Synopsis
Meat tenderness is definitely a very important market attribute and so describing the different processes concerning meat tenderness gives hints on enhancing the consistency and value of meat. It is a complicated natural mechanism that makes meat tender. The production of tenderness is based on the architecture of the skeletal muscle cell, its integrity as well as on activities that modify its interaction. The mechanisms that modify proteins and meat tenderness have been explicitly defined for protein degradation and protein oxidation. An significant element that regulates these activities is the intracellular environment. In the end, the interplay of these activities determines the tenderization pace and scale. Postmortem management to increase beef tenderness can be classified as 1) biological—manipulation of natural physiological/biochemical processes; 2) mechanical—application of external automated procedures to physically impact tenderness; 3) chemical—use of certain exogenous tenderizing chemicals; or 4) shockwave—use of physical shock to break down muscle structure (Balumar et al., 2013).
Analysis
Biological Management
Cooling Rate
Locker and Hagyard (1963) demonstrated that myofibrillar shortening occurred when the pre-rigor muscle was cooled at either very rapid or prolonged rates. At quick rates of cooling, extensive shortening occurred and the subsequently increased toughness was termed “cold shortening.” Pearson and Young (1989) determined that cold shortening only occurred when the muscle was less than 10o C, when the muscle pH was more significant than 6.0, and when ATP was available for muscle contraction. Uruh et al. (1986) and Simmons, Cairney, and Daly (1997) demonstrated that at prolonged rates of cooling, some shortening also occurred which, in some cases, was related to an increased toughness. This phenomenon was termed “rigor or heat shortening” and was considered to be due to the combination of the slow rate of cooling and associated low pH causing early exhaustion of proteolytic activity (Dransfield, 1993; and Simmons et al., 1996) and increased drip loss (Denhertogmeischke et al., 1997). This research formed the basis of a pH/temperature window defining conditions for postmortem management designed to maximize tenderness (Thompson, 2002).
Hwang and Thompson (2001a, b) examined the relationships of glycolytic rate, protease activity, and meat quality when electrical stimulation was applied either immediately after slaughter or just prior to cooling. They found that electrical stimulation just after the slaughter was associated with a very rapid decline in pH and a related decrease in m-calpain activity concurrent with increased calpastatin activity resulting in tougher beef. These workers concluded that the rate of postmortem muscle pH decline has a large effect on eating quality. Strip loins having slower rates of pH declines during cooling responded to aging in terms of more significant WBSF improvement. They concluded that the main penalty of a rapid pH fall was reduced aging effect on tenderness and increased drip loss. The reduced impact of aging on the improvement of tenderness associated with rapid rates of pH decline during cooling resulted from alterations of m-calpain kinetics toward early enzyme exhaustion.
The optimum pH declines for the most tender beef after 14 d of aging appeared to be at a cooling temperature of 29–30oC at pH 6 (Hwang and Thompson, 2001a, b). This temperature was higher than estimates obtained in vitro studies reported by Devine, Wahlgren, and Tornberg (1996) and Locker and Hagyard (1963) who reported that the optimal rigor temperature is 15–18oC. Thompson (2002) attributed these differences to differences between the constant temperature regimes used in the in vitro studies and declining temperature gradient in muscle samples in situ. Meat Standards Australia implemented the cooling temperature of 29-30oC. Sørheim et al. (2001) reported that for a 2-d chilling time, carcasses cooled at a constant 2oC responded to tender stretch methods in terms of reduced WBSF (61.4 N for tender stretch vs 103.2 N for Achilles hung carcasses); whereas, carcasses cooled at 10oC for 7 h then at 2oC were uniformly tender and did not respond to tender stretch (50.5 N for tender stretch vs. 55.5 N for Achilles hung carcasses). Mohrhauser et al. (2014) reported that µcalpain activity was more influenced by temperature than by pH. Delaying chilling by 4.75 hr. improved µcalpain activity.
Postmortem Aging
Aging is defined as the improvement in tenderness associated with extended periods of cold storage post-mortem. Hopkins and Thompson (2002) reviewed the literature and concluded that aging is primarily a function of extending the action of the calpain system, not a function of extending the action of cathepsins in contributing to proteolysis. Starkey et al. (2015) contributed most of the variation in the tenderness of lamb to proteolysis that occurred during aging. Bouton et al. (1973) examined the effect of tender stretch and aging rate on the strip loin and outside and interior rump muscles. They found an initial advantage due to tender stretch relative to Achilles hung sides for all three muscles, although the subsequent aging rate was lower in the tender stretch samples. O’Halloran et al. (1998) achieved a similar result for strip loin samples.
