Effect of film ageing on the surface properties of lactoglobulin and lactoglobulin+sucrose stearate monolayers

Combined passive and active microrheology study of protein-layer formation at an air-water interface

We investigate the mechanical properties of layers of the protein beta-lactoglobulin during their formation at the air-water interface using a combination of passive and active microrheological techniques. The passive microrheology, which employs multiple particle tracking measurements using spherical colloids, indicates that the interfacial rheology evolves over time through three stages as protein adsorbs at the interface: (i) an increase in viscosity, (ii) a period of spatial heterogeneity in which the interface contains elastic and viscous regions, and (iii) the development of a uniformly rigid elastic film. Varying solution pH between pH = 5.2, the isoelectric point of beta-lactoglobulin, and pH = 7.0 has no qualitative effect on this mechanical evolution. The active microrheology, which employs ferromagnetic nanowires rotating in response to magnetic torques, similarly shows an increasing interfacial viscosity at early times and evidence of mechanical heterogeneity at intermediate times. However, at late times, the nanowire mobility becomes strongly pH dependent. For pH = 5.2, the layer responds as a rigid elastic film to the stress imposed by the wire. For pH = 7.0, it displays a viscous response that contrasts with the passive measurements. We associate this contrast with a nonlinear response to the wire at late times that reflects a low yield stress of the film at higher pH. This ability to compare passive and active measurements demonstrates the advantage of applying multiple microrheological methods to resolve ambiguity in any single approach.

Morphological changes in adsorbed protein films at the oil-water interface subjected to compression, expansion, and heat processing

Adsorbed films of milk proteins at the oil-water (O-W) interface have been imaged using a Brewster angle microscope (BAM). Special adaptations were made to the BAM to allow imaging of the O-W interface and to enable in situ heating and cooling of the adsorbed films. The proteins beta-lactoglobulin (beta-L) and alphas1-, beta-, and kappa-casein were studied over a range of bulk protein concentrations (Cb) and surface ages at pH 7 and for beta-L at pH 5 also. The adsorbed films were subjected to incremental compression and expansion cycles, such that the film area was typically varied between 125% and 50% of the original film area, and the resulting film structure was recorded via the BAM at 25.0 degrees C. Structuring of beta-L films (the formation of ridges and cracks) was more pronounced at pH 5 (closer to the protein's isoelectric point) than at pH 7 and for longer adsorption times and/or higher Cb. Structuring was also much more apparent at the O-W interface than at the A-W interface on compression/expansion/aging, especially at pH 7. After heating beta-L films adsorbed at low Cb (0.005 wt %) to 80 or 90 degrees C, an even greater degree of film structuring was evident, but beta-L films adsorbed at higher Cb (> or =0.05 wt %) showed fewer but larger fractures. The adsorbed caseins showed little evidence of such features, either before or after heating, apart from slight structuring for the heated films of alphas1- and kappa-casein films after 1 day. Changes in the dilatational elastic modulus of the beta-L films (Cb = 0.005 wt %) were correlated with the variations in the structural integrity of the films as observed via the BAM technique. In particular, there was a marked increase in the elastic modulus on heating, while the cycle of compression and expansion appeared to result in a net film weakening overall. The beta-L films adsorbed at higher Cb (> or =0.05 wt %) behaved as if an even stronger elastic skin completely covered the interface. The overall conclusion is that interfacial protein films subjected to these types of thermal and mechanical perturbations, which are typical of those that occur in food colloid processing, can become highly inhomogeneous, depending on the type of protein and the bulk solution conditions. This undoubtedly has implications for the stability of the corresponding emulsions and foams.

Structural Characteristics of Hydrolysates of Proteins from Extracted Sunflower Flour at the Air−Water Interface

The structural and topographical characteristics of a sunflower protein isolate (SPI) and its hydrolysates at different degrees of hydrolysis (DH = 5.62%, 23.5%, and 46.3%) spread at the air-water interface at pH 7 and 20 degrees C were determined from pi-A isotherms coupled with Brewster angle microscopy (BAM). The structural characteristics of SP hydrolysate spread monolayers depend on the degree of hydrolysis. We observed a significant shift of the pi-A(APPARENT) isotherms toward lower molecular areas as the degree of hydrolysis (DH) increased. This phenomenon was attributed to spreading of the protein at the interface, especially at DH 46.3%. A change in the monolayer structure was observed at a surface pressure of 12-15 mN/m. At a microscopic level, the heterogeneous monolayer structures visualized near the monolayer collapse and during the monolayer expansion proved the existence of large regions of protein aggregates. Reflectivity increased with surface pressure and was a maximum at the monolayer collapse. The monolayer thickness decreased as the degree of hydrolysis increased. These phenomena explain the poor functional properties for the formation and stabilization of a dispersion (emulsion or foam) of protein hydrolysates at high degrees of hydrolysis.

