Factors affecting Meat tenderness

Factors affecting meat tenderness 

Meat tenderness is determined by factors like background toughness, the toughening phase, and tenderization. Meat toughness depends on the nature, structure, and amount of different connective tissues like collagen and elastin (Lepetit, 2008). In general, the organization of the perimysium in muscle tissue seems to have an effect on the background toughness, as there exist a correlation between the perimysium and the tenderness of muscles (Strandine et al., 1949). It has been found that the toughening pattern is similar in all carcasses under similar processing conditions, however, there is a variation in the tenderization phase. Consumers are well aware of tenderness variability in different cuts of meat and relate it with economic value. The meat industry is aware of the problem of toughness in cuts and its financial implications (Koohmaraie and Geesink, 2006). Polkinghorne et al. (2008) is of the view that prime grilling cuts are less than 10% of a carcass and the remaining 90% can be improved by the use of tenderizing techniques. Meat tenderness has been one of the most important criteria in deciding quality attributes, and there are several techniques to evaluate it. Many sensitive tools have been used to analyze the tenderness of meat, such as enzyme activity estimation (Koohmaraie et al., 1988), the myofibrillar fragmentation index, (Olson and Parrish, 1977), hydroxyproline measurement (Ashie et al., 2002), scanning electron microscopic studies, etc.

Factors affecting Meat tenderness
Factors affecting Meat tenderness

The postmortem results in subsequent changes in membranes and sarcolemma of muscle tissue leads to another event of importance—tenderization during aging, as a lower pH favors the activity of proteolytic enzymes in meat. The cooling process during aging causes a substantial variation in meat tenderness which depends on the animals’ stress prior to slaughter, and time/temperature/pH combinations postmortem. During tenderization, proteolysis attacks all muscle proteins including connective tissue (Hwang et al., 2003). The postmortem proteolysis of myofibrillar and myofibrillar-associated proteins is a key factor in tenderization (Koohmaraie and Geesink, 2006). The biochemical changes during the postmortem phase influence meat tenderization, which is a well-established concept (Herrera-Mendez et al., 2006). A number of endogenous enzyme system have been identified in the postmortem tenderization process during the aging of meat. In addition to endogenous enzymes, several exogenous enzymes from plants and other sources have been found to have an appreciable tenderizing effect on meat.

Tenderizing Technologies

Meat tenderness depends on three main factors: (1) the degree of contraction of muscle sarcomeres, (2) the integrity/degradation of the myofibrillar structure and (3) the connective tissue content (“background toughness”) (Koohmaraie et al., 2002; Sentandreu et al., 2002). Some existing interventions can be applied to alter one or more of these three factors to improve meat tenderness.

The extent to which muscle is contracted when it sets into rigor mortis depends on the pH decline rate and temperature at that time point. Therefore, the ability to control the pH and temperature early post-postmortem is essential to obtain tender meat. Interventions such as electrical stimulation, optimization of carcass chilling and stretching of muscle pre-rigor impact on the degree of muscle shortening and as a result affect meat tenderness (Thompson, J., 2002; Bowker et al., 2010; Bolumar et al., 2013). Unfortunately, the outcome of using many of these technologies is not easy to control or predict in commercial practice due to the high dependence on controlling the metabolic rate that governs the pH decline, as well as factors that affect temperature decline in the carcass. Pre-slaughter factors (e.g., environmental conditions during livestock transport, animal stress) and postmortem factors associated with the carcass itself (e.g., muscle sizes, anatomical location within the carcass) all can affect meat quality as well as the ability of the meat to age (Thompson et al., 2006). Aging is widely used in the meat industry to improve meat tenderness; however, it involves considerable processing costs (associated with time, 14–21 days; storage space; energy) and low value cuts do not reach the minimum tenderness threshold acceptable to the consumer. The integrity and degradation of myofibrillar structure can be affected by endogenous enzymatic proteolysis (aging), addition of exogenous enzymes, addition of chemicals compounds (salts, phosphates; which solubilize proteins), and mechanical disruptions of the muscle structure (mincing, blade tenderization). The third component deals with the manipulation of connective tissue content and its heat stability. There are limited interventions that can affect connective tissue and its effect on tenderness (Purslow, 2005). Low temperature long time (LTLT) cooking of meat and mechanical tenderization can reduce the impact of the toughening effect associated with meat containing high amounts of collagen, or collagen that has a high degree of cross-linking. Blade tenderization of cuts is effective for many primal cuts but its use is outweighed by the potential risk of cross-contamination of the center of the cut with pathogens, and therefore, the associated higher chance of survival of such pathogens if the center of the meat is not inadequately cooked. In this context, shockwave tenderization provides a unique opportunity to tenderize meat in a mechanical manner without jeopardizing food safety.

