Physical Interventions to Manipulate Texture and Tenderness of Fresh Meat: A Review

Abstract

Meat tenderness is a major eating quality attribute that ensures consumer satisfaction and repeat purchase of red meat. The variability in meat tenderness is related to several factors that are spread across the production chain (biological, on farm, processing, and consumer factors), which can lead to inconsistent tenderness in fresh red meat products. The tenderization process is dictated by physical and biochemical factors, which appear to affect the proteases involved in protein degradation and, consequently, they regulate the rate and extend of tenderization in meat. Several physical, chemical, and biochemical interventions have been investigated to improve the tenderness of meat. The present review discusses the physical interventions used to manipulate the texture of meat and their mechanism of action, optimal tenderizing conditions, and their effects on other meat quality attributes (colour stability, lipid oxidation, and water holding capacity). Attention should be paid to other quality attributes for full evaluation of the differing interventions.

Keywords:

• Meat
• Texture
• Interventions
• Tenderization

INTRODUCTION

The quality of meat is a multi-dimensional concept where the value of meat and its products are determined based on organoleptic (e.g., colour, texture, flavour), ethical and social (e.g., organic vs. conventional, animal welfare), symbolic and cultural (religion, e.g., pork or beef vs. lamb; halal vs. non-halal), nutritional (health aspects), functionality (e.g., table cuts vs. processing), and many other factors (safety, availability, and price). Organoleptic (sensory) quality is a network of attributes that plays a major role in meat marketability with some factors playing a greater role at the point of sale (such as the amount of fat and the colour of the meat), while others become more important once the meat is cooked (such as flavour and tenderness). Tenderness is arguably the most important determinant in eating satisfaction and, therefore, the most significant challenge in terms of acceptability of meat purchased by consumers.[1] Unfortunately, due to various physiological and biological factors, the tenderness of meat within the carcass varies widely with usually less than 10% of a carcass classified as prime grilling cuts[2] with the remainder of the carcass being regarded as requiring interventions to improve the level of tenderness. Improving the tenderness of meat cuts and maintaining consistency in meat quality would result in attracting a larger consumer base, a higher retail price and more frequent consumption.[2 ⁻ 5] Therefore, enhancing meat tenderness is of significant interest to the meat industry.

Numerous investigations have examined various factors contributing to meat texture, including on-farm (breed, diet, and handling); physiological (genetic background, stress, muscle type, and location); and processing (post-mortem temperature regime, electrical inputs, and storage conditions); various publications and patents describing knowledge and processes to produce consistent tender meat are available. Methods used in reducing meat toughness and achievement of consistently tender meat involves cellular and structural changes that can lead to adverse effects with the oxidative processes in the meat and significant effects on other quality attributes (e.g., water holding capacity, colour, flavour). The present report reviews the physical intervention systems that have been investigated to improve the texture and tenderize meat. The main emphasis in the report will be on the mechanism of action, the level of improvement achieved, and the impact on meat quality properties. The aim of this review is to provide a useful reference for students, industry, and early career researchers on this important topic.

MEAT TOUGHNESS

Animal muscles become meat by going through rigor mortis after slaughter involving a complex range of physiological, biophysical, and biochemical changes. Two components in the meat determine the texture of meat. The first component is related to muscle fibres, which are the building block of striated muscle. The striated appearance of muscle is caused by the structure of the myofibrils. The cylindrical-shaped myofibrils, making up to 80% of muscle fibres, are composed of continuous repeats of sarcomeres that are separated by Z discs (lines). The post-mortem changes in the contractile apparatus of the muscle (the sarcomere), which undergoes a shortening phase during the development of rigor mortis, determines meat texture. Processing and post-mortem handling, such as the use of efficient chilling systems to increase production efficiency and a rigor temperature of <10°C while the muscle pH is >6.0, can induce muscle shortening and meat toughness. Shortening-induced toughness can be resolved to a certain degree by the actions of endogenous proteases.[6 ⁻ 8] Therefore, methods that can activate (pH and temperature control), enhance (Ca²⁺ induction), or extend the time course for endogenous protease action (aging) can improve the texture component of myofibrillar proteins. The second component is due to the connective tissue in the meat “commonly referred to as the background toughness,” which is covered in a recent review.[9] The contribution of connective tissue to meat toughness depends on the structure and/or the amount of different collagens and elastin in the meat.[9] This component of meat toughness is not affected much by post-mortem processing and handling practices, but cooking style and temperature can partially improve this component of toughness.[10]

BIOLOGICAL VARIATION IN MEAT TENDERNESS

The physiological function of muscles in live animals determines the fibre type composition (fast vs. slow twitch, aerobic vs. anaerobic) and the connective tissue content. Also, after slaughter, the rate of chilling of a muscle or part of a muscle will depend on its location in the carcass. These differences will lead to wide variation in the chemical and biochemical composition, and the eating and keeping qualities of the meat. As a result, some muscles are generally tender (Longissimus dorsi [LD] and Psoas major [PM]), or tough (Biceps femoris [BF] and Supraspinatus [SS]). Other muscles, such as M. triceps brachii, vary in tenderness depending on processing conditions used.[11, 12] Variation in the tenderness of a muscle amongst and within animals has been documented.[13 ⁻ 15]

