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Characterization and consumer acceptance of three muscles from Hampshire x Rambouillet cross sheep expressing the callipyge phenotype¹

Department of Animal and Food Sciences, Texas Tech University, Lubbock 79409-2162

	Abstract

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Eight Hampshire x Rambouillet crossbred wethers expressing the callipyge phenotype and eight Hampshire x Rambouillet half-sibling wethers with a normal phenotype were slaughtered when they reached 59 kg. The supraspinatus (SPM), longissimus (LM), and semitendinosus (STM) muscles were analyzed to determine callipyge effects on calpain and calpastatin activities, sarcomere length, percentage of muscle fiber types, and muscle fiber areas. After 14 d of aging, chops were frozen until analyses for trained sensory panel evaluations, Warner-Bratzler shear force values, and consumer perceptions of tenderness, flavor, juiciness, and overall satisfaction of chops were conducted. Calpastatin activity was 57% greater (P < 0.05) and m-calpain activity was 33% greater (P < 0.05) in muscles from carcasses of callipyge than normal sheep. Sarcomeres were shorter (P < 0.001) in the LM than the SPM or STM, regardless of phenotype. Muscle fiber area was 76% larger (P < 0.05) in the LM of callipyge than normal sheep, but muscle fiber area was not affected (P > 0.05) by phenotype in the SPM or STM. Phenotype had no effect (P = 0.12) on the percentage of slow-twitch, oxidative fiber types in any of the three muscles. In STM and LM from callipyge lambs, the percentage of fast-twitch, oxidative/glycolytic fibers was lower (P < 0.05) and that of fast-twitch-glycolytic fibers was higher (P < 0.05) than in their normal counterparts. Phenotype did not affect (P = 0.90) the fiber type percentage in the SPM. Callipyge LM were less tender and normal LM were more tender than other chops (P < 0.05). Callipyge loin chops had higher Warner-Bratzler shear force values than other chops (P < 0.001). Consumers rated fewer (P < 0.05) callipyge loin and shoulder chops acceptable in juiciness, tenderness, and overall acceptability than normal chops, but phenotype did not affect (P > 0.05) consumer acceptability of leg chops. These results indicate that LM from Hampshire x Rambouillet sheep displaying the callipyge phenotype had higher calpastatin activity and were less tender than the LM from normal sheep. In addition, consumer perceptions indicated that only one in 10 leg chops, one in five shoulder chops, and one in four loin chops from callipyge sheep were unacceptable.

Key Words: Consumer Preferences • Histology • Lambs • Tenderness

	Introduction

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Callipyge lambs convert feed more efficiently (Jackson et al., 1997a), have higher dressing percentages, are leaner (Koohmaraie et al., 1995; Jackson et al., 1997b), and have up to 42% more muscle mass (Koohmaraie et al., 1995; Jackson et al., 1997c) compared with normal phenotype lambs. Previous research has shown that meat from progeny of callipyge Dorset sires have larger muscle fiber areas (Carpenter et al., 1996), higher calpastatin activity (Koohmaraie et al., 1995; Freking et al., 1999; Duckett et al., 2000), and are less tender than sheep with normal phenotypes (Field et al., 1996; Shackelford et al., 1997). Previous researchers that have characterized the muscle histology, calpain/calpastatin activity, and sensory traits of callipyge sheep have used white-faced biological types (Dorset cross) for their model (Koohmaraie et al., 1995; Carpenter et al., 1996; Shackelford et al., 1997). Because different biological types have not been fully investigated, a need exists to characterize other biological type lambs and their response to expressing the callipyge phenotype.

Mendenhall and Ercanbrack (1979) reported that the criteria for selection consumers purchasing lamb cuts mentioned the most often was the amount of lean in the cut, and the amount of fat in the cut second most often. In addition, Busboom et al. (1999) reported that the prices required to recover meat costs for leg, loin, rack, and shoulder were 19.7, 14.4, 12.6, and 11.9% lower, respectively, in callipyge lambs compared with their normal counterparts. It appears that callipyge lamb would fit consumers’ visual criteria, and a cost advantage exists for producing callipyge meat, but the question remains whether meat from callipyge sheep is acceptable in tenderness, juiciness, and flavor. Therefore, the objective of this study was to evaluate the histochemical properties, calpain and calpastatin activities, and sensory traits of lamb from Hampshire x Rambouillet sheep expressing the callipyge phenotype.

