E. Allen Foegeding

The gelation of proteins

Publisher Summary This chapter describes the process of the gelation of proteins. Gelation is a phenomenon; therefore, its definition and that of its product, a gel, is dependent on the perspective of the observer and the technique(s) used to observe it. For example, gels may be defined by their ability to immobilize a liquid, their macromolecular structure, or their textural or rheological properties. The chapter reviews a variety of protein gel systems with a broad perspective stressing similarities between them, yet not without mentioning the important differences that make each unique. It describes the multicomponent protein gels. The rheological and optical properties of thermally irreversible gels are the outcome of two events. First, a change in protein structure is needed that permits protein–protein interactions. The subsequent aggregation process, which is highly influenced by the solvent environment, may produce the gel microstructure, which is responsible for the optical and rheological properties. The physical integrity of a gel is maintained by counterbalancing forces of attraction and repulsion among polymer molecules and between the polymer network and the surrounding solvent

Advances in modifying and understanding whey protein functionality

Abstract Whey protein ingredients are used for a variety of functional applications in the food industry. Each application requires one or several functional properties such as gelation, thermal stability, foam formation or emulsification. Whey protein ingredients can be designed for enhanced functional properties by altering the protein and non-protein composition, and/or modifying the proteins. Modifications of whey proteins based on enzymatic hydrolysis or heat-induced polymerization have a broad potential for designing functionality for specific applications. The effects of these modifications are demonstrated by discussing how they alter gelation and interfacial properties.

Factors that determine the fracture properties and microstructure of globular protein gels

An understanding of the chemical reactions and physical processes associated with fracture properties of gels provides a fundamental understanding of select mechanical properties associated with texture. Globular proteins form thermally induced gels that are classified as fine-stranded, mixed or particulate, based on the protein network appearance. The fundamental properties of true shear stress and true shear strain at fracture, used to describe the physical properties of gels, depend on the gel network. Type and amount of mineral salt in whey protein and β-lactoglobulin protein dispersions determines the type of thermally induced gel matrix that forms, and thus its fracture properties. A fine-stranded matrix is formed when protein suspensions contain monovalent cation chlorides, sodium sulfate or sodium phosphate at ionic strengths ≤0.1 mol/dm 3 . At ionic strengths >0.1 mol/dm 3 the matrix becomes mixed. The network appears as a combination of fine strands and spherical aggregates, and has high stress values and minimum strain values at fracture. Higher concentrations of monovalent cation salts cause the formation of particulate gels, which are high in stress and strain at fracture. The formation of a particulate matrix also occurs when protein suspensions contain low concentrations of divalent cation chloride salts or di-cationic 1,6-hexanediamine at pH 7.0. The divalent cation effect on β-lactoglobulin gelation is associated with minor changes in tertiary structure, increasing hydrophobicity and intermolecular aggregation. The type of matrix formed appears to be related to the dispersed or aggregated state of proteins prior to denaturation. Mixed and particulate matrices result from conditions which favor aggregation at temperatures which are much lower than the denaturation temperature. Therefore, general and protein-specific factors can affect the dispersibility of proteins and thereby determine the microstructure and fracture properties of globular protein gels.

Sensory and mechanical aspects of cheese texture

Producing high quality dairy products requires precise control over factors determining product appearance, flavor and texture. Food texture is analyzed by descriptive sensory analysis. This method uses terms that depict the textural sensations perceived from first bite through mastication and swallowing. One component of sensory texture is mechanical properties, which are determined by empirical or fundamental methods. However, if one wants to understand the molecular basis of texture, then fundamental tests are required. Fundamental rheological properties are linked to network models, such as those for rubber elasticity or filled gels. These models predict how network interactions will alter rheological properties, providing a link from molecular interactions to sensory texture. In general, sensory and rheological terms that relate to the overall firmness and resiliency of cheese are highly correlated. However, sensory terms that describe the breakdown pattern, adhesiveness and cohesiveness of cheese, are weakly, if at all, correlated with rheological properties.

Gelation properties of dispersions containing polymerized and native whey protein isolate

Whey protein polymers (WP-polymers) were prepared by heating whey protein isolate below the critical concentration for gelation at neutral pH and low salt conditions. The effects of WP-polymers and salt types (CaCl2 or NaCl) on rheological properties (large-strain and small-strain analysis), water holding properties, turbidity and microstructure of heat-induced whey protein isolate gels were investigated. Replacement of native whey protein isolate with WP-polymers increased fracture stress, fracture modulus, held water, and the translucency of gels. With both salt types, the addition of WP-polymers changed the gel structure from particulate to fine-stranded. However, the effect of WP-polymers on rheological properties was salt specific. Addition of 20‐100% WP-polymers in the presence of 10 mM CaCl2 caused a continued increase in fracture stress. In contrast, protein dispersions containing 30 mM NaCl did not form self-supporting gels when

Tenderization of beef with bacterial collagenase.

