Utilization of restructuring technology in the production of meat products: a review

Abstract

Meat restructuring technology enables the production of value-added meat products from low quality cuts and trimmings. In addition, this technology can improve products' characteristics such as texture, fat content, bind strength, and shape; meanwhile, it can also satisfy the increasing demand for convenience. The factors affecting the quality of restructured meat products including appearance, comminution and formation, particle size, mixing time, meat type, and fat content are analyzed; roles of non-meat ingredients such as salt, phosphate, soy protein, transglutaminase (TG) are discussed, protein binding and cooking are also mentioned. Salt and phosphates are traditional additives, which facilitate extraction of myofibrillar proteins and enhanced binding of meat particles. Salt is also associated with meat discoloration and rancidity development. Recently, there is a tendency towards reducing salt intake for health reasons. Binders such as κ-carrageenan, alginate, TG are widely and successfully used in restructured meat production. Some new techniques such as meat fiber alignment and high pressure treatment are also discussed in this review.

La tecnología de reestructuración de carne permite la producción de productos de carne con valor añadido partiendo de cortes y pedazos de baja calidad. Adicionalmente, esta tecnología puede mejorar las características de los productos como textura, contenido graso, cohesión y forma; al mismo tiempo, puede satisfacer la demanda creciente de productos de mayor conveniencia. Se analizan los factores que afectan la calidad de los productos de carne reestructurada incluyendo su pulverización y formación, tamaño de partículas, tiempo para mezclar, tipo de carne y contenido graso; se discute el papel que juegan los ingredientes que no provienen de la carne como sal, fosfato, proteína de soya y transglutaminasas; cohesión proteínica y cocción se mencionan también. La sal y fosfatos son aditivos tradicionales que facilitan la extracción de proteínas miofibrillares y una capacidad superior de cohesión de las partículas de carne. La sal también se asocia con la decoloración de la carne y el desarrollo de rancidez. Recientemente hay una tendencia a reducir la ingesta de sal por razones de salud. Agentes cohesivos como κ-carragenano, alginato y transglutaminasa se usan amplia y exitosamente en la producción de carne reestructurada. Algunas técnicas nuevas como alineamiento de fibras de carne y tratamiento a alta presión se discuten en esta revisión.

Keywords:

• restructuring
• meat products
• bind strength
• texture

Palabras clave:

• reestructurante
• productos de carne
• fuerza cohesiva
• textura

Introduction

In the early 1970s, restructuring technology appeared as a new concept to enhance meat utilization (Mandigo, 1988). The definition of restructuring refers to the utilization of manufacturing steps to create a consumer-ready product which resembles an intact steak, chop, or roast (Mandigo, 1988). When meat is restructured, the form of the meat is changed. Because consumers are concerned about intaking too much fat and salt, this creates an increased market for lean restructured meat products (Mandigo, 1988).

Restructured meat products include any meat products that are partially or completely disassembled and then reformed into the same or a different form (Pearson & Gillett, 1996). Therefore, this definition includes all sectioned and formed meat products, as well as all sausages and a variety of other products (Pearson & Gillett, 1996). In restructured meat products, several serious problems, i.e., fat oxidation and color instability, are encountered and they are the prime limitation to acceptance by consumers (Akamittath, Brekke, & Schanus, 1990).

Many factors including changes in lifestyle, smaller family size, both husband and wife working away from home, the requirement for more lean and less fat, and the desire for more convenience indicate that the need for restructured meats has been increasing.

In this review, firstly factors affecting restructured meat products quality are analyzed, followed by discussion of the role of non-meat ingredients. Subsequently protein binding is studied; in addition, cooking conditions are investigated.

Factors affecting restructured meat products quality

Appearance

Visual appearance is a major consumer concern with regard to restructured steaks (Mandigo, 1988). Good fresh colors, where products resemble intact muscle steaks and have a fine distribution of small fat particles are the essential requirements for restructured meat products (Mandigo, 1988). Mixing time has an effect on the color and will increase the deterioration of the desired color if conducted in excess of 12 min (Booren, Mandigo, Olson, & Jones, 1981a). Vacuum mixing was evaluated as a method of preventing color deterioration of fresh meat, whereas it produced less desirable surface color in finished steaks (Booren, Mandigo, Olson, & Jones, 1981b).