INTERVENTIONS TO MANIPULATE THE TEXTURE OF FRESH MEAT

Post-mortem interventions used in fresh meat tenderization can be classified into three main categories (physical, chemical, and enzymatic) based on their mode of action. Physical interventions are those that cause structural change in the meat through applying force or physical stimulus, which includes electrical stimulation (ES) of carcasses, aging conditions (temperature, wet vs. dry), freeze-thaw cycles, pressure treatments (hydrodyne/shock wave, ultrasound and high hydrostatic pressure), and mechanical tenderization (blade/needle tenderization, flaking, mincing), and contraction-prevention (stretching/tenderstretch/alternative hanging, tendercut^TM, wrapping, rapid crust freezing). The outcome of these methods is visualized as alteration to the connective tissue and the myofibrillar protein networks and stimulation of proteolysis through direct interactions between endogenous enzymes, cofactors, and substrates. The present review is limited to methods used to improve the tenderness of intact meat cuts. Information on disintegration of the meat structure (e.g., mincing, flaking) and restructuring/binding techniques have been recently reviewed.[16]

Chemical interventions include infusion, marination, or injection with calcium salts (e.g., chloride, lactate, ascorbate, and carbonate), sodium (e.g., chloride, acetate, citrate, and ascorbate), phosphate salts, commercial preparations containing maltodextrin plus starch or combinations of these compounds, and vitamin D. These methods can improve the texture of meat and the palatability through either stimulating proteolysis (Ca²⁺ activation of calpains), solubilisation of myofibrillar proteins (sodium and phosphate ions), and improve the water holding capacity (phosphate salts).

Enzymatic interventions include infusion, marination, or injection with exogenous enzymes from plant proteases (ficin, bromelain, papain, actinidin, zingibain); microbial proteases (collagenase from Clostridium histolyticum; aspartic protease from Aspergillus oryzae; fungal protease EPg222; thermophile enzyme E A.1 protease from Bacillus strain E A.1; 4-1.A protease from Thermus strain Rt4-1.A; caldolysin from Thermus strain T-351; elastase from Bacillus sp. EL31410, collagenase from Vibrio B-30); and animal protease (porcine pancreatin). These enzymes act on myofibrillar proteins and connective tissues to various degrees to produce a more fragmented and disintegrated structure.

PHYSICAL INTERVENTIONS USED FOR TENDERIZING MEAT

Electrical Stimulation

Electrical stimulation of carcasses is mainly applied to prevent excessive muscle contraction during the onset of rigor mortis as a result of rapid chilling (cold-shortening). This phenomenon may occur when the muscle temperature drops below 10°C while the muscles still contain sufficient energy (at pH > 6.0) to drive contraction.[17, 18] It is well known that energy in the form of an electric current leads to fast post-mortem glycolysis rates and, consequently, to an accelerated pH decline.[19] Therefore, electrical stimulation has been used in the meat industry to enable fast chilling conditions without risking excessive muscle contraction (cold shortening). Although this technique is considered to be a standard processing technique in the meat industry and it has prevented cold-shortening problems with rapid chilling in lambs,[20] a recent review on the topic questions the need for ES during processing of heavy beef carcasses.[18] As mentioned earlier, muscle contraction is minimised when carcasses enter rigor at 15°C. Given that heavy beef carcasses chill relatively slowly, electrical stimulation may promote muscle shortening when muscles enter rigor mortis (at pH < 6.0) and at a relatively high temperature (>35°C). This phenomenon is known as heat-shortening. Numerous publications have reported a positive impact for ES on meat tenderness, which was attributed to several suggested mechanisms (discussed below), but the actual mechanism is still in debate.[18, 21]

Commercial electrical stimulation is carried out by applying electrodes to different parts of the carcass to deliver an electric current that can take different forms; low (<150 V) and high voltage electrical stimulation (up to 1130 V), but also a wide range (2.5–9000 V) has been used at an experimental level.[22] The main difference between low and high voltage stimulation is the route of the stimulation. Direct muscle stimulation is achieved with high voltage, while low voltage stimulation relies on the nervous system to transfer the stimulus to the muscles.[22] ES treatment consists of several parameters that determine the efficacy of the treatment; namely, the frequency, waveform, burst length, and the duration of the stimulation. A full account of the parameters and their importance to ES is available.[21]

Upon applying ES to a carcass, vigorous muscle contractions occur with a rapid pH decline, which is dependent on the ES parameters used. Suggested mechanisms to explain the tenderizing action include: (i) physical disruption of the sarcomeres leading to weaker fibres resulting from severe muscle contractions,[23, 24] and (ii) accelerated proteolysis in ES-treated meat[19, 25] due to increased activation of the calcium activated protease μ-calpain. Given that the tenderization process starts upon the onset of rigor mortis,[18, 21] stimulated muscles will reach this stage at an earlier time post mortem and, moreover, at a higher temperature (promoting enzyme activity). Therefore, stimulated muscles have a head start for tenderization and, thus, at the same post mortem time the extent of tenderization in stimulated muscles will be higher than in non-stimulated ones.[18]

It is likely that both of the suggested mechanisms cause the documented tenderizing effect for ES. The system(s) involved and the extent of tenderization will vary greatly depending on the animals (stress, age), ES intensity, and chilling conditions post-mortem (discussed in detail).[26] Indeed, more insight is needed on the relationships between ES and farming and industrial practices since overstimulation reduces the beneficial effects of ES.[18]

Effective tenderizing conditions

The tenderizing effects of ES will depend on several factors, such as the intensity of the stimulation process as depicted in the input electrical parameters, post-mortem temperature, total electric input during processing, and pH-temperature profile. Given that several ES processing regimes are used worldwide,[21] and many factors will be involved, such as chilling rate and carcass size, it is difficult to have a set of parameters that have general use. However, it is much easier to point out the factors that reduce the efficacy of ES.