	Materials and Methods

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Carcass Characteristics
Eight Hampshire x Rambouillet crossbred wethers expressing the callipyge phenotype and eight Hampshire x Rambouillet half-sibling wethers with a normal phenotype were slaughtered when they reached 59 kg. Sheep were determined to be callipyge by evaluating the live animals, genotyping those animals that expressed the callipyge phenotype, and finally, by evaluating the carcass conformation. This screening process gives the highest probability that the animals express the callipyge phenotype. All live and carcass evaluations were performed by experienced Texas Tech faculty. Carcasses were ribbed between the 12th and 13th ribs 24 h postmortem and evaluated for longissimus muscle (LM) area; leg conformation score; subcutaneous fat thickness opposite the 12th rib; marbling score; percentage of kidney and pelvic fat; lean color, firmness, and texture (1 = extremely dark red, soft, and coarse to 8 = extremely bright cherry red, firm, and fine, respectively); and primary and secondary flank streakings according to the USDA (1992). Temperature and pH of the longissimus at the 12th rib were monitored 20 min postexsanguination, hourly for the next 12 h, and at 24 h postexsanguination, with an Orion handheld pH/temperature meter (model 230A; Orion Instruments, Inc., Boston, MA) fitted with a stab electrode (model H-05998; Cole-Parmer, Inc., Niles, IL).

Calpains and Calpastatin
Ten-gram samples from the semitendinosus (STM), LM, and supraspinatus (SPM) were taken from the left side of each carcass within 20 min after exsanguination. All visible fat and connective tissue were removed, and samples were analyzed for calpain and calpastatin activity per gram of extractable protein (specific activity; Koohmaraie et al., 1995) using ion-exchange chromatography according to Koohmaraie (1990). Extractable protein concentration was determined after homogenization and centrifugation and before dialysis by the Biuret method of Garnall et al. (1949).

Sarcomere Length and Muscle Fiber Histology
Samples (1 x 1 x 3 cm) were removed from the STM, LM, and SPM from the right side of each carcass 24 h postmortem, frozen in liquid nitrogen, and stored at -70°C. Samples were removed from the muscles at a point nearest the subcutaneous fat to best detect the presence of cold shortening. After fixation at room temperature, individual muscle fibers (n = 6) were removed from each muscle sample and placed in the beam of a Spectra-Physics helium-neon laser (wavelength = 632 nm; model 117A, Oroville, CA) to determine sarcomere lengths as described by Cross et al. (1981).

Additional 0.5- x 0.5- x 1.0-cm samples were removed from each muscle, mounted on a 4-cm² cork with the muscle fibers oriented perpendicular to the surface of the cork, frozen in 2-methyl butane submersed in liquid nitrogen, and stored at -70°C until processed. Then, 10-µm thick sections were sliced with a Leica cryostat (model CM1800, Nussburg, Germany) and stained for acid-stable ATPase activity according to Solomon and Dunn (1988). Photographs were taken of four random fields from each sample at 200x magnification with an American Optical (model 1036A, Albuquerque, NM) photomicroscope. In addition, a photograph of a micrometer slide was taken and measured with a caliper to obtain true magnification for measurement. Percentage of slow-twitch, oxidative (SO), fast-twitch, oxidative/glycolytic (FOG), and fast-twitch, glycolytic (FG) fibers were calculated by dividing the number of each fiber type by the total number of fibers. Muscle fiber areas were measured by tracing photographs of each fiber with a Tamaya digital planimeter (model PLANIX 7, Overland Park, KS) and dividing by the magnification coefficient.

Sensory Panel Scores and Warner-Bratzler Shear Values
Each carcass was fabricated into a bone-in leg (NAMP #233), bone-in, square-cut shoulder (NAMP #207), and boneless loin (NAMP #232B; NAMP, 1997) at 14 d postmortem. These wholesale cuts from the left side were cut into 2.5-cm-thick chops, identified, vacuum-packaged, and frozen at -10°C until sensory panel and Warner-Bratzler shear (WBS) evaluations were performed. Four chops from each of the shoulder, loin, and leg were used for sensory evaluation, and two chops from each primal cut were used for WBS evaluations.