Dry vs. Wet Aging
Dry and wet aging are the two most common forms of postmortem aging (Campbell et al., 2001; and Warren and Kastner, 1992). Wet aging is more common and refers to meat aged in a sealed anaerobic package at refrigerated temperatures, whereas unpackaged meat aged at controlled temperature and humidity is referred to as dry aging. Almost all beef in the U.S. is vacuum packaged and wet-aged; however, dry-aging specific subprimal at the retail level may enhance palatability while creating a unique product (Miller et al., 1997). Dry aging is a costly process requiring both time and cooler space while resulting in a large amount of shrink and generating a significant amount of excess dried waste called ‘‘crust’’ that must be trimmed (Parrish et al., 1991; Warren and Kastner 1992; and Smith et al., 2008c). Shrink and associated waste makes dry-aging more expensive compared to wet aging (Smith et al., 2008c). Wet aging can be executed by either aging subprimal or steaks cut from subprimal. Eastwood et al., (2016) reported that consumers could not distinguish between whether steaks or subprimal were wet-aged.
Dry-aging is a traditional process of maintaining whole carcasses or unpackaged primals or subprimal under controlled environments (i.e., temperature, humidity, and airflow) for some time (Kim, Kemp, and Samuelsson, 2016). Primarily due to its positive impact on flavor, dry-aging has been typically practiced by processors in upscale markets (Warren and Kastner, 1992). Although most dry-aging involves beef subprimal (particularly middle meats), conventional carcass dry-aging (by hanging whole carcass sides in a cooler for 10 to 35 d) continues to be practiced by many local meat processors as a value-adding process (Jeremiah and Gibson, 2003b). However, several studies have failed to detect advantages for dry-aging on the palatability of beef (Dikeman et al., 2013 and Smith et al., 2008c) despite considerable costs associated with yield loss due to excessive surface drying and the resultant additional trimming. Also, several studies have reported that wet-aging, either as vacuum-packaged primal, subprimal, or steaks in a sealed barrier package at refrigeration temperature, resulted in equivalent eating quality attributes of beef muscles compared to the dry-aged counterparts (Dikeman et al., 2013; Parrish et al., 1991; Smith et al., 2008c; and Eastwood et al., 2016).
Aging Time
Although dry-aging has been a successful process used by some high-end restaurants and specialty outlets to meet the desires of consumers who prefer this unique product, Smith et al. (2008) found no difference between dry-aged and wet-aged short loins aged for 14, 21, 28, and 35d in WBSF or consumer panel determined tenderness. Li et al. (2014), however, reported superior sensory traits for dry-aging as compared to wet-aging for the longissimus muscle. They are aged for 8 or 19 d finding lower aging weight loss, odor score, and microbial growth in meat aged in dry aging bags than after traditional dry aging. The sensory panel detected no differences for most of the sensory attributes between the two dry-aging methods. The dry-aged steaks had more umami and butter fried meat taste compared with vacuum-aged steaks. Aging time affected most of the sensory traits in this study, which improved as aging time increased from 8 to 19 d. In a consumer test, meat aged for 21 d in dry aging bags was preferred compared to meat wet-aged in a vacuum. This may be due to the higher tenderness and juiciness obtained during storage in dry aging bags than meat wet-aged in a vacuum (Li et al., 2014).
Guelker et al. (2013) reported the 2010/2011 U.S. National Beef Tenderness Survey which showed that post-fabrication aging times for subprimal cuts in commercial cold storage facilities ranged from 1 to 358 d and 9 to 67 d for retail and foodservice subprimal, respectively. As indicated above, Bratcher et al. (2005) and Gruber et al. (2006) concluded that the time necessary for aging beef varies with marbling, indicating that USDA Select muscles should be aged at least 14 d postmortem. In contrast, beef from carcasses in the upper two-thirds of USDA Choice was tender by 7 d postmortem.
For Holstein beef, aging for 14 d, tenderness improves (Shimada et al., 1992), free amino acid content increases through the actions of aminopeptidases C and H (Nishimura et al., 1988), but the flavor does not improve (Fumika Iida et al., 2016). In contrast, Campbell et al. (2001) reported improved tenderness, flavor intensity, and juiciness for Holstein lean beef dry-aged for 16 or 21 d. For Holstein beef aged 28 d, Dixon et al. (2012) reported improved softness, and Nishimura et al. (1988) reported decreased intramuscular connective tissue. For Holstein beef aged for 32 to 56 d, Yanagihara et al. (1995) reported improved flavor and tenderness, but the flavor worsened with further aging. Fumiko et al. (2016) summarized that in terms of tenderness, juiciness, umami intensity, and flavor intensity the best duration of dry aging of highly marbled beef is 40 d.
