Interfacial rheology of mixed food protein and surfactant adsorption layers with respect to emulsion and foam stability

Influence of the presence of monoglyceride on the interfacial properties of wheat gluten

BACKGROUND

The physical stability of several food systems depends strongly on their interfacial properties, which may be modified by adding proteins and low-molecular-weight surfactants to their formulation. This study deals with the possibility of using wheat gluten to alter the surface and interfacial properties of an aqueous system, considering the effects of protein concentration, pH and the presence of monostearin.

RESULTS

It was generally found that the surface tension decreased as the protein concentration increased, reaching a minimum value at 0.5 g kg(-1). The influence of protein concentration on surface tension was much greater than the effect of pH owing to the low ionic character of wheat gluten protein. At acidic and alkaline pH values the interfacial viscosity of the protein system underwent a significant increase with time. The addition of monostearin either promoted the displacement of protein molecules at the interface or generated an interfacial mixed film with surface tension values lower than those of both single components, depending on the pH.

CONCLUSION

The results obtained indicate that gluten can contribute to the stabilisation of air/water and oil/water interfaces in some food systems (emulsions, foams, etc.).

Interfacial shear rheology of protein-surfactant layers

The shear rheology of adsorbed or spread layers at air/liquid and liquid/liquid phase boundaries is relevant in a wide range of technical applications such as mass transfer, monolayers, foaming, emulsification, oil recovery, or high speed coating. Interfacial shear rheological properties can provide important information about interactions and molecular structure in the interfacial layer. A variety of measuring techniques have been proposed in the literature to measure interfacial shear rheological properties and have been applied to pure protein or mixed protein adsorption layers at air/water or oil/water interfaces. Such systems play for example an important role as stabilizers in foams and emulsions. The aim of this contribution is to give a literature overview of interfacial shear rheological studies of pure protein and protein/surfactant mixtures at liquid interfaces measured with different techniques. Techniques which utilize the damping of waves, spectroscopic or AFM techniques and all micro-rheological techniques will not discuss here.

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.

Whey protein soluble aggregates from heating with NaCl: physicochemical, interfacial, and foaming properties

Whey protein isolate was heat-treated at 85 degrees C for 15 min at pH ranging from 6.0 to 7.0 in the presence of NaCl in order to generate the highest possible amount of soluble aggregates before insolubility occurred. These whey protein soluble aggregates were characterized for composition, hydrodynamic diameter, apparent molecular weight, zeta-potential, surface hydrophobicity index, activated thiol group content, and microstructure. The adsorption kinetics and rheological properties (E', etad) of these soluble aggregates were probed at the air/water interface. In addition, the gas permeability of a single bubble stabilized by the whey protein soluble aggregates was determined. Finally, the foaming and foam-stabilizing properties of these aggregates were measured. The amount of whey protein soluble aggregates after heat treatment was increased from 75% to 95% from pH 6.0 to pH 7.0 by addition of 5 mM to 120 mM NaCl, respectively. These soluble aggregates involved major whey protein fractions and exhibited a maximum of activated thiol group content at pH > 6.6. The hydrodynamic radius and the surface hydrophobicity index of the soluble aggregates increased from pH 6.0 to 7.0, but the molecular weight and zeta-potential decreased. This loss of apparent density was clearly confirmed by microscopy as the soluble aggregates shifted from a spherical/compact structure at pH 6.0 to a more fibrillar/elongated structure at pH 7.0. Surface adsorption was faster for soluble aggregates formed at pH 6.8 and 7.0 in the presence of 100 and 120 mM NaCl, respectively. However, interfacial elasticity and viscosity measured at 0.01 Hz were similar from pH 6.0 to 7.0. Single bubble gas permeability significantly decreased for aggregates generated at pH > 6.6. Furthermore, these aggregates exhibited the highest foamability and foam liquid stability. Air bubble size within the foam was the lowest at pH 7.0. The coarsening exponent, alpha, fell within predicted values of 1/3 and 1/2, except for very dry foams where it was 1/5.

