Egg Albumin Proteins

The albumins (eg, ovalbumin, ovotransferrin, ovomucoid) are among the most widely used proteins in food formulation, owing to their exceptional gelling and whipping properties. Their functional diversity is directly attributable to their susceptibility to a variety of dénaturants (69). Denaturation of albumins is a discrete phenomenon, the extent of which is dependent on temperature, pH, salt, and moisture content (69). When used in food formulations, egg protein gels that have been thermally set support and bind other ingredients within the matrix and contribute to the texture of the product (5,10). The extent of egg protein gelation (ie, denaturation) is influenced by the degree of oxidation of free sulfhydryls to form inter- or intramolecular disulfide linkages. For example, strong oxidizing agents, such as the metallic cations Fe3+ and Cu2+, in addition to potassium iodate, enhance gel formation. However, hydrogen peroxide and potassium bromate, comparatively weak oxidizing agents, have little influence on gel strength. At alkaline pH values (eg, 9-10), reduction of disulfide linkages may occur that could also affect gel strength.

Egg albumin proteins are often pasteurized to facilitate their later use in formulated food products. Pasteurization generally does not affect their functional properties. However, ovotransferrin is more prone than ovalbumin to thermal denaturation. Ovalbumin is most stable at pH7.0, whereas ovotransferrin is least stable to thermal denaturation at this pH (5,10,69). At pH 6.8 and 50°C, up to 50% of the ovotransferrin is denatured in 4 to 5 min. In the presence of metals (eg, iron and aluminum), however, conformation of this protein is stabilized by a chelation-type mechanism. This protective effect is the impetus for the addition of aluminum and adjustment of pH to 7.0 with lactic acid before pasteurization of egg whites. Alternative pasteurization processes rely on lower temperatures and the addition of antibacterial agents (eg, hydrogen peroxide) (69). The native conformation of ovomucoid is resistant to extremes of pH and temperature. Prolonged exposure of ovomucoid to temperatures of 100°C does not alter its physicochemical properties. Its stability is attributed largely to intramolecular disulfide linkages (70). Denaturation of ovomucoid is suggested to be a three-stage process involving denaturation of each of three separate domains within the protein (71). If exposure to denaturing conditions is not prolonged, denaturation of ovomucoid is reversible (72).

Surface tension, important in such functional properties as foaming and emulsification, of egg and other proteins decreases markedly as denaturation proceeds. Thermal denaturation without coagulation improves surface properties (Table 1). This again implicates, as with meat proteins, the limited importance of solubility for functional (eg, surface) properties. Once sorbed at the interface, surface hydrophobicity plays a more dominant role in the foaming and emulsifying properties of egg proteins than does solubility.

Plant Proteins The impetus for utilization and development of (new) plant protein resources has been twofold: (I) to supplement, simulate, and/or replace muscle protein systems, and (2) to meet the increasing nutritional requirements of an increasing Third World population. Detailed reviews concerning denaturation of plant proteins in relation to their functional properties and food applications have been published (1,73).

Soybeans have been used in food systems, owing to their excellent functional and nutritional qualities. The major globulins of soy protein are conglycinin (7S) and glycinin

(11S). On heating, the subunits of both of these proteins can dissociate and reassociate in different ways. Formation of soluble aggregates of soy protein and the existence of sol, progel, gel, and metasol states and the roles of different forces in their formation have been discussed (74). The 11S globulin oligomer appears to undergo only minor changes on heating, retaining its quaternary structure and undergoing little obvious denaturation during formation of a gel when heated to 100°C (75). The importance of the overall hydrophobicity of unfolded proteins, rather than the surface hydrophobicity of undenatured proteins, for thermal functional properties has been noted for various proteins. High-heat treatment (121°C) of soy protein isolate at pH 5.5 led to a marked increase in overall hydrophobicity and a large reduction of solubility and emulsion stability (76). In contrast, the same heat treatment at pH 7.2 caused a moderate increase in overall hydrophobicity and a large increase in solubility and emulsifying properties. It is evident that environmental factors (eg, pH, heat) have a major impact on the functional properties of soy proteins.