Calculations in the MSA model assume the aging curve for muscle samples is linear from 5 to 21 d aging and thereafter declines exponentially. Aging rates for individual muscles are higher for muscles with lower levels of connective tissue. Wu et al. (2014) reported that the rapid degradation of large structural proteins (titin, nebulin, filamin, and myosin) took place within 48 h post mortem. The testing involved in MSA has indicated that hindquarter cuts from tender stretch sides are more palatable at the commencement of the aging period and are slower in the rate of aging than comparable meat from non-tender stretched sides. The palatability of muscles from the hindquarters from tender stretched, and usually processed sides tend to converge with extended aging (Thompson, 2002).
The effect of aging time on tenderness has been reported to be unrelated to changes in collagen content (Gruber et al., 2006; Silva, Patarata, and Martins, 1999; Sentandreu, Coulis, and Ouali, 2002; and Colle et al., 2015, 2016) for BF, SM, LL, LD, and GM. However, Colle et al. (2016), Nishimura et al. (1998), and Lewis, Purslow, and Rice (1991) found that the strength of intramuscular connective tissue in beef decreased from 14 to 35 d postmortem as long as the cooking temperature was less than 60 °C. Therefore, dependent upon cooking temperature, the lack of improvement in BF WBSF values with aging might be attributed to its relatively large level of background toughness caused by high levels of insoluble collagen.
Aging Temperature
As shown in this text, it is well accepted that the aging process improves tenderness through the process of denaturation of cytoskeletal myofibrillar proteins by endogenous proteolytic enzymes that alter the meat’s ultrastructure (Huff-Lonergan and Lonergan, 2005). Elevating aging temperatures has been reported to increase the rate of this degradation through the enhancement of proteolytic enzyme activity (Dransfileld, 1994). This observation is substantiated by reports that μ-calpain activity is positively correlated with aging temperature (Camou et al., 2007). Camou et al. (2007) found that μ-calpain was more active at higher chilling temperatures in five bovine muscles (longissimus lumborum, longissimus thoracis, psoas major, semimembranosus, and triceps brachii) under similar pH conditions. Choe, Stewart and Kim (2016) also reported that elevated aging temperatures of lamb loins at 3 °C accelerated myofibrillar protein degradation with concurrent increases in tenderness compared to loins aged at −1.5 °C.
King et al. (2009) reported that raising the aging temperature from −0.5 to 3.3°C reduced slice shear force values of beef longissimus lumborum (strip loin) 17.6 vs. 16.0 kg and gluteus medius (top sirloin butt) 17.9 vs. 15.2 kg, respectively, with concurrent requirements for decreasing aging time from 40 d to 12 d to attain the desired level of tenderness.
Aging Muscle Differentiation
Gruber et al. (2006) found that 17 USDA Select muscles aged for two d averaged 1.56 kg WBSF tougher than those aged 28 d. For the upper two-thirds of USDA Choice, the advantage was 1.34 kg. The muscle with the most remarkable improvement in tenderness with increased aging time was the toughest cut of the 17, the semimembranosus. Benefits in tenderness for ¾ Bonsmara Natural Beef, ¼ Natural Angus steers, was similar to that reported by Gruber et al. (2006). For the Bonsmara x, Angus steers, one of the toughest muscles (rectus femoris, d7 WBSF, 7.13 lb), had the most remarkable improvement in WBSF when aged for 28 d (19.4% improvement) (Holloway and Warrington, 2009). Also, one of the tenderest muscles, the psoas major (d7 WBSF, 3.72 lb), had the least improvement (1.3%) (Holloway and Warrington, 2009). For the Bonsmara x Angus crossbreds, after 28 d aging, six muscles had comparable WBSF to longissimus aged for 7 d (Holloway and Warrington, 2009), indicating that these muscles after 28 d aging could be candidates for dry cooking as steaks.
