Background 

Postmortem aging to improve palatability of cooked beef is a common practice. Many studies have evaluated the tenderness of various subprimal cuts, the longissimus dorsi muscle, or other individual beef muscles after various postmortem aging times (Smith et al., 1978; Lorenzen et al., 1998; Meis et al., 1998; Johnson, 2003). George et al. (1999) reported a range of 89 days in post -fabrication age of striploin and top sirloin steaks sold at retail stores in eight differing U.S. cities.   

As the industry moves towards merchandising individual beef muscles, standardized postmortem aging times for muscles must be determined. Unfortunately, the majority of studies have been unable to discern differences in the amount of postmortem aging time required for individual beef muscles varying by USDA quality grade. Furthermore, most previous studies have relied on Warner-Bratzler shear force (WBSF) measurements obtained following freezing and thawing of beef steaks. This study was designed to develop standardized wet-aging time recommendations for fresh (never-frozen) individual beef muscles. Thus, allowing packers, retailers, and food service operations to manage the palatability of a wide range of beef muscles differing in USDA quality grade. 

Methodology 

Product selection. USDA Select (n = 40) and upper two-thirds USDA Choice (n = 40) beef carcasses were selected over a seven month period from Swift and Company Greeley, CO (Table 1). Two days postmortem, the following subprimals were removed from the right side of each carcass by in-plant fabrication: shoulder clod (IMPS 114), chuck roll (IMPS 116 C), chuck tender (IMPS 116 B), top round (IMPS 168), bottom round (IMPS 171 B), eye of round (IMPS 171 C), knuckle (IMPS 167), sirloin (IMPS 181), tenderloin (IMPS 189), and striploin (IMPS 180); the shoulder clod (IMPS 114), chuck roll (IMPS 116 C), chuck tender (IMPS 116 B), knuckle (IMPS 167), and bottom sirloin (IMPS 185) were removed from the left side of each carcass (NAMP, 1997). Subprimals were then transported to the Colorado State University Meat Laboratory for further processing.   

Muscle fabrication and steak allocation. The following individual muscles were removed from subprimal cuts: (a) Biceps femoris – long head, (b) Gluteus medius, (c) Infraspinatus, (d) Longissimus dorsi, (e) Psoas major, (f) Rectus femoris, (g) Tensor facia latae, (h) Teres major, and (i) Triceps brachii – long head (Table 2). Following fabrication, individual muscles within a carcass were cut into one-inch-thick steaks and the seven most suitable steaks within a muscle-type were randomly assigned to one of the following aging periods: 2, 4, 6, 10, 14, 21, and 28 days. Steaks assigned to the 2-day aging period were immediately evaluated by Warner-Bratzler shear force, and the remaining six steaks per muscle-type were vacuum packaged and stored and 2°C until the assigned aging-period was complete.   

Warner-Bratzler shear force determination. Upon completion of designated aging-time, steaks were removed from storage (2°C, never-frozen) for shear force analysis. Steaks most similar in thickness were cooked in groups of three on electric grills (model GGR64, Salton,Inc., Mt. Prospect, IL) that heated steaks from both sides simultaneously to a final internal temperature of 70°C. A Type K thermocouple (Omega Engineering Inc., Stamford, CT) was placed in the geometric center of each steak, and the internal temperature of each steak was monitored during cooking using a microprocessor thermometer (model HH21, Omega Engineering Inc.,Stamford, CT). After cooking each steak was allowed to equilibrate to room temperature (22°C) and up 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, with an Instron testing machine fitted with a Warner-Bratzler shear head; peak shear force measurements of each core were recorded and averaged to obtain a single shear force value for each steak.   

Statistical methods. Analysis of Warner-Bratzler shear force (WBSF) was conducted using the least squares, mixed models procedure of SAS (SAS Inst. Inc., Cary, NC). The ANOVA model included the three-way interaction between the fixed independent effects of quality grade (grade), postmortem aging period (age), and muscle type (muscle). Random effects included carcass nested within grade, and the two-way interactions of carcass within grade × muscle, and carcass within grade × age. Because this preliminary analysis resulted in a significant interaction between grade, muscle, and age, WBSF data was subsequently analyzed within each of the two quality grades (upper 2/3 Choice and Select). The final statistical model included the two way interaction of muscle × age as a fixed effect. Random effects included carcass, carcass × muscle, and carcass × age. Peak internal steak temperature served as a covariate for all analyses of WBSF.   

