Limited coalescence and Ostwald ripening in emulsions stabilized by hydrophobin HFBII and milk proteins

Abstract

Hydrophobins are proteins isolated from filamentous fungi, which are excellent foam stabilizers, unlike most of the proteins. In the present study, we demonstrate that hydrophobin HFBII can also serve as excellent emulsion stabilizer. The HFBII adsorption layers at the oil/water interface solidify similarly to those at the air/water interface. The thinning of aqueous films sandwiched between two oil phases ends with the formation of a 6 nm thick protein bilayer, just as in the case of foam films, which results in strong adhesive interactions between the emulsion drops. The drop-size distribution in hydrophobin stabilized oil-in-water emulsions is investigated at various protein concentrations and oil volume fractions. The data analysis indicates that the emulsification occurs in the Kolmogorov regime or in the regime of limited coalescence, depending on the experimental conditions. The emulsions with HFBII are very stable-no changes in the drop-size distributions are observed after storage for 50 days. However, these emulsions are unstable upon stirring, when they are subjected to the action of shear stresses. This instability can be removed by covering the drops with a second adsorption layer from a conventional protein, like β-lactoglobulin. The HFBII surface layer is able to suppress the Ostwald ripening in the case when the disperse phase is oil that exhibits a pronounced solubility in water. Hence, the hydrophobin can be used to stabilize microcapsules of fragrances, flavors, colors or preservatives due to its dense adsorption layers that block the transfer of oil molecules.

Architectures and Mechanical Properties of Drugs and Complexes of Surface-Active Compounds at Air-Water and Oil-Water Interfaces

“…A particularly useful starting point for looking at therapeutic dispersion behavior and commercial-stability, therefore efficacy, has been to consider the behavior of protein (usually globular proteins) at air-water (A/W) and oil-water (O/W) surfaces or interfaces, respectively [25,26]. Globular proteins, such a bovine serum albumin (BSA), human serum albumin (HSA) and -lactoglobulin (LG), are widely studied across the board [27][28][29][30][31][32][33] by colloid scientists by virtue of their compact size, near ideal profile of the four classes of amino acids (aromatic-aliphatic, acidicbasic, sulfur-containing and polar), superb surfaceactive properties, non-toxic nature and ease of isolation from a biological source.…”

“…A convenient way of understanding the basics of protein adsorption at the interface is by evaluating the ease by which and time involved to achieve equilibrium surface tension values. A/W measurement are generally more convenient than O/W measurements using many forms of modern apparatus [28,32,34,40,41] and frequently having yielded similar data. In some approximate way, A/W measurements and O/W measurements double-up as 'air' can be viewed as a loose model of the oil-phase of emulsion by virtue of its dielectric constant, which is about half of most oils and nearly 80 times less than water [2] but with this distinct concept, as has always been soundly argued, lacking the increased density and viscosity may inhibit species inter-penetration into the oil-phase.…”

“…In some approximate way, A/W measurements and O/W measurements double-up as 'air' can be viewed as a loose model of the oil-phase of emulsion by virtue of its dielectric constant, which is about half of most oils and nearly 80 times less than water [2] but with this distinct concept, as has always been soundly argued, lacking the increased density and viscosity may inhibit species inter-penetration into the oil-phase. Recent studies have however indicated that liquid hydrocarbons may allow surfactant-like species considerable freedom to enter the oil phase unlike the gel phase, as fats used in SLNs [19,26,28,33,34] may constrain movement due to impenetrability.…”

“…Once adsorbed at the interface, inter-molecular interactions for the protein molecule of the type indicated in Fig. 2, such as ion-pair formation and entanglement [26,28,[43][44][45] and evidence of species linking, can serve to modify the mechanical properties of the adsorbed layer and its architecture.…”

“…The Part shows the ease of adsorbed layer disruption using the sharp-edge of an interfacial rheometer bicone to the two-dimensional "gelled" adsorbed layer formed by 67 kDa BSA, ovotransferrin and Tween20:BSA at a molar ratio of R = 0.9. The plot shows how the network formed by both protein only systems, resists the effect of shearing at low rates of deformation, largely as a result of inter-molecular interactions and crosslinking of material adsorbed in the plane of the interface [28,32,42], possibly compounded by the formation of multiple layers as seen with BSA on metal nanoparticles [29], and finally being broken at much higher shear rates [34,48,49]. The mix of protein and surfactant data is more complex in some sense, as initial deformation is permitted to much higher shear rates but at very high shear rate this appears to cause interfacial friction between "interfacial aggregates" and increase in measured shear stress.…”