper, controversial perspectives still remain, and the relationship between meat quality traits and muscle fiber characteristics is yet to be fully established. Because both quality traits and fiber characteristics are influenced by various factors including species, breeds, gender, age, birth weight, exercise, use of hormones, slaughter weight, muscle type and location, sampling sites within muscle, etc., further strictly controlled studies are needed to understand this uncertain relationship. Nevertheless, the present knowledge and technologies suggest that the progressive improvement in meat quality could be achieved by manipulation of muscle fiber characteristics in animals. This manipulation may be possible through breeding of animals and using specific genes or gene markers. Also, muscle fiber characteristics could be changed via control of growth performance and gender of animals. Finally, dramatic changes in FTC could be achieved by treatment with specific hormones, and energy level and nutrient balance during animal growth. In addition, the FTC in individual muscles can be manipulated by physical exercise combined with ambient temperature at conventional rearing systems. Of course, an approach combined with all potential factors may be the best way to achieve the progressive improvement in meat quality. This would ensure continued production of high quality meat through manipulation of muscle fiber characteristics in animals. References Aaslyng, M. D., Oksama, M., Olsen, E. V., Bejerholm, C., Baltzer, M., Andersen, G., Bredie, W. L. P., Byrne, D. V., & Gabrielsen, G. (2007). The impact of sensory quality of pork on consumer preference. Meat Science, 76, Acebron, L., & Dopico, D. (2000). The importance of intrinsic and extrinsic cues to expected and experienced quality: An empirical application to beef. Food Quality and Preference, 11, Adams, G. R., McCue, S. A., Zeng, M., & Baldwin, K. M. (1999). Time course of myosin heavy chain transitions in neonatal rats: Importance of innervation and thyroid state. American Journal of Physiology, 276, R954 R961. Ali, Md. S., Yang, H. S., Jeong, J. Y., Moon, S. H., Hwang, Y. H., Park, G. B., & Joo, S. T. (2008). Effects of chilling temperature of carcass on breast meat quality of duck. Poultry Science, 87, Barton-Gade, P. (1981). The measurement of meat quality in pigs post mortem. In T. Froysten, E. Slinde, & N. Standal (Eds.), Porcine stress and meat quality Causes and possible solutions to the problems (pp. 359). As, Norway: Agricultural Food Research Society. Becker, T. (2000). Consumer perception of fresh meat quality: A framework for analysis. British Food Journal, 102, Bee, G., Calderini, M., Biolley, C., Guex, G., Herzog, W., & Lindemannt, M. D. (2007). Changes in the histochemical properties and meat quality traits of porcine muscles during the growing-finishing period as affected by feed restriction, slaughter age, or slaughter weight. Journal of Animal Science, 85, Beermann, D. H., Butler, W. R., Hogue, D. E., Fishell, V. K., Dalrymple, R. H., Ricks, C. A., & Scanes, C. G. (1987). Cimaterol-induced muscle hypertrophy and altered endocrine status in lambs. Journal of Animal Science, 65, Beermann, D. H., Fishell, V. K., Roneker, K., Boyd, R. D., Armbruster, G., & Souza, L. (1990). Dose response relationships between porcine somatotropin, muscle composition, muscle fiber characteristics and pork quality. Journal of Animal Science, 68, Berri, C., Bihan-Duval, E., Debut, M., Sante-Lhoutellier, V., Baeza, E., & Gigaud, V. (2007). Consequence of muscle hypertrophy on characteristics of Pectoralis major muscle and breast meat quality of broiler chickens. Journal of Animal Science, 85, Bonneau, M., Mourot, J., Noblet, J., Lefaucheur, L., & Bidanel, J. P. (1990). Tissue development in Meishan pigs: Muscle and fat development and metabolism and growth regulation by somatotropic hormone. In M. Molenat, & C. Legault (Eds.), Chinese pig symposium (pp ). Jouy-en-Josas, France: INRA Press. Caiozzo, V. J., Herrick, R. E., & Baldwin, K. M. (1992). Response of slow and fast muscle to hypothyroidism Maximal shortening velocity and myosin isoforms. American Journal of Physiology, 263, C86 C94. Calkins, C. R., Duston, T. R., Smith, G. C., Carpenter, Z. L., & Davis, G. W. (1981). Relationship of fiber type composition to marbling and tenderness of