Thermodynamic and functional properties of legumin (11S globulin from Vicia faba) in the presence of small-molecule surfactants: effect of temperature and pH

We report on the effect of a set of water-dispersible small-molecule surfactants (the main and the longest-hydrocarbon components of which are a citric acid ester of monostearate, a sodium salt of stearol-lactoyl lactic acid, and a polyglycerol ester of stearic acid) on molecular, thermodynamic, and functional properties of the major storage protein of broad beans (Vicia faba) legumin in different molecular states (native, heated, and acid-denatured). The interaction between legumin and the surfactants has been characterized by a combination of thermodynamic methods, namely, mixing calorimetry and multiangle laser static and dynamic light scattering. It was found that hydrogen bonds, electrostatic interactions, and hydrophobic contacts provided a basis for the interactions between the surfactants and both the native and the denatured protein in aqueous medium. Intensive association of the protein molecules in a bulk aqueous medium in the presence of the surfactants was revealed by static and dynamic laser light scattering. In consequence of this, both the surface activity and the gel-forming ability of legumin increased markedly, which has been shown by tensiometry, estimation of protein foaming capacity, and steady-state viscometry. A likely molecular mechanism underlying the effects of small-molecule surfactants on legumin structure-forming properties at the interface and in a bulk aqueous medium is discussed.

Protein–lipid interactions at the oil–water interface

The distribution of proteins and lipids in food emulsions and foams is determined by competitive and cooperative adsorption between the two types of emulsifiers at the fluid-fluid interfaces, and by the nature of protein-lipid interactions, both at the interface and in the bulk phase. The existence of protein-lipid interactions can have a pronounced impact on the surface rheological properties of these systems. Therefore, these results are of practical importance for food emulsion formulation, texture, and stability. In this study, the existence of protein-lipid interactions at the interface was determined by surface dynamic properties (interfacial tension and surface dilational modulus). Systematic experimental data on surface dynamic properties, as a function of time and at long-term adsorption, for protein (whey protein isolate (WPI)), lipids (monoglycerides), and protein-lipid mixed films at the oil-water interface were measured in an automated drop tensiometer. The dynamic behaviour of protein+lipid mixed films depends on the adsorption time, the lipid and the protein/lipid ratio in a rather complicated manner. The protein determined the interfacial characteristics of the mixed film as the protein at WPI>/=10(-2)% wt/wt saturated the film, no matter what the concentration of the lipid. However, there exists a competitive or cooperative adsorption of the emulsifier (WPI and monoglycerides), as the concentration of protein in the bulk phase is far lower than that for interfacial saturation.

Dilatational rheology and foaming properties of sucrose monoesters in the presence of β-lactoglobulin

The foaming properties and the dilatational rheology of systems containing purified sucrose caprylate (SM800), caprate (SM1000), laurate (SM1200) and palmitate (SM1600) have been studied. Addition of beta-lactoglobulin (beta-lg) at a low concentration (0.050 wt.%) can aid the foam formation in the cases, where the surfactant concentration is insufficient to support foam formation. However, foams where both species co-existed exhibited poor stability. beta-lg was found to affect the dilatational properties of surfactant films even at low concentrations. It is thought that this could be related to the effect of the protein on the adsorption-desorption relaxation mechanism, or to the possible formation of a protein-surfactant complex in the bulk. The age of the protein film was also found to affect the kinetics of protein displacement by SM1000, as monitored by the change in the dilatational properties (Langmuir trough technique) and the relative reflectivity of the interface (Brewster angle microscopy) with time. An insoluble monolayer of sucrose stearate (Suc18) and beta-lg was also studied and it was found that the presence of small amounts of Suc18 in the protein film lead to a reduction of the interfacial elasticity. This is believed to be due to the disruption of the protein network. A possible mechanism could involve the obstruction of the hydrogen-bond intermolecular protein association by the strongly hydrated sucrose headgroup or the obstruction of the protein-protein hydrophobic interactions by the formation of an interfacial protein-surfactant complex.

Surface Activity and Critical Aggregation Concentration of Pure Sugar Esters with Different Sugar Headgroups

We have studied the surface properties of a series of enzymatically synthesized sugar monoesters of xylose, galactose, sucrose, and lactose with different hydrophobic chain lengths (C12-C16) and purified, chemically synthesized sucrose esters that, unlike the enzymatically synthesized samples, contain a mixture of isomers. Data obtained have been compared with those for dodecanoic glucoside and maltoside acetals, and also a commercial sucrose myristate. Nearly all of the sugar esters studied brought about a significant reduction of the surface tension of water (to 31.0-43.0 mN m(-1)). A reduction in the critical aggregation concentration (CAC) of the surfactants with increasing carbon chain length was observed. Surfactants with more hydrophilic headgroups exhibited higher CAC, though this trend was moderated by the alkyl chain length. Comparing the chemically synthesized sucrose esters with their enzymatically synthesized equivalents revealed only minor differences in the CAC and the surfactant efficiency, indicating that the exact point of esterification might not be critical for the surfactant's properties. The presence of 0.1 M NaCl, KCl, or CaCl(2) did not significantly alter the surface behavior of the chemically synthesized esters, indicating the absence of surface-active species with charged headgroups. Copyright 2000 Academic Press.