Utilizing High Pressure Processing  for Meat tenderization

Meat tenderness is a very important sensory trait for the consumer and high pressure processing (HPP) can be applied to packaged meat, to increase tenderisation. Across a number of studies, and using meta-analysis, it is evident that beef, which has the highest initial toughness, shows significant tenderisation with HPP. Also, moderate pressure of 100–150 MPa combined with temperatures of 38–80 °C produce significant tenderisation. Application of HPP at high (200–400 MPa) to very high (520–600 MPa) pressure has either no effect, or a toughening effect, respectively. Hence, HPP shows great potential for meat tenderization, potentially adding value to the meat and food industry, but only when the appropriate temperature and pressure are applied.

Proteome technology in meat industry

Meat tenderness is influenced by the biochemical properties of muscle fibers as well as connective tissue matrix and is improved by aging – primarily due to degradation of cytoskeletal proteins. Tenderness is the major factor contributing to eating satisfaction and consumer acceptance and, therefore, is considered an important trait influencing repurchase decisions. It has been estimated that the inconsistencies in tenderness lead to US$217 million annual revenue loss to the US meat industry. Postmortem degradation of several structural proteins is the major reason for improvement in meat tenderness. Nevertheless, the fundamental mechanisms through which these biochemical changes govern meat tenderization are yet to be completely elucidated.

Improving and predicting tenderness are two factors critical to profitability of the meat industry. Although improvements in tenderness can be made by aging, predicting this attribute is not straight forward primarily due to muscle- and species-specific variations in meat biochemistry. The improvement in tenderness is dependent on the activity of endogenous proteolytic enzymes in postmortem muscles. The interactions between the enzymes and their substrates, primarily cytoskeletal proteins, are complex and are often influenced by different intrinsic (protein oxidation, calcium and vitamin D concentrations) and extrinsic (packaging system, aging condition, brine injection, antioxidant, and electrical stimulation) factors. The fact that the substrates for these enzymes are also proteins makes proteomics an invaluable tool to interpret the biochemistry of tenderness.

Meat tenderness is a muscle-dependent attribute, and significant amount of research has been undertaken to explain the muscle-specific biochemistry of beef tenderness. Use of DIGE to examine the proteolytic changes in beef adductor (tough muscle) and longissimus (moderately tender muscle) revealed that proteins such as actin, myosin heavy chain 1 fragment, myomesin-2, and a-actinin-3 undergo muscle-specific changes during aging; the abundance of these proteins in sarcoplasmic and myofibrillar fractions was different in adductor and longissimus indicating the possibility of using them as potential candidates for further investigations in muscle-dependent mechanism of tenderization. Of specific interest is the fragment of myosin light chain 1, which was more abundant in the sarcoplasmic extracts of tender beef than in the tough beef.

The calpain system is a major enzymatic system responsible for postmortem tenderization of meat and has been extensively studied. Conventional protein chemistry has characterized various calpain isoforms and their biochemical modifications, whereas the use of proteomic tools revealed that µ-calpain undergoes oxidation by forming an intermolecular disulfide bond and that the oxidation results in the loss of proteolytic activity. This finding highlighted the adverse influence of protein oxidation on meat tenderness.

Analyses of muscle proteome identified several proteins potentially associated with the calpains that can influence tenderness in a multitude of ways. These identified proteins included structural proteins, mitochondrial proteins, and proteins associated with calcium and glucose metabolism. The potential association between calcium-regulating proteins and calcium-dependent calpains suggest a complex nature of the mechanisms governing postmortem proteolysis.