Discoloration of restructured steaks can be caused by salt. A decrease in color desirability with increased salt levels has been observed by some researchers (Huffman & Cordray, 1979; Schwartz & Mandigo, 1976). The raw color could be improved by sodium tripolyphosphate (STP), which helps to compensate for the effect of salt (Schwartz & Mandigo, 1976). Akamittath et al. (1990) indicated that discoloration and lipid oxidation of restructured steaks are related. They found color degradation in beef occurred much earlier in storage than did lipid oxidation, and the lipid oxidation may have been catalyzed by oxidized pigments. Serrano, Cofrades, and Jiménez-Colmenero (2006) however indicated that color degradation and lipid oxidation were not related.

Comminution and formation

The methods for producing the particles of meat used in restructured steaks have a substantial impact on the texture, cooking performance, and palatability of the cooked product (Mandigo, 1988). Three basic procedures, (1) chunking and forming, (2) flaking and forming, and (3) tearing and forming, are used in reducing particle size and production of restructured meat (Pearson & Gillett, 1996). The first two procedures have been extensively used and are increasing in importance. The third procedure needs special equipment that tears the meat fibers apart and then reforms them into shapes resembling intact cuts (Pearson & Gillett, 1996). The type, size, and shape of the particles produced vary significantly between the grinder, chopper, and flaker (Mandigo, 1988).

Maintaining more of the characteristic structure and texture of the original meat cuts is the advantage of chunked and formed meat products (Pearson & Gillett, 1996). Susceptibility to autoxidation, which is common to all restructured meats that are not cured using either nitrite or nitrate is a major disadvantage of chunked and formed fresh meats (Pearson & Gillett, 1996). Oxidation, which causes fading of meat color and is accompanied by development of oxidized flavors will be prevented by the addition of antioxidants during processing (Pearson & Gillett, 1996).

Flaked and formed meats have a texture similar, but not identical, to intact steaks, chops, or roasts. The chief disadvantages of flaked and formed meats are (1) greater labor requirements, (2) the cost of the equipment required, and (3) problems due to autooxidation (Pearson & Gillett, 1996). The procedure of tearing and forming has two advantages: (1) it causes less membrane damage, and is less susceptible to oxidation, and (2) the product has more structural integrity and more closely resembles to intact meat cuts in texture (Pearson & Gillett, 1996).

Raharjo et al. (1995) evaluated quality characteristics of restructured beef by six mechanical treatments. Their results indicated that chunked meat or mixtures of chunked and sliced meats in combination with salt/phosphate or Na-alginate/Ca-lactate resulted in steaks with acceptable physical and sensory properties. In poultry patties, Hollender, MacNeil, and Mast (1987) found the coarsely ground patties prepared from spent layer meat by flake cutting or grinding had higher shear values and lower resistance to tear, although bind strength was less than the flaked cut meat.

To attain a product that not only imitates but also possesses the attributes of a whole-tissue product is the common objective of restructuring. The muscle fibers/fiber bundles in the restructured steaks must be aligned to obtain the texture and appearance of restructured steaks that imitate closely that of a real steak (Guenther, 1989). The results of Farouk, Zhang, and Cummings (2005) demonstrated that no effect on the color of raw or the bind strength of both raw and cooked steaks was caused by fiber alignment. Perpendicular (PP) or mixture of parallel (PR) and PP (MX) steaks had lower drip loss and visual appeal than raw PR steaks; however, cooked PR had lower visual appeal and higher yield than PP or MX steaks. The overall acceptability, appearance, texture, and tenderness of cooked PR were less acceptable than that of PP and MX steaks and the eating quality of the PR steaks has ranked significantly lower than the others.

Particle size

An important factor affecting the texture of restructured meat products is their particle size. An increase in meat surface area and an increase in the availability of myofibrillar proteins for binding is the net consequence of comminution (Acton, 1972). The percentage of cookout significantly decreased as particle size became smaller (Chesney, Mandigo, & Campbell, 1978). Berry, Smith, and Secrist (1987) indicated that thickness of the flake particle was as important as width of the flake particle in affecting texture. Marriott, Phelps, Costello, and Graham (1986a) showed that particle size had a minimal effect on muscle cut resemblance, cooking loss, shear value, and sensory attributes.