Geesink et al.[27] examined the effect of ES intensity on the quality of beef. They concluded that mild ES input was sufficient to stimulate beef LD, and higher ES intensity resulted in more rapid decline in pH. Their findings were in agreement with earlier reports[28, 29] suggesting the tenderizing effects of ES is achieved best with intermediate glycolysis rates. The study also highlighted the potential negative impact of cumulative electrical input along the production line on meat quality. The tenderizing effects of ES are completely diminished with aging and or incubation at temperatures >15°C.[18]

Effects on other meat quality traits

The dramatic effect of ES on pH decline in early post-mortem can have a great impact on other enzymatic systems and proteins that control meat quality attributes, such as water holding capacity, meat colour, and stability, and possibly antioxidative enzymes in the muscles. The literature offers diverse and often conflicting results, which are largely generated by employing different experimental conditions (species, age, maturity, muscles, ES intensity) that can affect the temperature-pH profile post-mortem. For example, while negative effects for ES on water holding capacity and colour stability have been reported for beef,[29, 30] a slight improvement in the initial colour was reported for lamb LD.[31] A real concern for ES treatment, especially in large carcasses or carcasses with a thick fat layer, is the increased drip loss resulting from protein denaturation when the chilling rate is low.

High Temperature Conditioning

High temperature conditioning (HTC) is normally referred to as the process of holding carcasses or meat cuts at temperatures above 5°C to improve the eating qualities (especially tenderness and flavour) of the meat. Interest in HTC was intensified after reporting the role of muscle shortening in manipulating meat tenderness[32] and the relationship between muscle shortening and pre-rigor temperature.[33]

Locker and Hagyard[33] reported that minimum contractions occurred when muscles are incubated within a temperature range of 14–18°C. The effect was demonstrated for several muscles (semitendinosus [ST], vastus lateralis [VL], and BF),[34] suggesting it can exist in several muscle types. HTC can be carried out by holding carcasses at relatively high temperatures either pre-rigor or post-rigor (after the muscles have gone through rigor under conventional chilling conditions). Clearly, the differences in the post-mortem time and the biochemical status of the muscle means that the energy added through thermal conditioning may act on different systems. The process can be performed on whole carcasses or meat cuts; vacuum packed or not packed; in controlled air or vacuum packed and immersed in circulated water at the desired temperature. Earlier studies reported significant tenderization at different temperatures,[35, 36] but the effect seems to be dependent on whether the conditioning is carried out pre- or post-rigor and the length of time, which seems to affect the muscle shortening.

The tenderizing effect of HTC is dictated by the post-mortem time (pre-rigor vs. post-rigor) and the temperature at which the muscle will be stored. The optimal tenderization was found to occur within a temperature range of 14–18°C.[37, 38] The mechanism for the tenderizing action of HTC can be summarized as follows: (i) reduction in muscle shortening when pre-rigor muscles are stored at a temperature close to 15°C and (ii) increased tenderization rate at intermediate post-mortem temperature (e.g., 15°C) due to increased proteolysis as a result of activation of μ-calpain and degradation of calpastatin.[39]

The meat tenderization rate is negatively compromised when muscles are stored at elevated pre-rigor temperatures (>25°C) due to muscle shortening[37, 38] and decreased μ calpain activity.[37, 40] By eliminating the effect of muscle shortening in meat incubated at 18°C and at 35°C, Devine et al.[38] discounted muscle shortening as a cause of low tenderization rate at the higher pre-rigor temperature (35°C) compared with 18°C, since eliminating the differences in the sarcomere length from both treatments did not change the differences in the shear force values. However, high pre-rigor also diminished calpastatin activity[37] and resulted in a higher rate of troponin T degradation (degradation of this structural protein has been implicated as a key step in meat tenderization[41]) at 1 day post-mortem.[35, 37] Wheeler and Koohmaraie[42] reported that muscle shortening did not affect the extent of troponin-T or desmin degradation (important structural myofibrillar proteins, their degradation of which is commonly used as indication of proteolysis) in either Longissimus or Psoas major. However, when the differences in shear force were compared with the extent of protein degradation (Table 1), it is clear that large differences existed at 1 and 10 days post-mortem between control and shortened muscles and these differences cannot be explained by the degree of proteolysis. This indicates that shortening had a persistent effect during aging, either by increasing the density of the sarcomeres causing more resistance to shearing or by preventing access of proteases to other structural proteins. Taken together, it seems that ensuring access to substrates within longer sarcomeres and the availability of and adequate level of μ-calpain, regardless of higher calpastatin activity, can lead to a high tenderization rate. Therefore, both the degree of muscle contraction and calpain activity appear to play a major role in HTC. An increase in free lysosomal enzyme activities under low pH and high pre-rigor temperature,[43] such as that available in HTC, can contribute to myofibrillar protein degradation and affect the thermal stability of collagen[44] leading to tenderizing action.

Table 1  Effects of treatment, muscle, and post-mortem aging on Warner-Bratzler shear force and Western blot analyses in lamb (adopted from Wheeler et al.[42])

Muscle	Treatment	Difference in shear force (N)	Difference in Troponin-T degradation (%)	Differences in Desmin degradation (%)
LD	Control-shortened* 1 d PM	−35.5	5.0	5.4
	Control-shortened* 10 d PM	−33.9	8.2	1.3
	Control 10 d PM–1 d PM ^#	−20.8	40.5	60.5
	Shortened 10 d PM–1 d PM ^#	−22.3	37.3	64.6
PsM	Control-shortened* 1 d PM	−27.6	− 6.9	1.1
	Control-shortened* 10 d PM	−21.6	15.6	6.6
	Control 10 d PM–1 d PM ^#	−6.5	34.8	46.6
	Shortened 10 d PM–1 d PM ^#	−12.5	11.7	41.1
*Differences between shortened and control samples at 1 day post-mortem.
^#Differences between 1 day and 10 days post-mortem samples.
PM: post-mortem; LD: Longissimus dorsi; PsM: Psoas major.

