J. Anim. Sci. 2002. 80:2390-2395
© 2002 American Society of Animal Science
Shear gradient in longissimus steaks1
C. R. Kerth2,
J. L. Montgomery3,
J. L. Lansdell4,
C. B. Ramsey and
M. F. Miller5,
Animal Science and Food Technology Department, Texas Tech University, Lubbock 79409-2162
5 Correspondence:
Box 42162 (phone: 806/742-2804; fax: 806/742-0169; E-mail:
mfmrraider{at}aol.com).
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Abstract
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Boneless top loin subprimals (n = 320) from Slight and Small marbled carcasses were fabricated into 2.54-cm thick steaks to determine core location effects on tenderness. In Exp. 1, top loins were aged to 7 d before steaks were cut and cooked to an internal temperature of 71°C. After cooking, a maximum of 15 1.27-cm diameter cores were removed and sheared with a Warner-Bratzler shear force (WBSF) device. There was not a marbling score x core location interaction (P = 0.36). However, there was a main effect of core location (P < 0.01). Cores from the medial, middle, and lateral portion of the longissimus muscle (LM) aged for 7 d differed, with less resistance (P < 0.05) in the medial than the lateral end. Also, there was an effect of marbling score on WBSF, with Small-marbled steaks having lower (P < 0.02) WBSF values than Slight-marbled steaks. In a second experiment, steaks were removed from the middle of the top loin subprimals and aged an additional 7 d to produce 14-d aged steaks. Shear values decreased (P < 0.05) from Exp. 1 to 2 for all core locations. Neither the main effect of marbling score nor the core location x marbling score interaction was significant (P > 0.40); however, the same lateral to medial gradient in WBSF values was discovered again in Exp. 2. Both experiments indicated there were regions of WBSF values that differed (P < 0.05) across the cross section of the LM producing a shear-force/tenderness gradient, with the most medial cores having the lowest WBSF values in both experiments independent of marbling score. Regression analyses indicated the middle and center portions of LM steaks tended to have the most predictive capacity of average WBSF. Because of the variability in tenderness caused by location within the LM, care should be exercised when selecting sampling areas for the measurements of tenderness using the WBSF measure.
Key Words: Beef • Meat Quality • Tenderness
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Introduction
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Morgan et al. (1991) and Brooks et al. (2000) reported that tenderness is unacceptably inconsistent. Many factors affect beef tenderness of the longissimus muscle (LM) including cookery (Cover and Hastetler, 1960; Machlik and Drandt, 1963; Hedrick et al., 1968), degree of doneness (Visser et al., 1960; Cover et al., 1962), measurement techniques (Bratzler and Smith, 1963; Sharrah et al., 1965; Wheeler et al., 1996), and muscle sample location within a single muscle (Ginger and Weir, 1958; Taylor et al., 1961).
A number of researchers have investigated potential tenderness gradients across the LM (Alsmeyer et al., 1965; Hostetler and Ritchey, 1964; Crouse et al., 1989). Increased marbling in the LM also has been reported to improve the palatability and tenderness and decrease Warner-Bratzler shear force (WBSF) values (McBee and Wiles, 1967; Smith et al., 1984; Wheeler et al., 1994). However, Berry (1993) is the only study to these authors’ knowledge to determine Slight and Small marbling effects and core location effects on beef tenderness. Unfortunately, Berry (1993) did not determine differences in WBSF within the entire cross section of the LM, just the dorsal edge of steaks. Therefore, the objectives of the present study were to elicit the impact of core location within steaks with marbling scores of Slight and Small on WBSF values of beef LM. A secondary objective was to determine how individual core locations would predict average WBSF values of beef LM steaks.
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Materials and Methods
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Selection
Three hundred twenty boneless top loin subprimals (IMPS #180; USDA, 1990) were fabricated at the Excel, Inc., beef processing plant at Plainview, TX (n = 160), and the IBP beef processing plant at Garden City, KS (n = 160). Carcasses were chilled at 2°C ± 2°C for 36 h at the Excel plant and for 48 h at the IBP plant. The carcasses ranged from yield grade 2.0 to 3.9. Two carcasses at each slaughter plant were selected with USDA (1989) marbling scores that were determined by USDA personnel using 10 degrees segments from Slight00 to Small90 (a total of 20 marbling degree segments). Each top loin strip collected from the left side was vacuum packaged and transported to the Texas Tech University Meat Laboratory where they were stored at 2°C for 7 d. At 7 d, the subprimals were unwrapped and cut in half. A 2.54-cm thick steak was cut from the anterior end, packaged in a vacuum bag, and frozen at -10°C. These steaks are denoted as Exp. 1.
In Exp. 2, the other half of the top loin was repackaged in a vacuum bag and stored at 2°C for another 7 d to produce the 14-d aged samples. Thus, the steaks in Exp. 1 were from the anterior portion of the strip loin and aged 7 d, whereas the 14-d aged steaks were from the middle portion of the strip loin.