Although the role of oxidation in loss of protein functionality has been known for several years, use of diagonal PAGE and MS demonstrated that myosin heavy chains in postmortem muscles are susceptible to oxidation and that the exposure to an oxidizing environment promotes cross-linking between titin and myosin; both phenomena lead to protein aggregation that subsequently results in an increase in toughness of meat.

Food security, nutrition and health

Meat tenderness is a complex attribute influenced by many structural and metabolic factors, mainly connective tissue concentration, final pH, muscle contraction during rigor mortis, and probably the most important, activity of proteolytic enzymes, calpains and cathepsins (Kusec et al., 2016). The calpain activity is regulated by calpastatin (Dransfield, 1994), which activity was proven to decrease with the pork frozen storage duration (Kristensen et al., 2006). This, and the structural damage due to the ice crystal formation during freezing (Shanks et al., 2002), most likely influenced the observed improvement of shear force. Furthermore: freezing, frozen storage and thawing could allow cathepsins to act on myofibrillar proteins and collagen increasing meat tenderness.

There is general consensus in the literature that meat tenderness increases with freezing, frozen storage and thawing when measured with peak force (Farouk et al., 2003; Lagerstedt et al., 2008; Shanks et al., 2002; Wheeler et al., 1990). Several factors could affect this important quality parameter: the extension of aging before freezing (Vieira et al., 2009), the ice size and location during freezing (Lagerstedt et al., 2008) and frozen storage (Farouk et al., 2003) both highly dependent on freezing conditions mainly speed and temperature and length and temperature of frozen storage (Alonso et al., 2016; Fernández et al., 2007; Muela et al., 2016). A decrease in shear force values through freezing/thawing has been reported by several studies (Crouse and Koohmaraie, 1990; Grayson et al., 2014; Kim et al., 2015; Lagerstedt et al., 2008; Muela et al., 2015; Shanks et al., 2002; Wheeler et al., 1992) and can likely be attributed to the physical destruction of muscle tissue and structural integrity by ice crystals (Petrović et al., 1993).

However, there have been some discrepancies between studies in that whether the reduction of shear force values through freezing/thawing would be translated into improvement in tenderness determined by trained assessors and consumer evaluations (Destefanis et al., 2008). Contradictory results were obtained in the literature related to sensory assessment of freeze/thawed meat texture, Muela et al. (2012) did not find any statistical differences in some texture sensory variables (tenderness, juiciness and fibrousness) evaluated by a trained panel in light lamb meat freezing with three industrial methods (air blast freezer, freezing tunnel and nitrogen chamber). In contrast, Lagerstedt et al. (2008) found that freeze/thawed beef meat was significantly less tender than chilled meat using a trained sensory panel, this sensory result was attributed to the loss of fluid during thawing that resulted in less water available to hydrate the muscle fibers. In the study of Muela et al. (2012) with light lambs stored frozen up to 6 months and 3 d of total ageing did not show any significant differences in texture parameters. Also, in the study of Vieira et al. (2009) with beef frozen stored up to 3 months, trained evaluators showed no significant differences in tenderness between fresh and 1 month frozen stored. Besides, in the study of Bueno et al. (2013) tenderness did not differ among freezing treatments having fresh meat intermediate values among thawed meats. When frozen storage of lamb meat was 1, 9 or 15 months and it was followed by short-term modified atmosphere packaging once thawed, meat taste properties were briefly affected and neither trained nor untrained sensory panel distinguished fresh meat from thawed meats. The low intensity or absence of negative descriptors in the test with trained sensory panel pointed the small detriment effect of frozen storage when preservation is performed under these experimental conditions (Muela et al., 2016).

Some studies have found that aging combined with freezing could result in better tenderness compared to frozen/thawed only meat without aging (Coombs et al., 2017; Crouse and Koohmaraie, 1990; Holman et al., 2017; Kim et al., 2011; Kim et al., 2015; Shanks et al., 2002). Kim et al. (2018) suggest that aging first then fast-freezing could be one of the most effective ways to minimize quality defects associated with freezing/thawing in pork meat.