Effective tenderizing conditions

Holding carcasses at 15–16°C pre-rigor for 10–48 h improved the tenderness by 40 to >50%[45] compared with controls held at a lower temperature (0–1°C) and an increased rate of tenderization occurred at this temperature.[46] More recently, Geesink et al.[37] reported a 30% increase in tenderness at 1 day post mortem in lamb LD incubated at 15°C compared with those incubated at 5°C. Pre-rigor incubation at higher temperatures (35–37°C) for a relatively short time (3 h) was reported to improve tenderness.[36, 47] In the experiment, this shorter time avoided the heat-shortening that normally occurs after 5 h at the same temperature. Extended periods of HTC can result in tougher meat after post-mortem aging.[37]

Generally speaking, tenderization is adversely affected when muscles go through rigor at temperatures ≥30°C.[18, 37, 38] The tenderness data reported[38] for lamb LT at 8, 26, and 72 h post-mortem and for beef LD at 1 and 5 days post-mortem[18] show that ES does not prevent the toughening effect of pre-rigor high temperatures (35°C and 37–40°C, respectively). However, Rosenvold et al.[48] found a protective for ES on beef Longissimus lumborum (LL) muscles that had gone through rigor at 35°C. The level of electrical input and animal age may be contributing to the seemingly contradictory results. Again, the reports warrant more research on the interactions of different physical inputs used in the meat industry and the physiological factors of the slaughtered animals.

Post-rigor HTC, on the other hand, can be very effective in increasing the tenderness of meat since the critical period for the development of shortening is passed. The rate of tenderization exponentially increased with the increase of post-rigor HTC up to 60°C.[49] Even if the muscles were subjected to shortening, HTC at 37°C can alleviate the shortening effect and produce meat with equivalent tenderness as that subjected to rigor at 15°C.[50] This principle has been used to reduce the aging time of beef from 2 weeks to around 1–2 days in a process called “Tenderay” where the meat is kept at a temperature as high as 43°C under a system that ensures reduced microbial growth, such as UV irradiation, or a source of ozone, or the use of antibiotics.[51] Intermittent thermal treatment (heating-cooling cycles over the range of 4–20°C or 4–25°C) could improve the rate of tenderization in ST and SM muscles from 24 h PM meat.[52]

Effects on other meat quality traits

Colour and colour stability have been reported to be affected by the temperature at which the muscle enters rigor. For example, significant early post-mortem improvements in colour parameters (chroma and L*) at elevated temperatures (30–35°C) have been reported.[37, 53, 54] The increase in lightness (L*-value) is likely to be due to increased protein denaturation and decreased water holding capacity at that level of temperatures, which will lead to higher amounts of moisture freely available on the meat surface, resulting in a lighter colour. HTC can affect the activity of mitochondria and decrease the oxygen consumption rate (OCR) which leads to higher oxygen content and oxymyoglobin content in meat upon retail display, hence higher chroma. However, the effect is lost with vacuum packaging and aging[37, 54] due to the loss of excess moisture as purge and loss of the advantage of reduced OCR with conventionally processed meat having a similar level of OCR. Both the temperature at which a muscle goes into rigor and the time it is held at a given temperature could affect MetMb formation.[53] Higher temperatures result in increased MetMb accumulation probably due to the denaturation of enzymes responsible for meat reducing activity and/or rapid loss of reduced nicotinamide adenine dinucleoti (NADH).

Apart from effects on meat colour, HTC may also affect water holding capacity of the meat as significant increases in drip and cooking losses have been reported.[37, 38] The likely explanation for these observations is increased protein denaturation at elevated temperatures. While elevated temperature could support increased microbial activity, under good hygienic conditions, elevated temperature conditioning (37°C for 3 h) did not significantly affect the microbiological quality of beef at 24 h post-mortem.[55]

Aging

Aging, also known as conditioning, is the process of refrigerated storage of meat to benefit from the tenderizing effect of endogenous proteases. The mechanisms of aging and the biochemical factors affecting it have been the subject of a number of recent reviews.[56, 57]

The process is normally carried out in the form of either wet aging or dry aging. Wet aging is the process where the meat cuts are vacuum packed and stored at temperatures just above freezing to prevent significant microbial activity (between −1 to 4°C, depending on the desired storage time and targeted market) to achieve a required level of tenderization. Dry aging refers to the process of refrigerated storage of complete carcasses or joints without vacuum packaging. Both systems have a significant impact on meat quality (flavour, colour, texture). The success of aging as a process depends on many factors (meat cut, pH, aging temperature, aging time). The key mechanism for tenderization during aging is the breakdown of the muscle structural proteins by endogenous enzymes with μ-calpain being the major enzyme involved in the process,[9] although the involvement of other proteases has been suggested.[8, 58]

Effective tenderizing conditions

At an industrial level, the conditions governing the aging process will depend on the targeted markets (local or international) and, therefore, whether slow or fast tenderization is required. In all instances, subprimal cuts are normally fabricated from carcasses, vacuum packaged, and maintained at temperatures above freezing. The aging time will vary greatly, with the aging temperature being shorter with higher temperatures. In the US, the average aging time for meat cuts varies between 2–61 days with an average range of 17–19 days.[59] Steaks that are intended to be quality guaranteed and destined for the food service industry will need longer (average 32 days) aging time.[59] The level of tenderness achieved with a certain aging time will vary greatly among different meat cuts[60] reflecting the differences in the level of endogenous enzymes, contraction status, and connective tissue content. Post-mortem aging is a standard industrial option for improving tenderness, but it has several problems associated with the extra cost of packaging, labour, chilling and the risk of meat spoilage, and the waste management of packaging material.

