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ANIMAL PRODUCTS |
* Department of Animal Sciences, Colorado State University, Fort Collins 80523-1171;
and
Department of Food Science and Human Nutrition, Iowa State University, Ames, 50011; and
Elanco Animal Health, Greenfield, IN 46140
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: aging • beef • biological type • palatability • ractopamine • tenderness
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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); however, limited information documenting the effects of RAC on beef tenderness has been published. The FDA (2003) reported that supplementing RAC (10 or 20 ppm) during the final stages of finishing did not affect (P > 0.05) palatability of beef from mixed-breed steers and heifers.
Since 2000, RAC (Paylean; Elanco Animal Health) has been available for use in US swine diets. Several studies have shown that RAC decreases pork tenderness (Aalhus et al., 1990; Uttaro et al., 1993; Carr et al., 2005), whereas others have reported no effect of RAC supplementation on pork tenderness (McKeith et al., 1988; Stites et al., 1994; Stoller et al., 2003). Recently, Xiong et al. (2006) suggested that inconsistent results from RAC pork palatability studies could be caused by differences in postmortem aging time of meat samples. Xiong et al. (2006) compared control and treated pork LM samples aged from 2 to 21 d postmortem and showed that chops from RAC-treated pigs had greater shear force values than control chops only when aged fewer than 10 d.
Surveys of US retail markets have documented substantial variation in length of postmortem aging time for beef retail cuts (Morgan et al., 1991; Brooks et al., 2000). Furthermore, research has shown that use of growth modifiers (Wheeler and Koohmaraie, 1992) and biological cattle type (Wheeler et al., 1990; O’Connor et al., 1997) often affect postmortem beef tenderization.
Therefore, this study was conducted to characterize the effects of RAC supplementation and postmortem aging on shear force and sensory properties of LM steaks from cattle representing 3 distinct biological types.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Experimental Design
Cattle management history, design of the experiment, and treatments were described in detail by Gruber et al. (2007). Briefly, 420 weanling steer calves were selected to equally represent 3 biological cattle types: British, Continental crossbred, and Brahman crossbred. Steers were weighed at reimplantation and sorted into 7 BW blocks, each block consisting of 2 pens (10 steers/pen) of each type. Each pair of pen replicates within a block x type subclass was randomly assigned to RAC supplementation treatments, as follows: 1) control, 0 mg·steer–1·d–1 of RAC; 2) treatment, 200 mg·steer–1·d–1 of RAC. Dietary treatments were administered to paired pens during the final 28 d before slaughter. The experimental design used in this study resulted in 7 BW blocks, 21 pen replicates/RAC treatment, 14 pen replicates/type, and 7 replicates of each RAC x type subclass.
Slaughter and Carcass Sampling
After conventional, humane slaughter procedures, prerigor carcasses traveled through 4 zones of electrical stimulation: 1) 16 V, 60 Hz, 15 s (1 s on, 1 s off); 2) 20 V, 60 Hz, 15 s (1 s on, 1 s off); 3) 24 V, 60 Hz, 20 s (1 s on; 1 s off); 4) 28 V, 60 Hz, 13 s (2 s on, 1 s off) and, then, were chilled (air temperature 2°C) for 36 h. For the first 8 h of the chill period, carcasses were sprayed intermittently (2 min on, 8 min off) with a mist of 2°C water.
A subsample of 7 carcasses was randomly selected from each pen-replicate for shear force determination and sensory panel analysis. Carcasses that were excessively trimmed, or that had other abnormalities, such as "fat-pulls" over the LM, were excluded from selection. At 48 h postmortem, striploins (IMPS 180; USDA, 1996) were removed from the right sides of 294 carcasses and transported immediately (under refrigeration) to the Colorado State University Meat Laboratory. At the Meat Laboratory, each striploin was assigned to a sampling scheme that randomly specified anatomical sections of the LM that would be assigned to each of 4 postmortem aging periods (3, 7, 14, and 21 d). The length of each LM section was determined by the number of steaks that was required to represent each aging period (n = 2, 1, 3, and 1 steaks for LM sections aged 3, 7, 14, and 21 d, respectively). Based on the sampling scheme, LM sections of appropriate length were removed sequentially starting from the anterior end of the striploin. Longissimus muscle sections were then placed into vacuum-sealed bags and stored at 2°C. Following completion of the appropriate aging time, LM sections were frozen and stored at –20°C.
