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Mechanisms of meat batter stabilization: A review a

b

A. Gordon , S. Barbut & Dr. Glenn Schmidt

c

a

Product Development Division, Grace Kennedy & Co. Ltd., 64 Harbour St., Kingston, Jamaica, W.I. b

Food Science Department, University of Guelph, Guelph, Ontario, N1G 2W1, Canada

c

Colorado State University, Dept. of Animal Science, Fort Collins, CO, 80523 Published online: 29 Sep 2009.

To cite this article: A. Gordon , S. Barbut & Dr. Glenn Schmidt (1992) Mechanisms of meat batter stabilization: A review, Critical Reviews in Food Science and Nutrition, 32:4, 299-332, DOI: 10.1080/10408399209527602 To link to this article: http://dx.doi.org/10.1080/10408399209527602

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Critical Reviews in Food Science and Nutrition, 32(4):299-332 (1992)

Mechanisms of Meat Batter Stabilization: A Review A. Gordon

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Product Development Division, Grace Kennedy & Co. Ltd., 64 Harbour St., Kingston, Jamaica, W.I.

S. Barbut Food Science Department, University of Guelph, Guelph, Ontario N1G 2W1, Canada Referee:

Dr. Glenn Schmidt, Colorado State University, Dept. of Animal Science, Fort Collins, CO 80523.

ABSTRACT: Comminuted meat products are a complex mixture of muscle tissue, solubilized proteins, fat, salt, and water. The two theories that have been presented to explain meat batters stabilization are reviewed. The emulsion theory explains stabilization by the formation of a protein film around fat globules, whereas the physical entrapment theory emphasizes the role of the protein matrix in holding the fat in place during chopping and subsequent heating. However, some aspects of stabilization cannot be explained adequately by either one of these theories. In this article the role of meat proteins, aqueous phase, and lipid phase are examined in light of past and recent research findings. KEY WORDS: meat batters, emulsion stability, muscle proteins, processed meats, protein gelation.

I. INTRODUCTION A. General Description of Meat Batters A meat batter can be described as a finely comminuted mixture of muscle proteins, fat particles, water, salt, and other ingredients that results in a fairly homogeneous meat product upon heat denaturation of the proteins (cooking). These meat products include frankfurters, bologna, vienna sausages, some meat loafs, and specialty items. Frankfurters and bologna are the most popular comminuted meat products. They are consumed at home and used by the food service industry, especially in fast food outlets.80 An idea of their popularity can be derived from the consumption figures for 1988; frankfurters (11.9%)

and bologna (16.4%) comprised 28.3% of the total federally inspected sausage production in the U.S. 163 Taken as a group, approximately 5 billion pounds of finely comminuted products are consumed annually in the U.S. alone. Consequently, these products are an integral part of our diet and have great economic importance. Finely comminuted meat products have a complex structure (Figure 1) and consist of numerous components that are held together by a variety of attractive forces.79 These components are combined to form what has been referred to as either a meat emulsion158 or a nonemulsion meat batter.101-131 A classic emulsion consists of two immiscible liquid phases, one of which is dispersed in the other in the form of a colloidal suspension.93 In meat batters, fat globules con-

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Muscle

Fiber

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Connective tissue Fiber FIGURE 1. Schematic illustration of a meat batter. (Adapted from Judge, M. D. et al., Principles of Meat Science, 2nd ed., Kendall/Hunt Publishing, Dubuque, IA, 1989, 146. With permission.)

stitute the dispersed phase,151 but are sometimes larger than the size required to form a true emulsion (dispersed particles not larger than 20 |xm).101

B. Preparation and Stabilization of Batter-Type Meat Products In the preparation of meat batters, salt is added to lean meat and comminuted to extract the myofibrillar proteins.12-109 Water is then added and comminution creates a protein-rich slurry capable of binding moisture and fat. The proteins form a film or membrane around fat globules that helps to stabilize the fat. Gums and other non-meat proteins can also be used with salt to increase binding and reduce cooking losses.30100 The nonmeat proteins can be either added directly with all the ingredients or may be chopped with the fat and water to form a preemulsified fat system.104135 On cooking, coagulation of proteins takes place, thereby immobilizing the fat, water, and other constituents.62146 This gives the characteristic texture to comminuted meat products. Consequently, the stabilization of fat and water within the system is important to the sensory acceptability of the product. Sodium chloride (NaCl) has been utilized in the production of finely comminuted sausages for centuries because it imparts desirable flavor and texture characteristics to meat products. However, the association of sodium with hyperten-