Within each quality grade, least squares means were generated for each muscle by aging period subclass, and aging curves were fit for each muscle using the least squares, nonlinear models procedure of SAS (SAS Inst. Inc., Cary, NC.). Aging time was used as the independent variable in each regression equation.   

Findings 

Aging response 

Least squares means of WBSF for each aging period by muscle subclass within a quality grade are presented in Tables 3 and 4. The non-linear equations fit to each muscle within a quality grade are illustrated in Figures 1 – 18.   

To estimate an optimal aging time for each muscle within a grade the following must be examined: a) WBSF at two days post-mortem, b) the maximum decrease in WBSF that can be expected (“aging response”)(Table 5), and c) the time that it takes for this reduction to occur. Based on the non-linear models fit to each muscle, within a quality grade, a predicted shear force value at two days postmortem, the change in shear force from day 2 to day 28, and the proportion of that change that has occurred at each of 6 aging periods is displayed in Tables 6 and 7. Estimates of decrease in shear force past twenty-eight days are not warranted in this data set and should not be extrapolated; however, the predicted instantaneous rate of change in WBSF for each muscle was ≤0.05 kg on day 28. The combination of these data determines the extent to which aging can improve tenderness (as determined by WBSF), and the time required for each muscle to obtain this improvement.   

Although postmortem aging will decrease WBSF values, if consumers cannot detect a change in beef palatability, postmortem aging has not added value to a muscle. Several studies have attempted to examine the impact of changes in WBSF on consumer acceptance of steaks (Miller et al., 2001; Lorenzen et al., 2003); Platter et al. (2003) reported predicted probabilities of consumer steak acceptance of 50 and 68% at approximate WBSF values of 4.4 and 3.7 kg, respectively. Although these probabilities of consumer acceptance are based on WBSF values of longissimus dorsi steaks, the WBSF values associated with them could be used as conservative thresholds. These thresholds could prove valuable in determining the length of time in days a beef muscle should be aged.  
Regardless of quality grade, biceps femoris muscles never reached WBSF values (4.4 kg) corresponding to 50% consumer acceptance. Upper 2/3 Choice biceps femoris muscles had the smallest response to postmortem aging (0.5 kg) and approximately 100% of this decrease in shear force was complete 14 days postmortem. Select biceps femoris muscles decreased 1.1 kg by day 28 postmortem, but only 82% of this response was complete after 21 days of aging.   

Although popular at food service operations and supermarkets because it is often more affordable than longissimus dorsi and psoas major steaks, according to Platter et al. (2003), neither top choice nor Select gluteus medius steaks reached WBSF values corresponding to 68% consumer acceptability. Gluteus medius muscles removed from upper 2/3 Choice carcasses in the present study reached 4.4 kg (50 % consumer acceptance) by day 15. Select gluteus medius muscles in our study had a 1.6 kg aging response after 28 days of postmortem aging, but only 78% of this total reduction in WBSF value was complete after 21 days of aging.   

The infraspinatus is often reported as the second most tender muscle in beef the carcass (Ramsbottom et al., 1945; McKeith et al., 1985), both Select and premium choice “top blade” steaks had a 1.4 kg aging response, and reached shear force values (3.7 kg) projected to correspond to 68% consumer acceptance of steaks. High grade infraspinatus steaks reached this threshold at 7 days postmortem, and Select top blade steaks required an additional 10 days of aging to obtain shear values corresponding to 68% of consumer acceptance.   

The aging response of the longissimus dorsi muscle varied greatly by quality grade. Top choice longissimus dorsi muscles decreased 2.0 kg in 28 days, and approximately 99% of this change was complete by day 21. Top choice longissimus dorsi muscles reached a 68% consumer acceptance (3.7 kg) by day 16. Conversely, longissimus dorsi steaks from a Select carcass reached WBSF levels of 4.2 kg after 28 days postmortem, and only 87% of this reduction in shear force was complete by day 21.   

