A gel consists of a three-dimensional lattice of large molecules or aggregates capable of immobilizing solvent, solutes, and filling material. Food gels may be formed by proteins and polysaccharides that may participate in gel formation in the form of solutions, dispersions, micelles, or even in disrupted tissue structures, as in meat and fish products.
Generally gelation is a two-step phenomenon (Damodaran, 1989). The first step usually involves dissociation of the quaternary structure of the protein, followed by unfolding. In several proteins heating to about 40°C is sufficient, and some fish protein sols turn slowly into gels, even at 4°C. Preheating at 25–40°C, called “ashi”
or setting, is applied prior to cooking in manufacturing gelled, elastic fish meat products. During setting the endogenous transglutaminase leads to formation of cross-links between myosin heavy chains. In ovalbumin solutions gelling starts at 61–70°C. In the second step, at higher temperatures, the unfolded molecules rear-range and interact, initially usually with their hydrophobic fragments, forming a lattice. Ovalbumin gels increase in firmness when heated to about 85°C. Subsequent cooling generally stabilizes the gel structure. If the rate of the structuring stage is lower than that of denaturation, the unfolded molecules can rearrange and form an ordered lattice of a heat-reversible, translucent gel. Too rapid interactions in the denatured state lead to an irreversible coagulum, due to random associations to insoluble, large aggregates.
In the gel network there are zones, where the polymers interact, and large segments, where the macromolecules are randomly extended. The lattice is respon-sible for the elasticity and the textural strength of the product. In multicomponent gels all constituents may form separate or coupled networks, or else one component, not involved in network formation, may indirectly affect the gelling by steric exclu-sion of the active molecules. Such excluexclu-sion increases the concentration of the active component in the volume of the solution where the gel is formed. In gels made from the mince of squid meat at 1.5% NaCl, the added carrageenan and egg whites form separate networks that support the structure made of squid proteins, while added
starch fills the lattice and retains water (Gomez-Guillen et al., 1996). Proteins and polysaccharides that have opposite net charges, when in mixed solutions, may form different soluble and insoluble complexes held by ionic bonds. Lipid-filled milk protein gels containing small fat globules with a narrow particle size distribution have a smooth texture and a high shear modulus.
A three-dimensional network of partially unfolded molecules is also in protein-aceous films. These films are usually made by preparing a protein solution at pH values far from the pI, controlling denaturation of the molecules due to heating or shear, adding plasticizers, degassing, casting or extruding through a nozzle, and drying to evaporate the solvent.
7.3.4.2 Interactions of Components
The structure of gels depends on the components and the process parameters. Proteins containing over 30% hydrophobic residues form coagulum-type gels, e.g., hemoglobin and egg white albumin. The gelling-type proteins contain less hydrophobic residues and are represented by some soybean proteins, ovomucoid, and gelatin.
The interaction of different macromolecules may decrease the gel strength, may have no influence on the rheological properties of the gel, or else may have a synergistic effect. Casein micelles in a whey protein matrix may enhance or decrease the gelling, depending on pH. Heat coagulation of sarcoplasmic proteins impairs the gelation of actomyosin in gels made from the meat of pelagic fish.
The proteinase-catalyzed softening known as modori in minced heated fish products may be decreased by adding protease inhibitors from potato, bovine plasma, porcine plasma, or egg white. Inhibitors from various legume seeds are also effective against fish muscle proteinases (Benjakul et al., 2001a, 2001b; Matsumoto and Noguchi, 1992). The impact of other factors may be controlled by applying optimum processing parameters.
7.3.4.3 Binding Forces and Process Factors
The hydrophobic interactions prevail at higher temperatures and probably initiate the lattice formation, while hydrogen bonds increase the stability of the cooled system. The electrostatic interactions depend on pH, charge of the molecules, ionic strength, and divalent ions. Intermolecular -S-S- bridges, as well as covalent bonds formed due to the activity of transglutaminases, may also add to the gel formation.
In gelled fish products -S-S- bonding occurs during cooking at about 80°C (Hossain et al., 2001). Gels stabilized mainly at low temperatures by hydrogen bonding are heat reversible, i.e., they melt due to heating and can be set again by cooling, while gels stabilized by hydrophobic interactions and covalent bonds are heat stable.
Depending on the properties and concentration of the protein, ionic strength, and pH, even a coagulum-type gel like that of ovalbumin can be melted by repeated heating and set again when cooled (Shimizu et al., 1991). Heat-induced gels may melt under increased pressure at room temperature, while cold-set gels of gelatin are resistant to such conditions (Doi et al., 1991). Optimum ionic strength and concentration of Ca2+
is required for producing well-hydrated heat-set gels from whey proteins.