Frozen LM samples were fabricated into 2.54-cm-thick steaks using a band saw (model 400, AEW, Norwich, UK). Individual steaks were placed in vacuum-sealed bags and designated for Warner-Bratzler shear force (WBSF), slice shear force (SSF), or trained sensory panel (TP) evaluations. Sections aged for 14 d were fabricated into 3 steaks for WBSF, SSF, and TP evaluations. Sections aged for 3 d were fabricated into 2 steaks that were packaged in vacuum-sealed bags and designated for WBSF or SSF. A single steak to be used for WBSF determination was removed from each 7- and 21-d LM section. Frozen steaks identified for TP analysis were transported to the Food Science and Human Nutrition Department at Iowa State University for further evaluation.
Shear Force Measurements
Frozen steaks used for WBSF measurements were thawed for 36 to 40 h at 2°C (precooking, internal steak temperatures were monitored to ensure that steak temperatures were between 1 and 5°C), and cooked on an electric conveyor grill (model TBG-60 MagiGrill, Magi-Kitch’n Inc., Quakertown, PA) for a constant time of 6 min, 5 s at a setting of 163°C for the top and bottom heating platens, to achieve a targeted peak internal temperature of 71°C. Peak internal temperature measurements were obtained by inserting a Type K thermocouple (model 39658-K, Atkins Technical, Gainesville, FL) in the geometric center of each steak. After cooking, WBSF steaks were allowed to equilibrate to room temperature (22°C) and 6 to 10 cores (1.27 cm in diameter) were removed from each steak parallel to the muscle fiber orientation. Each core was sheared once perpendicular to the muscle fiber orientation using a universal testing machine (model 4443, Instron Corp., Canton, MA) fitted with a Warner-Bratzler shear head (cross-head speed: 200 mm/min). Peak shear force measurements of each core were recorded and averaged to obtain a single WBSF value for each steak.
Steaks designated for SSF determination were thawed and cooked using procedures identical to those detailed for WBSF steaks. Within 5 min of recording the cooked temperature, a 1-cm-thick, 5-cm-long slice was removed from each steak parallel to the muscle fibers (Shackelford et al., 1999). The slice was then sheared perpendicular to the muscle fibers using a universal testing machine (model 4443, Instron Corp.) equipped with a flat, blunt-end blade (crosshead speed: 500 mm/min) resulting in 1 SSF measurement for each steak.
Trained Sensory Panel Evaluation
Steaks designated for TP evaluation were thawed for 48 h at 2°C and cooked on electric grills (model GGR62, Salton Inc., Lake Forest, IL). A thermocouple (Chromega/Alomega, Omega Engineering, Stamford, CT) was placed in the geometric center of each steak, and the internal temperature was monitored during cooking using a hand-held thermometer (Model DP41-TC-MDSS, Omega Engineering, Stamford, CT). Steaks were removed from the heat when a targeted internal steak temperature of 70°C was achieved before being cut into 1.3 cm x 1.3 cm x 2.54 cm cubes.
A 10-member TP evaluated each 14-d cooked LM steak for tenderness, juiciness, beef flavor, and off-flavors using a 15.0-cm, unstructured line scale. The line scale was anchored on the left (0 cm), with a term representing a low degree of juiciness, tenderness, beef flavor, and off-flavor. On the right end (15.0 cm) of the scale was a term representing a high degree of each sensory characteristic. Each trained panelist was presented with 1 warm LM cube in a preheated glass Petri dish. Panelists were seated at individual booths with red lighting overhead. Deionized distilled water (served at room temperature) and unsalted crackers were used to cleanse the palate between samples. Panelists participated in 25 sessions and received no more than 12 samples per session. Individual panelists’ observations were averaged to obtain a single value for each sensory trait for each 14-d LM steak.