Psoas major muscles, regardless of grade, had the lowest WBSF values at all but one postmortem aging period. Capable of achieving shear values corresponding to 68% consumer acceptance, both top choice and Select psoas major steaks had WBSF of 3.2 kg at 28 days postmortem.   Upper 2/3 Choice rectus femoris muscles decreased 1.0 kg after 28 days of postmortem aging, and approximately 98% of this change was complete after 21 days; however, according to Platter et al. (2003) the upper 2/3 Choice rectus femoris was only capable of obtaining shear force values that would correspond to 50% of consumer steak acceptance. In our study, WBSF values corresponding to this acceptance (4.4 kg) were reached 6 days postmortem. Rectus femoris muscles from USDA Select carcasses had an aging response of 1.3 kg, and reached approximately 87% of this reduction 21 days postmortem. WBSF values of approximately 4.4 kg were reached at 15 days postmortem. Premium choice tensor facia latae muscles decreased 1.0 kg after 28 days of postmortem aging, and greater than 90% of this change was complete day by 14. Select tensor facia latae muscles had a 28 day aging response of 1.1 kg, 94% of which was complete by 21 days postmortem. Upper 2/3 choice tensor facia latae muscles reached WBSF values corresponding to 68% consumer acceptance (3.7 kg) at 13 postmortem.   

Premium choice teres major muscles at two days postmortem were observed to have WBSF levels below those corresponding to 50% consumer acceptance of steaks. The 3.7 kg threshold for 68% consumer acceptance of steaks was reached approximately 11 days postmortem. A regression line was unable to be fit to least squared means of Select teres major muscles (Figure 16). Inability to fit a regression line to the Select teres major could be attributed to sampling difficulties associated with small muscles. 

Upper 2/3 Choice triceps brachii muscles decreased 1.4 kg after 28 days of postmortem aging, and approximately 91% of this change was complete after 14 days; however, according to Platter et al. (2003), the triceps brachii was only capable of obtaining shear force values that would correspond to 50% of consumer steak acceptance. WBSF values in our study corresponding to this acceptance (4.4 kg) were reached 7 days postmortem. Triceps brachii muscles from USDA Select carcasses had an aging response of 1.6 kg, and reached approximately 95% of this reduction 21 days postmortem. WBSF values of approximately 4.4 kg were reached at 13 days postmortem.   

In general, muscles from USDA Select carcasses had higher initial (two days postmortem) WBSF values, exhibited greater decreases in shear force values after 28 days of postmortem aging, and required more days of postmortem aging to complete the majority of this decrease in WBSF than did muscles from upper 2/3 USDA Choice beef carcasses.   

Muscle tenderness
A rank of beef muscles by Warner-Bratzler shear force (WBSF) within a quality grade at each of seven postmortem aging periods is located in Tables 8 - 21. Although numerous studies have characterized the tenderness of individual beef muscles (Ramsbottom et al., 1945, McKeith et al., 1985, Johnson et al., 1988; NCBA, 2000), properties of the longissimus dorsi muscle are still the most widely recognized. Therefore, an index value was created in this study for each muscle to facilitate comparisons in tenderness (as determined by WBSF). Within a quality grade, at each postmortem aging period, this index (Tables 8-21) identifies muscles that can be expected to perform similarly to the longissimus dorsi, in terms of tenderness, as well as those muscles that can be expected to out-perform the longissimus dorsi (Gruber et al., 2003). Also, an index has been created that compares the tenderness of individual muscles at each aging period to the tenderness of the longissimus dorsi aged 14 days (Tables 22 and 23). Caution, however, should be used when comparing muscles by index value alone. Because the index value is a proportion ((WBSF of the longissimus dorsi /WBSF of the individual muscle) * 100), small differences are magnified, and index values that may appear different are not significantly different (P < 0.05). Mean WBSF values are labeled with superscript letters identifying significant differences, and should always be used to test for tenderness differences between muscles. 

Implications   

Postmortem aging can be used to maximize tenderness potential of individual beef muscles. Combining initial WBSF measurements, aging responses within a 28-day period, and the rate at which this response occurs can facilitate estimation of optimal aging time of individual beef muscles differing in quality grade. The conservative use of WBSF thresholds in consumer acceptability also may prove useful in identifying muscles that can be marketed as steaks, and distinguish palatability improvements throughout postmortem aging.