Effects on other meat quality traits

Drip loss increases with the increase of aging time and temperature.[37, 61] Cooking losses tend to decrease with increasing aging time,[38, 60] but some reports found no change in cooking losses with aging time.[62, 63] Intermediate aging periods (4–7 days) may cause more cooking losses.[64] Aging can greatly improve the overall palatability of meat cuts that have relatively small amounts of connective tissue and that have not cold-shortened.[60, 65]

Upon aging, several biochemical changes take place that can play a very important role in controlling meat colour and colour stability. For example, the oxygen consumption rate (OCR) and mitochondrial respiration decreases with an increase in aging time; the reserve of NADH is completely depleted but NAD⁺ does remain for some time. The autooxidation rate is limited during vacuum packing but increases at a faster rate during subsequent meat display. These main events collectively will regulate the amount of O₂ uptake and the consumption of O₂. This will affect the level of oxidation during display and the activity of metmyoglobin reducing activity, leading to different colour profiles.

It is well known that the colour of fresh meat deteriorates with aerobic storage time and that vacuum packaging improves the colour of raw meat upon exposure to air. However, any improvements in the colour are short lived and the rate of change in colour of meat previously vacuum packed during aerobic display is higher than non-vacuum packaged meat. Studies by O'Keefe and Hood[66] on the OCR of LD muscle showed that OCR falls to 30% of the initial rate at 2 days post-mortem and to 15% by 10 days post-mortem. Later, Feldhusen et al.[67] found that aging of intact meat resulted in higher O₂ penetration and therefore there was higher O₂ pressure present below the surface of the meat as the aging time increased. They reported that 29% of Mb was OxyMb at 1 day post-mortem when meat was exposed to air for 5 h, whereas at 3–11 days post-mortem 51–64% of Mb was OxyMb after 5 h of exposure to air. However, the amount of OxyMb decreased after 13 days of aging although they found higher O₂ penetration and O₂ partial pressure after exposure to air. In agreement with Echevarne et al.,[68] they explained the effect of prolonged aging by the increase in MetMb reducing activity.

Freezing/Thawing

Exposing the meat to a freeze-thaw cycle can improve tenderness through physical and biochemical processes that are greatly influenced by the rate of freezing (dependent on the freezing medium/method and the product size), the post-mortem time of application, and the conditions during processing (e.g., ES, HTC). Meat cuts or whole carcasses can be subjected to cycles of freezing and thawing. Several options for freezing (conventional, air blasting, plate freezers, and cryogenic systems) and for thawing (under static pressure, microwave, circulated air or water) are available and the choice of any combination of these technologies will have an impact on the meat quality.[69] Significant levels of tenderization have been reported.[70, 71] For example, a 34 and 13% reduction in the shear force of lamb LD (at 1 and 7 days post-mortem, respectively)[27] and 14% in beef LD[72] have been observed due to one freeze/thaw cycle. However, the effect seems to be dependent on the aging post-mortem time.[73]

Exposing the meat to temperatures below the freezing point of meat (<−1.5°C) will cause the formation of ice crystals in the meat. Depending on the rate of freezing, the ice formation can be rapid in extracellular and intracellular compartments of the muscle (during very rapid freezing rates) with little chance of establishing an osmotic gradient across the cell wall. This will consequently prevent the migration of moisture across the cell wall and the cells will maintain their integrity. On the other hand, when the freezing rate is slow, the ice formation will be slow and large ice crystals will form outside the cells causing an increase in the solutes outside the cells. This eventually will lead to an osmotic gradient across the cell wall and migration of moisture from inside the cells to the outer side, which causes dehydration and collapse of the cell wall. With the phase change (water to ice) the density of the ice is lower than water and consequently the ice will occupy more molecular space than water, which imposes pressure on the cell wall and causes fragmentation. It could be assumed that upon thawing the cellular components will be freely available for reactions as direct interaction with substrates will be possible.

Effective tenderizing conditions

The tenderizing effect seems to be dependent on the time post-mortem, meat cut, and possibly the animal species.[27, 70 ⁻ 73] The tenderizing effect is also dependent on processing conditions since freeze-thawed lamb from mildly stimulated carcasses was more tender than lamb from intensely stimulated carcasses.[27]

Effects on other meat quality traits

Freeze-thawing can affect the colour of meat, depending on the freezing rate used, with a lighter colour in slowly frozen meat,[74] however, the redness and the colour stability is negatively affected.[61] A significant drawback of this technology is the increased drip loss upon thawing.[61] Furthermore, interactions with other processing steps can have a great effect on drip loss. For instance, freezing rate had a greater effect on drip loss in ES meat compared with none ES (NES) (range 7.5–10.9% for ES and 3.3–5.5% for NES, respectively) with higher losses in commercial systems (walk-in freezer, domestic home, blast air freezer, range 9.3–10.9%) compared with cryo-freezing (range 7.6–7.8%).[75] It is worth mentioning that while instrument-determined tenderness of beef treated with these technologies was significant, the sensory assessment of the meat was contradictory to the instrument measurements.[76] Furthermore, increased oxidative processes have been shown to occur in pork[77] but are yet to be confirmed in other species.