60% WP-polymers were added. Dispersions containing 200 mM NaCl formed self supporting gels at all levels of WP-polymer addition but fracture stresses for gels containing 20‐100% WP were similar. Dispersions containing 80% WP-polymers and 200 mM NaCl had lower gel points (time and temperature) than dispersions with 80% WP-polymers and 10 mM CaCl2. It appeared that CaCl2 was more effective in increasing gel fracture stress while NaCl was more effective in decreasing gelation time. Different gel properties may be prepared by altering the amount of WP-polymers and salt types. q 2001 Elsevier Science Ltd. All rights reserved.

Effects of anions on thermally induced whey protein isolate gels

The feasibility of using a purified collagenase produced by Clostridium histolyticum as a meat tenderizer was studied. Experiments were conducted with enzymes in model systems to compare collagenase with the currently used plant proteinases, papain, bromelain and ficin. Collagenase was shown to have a greater activity in hydrolyzing insoluble collagen than salt-soluble-protein (SSP) and highest activity between 40° and 60°C, with little to no activity above 60°C. Collagenase was added to raw steaks and steaks were placed in bags and cooked in a water bath to 6.5°C. Tenderness was evaluated by analyzing components of Warner-Bratzler shear-deformation curves. The results suggested that addition of NaCl or a combination of CaCl(2), NaCl and collagenase would cause equivalent tenderization. The lack of a significant tenderization due to collagenase could be related to a lack of sensitivity in the shear evaluation or an effect on the enzyme activity due to the meat environment.

Polyacrylamide gels as elastic models for food gels

Fundamental fracture, water-holding and microstructural properties of thermally induced whey protein isolate gels containing one of two divalent, stabilizing anions (sulfate and phosphate) or one monovalent, chaotropic anion (thiocyanate) were investigated. All of the salts produced similar overall trends in stress and strain; however, transition points were salt specific. At ionic strengths 0.09 ≤ μ ≤ 0.50, Na2SO4 and NaSCN produced strain values of very similar magnitudes. At low salt concentrations, gel matrices were fine-stranded and held water well. As salt concentrations increased, gel matrices became particulate and water-holding capacities decreased. Promotion of the particulate matrix formation followed the Hofmeister series (S42−>HPO42−>>>SCN−). The calcium chelating ability of phosphate was responsible for inhibiting particulate matrix formation. Optical density trends, reflecting stabilizing or chaotropic properties of each anion, indicated that differences between sulfate- and phosphate-containing gels could not be explained by Hofmeistertype stabilizing propensity.

Heat-induced phase behavior of β-lactoglobulin/polysaccharide mixtures

Abstract The physical properties of Polyacrylamide gels (10% w/v) were evaluated by dynamic oscillatory testing, stress relaxation and torsional fracture analysis. Gels had a linear relationship between shear stress and strain when deformed to fracture; thus small strain (storage modulus, G′) and large strain (fracture modulus, Gf) moduli were equivalent. Non-fracture and fracture tests showed that gels maintained an entropy (rubber) elastic response over all testing temperatures (10–80°C). Shear stress at fracture was independent of temperature, whereas shear strain at fracture decreased as temperature increased. This was associated with increased thermal motion strains decreasing the amount of mechanically induced strain required for fracture. Our results have shown that Polyacrylamide gels are a suitable elastic model for understanding the molecular mechanisms of food texture.

Effects of Caseins on Thermal Stability of Bovine β-Lactoglobulin

The influence of polysaccharides on the thermal stability of β-lactoglobulin at pH 6.8 was investigated regarding polysaccharide type, concentration and size. Two kinds of polysaccharides, sulfate-containing polysaccharides (carrageenans and dextran sulfate with different molecular mass) and neutral polysaccharides (dextran with different molecular mass), were investigated. At low ratios of sulfate-containing polysaccharide to β-lactoglobulin, heat-induced aggregation was decreased as shown by lower turbidity. Increasing the ratio induced a significant increase in turbidity, leading to segregative phase separation. Phase diagrams were established by centrifugation, chemical assays and visual observation for β-lactoglobulin/kappa-carrageenan and β-lactoglobulin/dextran sulfates. Significant phases (stable, separated and gel) were found, indicating the varieties of phase behavior and a strong competition between phase separation and gelation caused by thermal treatment. Moreover, gelation was reversible in β-lactoglobulin/dextran sulfate systems depending on the polysaccharide concentration.