Warner-Bratzler Shear Force Value Determination
Within 50 d, boneless top loins were selected at random, thawed at 4°C for 24 h and broiled on a Farberware Open Hearth electric broiler (Farberware, Bronx, NY, USA). Steaks were cooked to an internal temperature of 40°C, then turned over, and steaks were removed from the grill when the internal temperature reached 71°C in the geometrical center of each steak. Temperature was monitored with a Cooper Instruments (model SH66A, Middlefield, CT, USA) digital meat thermometer. Cooked steaks were placed on plastic trays and overwrapped with a polyvinyl chloride film and chilled for 24 h at 2°C. For each of the 320 steaks a total of 15 core locations were identified for shear force evaluation (Figure 1
). After chilling, 1.3-cm diameter cores were removed parallel to the muscle fiber orientation. The cores were taken in an arrangement of three columns from the dorsal to the ventral side of the steak and five rows from the lateral to medial region of the steak (Figure 1). While each steak had a total of 15 different core locations identified, the total number of possible cores removed varied from 10 to 15 due to connective tissue and total LM cross-sectional area (see Table 1). Thus, there was variation in the potential number of available cores from each steak. Cores that contained connective tissue or were not considered acceptable due to shape were discarded and not sheared. Each acceptable core was center-sheared across the muscle fibers with a Warner-Bratzler shear (WBSF) machine (G-R Elec. Mfg. Co., Manhattan, KS) according to AMSA (1995) guidelines.
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Table 1. Mean, standard deviation, and number of cores, measured in kg, sampled for each of the individual core locations from longissimus steaks from Exp. 1 (postmortem age 7 d) and Exp. 2 (postmortem age 14 d)
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Data Analysis
Both Exp. 1 and 2 were analyzed separately. Data were analyzed using GLM procedures of SAS (SAS Inst. Inc., Cary, NC) with a split-plot arrangement for repeated measures. The experimental unit was strip loin, and blocking was based on packing plant. The whole plot was comprised of marbling score (Small vs Slight marbling) and the block (packing plant location), and the error term used to test whole plot effects was marbling score nested within block and strip loin. The subplot consisted of core location and the core location x marbling score interaction, with the residual error term used to test subplot effects. Least squares means were computed, and, because of unbalanced replication, standard error of the means (SEM) was calculated according to Steel and Torrie (1980). Mean separation was conducted using a Fisher’s protected least significant difference, with an 
-level of 5% used to detect significant differences.
Linear regression analyses were conducted using the Proc Reg procedure of SAS to determine R2 values between the different core locations within individual experiment and quality grade treatments to predict average WBSF values. Multiple comparisons regression analysis was conducted using the Proc Reg procedure and MaxR selection function in SAS to determine R2 values between different core locations and average WBSF values within each experiment.
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Results and Discussion
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In Exp. 1 (samples were from the anterior portion of the strip loin and aged to 7 d) the main effect of marbling score was significant (P < 0.01) with steaks from Small carcasses being more tender than steaks from Slight carcasses (data not shown). The WBSF values of steaks with Small marbling were less (P < 0.02) than steaks with Slight marbling. USDA Choice steaks have been shown to be more tender and less variable than USDA Select steaks (Savell et al., 1987; Smith et al., 1984; Wheeler et al., 1994); however, in Exp. 2 there was no effect (P = 0.54) of marbling score on WBSF.
In both experiments, core location did not interact with marbling score (P = 0.36); however, core location had a noticeable effect (P < 0.01) on WBSF in both experiments. The impact of core location on WBSF in Exp. 1 and 2 are presented in Figures 2 and 3, respectively. In both experiments there were regions of WBSF values that differed (P < 0.05) across the cross section of the LM producing a tenderness gradient. In general there was a lateral to medial WBSF-gradient across the LM steaks (P < 0.05). While a dorsal to ventral gradient was evident in both experiments, the lateral-medial gradient was the most predominant. The furthermost lateral region of the LM steaks consistently produced the highest WBSF values (P < 0.05) than any other region of the LM steaks, regardless of experiment (aging) or marbling score. A number of researchers have reported a tenderness gradient within the cross section of the LM (Hostetler and Ritchey, 1964; Sharrah et al., 1965; Smith et al., 1969). Opinions concerning the direction of the tenderness gradient vary widely, but have prompted further research into developing guidelines concerning core location and assessment of WBSF and tenderness. The present study indicates that researchers should try to maintain a consistent core location when sampling steaks, especially when steaks are aged for less than 14 d. It is important to note that sampling 15 different core locations to maximize core variability is not recommended because Wheeler et al. (1996) demonstrated that very little improvement in repeatability was achieved when more than six cores from an individual steak were sheared.
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Figure 2. Experiment 1. Shear gradient of longissimus muscles aged to 7 d postmortem as measured by Warner-Bratzler shear force (kg). Locations with different markings differed (P < 0.05) and correspond with the critical difference of the least significant difference. Standard error of the means ranged from 0.12 to 0.14.
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Figure 3. Experiment 2. Shear gradient of longissimus muscles aged to 14 d postmortem as measured by Warner-Bratzler shear force (kg). Locations with different markings differed (P < 0.05) and correspond with the critical difference of the least significant difference. Standard error of the means ranged from 0.08 to 0.09.