Mechanical Tenderization

This method relies on achieving better texture through applying force (compression or shear) that causes deformation or damage to the network of meat structural proteins.

Blade Tenderization

Tenderization by applying a set of needles or blades has been of interest for a long time with the first patent granted for a blade tenderization (BT) prototype at the start of the 20th century.[78] The technique has been intensively investigated[79 ⁻ 83] and used at commercial scale. A set of needles or blades are spaced at a given interval distance, depending on the meat size (roast vs. steak), that are driven in the meat by either a manual or an automated system. The penetration disrupts the structure of the meat and causes weakening in the meat protein network. Blade tenderization can be carried out in conjunction with tumbling in marinades containing proteases (section 3) or enhancement mixture solutions (section 4). Manual blade tenderizers are available for home use or small scale processing (e.g., Jaccard™, Norpro™, and Deni™ meat tenderizers) but large operations require more sophisticated machinery (e.g., Ross and GMC Tenderit Tenderizers). The meat is passed through a machine 1 to 4 times depending on the level of tenderization required, but higher numbers of passes may not produce increased tenderization in some meat cuts.

During the penetration of needles or blades in meat, the myofibrillar protein network and connective tissue are severed, which causes weakening to the meat structure.[84] This suggests that the main action of BT is physical in nature. However, an increased myofibril fragmentation[85] and increased protein content of the uncooked meat exudate[86] as a result of blade BT could potentially indicate an indirect biochemical basis for the observed improvement in the meat tenderness with this treatment. This method has the advantage of achieving effectively an instant tenderization without the need to have holding time, exposure to relatively high temperatures, or the addition of non-meat components compared to many of the other meat interventions discussed in this article. However, little or no improvement appears to occur with further storage.[87]

Effective tenderizing conditions

Most BT studies and commercial application of the technology were carried out using post-rigor meat. Earlier research demonstrated that cold-boned BT-treated meat was superior in tenderness compared to hot-boned BT-treated meat despite the lower juiciness scores associated with cold-boned meat[88] and due to the convenience of processing and handling post-rigor meat. BT is more beneficial for meat cuts that contain a high level of collagen connective tissue.[81, 84, 88, 89] Although some improvement can be found with BT, not all meat cuts will be significantly affected by this technology. Jeremiah et al.[84] examined the effects of BT on the palatability attributes of 12 beef meat cuts and they found that BT significantly improved the initial and overall tenderness of half of the meat cuts with the cuts, such as rib-eye, short rib, blade eye, cross rib, and brisket were unaffected. A higher number of BT passes can achieve further improvement in tenderness but the effect was dependent on the muscle, with better tenderness levels achieved with a higher number of BT in Semimembranosus and Biceps femoris but not Longissimus[81] and in Semitendinosus but not in Psoas major.[80] The use of BT in addition to other treatments, such as injections with enhancement solutions and tumbling[83] and hydrodynamic pressure,[85, 87, 89] can have synergistic effects on the tenderization process.

Effects on other meat quality traits

BT has no effect on cooking losses[84, 88, 89] but decreased the juiciness of blade eye and top sirloin.[84] BT can affect the flavour of the meat, but the effect seems to be muscle/meat cut dependent, with a significant reduction in the flavour intensity of the top sirloin and rib-eye, and improvement in the flavour desirability of the inside round have been reported.[84]

Wrapping

This technology involves wrapping the muscles pre-rigor with polyethylene film to prevent the muscles from shortening during rigor development.[90, 91] Experimental work demonstrated that wrapping decreased the shear force of beef LD muscles and improved the sensory tenderness of the meat by 53 and 41% after aging for 7 and 14 days, respectively.[90] Hildrum et al.[91] found the tenderizing effects of wrapping on beef LD was equivalent to aging the meat at 10–12°C at 2 days post-mortem.

An industrially applicable form of this technique is the Pi-Vac Elasto-Pack system.[92] In this system, hot-boned meat is inserted into stretched tubes of highly elastic material and upon release of the stretching force, the tube contracts to its original shape exerting pressure on the meat. Wrapping is an effective way to prevent shortening, which is problematic especially in hot-boned meat that lacks the support of the skeleton in reducing muscle contraction.

The principle of this technology is similar to a patent by Cason et al.,[93] where muscle contraction and shortening is prevented by means of metal plates pressing against the meat through clamping. In the wrapping technology, muscle contraction is prevented by obstructing the diametrical muscle expansion through the compression force generated from the elastic film on the meat. The tenderizing effect obviously results from the prevention of the toughening effect of muscle contraction during rigor development.

Effective tenderizing conditions

Wrapping is an effective technique when potential muscle contraction can occur, such as in hot-boned meat and when rapid chilling is required. Some researchers found that in situations where muscle shortening is minimal (such as rigor development at about 15°C), wrapping was ineffective in increasing the tenderness level in meat.[90, 91] Others reported an increase in wrapped beef LD tenderness after it had gone through rigor at 15°C[62] possibly due to the effect of ES in the latter study. The effect of wrapping may depend on the fibre organisation of the muscle as the technique was ineffective in improving the tenderness of beef SM.[91]

Effects on other meat quality traits

Wrapping was reported to improve the eating quality of beef with higher acceptability scores compared with unwrapped.[90] The technique does not affect drip loss,[62] but may change the shape of the muscles.[91]