Gruber et al. (2006) also found that aging longer than 28 d had little impact on WBSF. USDA Select grade of cuts required about 7 d more aging to achieve the same proportion of tenderness than cuts grading USDA Choice (Gruber et al., 2006). Kerth et al. (2002) reported a larger medial to the lateral gradient in the WBSF of longissimus for 7 d of aging compared to 14 d postmortem aging. Gruber et al. (2006) concluded that most USDA Select muscles require longer aging times than those from carcasses grading in the upper two-thirds of Choice. Colle et al. (2015) aged USDA select beef strip loin and top sirloin for up to 63 d and concluded that the strip loin is adequately tender at 14 d aging, but the top sirloin increases tenderness to 21 d aging. As stated above, MSA assumes that the aging response is linear from 5 to 21 d and then decreases exponentially, but cuts differed according to the amount of connective tissue (Thompson, 2002).
Those having low levels of connective tissue have a higher aging response than those with high connective tissue. Biceps femoris and semimembranosus steaks excised from USDA Select beef rounds have been reported to respond well to postmortem aging, having potential for continued tenderization beyond 28 d of aging (Gruber et al., 2006). Both the 1991 (Morgan et al., 1991) and 2010/2011 (Guelker et al., 2013) U.S. National Beef Tenderness Surveys noted that improvement in tenderness of the round muscles is needed because they are consistently less tender than other muscles. Gruber et al. (2006) substantiated these findings by reporting that consumer panel tenderness scores for SM were higher and WBSF values were lower on d 42 and 63 than on d 2 and 14 of postmortem aging. Consumer panel tenderness scores for the BF were more outstanding when aged 21 d or more extended than when aged for 2 d.
However, BF WBSF values could not be shown to improve with aging time. Gruber et al. (2006) found that WBSF values of Select BF and SM muscles did not improve past 21 d postmortems. Smith, Culp, and Carpenter (1978) found WBSF values of USDA Choice BF did not improve after 11 d of aging. The continued improvement of tenderness scores for these muscles with longer aging periods may reflect interrelationships among sensory scores (i.e., the correlation often reported between tenderness and overall like in hedonic scores).
Aging and Growth Promotents
An illustration of postmortem management should be configured to premortem management to obtain the most tender beef possible is the interaction between the aggressiveness of the growth promotent implant program and postmortem aging times (Schneider et al., 2007; and Garmyn and Miller, 2014). After 3 d aging, a majority of implant treatments had WBSF values more significant than the nonimplanted control. Still, as the period of postmortem aging increased, the effects of implanting on WBSF gradually decreased so that only the most aggressive implant treatment resulted in tougher meat than the nonimplanted groups by 28 d of aging (Schneider et al., 2007; and Garmyn and Miller, 2014). Also, Van Donkersgoed et al. (2011) has reported that ractopamine and zilpaterol do not respond to postmortem aging in the same way (Van Donkersgoed et al., 2011; and Garmyn and Miller, 2014). Eastwood et al. (2016) failed to show that aging benefited the tenderness of beef from zilpaterol fed cattle.
Breed by Aging Interactions
Marino et al. (2014) performed a proteomic analysis showing that during muscle aging the solubility of some sarcoplasmic proteins decreased and some myofibrillar proteins were fragmented and released in the soluble fraction. A greater release of myofibrillar proteins was observed in the soluble fraction of the Italian breed. Podolian meat indicates more intense postmortem proteolysis in this breed than for Romagnola and Romagnola x Podolian.
Conclusion
Much because overall muscle cells are not reliant on a single protein, postmortem muscle cell structural weakening does not rely on alteration of a single myofibrillary or other cytoskeletal protein. Degradation and degradation of protein after postmortem may have significant implications for myofibril integrity and the whole muscle fiber. The proteins discussed in this study are found in various muscle cell regions and most are involved in preserving the structure and function of the muscle cell in several ways. The protease, μ-calpain, will catalyze titine, nebulin, filamine, desmin, and troponin-T into much of the same degradation products created from natural meat in myofibrils. This entails μ-calpain as a trigger for at least some of the postmortem muscle adjustments.
Those findings contribute to the possibility that μ-calpain postmortem activity can last for longer than originally suggested by μ-calpain activity, but it has been hard to analyze this. Protein oxidation is a key concern in developing meat consistency. Myosin oxidation can lead to reduced sensitivity of the meat. Cross-links between the heavy chain of myosin and titine can also minimize protein solubility and meat tenderness, conceivably. Calpain oxidation is believed to prevent the degradation of calpain catalyzed proteins and pro-oxidants thereby theoretically influence the sensitivity of the meat by promoting the aggregation of protein and inhibiting the degradation of the protein. Further study is required to evaluate the factors regulating these activities within the post-mortem skeletal muscle to establish strategies that efficiently modify or avoid tenderness.