Peanut and soy proteins that are heat denatured without any accompanying precipitation show enhanced foam-ability (77). Factors affecting foaming of these proteins include structural properties of the proteins per se (eg, flexibility, exposure of hydrophobic/hydrophilic groups, surface charge), ease of unfolding, and the Marangoni effect (ie, the ability to concentrate rapidly at a stress point). Also important are environmental factors such as temperature, pH, ionic strength, viscosity, and the presence of other components such as dénaturants that could affect the intrinsic properties of the proteins (53).

The unique ability of wheat flour to form a cohesive and viscoelastic paste or dough when mixed and kneaded is due primarily to the properties of the two classes of principal storage proteins, gliadins and glutenins. These are collectively referred to as gluten proteins. Glutenins are responsible for the elasticity, cohesiveness, and mixing tolerance of the dough, whereas gliadins facilitate fluidity, extensibility, and expansion of the dough, thus contributing to loaf volume. A proper balance of both gluten proteins is essential for bread making. During the mixing and kneading of hydrated wheat, the gluten proteins orient, align, and partially unfold (ie, denature). Protein unfolding enhances both hydrophobic interactions and the formation of disulfide cross-links through disulfide interchange reactions, resulting in establishment of a three-dimensional matrix that serves to entrap starch granules and other dough components. The cleavage of disulfide cross-links by reducing agents such as cysteine destroys the cohesive structure of the hydrated dough. The addition of oxidizing agents such as bromate increases toughness and elasticity by promoting disulfide linkage formation. In addition to the gluten proteins, soluble proteins (ie, albumins and globulins) that are found in minor quantities denature and aggregate to aid in gel formation, thus contributing to setting of the bread crumb (78).

Drying, roasting, and cooking are thermal treatments routinely used to process plant proteins. These processes can cause substantial protein denaturation. Oat globulin, the major protein fraction in oats, has a quaternary struc ture similar to that of soy 11S globulin (glycinin), a heat-coagulable protein. Differential scanning calorimetry showed that oat globulin, heated under conditions inducing gelation, was not extensively denatured and exhibited highly cooperative transition characteristics (79). When 1% oat globulin was heated, aggregation and precipitation occurred. Ultraviolet and fluorescence spectra of soluble and insoluble fractions indicated no marked protein unfolding in the former fraction, but extensive denaturation in the insoluble aggregates. The insoluble fraction had significantly higher surface hydrophobicity than the soluble fraction and unheated protein (80).

Sonication has been used as a means to solubilize plant proteins and isolates/meals. Heat-treated, acid-precipitated soy proteins, intermediates in the commercial production of isolated proteins from defatted soybeans, have been dispersed by sonication (81). Changes in flow properties may have been derived from ultrasonic-induced dissociation of protein aggregates formed during thermal treatment. This was considered to be associated with partial cleavage of intermolecular hydrophobic interactions, but not cleavage of peptide or disulfide bonds. It was postulated that ultrasonically exposed hydrophobic regions are subsequently buried through rearrangement of the molecular structure, conferring greater hydrophilicity to the protein (81). However, it has been shown that sonication leads to agglomeration and aggregation, particularly of the 7S fraction (82). The ultrasonic action may have (1) promoted hydrophobic interactions between globular proteins; (2) induced formation of complex mixtures, as in the case of apolipoproteins; or (3) altered the equilibrium protein-protein and/or protein-lipid interactions, thereby favoring the formation of a cluster-type structure.

Thermoplastic extrusion technology has been used to texturize many plant proteins, especially soy, to produce fibrous structures that simulate meatlike products (83). The process begins with moistened, defatted soy flour, which is fed into an extruder where it is worked and heated, causing the protein molecules to denature and form new cross-linkages that result in a fibrous structure. The heated plasticized mass is forced through a die to form expanded texturized strands of vegetable proteins that have meatlike characteristics on rehydration (84). Extrusion of plant proteins has been reviewed (46).

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