Stretching

Since the relationship between toughness and muscle shortening was reported,[32] the interest in improving meat tenderness has led to experimentation with different methods to prevent muscle contraction by means of fixation (clamping), adding weights, or manipulation of hanging methods (stretching).[60, 94 ⁻ 96] Several carcass hanging manipulations (horizontal, neck-tied, hip-tied, and hanging from pelvic or aitch bone) have been examined and the most effective (increasing the sarcomere length and tenderness) resulted from pelvic suspension.[95] The research has evolved into two main technologies, namely, tender stretch (pelvic suspension) and tenderCut^TM, which demonstrated beneficial impacts on the meat quality. Similarly, a dramatic increase in the tenderness and decrease in cooking loss have been reported using a stretching machine developed by Australian scientists.[97]

Tenderstretching

In this method, the carcass is hung from the pelvic bone (obturator foramen or aitchbone) instead of the traditional hanging from the achilles tendon. This suspension position causes the hind limb to fall into the walking position and the weight and tension generated from this leads to more tension to be imposed on the back of the carcass. While this method adds more tension on the back of the carcass, less tension will be applied on PsM and some hind limb muscles than the Achilles hanging position, which results in various tenderizing effects depending on the muscle location on the carcass.

Tendercut^TM

The exploitation of the carcass weight to generate tension/stretching action on certain muscles was the driving force in developing this technology. Stouffer et al.[98] were granted a patent for a tenderizing method that describes severing the backbone of the split carcass so that the weight of the carcass was sufficient to extend and maintain tension on LD during the development of rigor. The method was further developed by scientists at Virginia Polytechnic Institute and State University[99, 100] with several subsequent studies describing the effects on meat quality. This technology consists of making cuts in the skeleton at specific locations that can maximize the tension on the longissimus and lead to the stretching without the need to alter the carcass suspension. The process involves cutting the vertebral column at the 12th/13th rib junction and at the junction between the 4th and 5th sacral vertebrae. The adipose fat, adjacent connective tissue, and the multifidus dorsi are cut to expose the epimysium and ensure sufficient extension. The process increased the tenderness of LT by 18–28% and BF, gluteas medius (GM), and rectus femoris (RF) by 2–4%.[101] Other versions of the process have been reported by Sørheim et al.[102]

Before the onset of rigor, the muscle fibres are flexible and can be stretched. A 50% increase of ST tenderness was reported,[103] and twice the sarcomere length was achieved as a result of stretching. The physical prevention of shortening by stretching influences the effect of myofibrillar structure on tenderness.[94, 95] Reduction in the muscle fibre diameter and an increase in sarcomere length due to pelvic suspension[104] and tendercut[100] have been reported for longissimus and round muscles. This resulted in low shear force compared with non-stretched muscles.[100, 104] An added benefit of these techniques is that they reduce the variation in tenderness.

Effective tenderizing conditions

As mentioned earlier, stretching can improve the tenderness of beef LD after 2 days to a level equivalent to 12 days of aging.[105] However, the effect will depend on the muscle location in the carcass with improvement in LD and BF, but not semimembranus (SM), semitendinosus (ST), and tensor fascia latae (TF).[105] An additive effect for stretching with aging has been reported.[105, 106] Tenderness from stretching seems to be dependent on the degree of muscle stretching. A tenderizing effect was achieved when beef LT was stretched to 120% of the original length and no further tenderization was found at higher stretching extents (140 and 160%).[107] Similarly, Buege and Stouffer[108] reported an increased tenderness in lamb LD at 5% stretching but no further tenderizing effect was found with 15% stretching. The use of weights can reduce the shear force by 30%.[108] Stretching was found to be especially effective to improve the LT tenderness under fast chilling conditions,[60, 96, 102, 109, 110] but under slow chilling conditions there was no difference in tenderness between tender stretched and non-stretched muscles.[60, 96, 102, 110] The muscle location affects the tenderization level achieved by tenderstretching.[60, 96, 109] In PsM, tenderstretching produced a slight toughening and shortening of sarcomeres but slow chilling had no effect with a slight improvement in shear force with aging. In BF, tenderstretching greatly lengthened the sarcomeres but produced only a small tenderizing effect. Slow chilling and aging had no effect on tenderness. In ST, tenderstretching increased sarcomere lengths with a slight improvement in shear force with aging. In SM, tenderstretching increased sarcomere length and produced meat that was more tender at 2 days post-mortem. Slow chilling had no effect on tenderness but aging improved the overall tenderness. In GM, tenderstretch suspension produced a marked persistent tenderizing effect.[109] Pelvic suspension improved the tenderness of pork,[111, 112] but the degree of tenderization is dictated by the genetic background of the animals.[111] The best tenderizing effect was achieved when the tender stretch was carried out 45 min post stunning at pH 6.1.[110]

Effects on other meat quality traits

Stretching through TenderCut^TM has no effect on cooking loss and slightly decreased a* values of USDA choice ribeye.[101] Tender stretch has been reported to decrease drip and cooking losses in beef but not in pork.[110]

Hydrodynamic Pressure (HDP)

Hydrodynamic pressure (HDP), also known as hydrodyne, is a post-mortem meat processing treatment originally patented by Godfrey[113] and improved by Long.[114] The treatment involves the use of a small amount of explosive to generate a hydrodynamic shockwave in a fluid medium (usually water) in which vacuum packaged meat is immersed. HDP is distinguished from a related technology called high hydrostatic pressure (HHP) in that HHP exploits the pressure of fluid at rest by pumping fluid into a pressure vessel in which vacuum packaged meat is placed.

With HDP, the shockwave generated following underwater detonation creates pressure fronts in the 70 to 100 MPa range. As the shockwave passes through the meat placed at the bottom of the container in fractions of a millisecond, a sudden increase in pressure is exerted on the meat, which results in disruption of muscle and connective tissue structures. Physical forces exerted on meat are created by pressure from both the compression wave and gas bubble contraction resulting from the underwater detonation.

