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* ARS, USDA, Subtropical Agricultural Research Station (STARS), Brooksville, FL 34601;
and
University of Georgia, Athens, 30602; and
and
University of Florida, Gainesville, 32611
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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0 + 1 exp{2 ti} + 
i, where ti represents aging in days, 0 is ultimate MFI after aging, 1 is the difference between initial and ultimate MFI, 2 is the rate of increase in MFI, and i is the error term associated with the ith observation, assumed to be independent and identically distributed normally). Sires had different estimates and combinations of estimates, which were used to plot MFI change with time. These curves visually differed for sires and suggested that postmortem tenderization extent and rate differ as well. Use of a combination of these estimated parameters in a selection/carcass sorting program represents an alternative consideration for tenderization improvement programs.
Key Words: Brahman • Palatability • Postmortem Changes • Proteolysis • Tenderness
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Although Brahman-cross cows are highly productive in many environments, beef from Brahman cattle has been characterized as less tender than that from Bos taurus cattle (Crouse et al., 1989
), and cattle buyers often discount the price of calves that have the overt appearance of Brahman background for that reason. Selection for beef tenderness would be beneficial for producers who value the adaptability and productivity of Brahman cross cattle. Various traits related to Bos indicus tenderness have been studied, primarily in crossbred animals (Johnson et al., 1990a; Whipple et al., 1990a; O’Connor et al., 1997). Although the details related to specific timing and actions of the proteolytic enzyme activity in the postmortem cellular environment are still being clarified, it is accepted that increased calpastatin activity is responsible for reduced proteolysis in beef from Bos indicus cattle (Johnson et al., 1990b; Wheeler et al., 1990; Whipple et al., 1990b). Furthermore, higher levels of calpastatin activity have been associated with higher percentages of Brahman in cattle (Pringle et al., 1997).
Straightbred Brahman have been studied in breed comparisons (Wheeler et al., 1990; Pringle et al., 1997; Ferguson et al., 2000), but the potential for effective selection of enzyme activity in straightbreds has not been adequately addressed. The tenderization process, including the rate and extent of proteolysis, occurs unequally in different breeds (Wheeler et al., 1990), and in different animals within a breed (Koohmaraie, 1996); knowledge of the genetic influence over this process is limited. The objectives of this study were to assess the influence of sire on µ- and m-calpain activities, to evaluate the relationships of activities of these enzymes to other traits related to beef palatability, and to assess the influence of sire on the rate of tenderization (as measured by myofibril fragmentation index) in Brahman longissimus muscle.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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. Calves were spring-born in 1995 (n = 24) and 1996 (n = 63) at the Subtropical Agricultural Research Station (Brooksville, FL). One bull sired progeny in both years, and the number of calves per sire ranged from five to 16. Calves were weaned at approximately 7 mo of age, and after a 2-wk preconditioning period, they were sorted by weight and gender into feedlot pens and gradually adjusted to a 72.5% corn diet. When the median 12th-rib fat thickness (determined by real-time ultrasound) of the animals in a pen was 10 mm, the entire pen was slaughtered at a commercial beef packing plant (Center Hill, FL). Carcasses were not electrically stimulated in this project. Approximately 18 h after slaughter, the strip loin from the left side of each carcass was removed; shipped to the University of Florida Meats Laboratory, and fabricated into 2.54-cm-thick steaks that were vacuum-packaged and subsequently aged for 1, 7, 14, or 21 d. Steaks used for myofibril fragmentation index (MFI) and Warner-Bratzler shear force (WBSF) determinations were frozen at -40°C after the appropriate aging time.
Measurement of longissimus muscle µ-calpain, m-calpain, and calpastatin activity was done on fresh samples collected approximately 24 h after slaughter. Homogenized samples (50 g) were shipped to the University of Georgia for activity determination using the procedures described by Pringle et al. (1997). Myofibril fragmentation index was determined on thawed muscle at 1, 7, 14, and 21 d postmortem, also at the University of Georgia, by the method of Culler et al. (1978).
Warner-Bratzler shear force determination, sensory panel assessment of juiciness, tenderness, and detectable amount of connective tissue, and proximate composition analyses were conducted at the University of Florida, according to previously described procedures (Riley et al., 2003). Cattle in the present study are a subset of those reported in Riley et al. (2003), and these data (WBSF, sensory panel traits, and proximate composition) were included for this subset only to determine their relationship with µ- and m-calpain activity and MFI of the different aging periods. Calpastatin activities also were determined separately at the University of Florida for all carcasses reported in Riley et al. (2003), yet only activities assayed at the University of Georgia were included in this study, primarily for the determination of the relationship of calpastatin with µ- and m-calpain activities. The unadjusted mean (2.23) and SD (0.85) of calpastatin activity assayed at the University of Florida laboratory were slightly lower than those assayed at the University of Georgia (Table 1). The correlation coefficient (unadjusted) for calpastatin activity assayed at the two laboratories was 0.70 (P < 0.001).