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sion87 has led to changing consumption patterns that have created a market for low-sodium products. Fat, traditionally an important part of frankfurter-type products, has also been linked to heart disease171 and atherosclerosis. Although the relationship between fat/salt and hypertension/heart disease is not a simple one, meat processors are faced with a growing demand for low-salt and low-fat meat products. In order to satisfy these consumer demands, changes in formulations and/ or processing procedures must be made. This is being hampered by insufficient knowledge of the interactions that result in stabilizing the complex mixture of components that comprise a meat batter. The mechanism by which batter-type meat products are stabilized has been studied extensively. Traditionally, the most widely accepted theory viewed these products as behaving like a "classic" oil-in-water emulsion. However, much of the recent research work has indicated that the gel-forming ability of the meat proteins is the factor of major significance.41101-131 Nevertheless, many aspects of the behavior of meat batters cannot be explained adequately by either of these two theories in their current form. Two main models have been proposed to explain fat stabilization in meat batters. The first favors the formation of an interfacial protein film around the fat globules that stabilizes them, whereas the second claims that the fat globules are physically entrapped within the protein

matrix.101 So far, the mechanism that is the most important in determining the overall properties of these meat products has not been determined precisely.24>30>136 Despite this, comminuted meat products are produced successfully around the world using a variety of chopping techniques.

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C. Preparing Meat Batters Theno and Schmidt158 showed that commercial frankfurters in the U.S. vary widely in their microstructure from a fine protein matrix structure to a very coarse matrix containing large fat globules and intact muscle fibers. The overall quality of the product is dependent on processing conditions such as chopping time, final chopping temperature, and equipment design. The effect of chopping temperature and time have received more attention than any of the other fact o r s 78,86,ioi.i6o However, studies on the effects of different mincing equipment on the microstructure and functional properties of comminuted products68 and the use of different chopping methods6 have also been presented. Processors like to be able to optimize their use of "classic" sausage ingredients or be able to substitute some of these with some of the nonmeat ingredients now available for use in the meat industry. These ingredients mainly include dairy proteins (i.e., whey), plant proteins (i.e., soy), hydrocolloid gums, starches,30 and fish protein in the form of surimi or surimi-based products.95 These ingredients can assist in improving the quality of comminuted meat products, help in increasing profitability and efficiency of production, and also reduce the chances of occasional losses due to product failure. Recently, much attention has been focused on the production of low-fat meat products to satisfy consumer demands. The technology that has been developed generally involves the use of carbohydrates such as maltodextrins, oat bran and fiber, iota carageenan, modified starches, and functional blends containing several ingredients. Blends based on non-meat proteins such as whey protein isolate (WPI), soy protein concentration (SPC), and soy protein isolate (SPI) have also been developed.45 There are currently several formulated ingredients on the market that, when

used as directed, can result in various degrees of fat replacement, generally by increasing water binding. These include Oatrim, Leanmaker™ and Prime-O-Lean™,45 which are beginning to find wide application in the meat-processing industry. Low-fat frankfurters have also been made with precooked carbohydrate and WPI filler gels and have shown good instrumental and sensory textural correlations with the regular product.66-181 However, the addition of precooked gel particles that can hold moisture and mimic the mouthfeel of fat is different from the addition of uncooked non-meat proteins that can physically participate in the formation of the IPF or contribute to matrix formation. Overall, in order to increase its market share in an increasingly competitive market, the meat industry must be able to manipulate its production processes to address those needs.16137 A better understanding of the mechanisms by which comminuted products are formed and stabilized will assist in this endeavor. This review will first discuss the two major theories regarding meat batter stabilization. The role of the meat proteins (specifically myofibrillar proteins) within the matrix, aqueous phase, and lipid phase will be examined, together with the role of salt in meat batter manufacturing. Finally, the two theories will be examined in light of past and recent research findings that include studies involving chemical modification of muscle proteins and different processing conditions.

II. DISCUSSION A. Current Theories Over the last 20 to 30 years, researchers involved in meat batter research have developed two main theories to explain the mechanisms involved in comminuted meat product stabilization. These are the emulsion theory in which meat batters are basically described as an oil-in-water emulsion, and the physical entrapment theory, which suggests that the fat particles do not separate from the batters because of the forces exhibited by the gel matrix. The following discussion reviews the work done in this area and how it relates to the two theories. As will quickly

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become evident, this matter is quite complex because not only do commercial comminuted products exhibit different microstructures,158 they also are dealing with semiliquid material in the raw batter (i.e., a paste-like texture) that is transformed into a rigid gel upon cooking.