The particle sizes of 2.5 and 5.0 mm in pre-rigor and aged beef steaks were studied by Seideman, Durland, Quenzer, and Michels (1982), and it was indicated that larger particle sizes were associated with lower texture desirability ratings and less tender beef steaks. Whereas Penfield, Swanson, Mitchell, and Dorko (1992) indicated that larger flake size (1.905 cm) and salt improved quality and acceptability of restructured reindeer steaks in comparison with smaller flake size (1.295 cm). Noble, Seideman, Quenzer, and Costello (1985) indicated that slice thickness had little impact on sensory properties and meat mixed for 5 or 10 min was tenderer than meat mixed for 15 or 20 min. Small, Claus, Wang, and Marriott (1995) investigated particle size and mixing time effects on sensory and physical properties of low-fat, high-moisture pork frankfurters, and indicated that changes in particle size affected measured characteristics more than changes in mixing time. Products obtained with a 2.0-mm plate compared to a 1.4-mm plate had higher hardness values and total energy to shear. Cooking loss, springiness, cohesiveness, or chewiness were not affected by particle size.

In the study of Sen and Karim (2003), the mutton meat was sliced to 7 cm length and 8 mm thickness (G1) and reduced in a meat mincer with an opening size of 20 mm (G2), 12 mm (G3), and 5 mm (G4). The results showed that purge loss percent was significantly lower in G3 and G4 than in G1, and restructured mutton steaks had greater shear force value in G3 and G4 (smaller particles) than G1 (sliced meat). In addition, a consumer panel showed similar preference for the products prepared by the methods of particle size reduction. Thus they concluded that cook yield of restructured mutton steakettes were significantly affected by meat particle size.

Cofrades et al. (2004) studied microstructure, texture, and sensory properties of precooked restructured beef made with different proportions of walnut (0, 5, 10, 15%) in relation to meat particle size (grinder plate hole: 0.6 and 1.4 cm). They indicated that increasing amounts of walnut were matched by decreasing Kramer shear force (KSF), bind strength, and elongation values. When walnut (5 and 15%) was added, products made with coarsely ground meat presented higher KSF values than those made with finely ground meat. Sensory properties of the products scored highest with 10% walnut in finely ground products and 5% walnut in coarsely ground products.

Mixing time

In a study to evaluate mixing time on the binding effect of restructured meat, Booren, Mandigo, Olson, and Jones (1982) found that there was a significant linear increase in binding strengths up to 12 min of mixing at 2°C. The cooking yields were increased with beef rolls by increasing the mixing time at 2°C, whereas binding strength was less affected (Pepper & Schmidt, 1975; Schmidt, 1986). For sectioned and formed beef steaks, binding effect increased and tenderness improved by increasing the mixing time from 8 to 16 min at 2°C (Booren, Jones, Mandigo, & Olson, 1981c). However, the results of Ahmed, Miller, Lyon, and Reagan (1989) indicated that the lamb shoulder roasts got less tenderness and more springiness scores with increased vacuum mixing time at 2°C.

Wiebe and Schmidt (1982a,b) reported increased binding strength of restructured beef steaks due to vacuum mixing at 3–5°C, although it might not justify the extra cost of the vacuum equipment. Ghavimi, Rogers, Althen, and Ammerman (1986) assessed vacuum, non-vacuum, or nitrogen back flush processing conditions at 1–3°C during tumbling of restructured cured beef and indicated that meat had higher cooked yields in a non-vacuum atmosphere; while tumbled in nitrogen atmospheres, meat had higher protein extraction values. The results of Sylvia (1992) showed that extended mixing of postminced batter at 2°C resulted in (numerically) higher hardness values in cooked frankfurters. Small et al. (1995) also found that additional mixing tended to increase springiness and chewiness of frankfurters although the mixing temperature was not mentioned. In general, few articles have been published on mixing time in relation to sensory and structure characteristics of restructured meat products in recent 10 years.

Meat type

Besides beef and pork, mutton and lamb were also used to produce restructured roast (Prasad, Field, Miller, Williams, & Riley, 1987). Paterson, Jones, Gee, Costello, and Romans (1987) found that bull meat was less prone to oxidative rancidity than steer meat, even though in the former there were higher cooking losses and shear values. Ensor, Sofos, and Schmidt (1990) concluded that the use of high-connective-tissue meat or addition of concentrated forms of connective-tissue in algin/calcium gel restructured meats could improve product texture and reduce formulation costs. Penfield et al. (1992) demonstrated the feasibility of producing restructured steaks from reindeer forequarters. Motzer, Carpenter, Reynolds, and Lyon (1998) successfully used pale, soft and exudative pork to manufacture restructured hams.