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Data were analyzed using the GLM procedures of SAS (SAS Inst., Inc., Cary, NC). The basic model investigated included sire, sex, year, year x sex, and group nested within year-sex as fixed effects. All traits were adjusted to the overall mean adjusted 12th-rib fat thickness (13.95 mm), regardless of the significance of the continuous variable. Groups (n = 16) were calves of one sex that were fed in the same pen and slaughtered on the same day. These models were used to generate residuals for further analyses. Any effect with a P-value of 0.25 or less was retained in final models of individual traits. In analyses where sire did not explain substantial variation (P > 0.25), it was still retained in final models as the fixed effect of interest in order to generate and compare least squares means from models for each trait. Simple and residual correlations between pairs of traits were estimated using PROC CORR of SAS. Residual correlations represent associations of traits within sire, sex, group, and from calves slaughtered at a constant 12th-rib fat thickness. Relationships involving µ-calpain, m-calpain, and calpastatin activities were explored by evaluation of Spearman’s rank correlation coefficient of the ranked sire least squares means (for all traits, the numerically highest and lowest were ranked 1 and 9, respectively) for each trait.
Postmortem tenderization associated with aging was evaluated by fitting residuals from MFI models to an exponential function:
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where, 0 represents the maximal MFI after prolonged aging, 1 represents the difference between the initial and maximal MFI, 2 represents the rate of increase, ti is aging period in days of the ith observation, and i is the error term associated with the ith observation (assumed to be independent and identically distributed normally). This model was fitted using NLIN procedures of SAS. The data from all sires were fitted to a single curve, and then data from each sire were fitted separately to individual curves. Estimated parameters and plotted functions of each individual sire and the combined data were visually compared to assess potential genetic differences.
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Results and Discussion |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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The numbers of observations and simple statistics for the traits evaluated in this study are presented in Table 1
. The mean age at slaughter was 456 d, and the mean adjusted 12th-rib fat thickness was 13.95 mm.
Significance levels (probability values) for effects in the various models of traits are shown in Table 2. Sire was a source of variation for µ-calpain activity (P < 0.04) but not (P < 0.75) for m-calpain activity. Sire was significant (P < 0.05), or approached significance (P < 0.07), for all other traits except d-7 MFI (P > 0.37), d-21 MFI (P > 0.65), d-21 WBSF value (P > 0.35), sensory panel tenderness score (P > 0.96), sensory panel evaluated connective tissue amount (P > 0.93), and percentage of cooked lipids (P > 0.10). It was unexpected that sire was a significant source of variation for only two (MFI and WBSF) of three tenderness traits after 14 d of aging. This may be due to sampling error or to the differences between subjective and objective measurements of the different traits. Sensory panels did not detect as much overall variation in tenderness score (unadjusted CV = 16.7) as that found for shear force or d-14 MFI (unadjusted CV = 23.1 and 25.5, respectively). Other than the sex x year interactions for µ-calpain activity, calpastatin activity, d-7 MFI, d-21 MFI, sensory panel tenderness score, sensory-panel evaluated connective tissue amount, and percentages of raw and cooked lipids, no other interactions were important (P > 0.25) in these analyses. Group effects were important (P < 0.25) for all traits except d-21 MFI, d-7 WBSF, and juiciness score.
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illustrated maximal sire differences. There was no clearly "best" sire in terms of tenderness, but Sire 3 frequently had the best, or near best, mean for tenderness traits. There was less ambiguity about the least desirable sire, since Sire 1 had unfavorable means for MFI, shear force, and tenderness score. This sire also had the most unfavorable mean for µ-calpain activity and for sensory-panel evaluated connective tissue amount; however, Sire 1 possessed the most favorable (highest) means for percentage lipids. It is generally accepted that there is a small favorable relationship between intramuscular fat (measured in various ways) and tenderness traits (Shackelford et al., 1994; Wulf and Page, 2000).
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; Pringle et al., 1997; 1999). Reasons for this difference are unclear; however, the calpastatin activity means of calves in the present study were much higher than those reported by Whipple et al. (1990b) and Pringle et al. (1997; 1999). Autolysis may be responsible for the low µ-calpain activity means (Koohmaraie, 1992), and McDonagh et al. (2001) reported that most of the observed change in calpain activity at 24 h postmortem had actually occurred during the first 8 h. The observed sire differences for µ-calpain activity suggest genetic control; sire differences might have been more pronounced if they had been measured earlier postmortem. However, if autolysis is responsible for the low means, there may be confounding animal differences in the rate or causes of autolysis. Changing conditions associated with the onset of rigor may reduce the magnitude of genetic control of postmortem enzyme activity, since breed differences were reported in prerigor (not postrigor) levels of µ-calpain activity (Pringle et al., 1999).