bovine muscle. Journal of Food Science, 46, Candek-Potokar, M., Lefaucheur, L., Zlender, B., & Bonneau, M. (1999). Effect of slaughter weight and/or age on histological characteristics of pig longissimus dorsi muscle as related to meat quality. Meat Science, 52, Carter, E. E., Thomas, M. M., Murynka, T., Rowan, S. L., Wright, K. J., Huba, E., & Hepple, R. T. (2010). Slow twitch soleus muscle is not protected from sarcopenia in senescent rats. Experimental Gerontology, 45, Cho, S. H., Kim, J., Park, B. Y., Seong, P. N., Kang, G. H., Kim, J. H., Jung, S. G., Im, S. K., & Kim, D. H. (2010). Assessment of meat quality properties and development of a palatability prediction model for Korean Hanwoo steer beef. Meat Science, 86, Choe, J. H., Choi, Y. M., Lee, S. H., Shin, H. G., Ryu, Y. C., Hong, K. C., & Kim, B. C. (2008). The relationship between glycogen, lactate content and muscle fiber type composition, and their influence on postmortem glycolytic rate and pork quality. Meat Science, 80, Choi, Y. M., Ryu, Y. C., & Kim, B. C. (2007). Influence of myosin heavy- and light chain isoforms on early postmortem glycolytic rate and pork quality. Meat Science, 76, Condon, K., Silberstein, L., Blau, H. M., & Thompson, W. J. (1990). Development of fiber types in the prenatal rat hind limb. Developmental Biology, 138, Decker, E. A., & Park, Y. (2010). Healthier meat products as functional foods. Meat Science, 86, Depreux, F. F. S., Grant, A. L., & Gerrard, D. E. (2002). Influence of halothane genotype and body-weight on myosin heavy chain composition in pig muscle as related to meat quality. Livestock Production Science, 73, Dransfield, E., Nute, G. R., Roberts, T. A., Boccard, R., Touraille, C., Buchter, L., Casteels, M., Cosentino, E., Hood, D. E., Joseph, R. L., Schon, L., & Paardekooper, E. J. C. (1984). Beef quality assessed at European research centres. Meat Science, 10, Dreyer, J. H., Naudé, R. T., Henning, J. W. N., & Rossouw, E. (1977). The influence of breed, castration and age on muscle fibre type and diameter in Friesland and Afrikaner cattle. South African Journal of Animal Science, 7, Du, M., & Dodson, M. V. (2011). Advanced techniques to enhance marbling in meat. Control of meat quality (pp ). : Research Signpost. Du, M., Yin, J., & Zhu, M. J. (2010). Cellular signaling pathways regulating the initial stage of adipogenesis and marbling of skeletal muscle. Meat Science, 86, Essén-Gustavsson, B., & Fjelkner-Modig, S. (1985). Skeletal muscle characteristics in different breeds of pigs in relation to sensory properties of meat. Meat Science, 13, 33. Faustman, C., Sun, Q., Mancini, R., & Suman, S. P. (2010). Myoglobin and lipid oxidation interactions: Mechanistic bases and control. Meat Science, 86, Fitts, R. H., Winder, W. W., Brooke, M. H., Kaiser, K. K., & Holloszy, J. O. (1980). Contractile, biochemical, and histochemical properties of thyrotoxic rat soleus muscle. American Journal of Physiology, 238, C15 C20. Fitzsimons, D. P., Herrick, R. E., & Baldwin, K. M. (1990). Isomyosin distribution in rodent muscles: Effects of altered thyroid state. Journal of Applied Physiology, 69, Glitsh, K. (2000). Consumer perceptions of fresh meat quality: Cross-national comparison. British Food Journal, 102, Gondret, F., Lefaucheur, L., Juin, H., Louveau, I., & Lebret, B. (2006). Low birth weight is associated with enlarged muscle fiber area and impaired meat tenderness of the longissimus muscle in pigs. Journal of Animal Science, 84, Grandin, T. (2010). Auditing animal welfare at slaughter plants. Meat Science, 86, Grindflek, E., Meuwissen, T. H. E., Aasmundstad, T., Hamland, H., Hansen, M. H. S., Nome, T., Kent, M., Torjesen, P., & Lien, S. (2011). Revealing genetic relationships between compounds affecting boar taint and reproduction in pigs. Journal of Animal Science, 89, Harrison, A. P., Rowlerson, A. M., & Dauncey, M. J. (1996). Selective regulation of myofiber differentiation by energy status during postnatal development. American Journal of Physiology, 39, R667 R674. Hausman, G. J., Dodson, M. V., Ajuwon, K., Azain, M., Barnes, K. M., Guan, L. L., Jiang, Z., Poulos, S. P., Sainz, R. D., Smith, S., Spurlock, M., Novakofski, J., Fernyhough, M. E., & Bergen, W. G. (2009). Board-invited review: The biology and regulation of preadipocyte and adipocytes in meat animals. Journal of Animal Science, 87,