Chops were thawed at 4°C for 24 to 36 h and cooked on a Farberware Open Hearth Broiler (Farberware, Inc., Bronx, NY) to an internal temperature of 40°C, turned, and removed when they reached an internal temperature of 70°C. Temperature was monitored with a Cooper Instruments (model SH66A, Middlefield, CT) digital thermometer placed in the geometric center of the muscle. Chops from the shoulder and leg were cooked whole, and then the SPM or STM was removed from each chop, respectively. All visible connective tissue was removed and each muscle was cut into 1-cm cubes, and stored (no longer than 5 min) in pans over sand preheated to 70°C until served. An eight-member trained sensory panel (Cross et al., 1978), consisting of meat science graduate students and faculty, scored each sample using an eight-point scale for initial and sustained juiciness, initial and sustained tenderness, and flavor intensity (1 = extremely dry, dry, tough, tough, and intense off-flavor to 8 = extremely juicy, juicy, tender, tender, and intense lamb flavor, respectively) according to AMSA (1995) guidelines.

Chops for WBS evaluations were cooked the same as for sensory evaluations. Cooked chops were cooled at 2°C overnight, and two 1.3-cm-diameter cores were removed from specified muscles parallel to the muscle fiber orientation, and sheared once perpendicular to the muscle fiber with a Warner-Bratzler Meat Shear (GR Electric Mfg., Co., model 3000, Manhattan, KS).

Consumer Acceptance Evaluation
Intact chops remaining after sensory panel, WBS, fiber type, enzyme activity, and sarcomere evaluations were vacuum-packaged, with each package containing one callipyge chop and one normal chop (shoulder, loin, and leg chops were packaged separately). A total of 384 chops were used, consisting of eight chops from three wholesale cuts (leg, loin, and shoulder) taken from each of the 16 lamb carcasses. Consumers (n = 300) were mailed a form asking them to participate in the study along with a questionnaire. Respondents 15 yr of age or older were contacted by phone and asked to obtain their lamb chops at the Texas Tech University Meat Science Laboratory. Of those that were eligible, 123 consumers (Table 1) returned information and were included in the study. Each member of each household evaluated every chop that was given to that household, resulting in more than one evaluation for each chop.

Statistical Analyses
Data were arranged in a 2 (normal vs. callipyge phenotype) x 3 (STM, LM, or SPM) factorial arrangement of a completely randomized design. Least squares means and SEM were calculated with ANOVA using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). Differences between means were separated using Fisher’s protected LSD. Percentages for consumer acceptability were analyzed using the frequency procedure with the ² option. Acceptable level for making a type-I error was 5%.

	Results

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Longissimus muscle area area tended to be 40.9% larger and fat thickness lower in callipyge compared to normal sheep (P = 0.08, Table 2). Leg scores were two units higher (P < 0.001) and marbling scores, percentage of kidney and pelvic fat, lean texture and color, and primary flank streaking scores were lower (P < 0.05) for callipyge carcasses than normal carcasses. Phenotype did not affect (P > 0.09) lean firmness or secondary flank streaking scores. Temperature of the longissimus at the 12th rib did not differ between callipyge and normal carcasses during chilling (data not shown, P > 0.05). Mean 0-h longissimus pH was 0.3 unit higher for callipyge (pH = 6.9) than normal (pH = 6.6) lambs (data not shown, P < 0.05); however, pH did not differ (P > 0.05) between phenotypes at anytime thereafter.

Initial and sustained juiciness scores were lower (P < 0.05) for callipyge chops compared with normal chops in the STM and longissimus, but phenotype did not (P > 0.05) affect juiciness scores in the SPM. No difference (P > 0.05) in tenderness scores was noted between phenotypes for the STM or SPM, but tenderness scores were lowest in the callipyge LM and highest in the normal LM compared with all others (P < 0.05). Flavor intensity scores were not (P > 0.1) affected by phenotype, muscle location, or their interaction. The WBS values were highest (P < 0.05) in the callipyge longissimus compared to the other combinations of muscles and phenotypes.