Fat content

The fat level clearly influenced the structure of the gel/emulsion network, as reflected by the differences in the type of protein molecular interactions involved in its formation, and this in turn affected the fat binding properties and the texture of the end product (Cofrades, Carballo, & Jimenez-Colmenero, 1997).

Petro et al. (1983) investigated restructured chunked and formed lamb roasts containing 10 or 30% mechanically separated lamb. In the reheated products there was little evidence of extensive lipid oxidation. Costello, Penfield, and Riemann (1985) found that when the microwave or broiled steaks were cooked at 73°C, cooking losses were greater for microwave-cooked steaks. They concluded that the problems of tenderness, juiciness, and off-flavors associated with grass-fed animals were minimized by the use of fat from grain-fed animals. Berry, Smith, and Secrist (1985) found that the steaks with higher fat levels (18 and 22%) were juicier, moister, and had greater mouth coating property than lower fat level (10 and 14%). In high fat formulation, cooked steak thickness was considered important owing to its effect on the sensory properties.

Carrapiso (2007) investigated the influence of fat content (5, 10, and 15%) of sausage on the release of 10 volatile compounds and found that increasing fat content caused a general increase in the volatile compound concentrations of the sausages (fat acted as a reservoir).

High pressure

High pressure processing (HPP, up to 1000 MPa) can influence meat protein conformation and induce protein denaturation, aggregation or gelation, depending upon the meat protein system, the pressure used, the temperature, and the duration of the pressure treatment. Although HPP on food system was first reported by Hite (1899) over 100 years ago, few studies were published until the 1980s because of technical difficulties and costs associated with HPP units and packaging of materials (Galazka, Dickinson, & Leward, 2000).

High pressure can modify the structure and function of meat proteins. For example, myosin from both meat and fish will be denatured by pressure and subsequently form a gel-like texture (Cheftel & Culioli, 1997). These structural changes will affect the texture of the muscle and induce binding effect.

Pressure-induced gelation of meat proteins depends upon the protein system and upon the HPP conditions (e.g. pressure level, time, and pressurizing temperature) (Jiménez Colmenero, 2002). Temperature is an important factor. Pressure and temperature are necessarily associated, given that the effect of temperature depends upon the pressure. Pressurization can either improve or reduce the effect of temperature in the system. The mechanism of protein denaturation differs based on the pressure/temperature combinations (Messens, Van Camp, & Huyghebaert, 1997). The studies that have been conducted on muscle protein gelation processes may be classified in several categories according to meat system conditions (raw or preheated) and pressure/temperature combinations (Jiménez Colmenero, 2002).

Farous and Zhang (2005) studied the effect of pressure at 1380, 4137, and 6895 kPa and meat homogenate (0, 2.5, 5, and 10%) on raw and cooked attributes of restructured beef steaks. They found that the bind strength of restructured steaks increased with increasing pressure and homogenate content. They concluded that by using pressure alone without binders the restructuring of beef may be achieved, and that by using pressure the use of meat homogenate tended to increase binding strength in restructured beef.

Iwasaki, Noshiroya, Saitoh, Okano, and Yamamoto (2006) found that the microstructure of pressure–heat-induced chicken myofibrillar gel was composed of three-dimensional fine strands. Pressurization at 200 MPa prior to heating increased the apparent elasticities of chicken myofibrillar gel and pork patty.

Role of non-meat ingredients

Salt

Salt not only plays an important role in processed meats but also demonstrates powerful function in restructured meats. The effect on color takes an additional importance because the restructured meat is a fresh meat system (Mandigo, 1988). Functionally, salt and phosphate helps to extract myofibrillar proteins which enhance binding of meat particles (Schmidt, 1986). In raw restructured beef steaks, the use of salt has been associated with discoloration (Chu, Huffman, Trout, & Egbert, 1987) and rancidity development (Andersen & Skibsted, 1991; Wheeler, Seideman, Davis, & Rolan, 1990). The amount of salt added is generally between 0.5 and 1.0% (Pearson & Gillett, 1996). Although some researchers indicated that generally when salt level decreases to below 2%, there is a negative effect on the functional and mechanical properties of meat products and 1.5% has been considered as the minimum salt level required for maintaining meat products without excessively decreasing functional and mechanical properties (Gómez-Guillén, Solas, & Montero 1997; Su, Bowers, & Zayas, 2000). In restructured steaks, the synergistic relationship between salt and phosphates has also been extensively studied (Mandigo, 1988).