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1. The Emulsion Theory Evidence for the involvement of myofibrillar proteins in film formation came from early studies.67-152 Hansen67 suggested that the salt-soluble proteins are drawn to, and concentrated at, fat globule surfaces, thus forming a stabilizing membrane in the raw state. Borchert et al.19 showed that the interfacial protein film (IPF) was also present in cooked meat batters and discovered the existence of small holes or "pores" in the protein film of the cooked product. Jones and Mandigo78 studied the occurrence of these pores by scanning electron microscopy (SEM). They found a large number of pores in the protein envelope surrounding larger fat globules and several small fat droplets in the vicinity of these pores. They therefore proposed that the pores act as "pressure release valves" that allow the thermal expansion of fat during cooking without disrupting the stabilizing protein film (Figure 2). The myofibrillar proteins involved in IPF formation begin to denature at 43°C.176-177 However, meat batters are cooked to 68 to 70°C, and in this temperature range the encapsulated fat would be expanding, while the surrounding protein membrane is in a semirigid state. The creation of weak spots in the protein film, resulting in the formation of pores that would allow the release of small amounts of fat while maintaining the integrity of the film against internal pressure increase is, therefore, a likely occurrence.78 Three major factors contribute to fat stabilization in meat batters: the biophysical properties of the IPF, the gelation properties of the protein matrix, and the physical characteristics and cell integrity of the fat.79 The gelation properties of the protein matrix appear to play a major role in fat stabilization. However, interactions between the encapsulated fat droplets, matrix proteins, and water also strongly influence the stability of the system.79 The role of the protein envelope in

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FIGURE 2. A proposed schematic illustration of the "pressure release" mechanisms in fat globules during cooking. (From Jones, K. W. and Mandigo, Ft. W., J. FoodSci., 47, 1930, 1982. With permission.)

mediating the lipid-protein interactions that stabilize these systems is therefore of importance. Besides the myofibrillar proteins, other proteins such as the sarcoplasmic proteins and collagen are also present in meat systems and may interact with the other components of the batter to influence the stability of the system. Sarcoplasmic proteins include pigments, enzymes, and other proteins required for normal cellular functions. Very little is known about the contribution of this group of proteins to the stabilization of meat batters.136 However, they are known to contribute to the T2 peak in the thermal denaturation profile of myofibrillar systems148176 but do not form rigid gels when heated under conditions similar to those extant in commercial meat processing; a floe is formed by the sarcoplasmic proteins under those conditions.136 Collagen denatures when heated to 65 to 67°C33 and forms gelatin. This leads to a softening of the texture of meat products in this temperate range, which is later superseded by the denaturation of actin at higher temperatures.7-107 In addition, high levels of collagen are thought to interfere with the adsorption of the myofibrillar proteins during IPF formation, resulting in batter

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failure due to the condition known as "short meat". 80 Ladwig et al.96-97 observed that high collagen levels resulted in increased cookout losses in meat batters, although the addition of phosphate was able to moderate this effect. Nevertheless, the exact mechanism by which collagen influences batter stability is not clear. The main protein involved in the formation of the IPF is myosin.40-152 Jones79 proposed that undernatured myosin formed a monomolecular layer around fat globules in uncooked emulsions to which other proteins are bound by various interactions (Figure 3). This is made possible because of a thin layer of oil on the surface of the fat globule, formed as a result of localized frictional heating during comminution. The myosin molecule is oriented at the interface with the heavy meromyosin (HMM) head toward the hydrophobic phase and the light meromyosin (LMM) tail protruding into the aqueous phase.79 The relatively high surface hydrophobicity in the HMM SI fragment of myosin20 supports this theory. The emulsification of oil by protein can be explained on the basis of protein hydrophobicity.81 Kato et al.82 correlated surface hydrophobicity to the emulsifying capacity of proteins.

OTHER PROTEIN MONOMOLECULAR LAYER FIGURE 3. Diagrammatic representation of events that may occur in the initial development of an interfacial protein film in a raw sausage batter. The hydrophobic myosin head region is interacting with the fat. Subsequent protein-protein interactions likely thicken and strengthen the film. (Adapted from Jones, K. W., Proc. Annu. Recip. Meat Conf., 37, 52, 1984. With permission.)