Statistical Analysis
Data for WBSF and SSF were analyzed using a REML-based, mixed-effects model for repeated measures (PROC MIXED; SAS Inst. Inc., Cary, NC). For all analyses, the statistical model included RAC, type, and aging period (AGE) as independent fixed effects. All relevant 3- and 2-way interactions of fixed effects were included and subsequently removed from the model and pooled within the residual if not significant (P > 0.05). The ANOVA model for WBSF included RAC, type, AGE, RAC x type, and type x AGE as fixed effects and block and block x type as random effects. A spatial power covariance structure was used, and the containment approximation was used to calculate denominator df in the analysis of WBSF. The ANOVA model for SSF included RAC, type, AGE, and type x AGE as fixed effects. Block, block x type, and block x type x RAC were included as random effects, and the Satterthwaite approximation was used to calculate denominator df. All shear force measurements were adjusted to a common peak internal temperature using analysis of covariance. For all shear force analyses, pen served as the experimental unit, AGE was treated as a repeated measurement, and means were separated using the PDIFF option at a significance level of P < 0.05.
The least squares, nonlinear models procedure of SAS was used to fit the following exponential decay model to least squares means for WBSF values corresponding to the RAC x AGE interaction: WBSF = b2 + b1 exp(–b0t), where b2 is the distance from zero to the asymptote, b1 is the distance from the asymptote to the y-intercept, b0 is a constant rate of change, and t is the time (d) postmortem (Gruber et al., 2006).
Trained sensory panel data were analyzed using a REML-based, mixed-effects model (PROC MIXED; SAS Inst. Inc., Cary, NC). The ANOVA model included RAC and type as independent fixed effects, and the RAC x type interaction was tested and subsequently removed if not significant (P > 0.05). Block x type was included as a random effect, and the Satterthwaite approximation was used to calculate denominator df. Pen served as the experimental unit, and means were separated using the PDIFF option at a significance level of P < 0.05.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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On average, daily supplementation of steers with 200 mg RAC increased (P = 0.001) LM WBSF and SSF by 0.38 and 1.4 kg, respectively (Table 1). Increases in LM SSF associated with RAC supplementation were of similar magnitude for all 3 biological types; however, the magnitude of the RAC effect on LM WBSF differed among cattle types (RAC x TYPE; P = 0.006). As shown in Figure 1, feeding RAC resulted in greater increases in LM WBSF among Brahman crossbreds (0.57 kg) than among Continental crossbred (0.20 kg) and British steers (0.35 kg).
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showed that effects of RAC supplementation on WBSF, though diminished slightly, were not completely counteracted by aging LM samples for 21 d postmortem (PRAC x AGE = 0.16).
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). Compared with Continental crossbred and British steers, Brahman-crossbred steers produced LM steaks that had the greatest (P < 0.05) WBSF values at all postmortem aging times, coupled with the largest change (1.73 kg) in WBSF from 3 to 21 d PM (Table 2). Moreover, LM samples from Brahman crossbred steers required 21 d of aging to have comparable WBSF values to those for LM samples from Continental crossbred and British steers aged for 3 or 7 d (Table 2). Warner-Bratzler shear force values of steaks from Continental type and British steers did not differ (P > 0.05) at any of the postmortem aging times (Table 2). At 3 d postmortem, SSF measurements were greatest (P < 0.05) for steaks from Brahman crossbred cattle (Figure 3); however, aging diminished differences in SSF among the biological types, so that at 14 d postmortem, LM SSF values for Brahman crossbred steers were similar (P > 0.05) to those for British steers.
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Least squares means for TP ratings of LM steaks aged 14 d postmortem are shown in Table 1
. Steers fed RAC produced steaks that received lower (P < 0.05) ratings for tenderness and juiciness, along with slightly lower (P = 0.06) ratings for beef flavor, compared with steaks from control steers (Table 1). No differences (P = 0.30) in off-flavor were detected between treatment groups (Table 1).
Steaks produced by British steers received the highest (P < 0.05) sensory ratings for tenderness, juiciness, and beef flavor, whereas steaks from Brahman cross-bred steers received the lowest (P < 0.05) ratings (Table 1). Sensory panelists also detected differences in tenderness, juiciness, and beef flavor between the British and Continental crossbred groups; steaks from British steers received greater (P < 0.05) ratings for all 3 traits (Table 1).