Consumer acceptability of chops is shown in Table 4. Juiciness acceptability of leg chops did not (P = 0.4) differ between phenotypes. However, percentages of chops rated acceptable for juiciness, from either the loin or shoulder, were higher (P < 0.05) in the normal phenotype than the callipyge phenotype. Percentage of leg chops rated acceptable for tenderness did not (P = 0.4) differ between the normal (95.9%) and callipyge (92.2%) phenotypes. In contrast, loin chops from the normal phenotype were rated acceptable for tenderness 27.9% more often, and shoulder chops 20.0% more often, than chops from the callipyge phenotype (P < 0.001).

Consumers were asked whether tenderness, juiciness, or flavor was most important to them in providing an acceptable eating experience. Of those responding, 39% selected tenderness, 54% selected flavor, and only 7% selected juiciness as the most important factor they considered when eating lamb.

	Discussion

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Previous studies have demonstrated that callipyge sheep had more muscling and less fat compared to normal sheep (Koohmaraie et al., 1995; Jackson et al., 1997b; Freking et al., 1998). In agreement with these observations, results from the present study indicated that the LM area was 41% larger and fat thickness was 19% less in callipyge carcasses compared to the normal phenotype.

Protein accretion in muscles affected by expression of the callipyge gene has been largely attributed to decreased protein degradation (Lorenzen et al., 2000; C. R. Kerth, K. J. Morrow, C. B. Ramsey, S. P. Jackson, and M. F. Miller, unpublished data) possibly because of a large increase in calpastatin activity (Koohmaraie et al., 1995; Delgado et al., 2000; Duckett et al., 2000). Unlike the previous work that found these increases in calpastatin only in the LM and leg muscles, results of the present study found calpastatin activity elevated in SPM muscles from callipyge sheep. The reason for this discrepancy is not known.

Although the percentage of FOG fiber types was lower in the LM in the present study, the resulting increase in FOG fibers caused by expression of the callipyge phenotype was similar to that reported by Carpenter et al. (1996). Callipyge fiber areas were larger in LM compared to normal sheep, but phenotype did not affect fiber area in the SPM. These results parallel those reported previously (Carpenter et al., 1996) and reflect the differences in expression between muscle locations (Koohmaraie et al., 1995; Jackson et al., 1997c).

Trained sensory panel ratings and WBS values in this study and others (Koohmaraie et al., 1995; Duckett et al., 2000) indicate that LM muscle from callipyge sheep is less tender than normal sheep. The extent to which this decreased tenderness would affect consumer acceptability had yet to be determined. In the present study, calpastatin activity was elevated in all three muscles studied, indicating a decrease in proteolysis; however, sarcomere length was longest in the STM and shortest in the LM. Trained sensory panel tenderness, especially consumer tenderness acceptability, appears to be directly related to sarcomere length in the absence of proteolysis (as determined by elevated calpastatin activity), similar to results of Koohmaraie et al. (1996) and Wheeler and Koohmaraie (1999).

The data presented in the present study indicate a significant decrease in consumer acceptability for tenderness (93 vs. 65% acceptable) of loin chops from callipyge sheep. Still, more than three-fourths of consumers found callipyge loin chops to be acceptable overall. In addition, the expression of the callipyge phenotype did not affect consumer acceptability of leg chops, indicating no marketing disadvantage for callipyge leg chops compared to normal leg chops.

	Implications

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This study supports the theory that the toughening of muscle from callipyge is a result of a combination of short sarcomeres in the absence of normal muscle proteolysis. Although a majority of consumers found all lamb chops from callipyge sheep to be acceptable, consumer perceptions indicated that, only one in 10 leg chops, one in five shoulder chops, and one in four loin chops from callipyge sheep were unacceptable. Cuts from the leg of callipyge sheep could be marketed without further processing, whereas cuts from callipyge loins may need to be enhanced for tenderness to improve consumer tenderness ratings.

	Footnotes

 
¹ Brand names are necessary to report factually on available data; however, Texas Tech University does not guarantee nor warrant the standard of the product, and the use of the name by Texas Tech University implies no approval of the product to the exclusion of others that also may be suitable.

² Correspondence: Department of Animal Sciences, Auburn University, 209 Upchurch Hall, Auburn, AL 36849 (phone: 334-844-1503; fax: 334-844-1519; E-mail: ckerth{at}acesag.auburn.edu.

Received for publication December 17, 2001. Accepted for publication May 21, 2003.

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