Surface hydrophobicity is due to the hydrophobic groups exposed in the native protein. In a review on hydrophobicity, Nakai115 noted that good correlations with surface hydrophobicity were also found for emulsion stability and fat binding capacity. Therefore, the role of a myofibrillar protein-based IPF in fat binding must be considered if the mechanisms involved in stabilizing meat batters are to be fully understood. Interfacial film formation is an energy-driven process that reduces the free energy of the system, hence increasing its stability. The free energy of the system can be reduced by increasing its entropy. Hydrophobic interactions are almost entirely entropy driven.18 Water tends to form ordered clusters around nonpolar solutes, thereby decreasing entropy. Thus, when several hydrophobic molecules associate in aqueous solution, the ordering of water molecules around them decreases, hence entropy increases. This causes the reduction in free energy that is the driving force behind interfacial film formation. Galluzzo and Regenstein40^12 and Schut139 have used model systems to show that myosin is adsorbed to form a film during fat emulsification. Consequently, myosin appears to act as an emulsifier even in its native state and forms an interfacial film of defined viscoelastic and mechanical properties at the oil-in-water interface, thereby assisting in the stabilization of fat in the uncooked meat batter. The adsorbed IPF may present different morphologies associated with varying functionalities in meat batters. Using five chloride salt treatments, Gordon and Barbut51 have indicated that two basic types of surfaces exist on fat globules, smooth and rough, as was also described by Jones and Mandigo.78 These two types were found in all treatments (Figure 4); however, the globules with rough surfaces were more prevalent in the unstable meat batters such as the CaCL, treatment. Pores were consistently present in the protein film of rough globules and the rough areas of the protein envelope were often not evenly distributed around the fat globule. Some globules in all treatments had both smooth and rough coats and the pores appeared to be concentrated in the rougher sections of the protein coats.51 The rough areas appeared to represent points of continuity between the protein coats and the matrix (Figure 5). Barbut" also showed the presence of "rough" (wrinkled) surfaces in batters prepared with dif303

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ferent polyphosphates and NaCI and suggested that they were due to excessive fat loss from the globules during cooking. However, Gordon and BarbutSI subsequently have shown that globules that lost only some of their fat still remain round.

FIGURE 4. SEM of smooth- and rough-coated fat globules from a 2.5% NaCI cooked poultry meat batter: f, fat; m, protein matrix; p, pores (visible in the rough areas); b, point of fat globule binding to the matrix. (Bar = 1 urn.) (From Gordon, A. and Barbut, S.,Food Struct., 9, 77, 1990. With permission.)

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FIGURE 5. TEM of relatively stable protein membrane situated adjacent to an unstable globule with a disrupted membrane. From a cooked meat batter treated with 1.58% CaCI2. (Bar = 0.5 ji.m.) (From Gordon, A. and Barbut, S., Food Struct., 9, 77, 1990. With permission.)

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Gordon and Barbut51 also reported that the smooth fat globules were prevalent in the more stable batters and consisted of two subgroups: thickly coated globules with a few evenly distributed, tiny pores and globules with thin protein envelopes and larger pores. Generally, for smooth globules, it appeared that the smaller, round globules had a relatively thick protein coat with few or no pores, while larger, round globules were thinly coated and had several pores. The irregularly shaped large globules tended to be thickly coated and had a rough protein envelope (Figure 5). Lin and Zayas104 have also reported that large, irregularly shaped globules were thickly coated in frankfurters prepared with preemulsified fat. In the less stable batters (MgCl2 treatment, Figure 6), protein envelopes ranged from fairly thick (>0.05 |xm) to almost indiscernible (0.05). Both 2.5%

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TABLE 1 Proteins Extracted from Meat by Six Chloride Salt Treatments Protein content of components (mg/ml)**

Treatment*

Total protein (mg/ml)

157 kDa

240 kDa

260 kDa

300 kDa

425 kDa

520 kDa

1.5%NaCI (SD)

13.52d 0.12

0.55d 0.02

2.19" 0.18

1.67" 0.05

0.53' 0.04

7.05= 0.21

2.03° 0.06

2.5% NaCI (SD)

26.79a 0.22

0.86= 0.03

5.31° 0.38

1.83C 0.08

3.29a 0.15

9.05b 0.11

6.45a 0.39

1.35%MgCI2 (SD)

10.35* 0.20

0.86= 0.28

1.94° 0.26

1.866 0.15

1.17* 0.47

4.52d 0.20

0.00" 0.00

1.58%CaCI2 (SD)

4.55' 0.02

0.00° 0.00

1.74' 0.55

1.42' 0.25

1.41 = 0.32

0.00" 0.00

0.00" 0.00

3.19% KCI (SD)

19.30c 0.20

1.76b 0.12

2.35b 0.14

1.57" 0.38

1.39" 0.26

6.94= 0.31

5.29" 0.16

1.81% LiCI (SD)

25.78" 0.18

2.64a 0.13

2.31= 0.10

2.16* 0.48

2.19" 0.32

9.92a 0.91

6.57a 0.27

Note: a-f in the same column with different superscripts are significantly different p

Mechanisms of meat batter stabilization: a review.

Comminuted meat products are a complex mixture of muscle tissue, solubilized proteins, fat, salt, and water. The two theories that have been presented...
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