An increased amount of myofibrillar proteins and greater binding were associated with the concentrations of salt (Booren et al., 1981b). In other products, the role of salt is well documented in restructured beef rolls (Pepper & Schmidt, 1975), and flaked and formed beef patties (Huffman, Cross, Campbell, & Cordray, 1981a). The results of Huffman, Ly, and Cordray (1981b) showed that addition of salt at all levels increased thiobarbituric acid (TBA) values and decreased color levels, but flavor, juiciness, and the textural properties were improved. Salt levels between 0.5 and 1.0% were recommended.

The potential use of chloride salts of various metals in restructured beef steaks was studied by Miller, Davis, Seideman, Ramsey, and Rolan (1986b). The results indicated that KCl and CaCl₂ were more desirable than NaCl. Marriott et al. (1986a) found that in relation to the reduction in color and flavor, the storage time and the presence of salt were more critical than the grade of salt.

Because salt directly relates to high blood pressure and other diseases, the consumer demand for a reduction in dietary sodium intake in the past several decades led to development of low salt content restructured meat products (Ruusunen & Puolanne, 2005). A variety of approaches to replacement or substitution of sodium chloride are available for meat processing, which included using transglutaminase (TG), KCl, Dietary fiber, and caseinate as salt replacer (Jiménez Colmenero, Ayo, & Carballo, 2005; Kuraishi, Sakamoto, Yamazaki, Susa, Kuhara, & Soeda 1997).

Phosphates

Phosphate enhances the effect of salt and with its addition, water binding capacity can be increased at a rather low salt content of about 1.5% NaCl (Knipe, Olson, & Rust, 1985). Miller, Davis, Seideman, Ramsey, and Rolan (1986a) found that in restructured beef steaks phosphate increased shear values, which was attributed to increased binding or cohesiveness. The study of Lamkey, Mandigo, & Calkins (1986) showed that the amount of oxidation was reduced by phosphate; adding phosphate to restructured beef steaks containing salt improved texture with no detrimental effects on color. Trout and Schmidt (1986) indicated that binding strength increased with increasing ionic strength. They found as the chain length of the phosphate increased, the extent of the synergistic effect between the salt and the phosphates decreased linearly. Marriott, Korzon, Boling, and Graham (1986b) found that storage time affected the color and other visual traits more than the adjuncts in frozen storage tests. Miller, Davis, Seideman, Ramsey, and Rolan (1986a) studied seven blends of restructured beef steaks, with different level of phosphate salts. They found the shear force values of steaks made with STP were the highest among all other restructured steaks, indicating better binding.

A variety of phosphates in different combinations, concentrations, and with concomitant salt concentrations were evaluated by Trout and Schmidt (1984). They found that tetrasodium pyrophosphate had the greatest binding effectiveness, which was followed by sodium tetrapolyphosphate, and then sodium hexametaphosphate. They concluded that most of the changes in binding could be explained by the ionic concentration of the phosphates. STP also delays development of rancidity and is added at a level of about 0.25% for adequate protein extraction and flavor development (Pearson & Gillett, 1996). Nielsen, Peterson, and Møller (1995) observed optimum effects of STP on the texture at a concentration of 0.2%.

Although phosphates play an important role in restructured meats to enhance water-binding capacity and reduce shrinkage, the presence of excessive amounts of phosphates in the diet may influence the calcium, iron, and magnesium balance in the human body, and can increase the risk of bone diseases (Shahidi & Synowiecki, 1997). Therefore, there is an increasing tendency for reducing phosphates content in meat products (Ruusunen et al., 2003).

In general, the role of salt and phosphates were extensively studied several decades ago and the interaction mechanism with meat protein is very clear, therefore they do not attract much attention recently.

Soy proteins

A variety of isolated proteins have been used as functional ingredients in restructured meat products, of which soy proteins are probably the most widely used (Pietrasik & Li-Chan, 2002a). Because of their superior nutritional and functional properties, soy proteins were widely researched and developed as meat and other food products' additives in last century. Soy proteins are utilized in processed meats because of the specific functionalities they are able to impart, for example, as binders to improve the product's yield and texture, as possible gelling agents to enhance the emulsion stability upon heating (Renkema & van Vliet, 2002), improve water absorption, and binding properties, have shown to have antioxidant activity (Pratt & Birac, 1979) which has been attributed to isoflavones and phenolic acids (Pratt, Pietro, Porter, & Giffee, 1981), and as a meat replacement to reduce the formulation costs (Chin, Keeton, Longnecker, & Lamkey, 1999).