Simple and residual correlations are shown in Table 4. Correlations between µ- and m-calpain activities were larger and of opposite sign with respect to those reported by Whipple et al. (1990a), but correlations of calpastatin activity with these two traits were similar for both studies. Rhee and Kim (2001) reported all three correlations (among calpain and calpastatin activity) were greater than 0.7. Conversely, relationships between enzyme activity and various tenderness measures (WBSF values and sensory panel scores) were of the same sign (negative for MFI and tenderness score; positive for WBSF values) and of similar magnitude as other reported correlations (Johnson et al., 1990b; Sherbeck et al., 1996; Wulf et al., 1996), but were lower than those reported by Wulf et al. (1997). Pringle et al. (1997) reported nonsignificant correlations of calpastatin activity with WBSF values and tenderness score. All of these studies, except Wulf et al. (1996), involved crossbred or composite cattle with Brahman background, whereas Pringle et al. (1997) had both straightbred Brahman and Angus represented in their data set. In the present study, correlations of calpastatin activity with tenderness traits were of greater magnitude than those involving tenderness traits with either µ- or m-calpain activity, and correlations of tenderness traits (WBSF values, MFI, and sensory panel tenderness scores) with m-calpain activity were of slightly greater magnitude than those with µ-calpain activity. McDonagh et al. (2001) reported correlations of 0.29, 0.25, and -0.41 for MFI with µ-calpain, m-calpain, and calpastatin activities, respectively. Moreover, Pringle et al. (1997) observed significant correlations of µ-calpain activity (but not m-calpain activity) with WBSF and tenderness scores. McDonagh et al. (2001) reported correlations of WBSF values with µ-calpain, m-calpain, and calpastatin activity that did not differ from zero. Simple correlations indicated that a higher percentage cooked lipids were associated with lower MFI after all aging periods, and that higher percentage raw lipids were associated with lower MFI after 14 and 21 d of aging. This was inconsistent with other reported tenderness–intramuscular fat relationships, but this relationship frequently appears to be a characteristic of the population sampled (Shackelford et al., 1994) or of sampling error. There were moderate, positive simple correlations of enzyme activity with percentage of cooked lipids (r = 0.43, 0.24, and 0.35 for µ-calpain, m-calpain, and calpastatin activities, respectively), but the only significant residual correlation involving percentage cooked lipids was with calpastatin activity (r = -0.25).
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; Wulf et al., 1997; Wulf and Page, 2000). There was a strong negative relationship of calpastatin activity with juiciness scores, which has not been reported by other researchers (Wulf et al., 1996; 1997; Wulf and Page, 2000). Strong relationships between sensory panel assessment of connective tissue amount and essentially all traits were evidenced by simple and residual correlation coefficients. These results, especially the correlation coefficients involving the different tenderness measures, appear to be consistent with others who have reported that greater amounts of detected connective tissue were associated with the longissimus muscle from cattle with a greater percentage of Brahman inheritance (Crouse et al., 1989; Whipple et al., 1990b; Pringle et al., 1997). However, Johnson et al. (1990a) reported no differences in amounts of detected connective tissue among 1/4, 1/2, and 3/4 Brahman steers.
Sires that were ranked favorably for µ-calpain and m-calpain activity also were ranked favorably for most of the other traits related to tenderness. Sire rank correlations of calpastatin activity means with µ-calpain and m-calpain activity were highly significant (Table 5). Rank correlations were significant for µ-calpain and m-calpain activity paired with MFI of steaks aged 1, 14, and 21 d and with sensory panel tenderness scores (r > 0.7). The rank correlation for calpastatin activity with sensory panel tenderness scores (r = 0.68) was significant; however, there were no significant rank correlations of enzyme activity with WBSF values at any aging period.
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Overall and individual sire parameter estimates (ki estimates j, j = 0, 1, and 2) of the fitted curve (MFIj = 0 + 1 exp{2 ti} + i) are shown in Table 6. All analyses converged in nine or fewer iterations. Figure 1 pictorially describes the curve for all the data combined (± one SE confidence band), and those for Sires 4 and 6. The curves in Figure 1 represent, within a single sire or the entire group, the plot of expected change in MFI associated with aging time. Although the model fitted is certainly an oversimplification of the true process, comparison of these curves suggests that progeny of different sires have different MFI improvement trajectories, or aging patterns. The curves shown in Figure 1 are presented to illustrate estimated parameter differences, but are not the most extreme sires (for example, Sire 1 had a lower and much flatter curve than any of the other sires). The curve for Sire 3, which had among the most favorable means for tenderness traits (Table 2), would plot almost exactly on the upper bound of the confidence band for the curve constructed from fitting all data (upper dashed line).