Effective tenderizing conditions

Meat tenderization by HDP has been reported in several studies. Solomon et al.[115] showed that HDP significantly reduced the shear force of four different beef muscles by 49–72%. The study also illustrated that the magnitude of tenderness improvement achieved was largely dependent on the quantity of explosive and the number of detonations performed on the meat samples. For instance, a greater decrease in shear force was observed with detonation of 100 g explosive compared to that of 50 g explosive, however, meat samples exposed to two explosions with 50 g explosive each appeared to have a lower shear force than those exposed to one detonation by 100 g explosive. Tenderization by hydrodynamic shockwave was found to be effective in other studies on various meat and muscle types.[116] As mentioned above, HDP tenderizes meat by disrupting meat myofibril structure. A study by Zuckerman and Solomon,[117] using transmission electron microscopy, found that strip loins treated with HDP resulted in myofibril fragmentation of Z lines, A bands, and adjacent regions, which lead to an increase in intra-myofibril spaces, splits in the myofibril lattice, and a 37% decrease in shear force. Moreover, HDP has also been shown to be more effective than post-mortem aging at accelerating meat protein solubility, troponin T degradation, and accumulation of a 30 kDa polypeptide product,[117, 118] which is a marker of protein degradation during aging. The mechanism of meat tenderization by HDP remains unclear. However, various studies have shown that pressure treatment by the related meat processing method HHP resulted in an increased release of meat endogenous proteases (cathepsins and calpains)[119, 120] and was linked to an elevated level of sacroplasmic reticulum Ca²⁺.[121]

Effects on other meat quality traits

HDP treatment has been shown to have no effect on meat pH[117] cook loss, colour,[116] or flavour-related components.[122]

Ultrasound

Ultrasound is a very promising technology to tenderize meat, which has been extensively reviewed.[123] Ultrasound is a mechanical vibration at a frequency higher than the frequency range audible to the human ear. Ultrasonic energy can be transmitted in solids, liquids, or gases. The use of ultrasonic processing in meat has become more popular with various research publications showing that meat physical properties, such as pH, shear force, cook loss, and total loss, can be altered with ultrasonic treatment.[123] The use of ultrasound in food processing can be grouped into two categories: high power-low frequency (up to 10 kW and frequency range of 20–100 kHz) and low power-high frequency (up to 10 W and a frequency of up to 10 MHz).[123]

Ultrasound has been shown to disrupt muscle fibre by altering Z line structure and increase the level of cytosol Ca²⁺.[124] A study by Stadnik et al.[125] using electron microscope reported structural changes in muscle fibre treated with ultrasound (45 kHz). Significant disruption at the regions of A band and Z disk was observed after 24 h of aging. The level of structural changes in the muscle fibres increased with longer aging periods. Similarly, ultrasonic treatment in meat was shown to be capable of reducing the stability of collagen fibre structure and the muscle fibre diameter.[126 ⁻ 128] After a low frequency and high power ultrasound treatment, the collagen fibres became disordered and staggered significantly, leading to significant effects on meat textural properties.[128] The release of enzymes from lyzosomes due to ultrasound treatment has been suggested for the tenderizing effect observed in lamb.[129]

Effective tenderizing conditions

High power ultrasound treatment (24 kHz, 12 W/cm² for various times), was able to significantly reduce shear force and hardness of beef[124 ^,158] and lamb muscles.[129]

Effects on other meat quality traits

Low frequency and high power ultrasound has been shown to decrease yellowness and muscle fibre diameter.[128] On the other hand, high frequency ultrasound has been reported to increase water holding capacity of beef samples.[125] Ultrasound treatment was reported to reduce cooking losses.[126, 127]

CONCLUSION

The results from the reviewed work show that there are many physical methods to tenderize meat, but no generic solution to solve tenderness problems. The level of tenderization achieved by these different methods varies tremendously from no effect to over-tenderization depending on the muscle (myofibrillar and connective tissue contents), the meat form (individual muscles or whole carcass) with subsequent advantages and disadvantages in terms of flavour, textural and product acceptability. The reason for this is that meat toughness/tenderness is determined primarily by the amount and maturity of connective tissues, the level of muscle contraction, and the level of tenderization during aging (determined by the level of proteases and inhibitors). Depending on the muscle type, species, animal age, processing conditions, and cooking method, the relative contribution of these factors to toughness varies. Therefore, the optimal tenderization strategy may differ depending on the meat cut and recommended cooking method.

Hot-boning of meat has several advantages over cold-boning (lower weight loss, lower chilling costs, higher binding capacity, better control of chilling, and better and safer handling of meat). It also has some disadvantages, including the fact that hot-boned muscles are no longer attached to the skeleton, and therefore free to contract during the development of rigor-mortis. This factor can have a profound negative effect on meat tenderness. Some of the discussed interventions combined with enhancement can be useful to overcome this problem. Methods using injection and/or BT can potentially risk contamination and good hygiene practices should be used. Generally speaking, increasing texture softening and protein degradation can lead to increased oxidative processes, which can affect colour stability and meat flavour. The incorporation of antioxidants can stabilize the meat. Clearly, most of the reported studies focused on meat tenderness and cooking yield without paying much attention to other meat qualities (especially colour) and this needs to be addressed in future work since colour can greatly affect the marketability of the meat.

ACKNOWLEDGMENTS

Meat & Livestock Australia is acknowledged for funding this work through their Red Meat Innovation program.