Soy proteins have been widely used in restructured meat products and will not be discussed here intensively. The most recent research by Tsao, Kao, Hsieh, and Jiang (2002) indicated that the soy protein would have high potential in the production of binders for restructured meat products because of their high binding ability with muscle proteins. They also indicated that soy proteins with NaHSO₃ could be good substrates for transglutaminase (TG).

Transglutaminase

Transglutaminase (TG, glutaminyl-peptide:amine λ-glutamyltransferase, E.C. 2.3.2.13) is a widely distributed enzyme in nature. TG can modify proteins by catalyzing acyl transfer between a λ-carboxyamide of a peptide/protein bound glutamine and lysine forming an ε-(λ-glutamyl) lysine [ε-(λ-Glu)Lys] cross-link (Kuraishi, Yamazaki, & Susa, 2001). TG catalyzes conversion of soluble proteins to insoluble high-molecular-weight polymers through formation of covalent crosslinks (Motoki, Aso, Seguro, & Nino, 1987; Nino, Motoki, & Takinami, 1985). This cross-linking results in the polymerization of protein/peptide molecules with a subsequent increase in molecular mass. This enables TG have the potential to be a great binder in restructured meat products.

During the last decade microbial transglutaminase (MTG) has been used in the production of restructured meat products (Kolle & Savell, 2003; Kuraishi et al., 1997; Nielsen et al., 1995), chicken sausages (Muguruma et al., 2003), chicken döner kebab (Kilic, 2003), low-salt meat balls (Tseng, Liu, & Chen, 2000), and cooked ham (Müller, 2003).

In cooked restructured meat products, gel firmness and water-holding capacity (WHC) have been reported to increase by addition of TG in high-salt (2%) products but not in low-salt products (Pietrasik & Li-Chan, 2002b). TG was able to improve consistency (firmness) but not cooking loss of the product in a low-salt (1%) system (Dimitrakopoulou, Ambrosiadis, Zetou, & Bloukas, 2005).

Kuraishi et al. (1997) investigated the effect of salt on binding strength and indicated that provided there was addition of salt (NaCl), MTG treatment caused effective binding of meat pieces. Their result showed that an increase in binding strength caused by adding salt (1.0–3.0%) with MTG when compared to MTG alone. Therefore they concluded that MTG can be used in meat processing to improve rheological properties while reducing, or even eliminating, the need for salts.

Nielsen et al. (1995) indicated that binding of meat pieces containing 0.4% active enzyme of TG F XIIIa, 0.2% phosphate, and 1% salt showed a significant effect on the tensile strength. Nevertheless, when adding TG, color deterioration was observed. TG was also used in restructured fish, such as silver carp products (Ramírez, Uresti, Téllez, & Vázquez, 2002; Téllez-Luis, Uresti, Ramírez, & Vázquez, 2002).

In the study of Dimitrakopoulou et al. (2005), they found that TG level only affected the consistency and the overall acceptability and concluded that TG can be used at a level of 0.15% with reduced salt level (1%) and processing at 72°C/65 min to produce phosphate-free restructured cooked pork shoulder with acceptable sensory attributes.

Jiménez Colmenero et al. (2005) evaluated the effect of TG combined with caseinate, KCl, and dietary fiber as salt replacers in low-sodium frankfurter with added walnut and indicated that the combination of TG with caseinate, KCl, or fiber led to harder, springier, and chewier frankfurters with better water- and fat-binding properties (emulsion stability and cooking loss) than those made with TG only. Frankfurter with NaCl had a harder, springier, and chewier gel/emulsion network with lower cooking loss than those of NaCl-free.

In most studies, the MTG was allowed to act (reaction times vary from a few hours to around 24 h) (Kilic, 2003; Pietrasik, 2003; Pietrasik & Li-Chan, 2002a,b). The persistence of residual MTG activity after 24 h was assumed to create additional cross-linking reactions and therefore increase binding of meat particles (Carballo, Ayo, & Jimenez Colmenero, 2006).