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There is generally a pH range at which the gel strength in the given system is highest. It depends on the nature of the polymers participating in cross-linking and increases with protein concentration. At the pI of the proteins, due to the lack of electrostatic repulsion, the rate of aggregation is usually high, leading to less-ordered, less-expanded, and less-hydrated gels. In heat-induced gels made of water-washed chicken breast minced muscle at a pH of 6.4 and a low NaCl concentration, the myofibrils, insoluble at such conditions, form local networks of aggregates, with large voids between them. Increasing the pH to 7.0 results in a gel with an evenly distributed network of myofibrils and an additional network of fine strands, smaller intramyofibrillar spaces, increased stress and strain values, and higher WHC (Feng and Hultin, 2001).
The ovalbumin gel has optimum rheological properties at pH 9, while at pH <
6 it is brittle and has low elasticity. Transparent ovalbumin gel can be made by heating at a pH other than the pI at a certain salt concentration. In a two-step procedure transparent gels can be made from ovalbumin, bovine serum albumin, and lysozyme over a broad range of salt concentration by heating the protein solutions first without salt and after cooling by repeated heating in the presence of added salt (Tani et al., 1993). The pH range for gelation of whey proteins is 2.5–9.5, although near the pI, which is about 5, the gels are opaque, coarse, and may turn into curd-like coagulum. In the neutral to alkaline pH the gels made of fat-free whey protein isolates or purified β-lactoglobulin are translucent, smooth, and elastic. In acid conditions, if high shear force is applied for a short time at denaturation temperature, aggregation of the whey proteins to microparticles occurs. This leads to well-hydrated gels of a smooth, nonelastic texture, similar to that of a fat emulsion.
In a slightly alkaline environment at a temperature of above 60°C, insoluble aggre-gates are formed due to denaturation of β-lactoglobulin, the major component of whey proteins. The rheological properties of whey protein gels, at different pH values, depend also on the concentration of Ca2+.
The -S-S- bridges are responsible for the thermal stability of gels. Such bonds add to the elasticity of heat-set whey protein gels at neutral to alkaline pH values, but not in acidic conditions when the thiol has low reactivity (Jost, 1993). Ascorbic acid improves the formation of heat-set gels of ovalbumin and fish proteins by undergoing rapid oxidation to dehydroascorbic acid, which affects polymerization by intermolecular -S-S- bridges. In making edible films from wheat gluten, the exposed SH groups of the protein, heat-denatured in an alkaline solution, form -S-S- cross-links due to air oxidation during drying (Roy et al., 1999).
7.3.4.4 Importance in Food Processing
Gelling is important for the quality of comminuted-type, cooked sausages and gelled fish products. The gel strength of such commodities is mainly affected by the properties of myosin and processing conditions. Comminuting of the meat with salt results in unfolding of the myosin microfibrils and increases the surface hydropho-bicity. This leads to hydrophobic associations in the lattice structure. Heating to 50–80°C favors deconformation of the myosin heads (Figure 7.5) and their interac-tions. Although myosin has the highest gel-forming ability of all muscle proteins,
the whole myofibrillar protein fraction, the sarcoplasm, and the connective tissue proteins are also capable of gelation. The overall gel strength depends on the concentration and interactions of different proteins.
Edible films may be used for their barrier properties to prevent the migration of water, oil, oxygen, and volatile aroma compounds between food and the environment.
The coatings prevent oxidative browning of sliced fruits and vegetables due to their antioxidant properties. The oxygen radicals’ scavenging capacity of films made of commercial concentrated whey protein powder has been found to be higher than that of coatings based on calcium caseinate. The addition of carboxymethyl cellulose to the formulation increases the antioxidant capacity of the products (Le Tien et al., 2001).
Films can also find application as enzyme supports and carriers of food ingredients.
For edible sachets used for delivering premeasured quantities of ingredients in food processing, the heat seal ability of the material is important. Antimicrobial agents added to edible coatings used for food protection may affect the mechanical strength and barrier properties of the material (Ko et al., 2001). Different films have unique functional properties best suited to fulfill the needs of specific food applications.
The films can be made either of proteins or of composite materials with saccharides or lipids. For these purposes, collagen, gelatin, casein, total milk proteins, whey proteins, wheat gluten, corn zein, water-soluble fish proteins, and soy proteins are used. The barrier properties, appearance, tensile strength, thermal stability, and heat seal ability of the products depend on the characteristics and proportions of the gelling components, interactions and contents of plasticizers, and conditions of fabrication.
Films made of whey protein are transparent, bland, and flexible; have very high oxygen, oil, and aroma barrier properties in an environment of low humidity; and are poor protection against water vapor migration. Some of their characteristics depend on the degree of thermal denaturation of the protein before casting. Increasing the degree of denaturation by heating leads to higher tensile strength and insolubility and to lower oxygen permeability of the films (Perez-Gago and Krochta, 2001). Water vapor per-meability may be decreased by adding lipids, either by laminating a lipid layer over a protein film or by uniformly dispersing the lipid in the proteinaceous component.