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Rigor shortening of the sarcomere results in meat toughening (Wheeler and Koohmaraie, 1994). Muscles that shorten less during the onset of rigor result in longer sarcomeres and are more tender in comparison to muscles with shorter sarcomeres (Marsh, 1985; Smulders et al., 1990). Thus, meat toughening can be prevented or reduced if rigor shortening is prevented or reduced (Koohmaraie et al., 1996). Using lamb carcasses with different levels of subcutaneous fat, Smith et al. (1976) found that external fatness of carcasses serves as an insulation, reducing the rate of postmortem temperature decline. Differences in WBSF values along the dorsal axis of LM steaks observed in the present study may be a response to a visually unnoticeable degree of cold shortening. Although sarcomere length was not measured during either experiment of this study, Crouse et al. (1989) reported a reduction in sarcomere length at the dorsal (fat) side of LM steaks and elevated WBSF values as well.
In the present study, WBSF varied in different cross-sectional regions of the LM, producing a tenderness gradient from the lateral side to the medial side of steaks (P < 0.05). While differences existed from the dorsal to ventral region, differences were generally small.
Results obtained from the present study agree with previously published results (Hedrick et al., 1968; Smith et al., 1969; Berry, 1993) that a lateral to medial tenderness gradient exists within the LM, with the lateral area being the toughest. In contrast, however, McBee et al. (1967) reported that medial positioned cores were the least tender, and Crouse et al. (1989) and Cover et al. (1962) showed that lateral cores were the most tender. Similar to the present study, Alsmeyer et al. (1965), Hedrick et al. (1968), and Smith et al. (1969) reported that the medial portion of longissimus steaks were most tender. While many studies have evaluated differences in a lateral to medial direction, few studies have compared differences in shear values across the short axis (dorsal to ventral) of the LM as in the present study.
Simple linear regression equations were constructed to determine how much of the total variation in average WBSF values could be explained by the individual core locations within the individual experiments and marbling score. Correlation coefficients (R2) for the predictive ability of core location on average WBSF are displayed in Table 2. Core locations 3, 8 and 9 tended to explain the most variation in average shear force. Core 3 explained 51% of the variation in steaks with small marbling, whereas, core location 3 and 8 explained 46% and 56% of the variation in mean WBSF for steaks in Exp. 1 and 2, respectively. Because core 3, 8 and 9 are in the center and middle of the steak, it appears that core locations in these areas were the most representative of average WBSF.
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Table 2. Coefficients of determination from simple linear regression equations to predict average Warner-Bratzler shear force of longissimus steaks from Exp. 1 (postmortem age 7 d) and Exp. 2 (postmortem age 14 d)
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Multiple-variable linear regression equations were constructed to determine how much of the total variation in average WBSF values could be explained by the individual core locations (Table 3). The use of six to eight cores was found to explain a high percentage (90 to 95%) of the variation in average WBSF values. Placing cores 3 and 9 in the model statement explained 67% of the variation in average WBSF in Exp. 1 (7 d aged) and 77% of the variation in WBSF could be accounted for with cores 3, 7, and 9. Placing cores 8 and 12 in the model statement explained 71% of the variation in average WBSF in Exp. 2 (14 d aged), while cores 8, 10, and 11 accounted 80% of the variation in average WBSF. Wheeler et al. (1996) found the repeatability of shear force increased quadratically as the number of cores used to calculate the shear force mean increased, however, the overall improvement of repeatability declined dramatically after five cores. Results of the present study demonstrate the existence of a tenderness gradient in the cross section of the LM; therefore, careful consideration must be taken to control core location.
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Table 3. Coefficients of determination from multiple linear regression equations using MaxR selection to predict average Warner-Bratzler shear force (WBSF) of longissimus steaks from Exp. 1 (postmortem age 7 d) and Exp. 2 (postmortem age 14 d)
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Implications
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Results from two experiments indicate the existence of a Warner-Bratzler shear force gradient across longissimus muscle steaks from Slight- and Small-marbled carcasses aged for either 7 or 14 d postmortem, yet the magnitude of this tenderness gradient appears to be less in steaks aged for 14 d. Cores from the center and middle of steaks tended to have the most predictive capacity of average WBSF. Overall, these data indicate the need for consistent collection of core samples and careful matching of sample location from carcass to carcass to characterize the mean shear value and sensory tenderness rating of the longissimus muscle.
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Footnotes
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1 This research was partially supported partially by the Cattleman’s Beef Promotion and Research Board through the Beef Industry Council of the National Live Stock and Meat Board. 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 may also be suitable. Manuscript No. T-5-409 of the Texas Tech University College of Agricultural Sciences and Natural Resources. 
2 Current Address: Animal and Dairy Science Department, Auburn University, 140 Upchurch Hall, Auburn, Alabama 36849.
3 Current Address: Intervet Inc., 405 State Street, Millsboro, Delaware 19966.
4 Current Address: Continental Deli Food Inc., 2601 Northwest Expressway, Suite 1000W, Oklahoma City, Oklahoma 73112.
Received for publication October 2, 2001.
Accepted for publication May 13, 2002.
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Abstract
Introduction