Measurement of Aging

Meat tenderness is the way consumers see one aspect of the property of meat: cooked meat becoming more tender and acceptable (higher scores) as the meat ages. To replicate consumers, a common procedure is to train panellists to evaluate meat attributes. Meat is cooked to a standard end-point temperature and maintained at this temperature when served to the panellists under standard lighting conditions. Measuring appreciation of tenderness through the use of a consumer panel provides more information than just tenderness and is influenced by the type of cut, amount of fat, and connective tissue, juiciness, cooking procedure, and cooking temperature effects, such as the well-known differences that exist between a well-cooked and a rare steak. Some evaluations use untrained people but, in the absence of training, there is a very wide range of scores for the various attributes. A more objective measure is obtained by shearing meat following standardized cooking procedures, for example, 70 (American Meat Science Association) to 75 °C (Meat Industry Research Institute of New Zealand (MIRINZ)) in a water bath or an electric belt grill system – the lower the shear value, the more tender the meat. There are several types of ‘tenderometers’ in use, for example, the Warner–Bratzler tenderometer and MIRINZ tenderometer. The absolute shear values depend on the type of tenderness-measuring device used (e.g., Warner–Bratzler values are 0.6 times MIRINZ values), but the rate of change over time, i.e., the aging rate, will be similar for all devices.

Ideally, one would like to measure the tenderness of raw meat. Cytoskeletal proteins degrade over time and alter a meat’s integrity, so the consequences of protein degradation in raw meat can be determined by using the myofibrillar fragmentation index. This involves viewing myofibrillar length changes under a microscope or determining turbidity changes. The degradation of the cytoskeletal proteins is also associated with a decrease in their water binding and the increased water release changes the NIR spectra, and this can be correlated with tenderness. NIR has been developed as a nondestructive method to measure meat tenderness and can also measure other meat quality attributes on line. A corollary is that as meat ages (tenderizes) there is an increase in drip. When aging is rapid, such as after electrical stimulation and high temperatures that occur, drip increases and meat tenderizes more rapidly as expected but there is no more drip for the equivalent tenderness achieved.

Pulling all of the discussion points above, the shear force of meat decreases exponentially over time and is significantly modified by pHu and electrical stimulation as shown in Figure 7. In this figure, lamb meat was held at a constant 15 °C (this ensures more rapid aging than at lower temperatures). The mean shear force in the electrically stimulated group is significantly lower at all aging points than the nonstimulated lambs (P<0.05). The line drawn corresponding to shear values of 40 N (moderate acceptability) shows that acceptable values are reached in 25–30 h for nonstimulated, low pHu muscles and within 10 h for stimulated muscles. Because of the considerable variability, long aging is needed for all meat to become acceptable – consumers’ appreciation of tenderness is remembered by the tough outliers.

RTU imaging method for quality control of meat

Meat tenderness and juiciness are positively correlated with the proportion of fat in the carcass (Wood, 1990; Bruns et al., 2004). Marbling fat has no direct effect on meat tenderness (Renand et al., 2003; Thompson, 2004); however, it plays an important role in meat juiciness and overall eating satisfaction. In fact, marbling leads to greater palatability in panel scores (McPeake, 2001) and lower shear force values (Dolezal et al., 1982). Carcasses with higher marbling content also have a higher subcutaneous and intermuscular fat content, thus insulating the muscles during chilling and preventing the phenomenon of cold shortening.

The production of carcasses with excessive weight, excessive subcutaneous fat and only a small degree of marbling, as well as a lack of uniformity, is a common problem in meat production systems. The production of carcasses with the correct weight and an optimum amount of subcutaneous fat therefore ensures that the meat is protected during the cooling process and also maximizes the organoleptic properties.