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). Progeny from the high k1 group (Sires 2, 3, 5, and 8) would benefit from aging (i.e., greater difference between initial and maximal MFI). Progeny from the low k1 group (Sires 6 and 7) could be assigned to shorter (or no) aging periods, since even long aging periods would not yield improvements in MFI as large as those for the sires with higher k1 values. Sire k2 estimates could then be used to target progeny carcasses (high k1 group, Sires 2, 3, 5, and 8) to aging periods. For example, although fragmentation would be expected to occur at a slower rate for Sire 2, its large k1 estimate would indicate a considerable amount of improvement potential. Segregation of carcasses by such a method would be dependent on other biological and economic considerations. This system could facilitate or complement niche marketing and/or value assignment based on the readiness for presentation to the public.
A potential limitation of such a program is the lack of independence among the i parameters. High rates of fragmentation would be expected to be strongly associated with high maximal MFI (Sires 2 and 3) and vice versa, as indicated by asymptotic correlations between k0 and k2 values (-0.56 across all sires and a range of -0.88 to -0.22 for individual sires). Asymptotic correlations of 0.87 (across sires, a range of 0.77 to 0.91 for individual sires) of k0 and k1 values imply that larger ultimate fragmentation indices (k0 ) would be associated with lower differences between initial and ultimate MFI (k1). The weaker asymptotic correlations of k1 with k2 (-0.24 across sires, and -0.74 to 0.23 for individual sires) seems to indicate that a range of rates are possible, regardless of the fragmentation potential of a sire (compare parameter estimates of Sires 2 and 6 in Table 6).
Electrical stimulation of carcasses would likely have an effect on the parameters estimated by this procedure. It has been recently shown that electrical stimulation rapidly decreased levels of µ-calpain, m-calpain, and calpastatin (Rhee and Kim, 2001). Ferguson et al. (2000) provided similar results in carcasses of Brahman and Brahman x Hereford cattle when they concluded that electrical stimulation offers more benefit to cattle with higher percentages of Brahman background.
Parameter estimates (or the function describing the curve) would likely vary between breeds. In their review of work related to postmortem proteolysis, Oddy et al. (2001) contended that variation in the rate of tenderization in beef had both genetic and environmental components. Breed differences in postmortem tenderization (in most cases based upon evaluation of breed x aging period interactions) between Brahman crossbreds and Bos taurus cattle have been reported in studies using MFI (Whipple et al., 1990b; Shackelford et al., 1991) and in studies using WBSF values after different aging periods (Johnson et al., 1990a; Wheeler et al., 1990; O’Connor et al., 1997). However, O’Connor et al. (1997) failed to find these breed differences among carcasses of Braford, Red Brangus, and Simbrah steers and heifers, and Wulf et al. (1996) reported that sire differences in WBSF values after short aging periods persisted after extended aging periods. Riley et al. (2000) reported sire parameter estimates from a study that fitted curves to serial WBSF data (based on the premise that shear force practically decreases to an asymptote) in half-blood Brahman-British steers. In that study, one sire had a small number of extreme observations that resulted in a plotted curve that was essentially linear with large asymptotic SE for parameter estimates. However, the remainder of the sire curves from that study, and those of the present study, suggest the existence of sire differences in the postmortem tenderization process within the Brahman breed.
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Implications |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Sire differences in postmortem muscle µ-calpain activity may be useful in development of alternative programs for improvement of Brahman beef tenderness. There seem to be sire differences in the postmortem aging process in Brahman longissimus muscle as modeled with an exponential function. These may be indicative of additive genetic control of tenderization associated with aging of beef, and verification should be initiated with larger data sets. These findings could be important as a unique avenue for investigation for improvement of beef tenderness, especially in breeds of cattle with a reputation for inadequate tenderness. This could facilitate the appropriate feeding, product designation, and value assignment for cattle and their carcasses from different genetic lines.
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Footnotes |
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2 Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product to the exclusion of others that may also be suitable.
3 Appreciation is extended to Brahman producers for loaned bulls; to members and representatives of the Florida Brahman Assoc. and the American Brahman Breeders Assoc.; and to E. L. Adams, E. J. Bowers, V. E. Rooks, M. L. Rooks, and the STARS staff for technical assistance and animal care.
5 Present address: USDA, ARS, SAA, 950 College Station Rd., Athens, GA 30604.
4 Correspondence: 22271 Chinsegut Hill Rd. (phone: 352-796-3385; fax: 352-796-2930; E-mail: dgriley{at}mail.ifas.ufl.edu).
Received for publication January 29, 2003. Accepted for publication June 16, 2003.
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Literature Cited |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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