Although most of the studies using MTG for restructuring meat conducted by incubation meat at optimum temperature (37–50°C) of MTG or by cooking to obtain sufficient binding strength, some researchers obtained good binding effect by using cold binding (2–5°C), with the combination of MTGase and sodium caseinate, without addition of salt or cooking (Kuraishi et al., 1997; Serrano, Cofrades & Jimenez Colmenero, 2004). Kuraishi et al. (1997) indicated that the MTG reaction condition of 5°C for 2 h would not enable any bacteria present to increase much and discoloration of the meat was not observed in the raw, refrigerated state. This MTGase/caseinate cold binding system appears to be more practical and useful than previously reported incubation binding systems because it could inhibit rapid growth of bacteria and keep fresh color of the restructured meat which can be distributed in the raw, chilled state. However, there are hardly any studies on how the time in chilled storage can affect the characteristics of raw and cooked restructured meat prepared with MTG as a cold-set binder (Carballo et al., 2006).

Other ingredients

Besides the ingredients mentioned above, numerous other ingredients, primarily proteins and polysaccharides have been used as binders or fillers to improve functional and nutritional quality of restructured meat products and this is the most important tendency in meat industry in recent years.

Alginate, a polysaccharide extracted from brown seaweed, was widely used in restructured meat products. Raharjo et al. (1994) found that trimmings restructured with Na-alginate/Ca-lactate had lower binding scores than veal trimmings restructured with salt/phosphate. κ-carrageenan (KC) is another polysaccharide also extracted from seaweed. Shand, Sofos, and Schmidt (1994) indicated that addition of KC to ham has resulted in good water binding, decreased cooking losses, and improved texture.

Besides alginate, Fibrimex, a blood-based binding system can be used for binding comminuted and large pieces of meat (Boles & Shand, 1998, 1999). The binding mechanism of restructured meats is based on the blood clotting action between fibrinogen, thrombin, and TG. Cross-linking and gelation between fibrin itself and between meat collagen and the fibrin are induced by TG (Sheard, 2002).

Other ingredients were also used to increase binding strength, nutritional value, or to improve flavor. These ingredients include vital wheat gluten (Hand, Crenwelge, & Terrell, 1981), crude myosin extract, surimi (Chen, Huffman, & Egbert, 1992), egg white powder, raw egg white, egg powder, bovine, porcine, lamb, broiler plasma powders, broiler breast meat powder, gelatine (Lu & Chen, 1999), dried apples, corn crumbs, mushrooms (Marriott, Graham, Schaffer, & Boling, 1986c), rice bran oil and fiber (Kim, Godber, & Prinaywiwatkul, 2000), and walnut (Jiménez Colmenero et al., 2003; Serrano et al., 2006), and most non-protein ingredients have detrimental effects on texture properties of restructured meats.

Protein binding

In the early research, some physical methods were employed to improve protein binding ability. Among the early reports on flaking and on the role it could play in restructured meat products, the study of Randall and Larmond (1977) showed improved binding and cohesive properties in flake-cut meats compared with ground meat cut at the same particle size. Besides physical methods, more and more binders have been applied in restructured meat production. Tinney, Miller, Ramsey, and Braden (1995) tested 11 texture modifying agents/binders for their binding characteristics in restructured hams. The results showed that the products containing starch, whey and yeast binders tended to get higher overall palatability scores. Tsai, Unklesbay, Unklesbay, and Clarke (1998) concluded that the textural characteristics were highly affected by binders, heating temperature, and their interaction. Téllez-Luis et al. (2002) found it is feasible to obtain low-salt restructured silver carp production with improved textural and functional properties using 3 g/kg TG and 10 g/kg NaCl as protein binders.

Cooking

Cooking is an important factor affecting the quality of restructured meat products. Arganosa, Godber, Tanchotikul, McMillin, and Shao (1991) found it was unexpected that at 100°C STP treated roasts had highest shear force, as others have found that phosphates reduced shear values in reheated whole-muscle beef roasts (Paterson & Parish, 1988). Because of the adverse effect that higher cooking temperature had on cook yield and moisture attributes, at higher cooking temperature a higher shear value was obtained (Arganosa et al., 1991). Arganosa et al. (1991) also indicated that STP was more effective on oxidation inhibition at higher cooking temperatures and soy protein isolate (SPI) at lower temperatures. In general, cooking was not drawing more attention in recent decades.

In addition, some technologies were also applied in restructured meats production, for example, low-frequency ultrasound could improve restructured beef rolls by causing disruption of muscle fibers, including separation of fibers to approximately 1 cm in depth (Vimini, Kemp, & Fox, 1983).