Introduction

Aging is one of the most commonly used postmortem techniques for improving beef tenderness, which is one of the most important factors determining beef quality. Numerous researchers (^{Belew et al., 2003}; ^{Molina et al., 2005}; ^{Rhee et al., 2004}; ^{Von Seggern et al., 2005}; ^{Yadata et al., 2009}) have studied the tenderization process of several beef muscles at different postmortem aging periods. However, most of these studies consider beef from steers and heifers while data from young bulls, which are the main source of beef in some countries (^{Mendez et al., 2009}) are very scarce.

The limited number of studies on young bulls (^{Chávez et al., 2012}; ^{Hildrum et al., 2009}; ^{Rubio et al., 2007}; ^{Simões et al., 2005}; ^{Vieira et al., 2009}) have shown meat produced by these animals is tough. The high WBSF values reported in most of these studies indicate this type of beef is resilient to aging. In that sense, it is well documented that freezing of meat prior to aging reduces calpastatin activity (^{Crouse & Koohmaraie, 1990}; ^{Koohmaraie, 1990}). In fact, recent research has shown USDA Select beef previously frozen and thawed age faster than that which is never frozen (^{Grayson et al., 2014}). Therefore, we believe a freezing-thawing-aging strategy may be an effective means of tenderizing and adding value to muscles from young bulls as well. The latter could be a feasible alternative in some specific markets. Hence, the objective of this study is to estimate the effect of freezing, thawing and aging on the tenderness of chuck muscles from young bulls that are exploited in Mexico.

Materials and methods

Results and discussion

Muscle and aging time significantly interacted to affect WBSF of beef from Charolais (P = 0.0008) and Brahman (P < 0.0001) bulls. Thus, analyzing WBSF of individual muscles across aging times provides more accurate information. The combined effects of freezing, thawing and aging significantly decreased (P < 0.05) WBSF of muscles from both phenotypes except values of biceps brachii, brachialis, and supraspinatus (Table 1). These results are in line with previous reports documenting the limited effect of aging on the WBSF of brachialis (^{Belew et al., 2003}; ^{Von Seggern et al., 2005}) and supraspinatus (^{Hildrum et al., 2009}; ^{Rhee et al., 2004}). Conversely, present findings failed to support previous studies, in which the biceps brachii from USDA Choice and Select carcasses was classified as tender (^{Belew et al., 2003}; ^{Von Seggern et al., 2005}). It should be noted, however, that some bull muscles often have a poorer response to aging than those of steers and heifers (^{Claus et al., 2010}; ^{Mach et al., 2009}; ^{Rubio et al., 2007}). In aged triceps brachii (lateral head), WBSF remained high, which is comparable to results observed in previous studies with steers and heifers (^{Gruber et al., 2006}; ^{Stelzleni et al., 2007}; ^{Von Seggern et al., 2005}), and with bulls (^{Simões et al., 2005}).

Table 1 Least square means of WBSF (N) for Brahman and Charolais chuck muscles (n = 10) frozen for one week, thawed for 24-30 h at 2-4 °C, and aged for different periods 

a b Means with different superscripts in the same row are significantly different (P<0.05) * P < 0.05; *** P < 0.001

Interestingly, tenderization of muscles (except complexus from Brahman) occurred up to nine days while no further decrease in WBSF was observed at 16 or 23 days. The latter is difficult to explain since there is no agreement on what could be an appropriate aging time for this type of beef. Recommendations from other studies are quite variable (5 to 21 days) and usually depend on carcass quality grade and muscle (^{Bratcher et al., 2005}; ^{Gruber et al., 2006}; ^{King et al., 2009}; ^{Nelson et al., 2004}; ^{Von Seggern et al., 2005}). However, most of these studies consider beef from steers and heifers while data from young bulls, which are usually excluded from quality grading schemes, are very scarce. Moreover, the common methodology used to study meat tenderization follows an aging-freezing-thawing strategy. Under these circumstances, beef from young bulls has consistently exhibited high WBSF values (^{Hildrum et al., 2009}; ^{Li et al., 2012}; ^{Vieira et al., 2009}) even after relatively long aging periods. On the other hand, freezing of meat prior to aging has been shown to drastically reduce calpastatin activity (^{Koohmaraie, 1990}; ^{Crouse and Koohmaraie, 1990}). Consequently, the strategy used in this study, which comprises freezing and thawing of muscles before aging, may have accelerated the tenderization process. Hence, the studied muscles may have reached the lowest WBSF possible within nine days. This reasoning is supported by findings from a recent study (^{Grayson et al., 2014}) showing beef frozen, thawed, and aged for 14 days was more tender than that aged for 28 days. Therefore, the freezing-thawing-aging strategy appears to be a viable alternative to counteract the resilient response to aging of chuck muscles from young bulls.