The result depends on the properties and amount of added lipid.
7.3.5 EMULSIFYING PROPERTIES
7.3.5.1 Principle
Proteins help to form and stabilize emulsions, i.e., dispersions of small liquid droplets in the continuous phase of an immiscible liquid. The decrease in the diameter of
‘
FIGURE 7.5 Schematic representation of the myosin molecule. (From Sikorski, Z.E., Pro-teins, in Chemical and Functional Properties of Food Components, Sikorski, Z.E., Ed., Technomic Publishing Co. Inc., PA, 1997. With permission.)
the droplets due to agitation exponentially increases the interfacial area. The work (W) required for the increase in surface area (∆A) can be decreased by lowering the surface tension (z): W = z ∆A, due to attachment of proteins to the droplets. The protein film around the lipid globules, with its electrostatic charge and steric hin-drance, prevents flocculation, i.e., formation of clusters of globules, and thus more rapid creaming due to the action of gravitational force: V = 2 r2 g ∆P/9 µ, where V is the velocity of the droplet, g is the gravitational force, ∆P the difference in density of both phases, µ the viscosity of the continuous phase, and r the radius of the droplet or cluster of droplets.
Stable films around the fat globules also prevent the coalescence of the dispersed phase, i.e., joining of the fat globules to form a continuous phase. Furthermore, soluble proteins increase the viscosity of the dispersing phase, thus reducing the rate of creaming and coalescence.
The efficiency of proteins as emulsifiers depends on their surface hydrophobicity and charge, steric effects, elasticity and rigidity, and viscosity in solution. Globular proteins that have stable structures and are very hydrophilic are good emulsifiers only when unfolded. However, the emulsifying properties do not increase linearly with the hydrophobicity of the protein, as they depend on the hydrophile/lipophile balance (HLB), which is defined as: HLB = 20 Wh/Wt, where Wh is the weight of the hydrophilic groups and Wt is the total weight of the molecule.
The emulsifiers with HLB < 9 are regarded as hydrophobic; HLB = 11–20, hydrophilic; and HLB = 9–11, intermediate. There is an effect of protein solubility, as the molecules must be able to migrate to the surface of the fat globules. However, in comminuted sausage batters, in the presence of salt the insoluble proteins may also participate in the formation of fat dispersions. After a few minutes of homog-enization, about 90% of the initially insoluble meat proteins of the stroma can be found in the emulsion layer (Nakai and Li-Chan, 1988). The quantity of protein required for stabilization of an emulsion increases with the volume of the dispersed phase and with the decrease in diameter of the droplets. The concentration of proteins forming a monomolecular layer at the interface is of the order of 0.1 mg/m2, and the effective concentrations are in the range of 0.5–20 mg/m2. For a high rate of film formation, the required concentration of protein in the emulsion may be as high as 0.5–5%.
7.3.5.2 Factors Affecting Emulsification
The pH of the environment affects the emulsifying properties by changing the solubility and surface hydrophobicity of proteins, as well as the charge of the protective layer around the lipid globules. Ions alter the electrostatic interactions, conformation, and solubility of the proteins. However, in many foods, mainly comminuted meat batters, the concentration of NaCl is considerably high for sensory reasons. Thus small changes in the salt content within the accepted range may have no significant effect on the properties of proteins. Heating to about 40–60°C, causing partial unfolding of the protein structure without loss in solubility, may induce gelation of the protective layer, as well as decrease the viscosity of the continuous phase. Therefore moderate heating may improve the emulsifying properties of proteins.
7.3.5.3 Determination of Emulsifying Properties
Several procedures are used to determine the efficiency of proteins in emulsifying lipids and the stability that the proteins impart to the emulsions.
The emulsifying capacity is represented by the volume of oil (cm3) that is emulsified in a model system by 1 g of protein when oil is added continuously to a stirred aliquot of solution or dispersion of the tested protein. It is determined by measuring the quantity of oil at the point of phase inversion. The latter can be detected by a change in color, viscosity, or electrical resistance of the emulsion, or the power taken by the stirrer engine. The emulsifying capacity decreases with an increasing concentration of protein in the aqueous volume. It is affected by the parameters of emulsification, depending on the equipment, as well as by the prop-erties of the oil.
The emulsion stability is measured as the final volume of the emulsion after the initial volume has been centrifuged or standing for several hours at specified conditions.
It may also be determined as the quantity of oil or cream separated from the emulsion, or the time required for the emulsion to release a specified quantity of oil.