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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documented the effects of RAC supplementation (0, 10, 20, or 30 ppm for 28 or 42 d prior to slaughter) on the palatability of 14-d aged LM steaks from mixed-breed steers and heifers. In those studies, WBSF measurements and TP ratings for tenderness, juiciness, beef flavor, and off-flavor of LM samples from cattle in the 10 and 20 ppm treatment groups did not differ from those for control samples; however, cattle that received 30 ppm of RAC produced steaks with greater WBSF values (3.95 vs. 3.54 kg) and less desirable TP ratings for tenderness, than did steaks from control cattle (FDA, 2003). The RAC dosage administered in the current study (200 mg·steer–1·d–1) is comparable with 20 ppm of RAC; however, results from the current study do not agree with those reported by FDA (2003). To date, no other studies have documented the effects of RAC supplementation on beef tenderness as measured by SSF, and we found no other beef palatability studies that documented a significant interaction between RAC supplementation and cattle type. In a separate report (Gruber et al., 2007), no RAC x type interactions were observed for any growth performance or carcass traits measured for the steers utilized in the current study. Steers fed RAC had improved ADG (1.50 vs. 1.73 ± 0.09 kg) and produced carcasses that had heavier HCW (359 vs. 365 ± 4.9 kg) and larger (P = 0.046) LM areas (81.7 vs. 84.0 ± 1.1 cm2) compared with control steers (Gruber et al., 2007).
In the current study, the effect of RAC supplementation on beef LM WBSF was not dependent upon postmortem aging (Figure 2). In contrast, Xiong et al. (2006) noted that the effect of RAC supplementation (20 ppm for 28 to 30 d prior to slaughter) on WBSF of pork LM was affected by postmortem storage. In the latter study, differences in shear force between the control and treatment groups were evident at 2, 4, and 7 d postmortem, but WBSF of chops from the control and RAC-treated pigs did not differ following 10 d, or longer, of postmortem aging (Xiong et al., 2006).
The decrease in meat tenderness associated with β-agonist supplementation has been attributed to: 1) an increase in postmortem calpastatin activity (Koohmaraie et al., 1991; Wheeler and Koohmaraie, 1992; Geesink et al., 1993) and 2) a shift in the proportion of muscle fiber-types with a corresponding increase in muscle fiber diameter (Aalhus et al., 1992; Vestergaard et al., 1994). Beta-agonists, such as cimaterol, clenbuterol, and L-644,969, are commonly classified as β2-agonists, and it is hypothesized that the mechanism by which these compounds induce muscle hypertrophy is through an increase in protein synthesis, together with a decrease in protein degradation (Moloney et al., 1991; Moody et al., 2000). The decrease in cooked meat tenderness associated with supplementation of β2-agonists has, therefore, been attributed, in part, to increased postmortem calpastatin activities that result in decreased postmortem proteolysis (Koohmaraie et al., 1991; Wheeler and Koohmaraie, 1992; Geesink et al., 1993). Ractopamine, purported to be a β1-agonist, is believed to increase muscle mass in livestock primarily through an increase in protein synthesis with minimal effects on protein degradation (Bergen et al., 1989; Smith et al., 1990; Moody et al., 2000). Therefore, it would seem unlikely that the decrease in tenderness observed in the current study would have been caused by an increase in early postmortem calpastatin activity. However, there are currently no published reports of studies that have measured postmortem calpastatin activity in beef from RAC supplemented cattle. It also has been hypothesized that the decrease in meat tenderness associated with β-agonist supplementation may be associated with increased muscle fiber diameter (Aalhus et al., 1992; Vestergaard et al., 1994; Carr et al., 2005). In muscle from RAC-supplemented pigs, Aalhus et al. (1992) noted an increase in the proportion and diameter of white (type IIb) muscle fibers. White (type IIb) muscle fibers are associated with large fiber diameter, and it was suggested by Aalhus et al. (1992), that the increase in the percentage of white (type IIb) fibers and corresponding increase in muscle fiber diameter could contribute to increased shear values noted for meat from RAC-supplemented pigs (Aalhus et al., 1990, 1992). Identification of the mechanism by which RAC caused a decrease in tenderness of LM steaks (Table 1 and Figure 1) is beyond the scope of this study, but represents an interesting area for further investigation.
1 Corresponding author: J.Daryl.Tatum{at}Colostate.edu
Received for publication April 4, 2007. Accepted for publication September 6, 2007.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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