Most muscles from both phenotypes had 9 d WBSF means lower than those reported elsewhere. For instance, ^{Vieira et al. (2009}) reported high WBSF values (> 46 N) in muscle longissimus thoracis from young bulls aged for 10 days and then frozen for up to 90 days. ^{Likewise, Li et al. (2012)} reported WBSF > 60 N in longissimus lumborum from Chinese Yellow cattle subjected to different chilling regimens and aged for up to 21 days. Another study with Norwegian Red bulls (^{Hildrum et al., 2009}) reported WBSF values above 50 N for triceps brachii [long head] aged for 9 days (never frozen). This is roughly 15 N higher than the average 9 d WBSF we observed for the same muscle. Likewise, the 9 d WBSF reported by ^{Hildrum et al. (2009)} for infraspinatus is about 10 N higher than that observed here. Interestingly, muscles teres major, rhomboideus, and triceps brachii [long head] had average 9 d WBSF equal to or even lower than those reported elsewhere in steers for beefsteaks aged 14 d (^{Bratcher et al., 2005}; ^{Rhee et al., 2004}; ^{Stelzleni et al., 2007}).

Given these facts, it is worth mentioning that differences in methodology make direct comparisons of actual WBSF across studies very difficult. However, four muscles from Brahman bulls and seven from Charolais reached WBSF values < 38.22 N, which are typical of tender beef (^{Belew et al., 2003}). Despite the animals were slaughter in different slaughter houses, these results support findings documenting the superior tenderness of beef from B. taurus breeds as compared to the B. indicus ones (^{Bidner et al., 2002}; ^{Bonilha et al., 2008}; ^{Shackelford et al., 1995}). Moreover, current results confirm the widely documented superior tenderness of infraspinatus (^{Bratcher et al., 2005}; ^{Gruber et al., 2006}; ^{Rhee et al., 2004}; ^{Searls et al., 2005}; ^{Stelzleni et al., 2007}), as well as the potential for value addition of other beef forequarter muscles. Our results also support prior suggestions (^{Bratcher et al., 2005}) that fabrication and merchandizing decisions should be made on an individual muscle basis.

In summary, these findings show some forequarter muscles from young bulls are able to produce tender beef if frozen and thawed before aging. The latter does not seem to be affected by zilpaterol supplementation, which was administered to all of the animals used in the experiment. Hence, freezing and thawing before aging could serve as an effective means of improving the tenderness consistency of meat from young bulls. We believe this is a striking outcome since young bulls conform nearly 80% of the Mexican beef slaughter population (^{Mendez et al., 2009}) and zilpaterol is widely used in Mexican feedlots (^{Camponova, 2006}).

Even though it is unlikely that this strategy would be massively used by the meat industry in the future, it may be a viable alternative in some specific markets. Particularly, when dealing with tough meat that needs long aging times to tenderize. In the studied sample, several chuck muscles from young bulls reached WBSF typical of tender beef when kept frozen for one week and aged for nine days. Given these facts, we believe further research is needed in order to assess the effects of freezing on the sensory properties and other quality attributes of economical importance in beef. The latter is vital in order to optimize the tenderization strategy studied here.

Conclusions

Data from the present investigation show freezing, thawing and aging of several chuck muscles from young bulls significantly improves beef tenderness. Further research is needed to optimize the tenderization process and assess its potential impact on other meat quality attributes.