Fatter carcasses undergo a faster drop in pH, which is associated with more tender meat; and slower cooling of fatter carcasses contributes to an increase in the activity of ageing enzymes, leading to greater tenderness (Wood, 1995). Even under normal chilling conditions, carcasses with less than 13 mm of SFD over LTL display reduced tenderness due to the cold shortening effect (Wood, 1995). Ageing a carcass affected by cold shortening will not alleviate the detrimental effects on tenderness. Thus, the SFD is a very important attribute, because it protects the meat from thermal shock during refrigeration, which prevents cold shortening, oxidation of muscles, browning and microbial contamination of meat during skinning.

As stated previously, the ability to measure the SFD, LMA and marbling using ultrasound images taken from live animals provides an opportunity to study the relationship between animal growth and the development of various tissue types. Thus, for a known feeding strategy, it is possible to monitor the growth of SFD and LMA and the deposition of marbling as the animals grow and during the finishing phase. These data can then be used to project the slaughter date, for a pre-defined subcutaneous fat level (Brethour, 2000). For example, Delehant et al. (1996) showed that ultrasound measures, taken on cattle prior to feedlot feeding, combined with performance data collected during the finishing phase, could effectively predict LMA, SFD and IMF percentage at any point during the finishing phase. The ability to predict the optimum slaughter date of a particular animal is an attractive use of ultrasound technology (Lusk et al., 2003). The RTU can be used to develop models to predict the number of days necessary to reach a target carcass composition under a defined feeding regime (Hassen et al., 1999a), or to develop a feeding regime that maximizes the production of carcasses with a higher-yield or higher-quality grade (Basarab et al., 1999).

So far, the use of RTU to optimize the slaughter date has been focused on beef production; however, RTU can also be successfully used for meat species such as swine. RTU is also useful in predicting market-weight slaughter characteristics and in predicting the percentage of lean cuts in market-weight swine. The ability to predict market-weight slaughter characteristics was investigated by Robinson et al. (1992). Similarly, McLaren et al. (1989) studied 110 barrows and gilts, which were scanned every two weeks from 42 days old up to the point of slaughter to measure SFD at the first rib, last rib and last lumbar vertebrae, and to measure LMA at the 10th rib. They showed that ultrasound measurements were able to estimate lean gain a day early (up to 53 kg BW) immediately prior to slaughter. These authors (McLaren et al., 1989) concluded that ultrasound data were useful in early selection decisions and for selections made at market weight for carcass merit in swine. Olsen et al. (2007) also used ultrasound for online classification of swine carcasses and showed that live animal ultrasound measurements could predict retail product yield after slaughter.

The implementation of RTU in meat production systems can help to reduce the production of carcasses with either too little or too much subcutaneous fat. This is beneficial for producers – first, as it can lead to reduced feeding costs, and also because it improves the quality of the product presented to consumers.

Fresh meat texture and tenderness

Meat tenderness is affected by complex interactions of multiple antemortem and postmortem factors. Genetics determine an animal's potential for producing tender meat, and the interaction of genetics with ante- and postmortem environment and management will determine the ultimate tenderness of the meat from an animal. A mutation in the myostatin gene has been associated with the condition in cattle known as ‘double muscling’ (Grobet et al., 1997; Kambadur et al., 1997; McPherron and Lee, 1997; Smith et al., 1997). This syndrome is characterized by embryonic hyperplasia caused by inactive myostatin, which normally inhibits cell-proliferation. Carcasses of double-muscled cattle yield a greater percentage of retail product than carcasses of normal cattle (Arthur, 1995; Wheeler et al., 1997a). Additionally, meat from these animals is more tender, primarily due to reduced collagen concentration (Wheeler, unpublished data). Animals with one or two inactive myostatin alleles produced ribeye, top sirloin, bottom round, and top round steaks that received greater trained sensory panel tenderness ratings than animals with two normal alleles at the myostatin locus (Fig. 3.3; Wheeler et al., 2001b). In addition, bottom round steaks from animals with two inactive myostatin allele received higher tenderness ratings than steaks from animals with one inactive allele. In all muscles evaluated, increasing the number of inactive myostatin alleles decreased collagen concentration (Wheeler, unpublished data). This trend was more pronounced in the bottom round, which had the highest collagen content of the four muscles evaluated.

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