Disulfide-Mediated Polymerization of Whey Proteins in Whey Protein Isolate-Stabilized Emulsions

Abstract

The effects of protein polymerization in whey protein isolate-stabilized emulsions on emulsion properties were investigated. Polymerization, involving intermolecular disulfide bonds between whey proteins adsorbed at the oil-water interface, increased with increasing storage time following emulsion formation. Ageing resulted in increased aggregation of emulsion droplets, emulsion viscosity and susceptibility to creaming but these effects were lower when thiol-disulfide interchange reactions were inhibited by N-ethylmaleimide (NEM). Following heating to 75°C, disulfide-mediated polymerization of whey proteins increased as did droplet aggregation, emulsion viscosity and creaming. While NEM lowered the extent of disulfidemediated polymerization it did not affect the measured physical properties of the heated emulsions. Non-covalent interactions appeared to be the principal forces leading to aggregation of emulsion droplets but aggregates once formed were stabilized by disulfide bonds. 127S. Damodaran (ed.), Food Proteins and Lipids

Protein-Stabilized Nanoemulsions and Emulsions: Comparison of Physicochemical Stability, Lipid Oxidation, and Lipase Digestibility

“…The conventional emulsion is likely to have a monolayer of adsorbed globular proteins formed during homogenization, whereas the nanoemulsion is likely to have a multilayer of adsorbed proteins. After adsorption, globular proteins tend to unfold and form covalent cross-links (disulfide bonds) with their nearest neighbors − . Consequently, this cross-linked protein layer may become crinkled when the droplets in the nanoemulsions shrink during ethyl acetate removal.…”

“…Visual observation of the nanoemulsions indicated that they remained fairly transparent at holding temperatures less than 70 °C but became increasingly turbid at higher temperatures (data not shown). The increase in droplet aggregation at high temperatures can be attributed to an increase in the surface hydrophobicity of the droplets due to protein unfolding, which leads to a strong hydrophobic attraction between the droplets , . The overall attractive force was then presumably strong enough to overcome the net repulsive force, such as steric and electrostatic forces.…”

Factors Influencing the Freeze‐Thaw Stability of Emulsion‐Based Foods

“…Whey proteins contain a large amount of sulfur‐containing amino acids that form disulfide bonds, giving globular proteins their compact shape. These disulfide bonds are usually broken when proteins unfold at a surface (“surface denaturation”) or due to heating (“thermal denaturation”), and may be involved in intra‐ or intermolecular interactions with other sulfhydryl groups (Monahan and others ). The thermal denaturation temperature of β‐lactoglobulin and α‐lactalbumin are fairly similar being around 65 to 85 °C, with the precise value depending on pH, salt concentration, cosolvents, and measurement method (Peltonenshalaby and Mangino ; Boye and Alli ; Fitzsimons and others ).…”

Generation and Stabilization of Whey-Based Monodisperse Nanoemulsions Using Ultra-High-Pressure Homogenization and Small Amphipathic Co-emulsifier Combinations

“…Monahan et al. reported polymerization involving intermolecular disulfide bonds between adsorbed whey proteins takes place at the oil–water interface leading to an increased level of aggregation of emulsion droplets . Additionally, at protein concentrations above the critical limit, it is possible that unadsorbed proteins in the aqueous dispersion could undergo conformational changes at high UHPs causing association and consequent aggregates that can contribute to the light scattering during size analysis .…”