Spreading of monoglycerides onto beta-casein adsorbed film. Structural and dilatational characteristics

The effect of monoglycerides (monopalmitin and monoolein) on the structural, topographical, and dilatational characteristics of betacasein adsorbed film at the air-water interface has been analyzed by means of surface pressure (pi)-area (A) isotherms, Brewster angle microscopy (BAM), and surface dilatational rheology. The static and dynamic characteristics of the mixed films depend on the interfacial composition and the surface pressure. At surface pressures lower than that for the beta-casein collapse (at the equilibrium surface pressure of the protein, pi(e)(beta-casein)) a mixed film of beta-casein and monoglyceride may exist. At higher surface pressures the collapsed beta-casein is partially displaced from the interface by monoglycerides. However, beta-casein displacement by monoglycerides is not quantitative at the monoglyceride concentrations studied in this work. The protein displacement by a monoglyceride is higher for monopalmitin than for monoolein and for spread than for adsorbed films. The viscoelastic characteristics of the mixed films were dominated by the presence of beta-casein in the mixture. Even at the higher surface pressures (at pi > pi(e)(beta-casein)) the small amounts of beta-casein collapsed residues at the interface have a significant effect on the surface dilatational properties of the mixed films. The structural, topographical, and viscoelastic characteristics of the mixed films corroborate the fact that protein displacement for monoglycerides is higher for spread than for adsorbed mixed films.

Calcium-Induced Changes to the Molecular Conformation and Aggregate Structure of β-Casein at the Air−Water Interface

The influence of calcium on interactions of beta-casein at the air-water interface has been studied by several techniques, including interfacial rheology, atomic force microscopy (AFM), infrared reflectance-absorbance spectroscopy (IRRAS), and zeta potential measurements. In the absence of calcium, a weak interfacial gel forms after about 2.5 h. Also in the absence of calcium, the adsorbed beta-casein film exhibits some degree of both intra- and intermolecular structural organization. For example, IRRAS spectra show a measurable amount of alpha-helix content, and AFM images indicate the presence of interfacial aggregates with a characteristic lateral length scale of 20-30 nm, which we interpret as hemimicelles. Upon the addition of calcium, particularly at Ca:beta-casein molar ratios above approximately 5:1, a stronger interfacial gel forms more quickly; for example, the interfacial shear moduli increase twice as rapidly. Also under these conditions (5:1 Ca:beta-casein ratio) there is little evidence of structural organization; i.e., the alpha-helix peaks are very weak, and AFM images show a disordered, but continuous film, without distinct hemimicelles. On the basis of these findings, we hypothesize that calcium binding destabilizes the coupled intra- and intermolecular structural organization, and that the loss of organization permits more rapid interfacial gelation. These phenomena are characteristic of the air-water interface; they are not accompanied by analogous structural changes in bulk solution.

Shear Characteristics, Miscibility, and Topography of Sodium Caseinate-Monoglyceride Mixed Films at the Air−Water Interface

In this contribution, we are concerned with the study of structure, topography, and surface rheological characteristics (under shear conditions) of mixed sodium caseinate and monoglycerides (monopalmitin and monoolein) at the air/water interface. Combined surface chemistry (surface film balance and surface shear rheometry) and microscopy (Brewster angle microscopy, BAM) techniques have been applied in this study to mixtures of insoluble lipids and sodium caseinate spread at the air-water interface. At a macroscopic level, sodium caseinate and monoglycerides form an heterogeneous and practically immiscible monolayer at the air-water interface. The images from BAM show segregated protein and monoglyceride domains that have different topography. At surface pressures higher than that for the sodium caseinate collapse, this protein is displaced from the interface by monoglycerides. These results and those derived from interfacial shear rheology (at a macroscopic level) appear to support the idea that immiscibility and heterogeneity of these emulsifiers at the interface have important repercussions on the shear characteristics of the mixed films, with the alternating flow of segregated monoglyceride domains (of low surface shear viscosity, etas) and protein domains (of high etas) across the canal.

Structural, topographical, and shear characteristics of milk protein and monoglyceride monolayers spread at the air-water interface

In this contribution we are concerned with the study of structure, topography, and surface rheological characteristics under shear conditions of monoglyceride (monopalmitin and monoolein) and milk protein (beta-casein, kappa-casein, caseinate, and WPI) spread monolayers at the air-water interface. Combined surface chemistry (surface film balance and surface shear rheometry) and microscopy (Brewster angle microscopy: BAM) techniques have been applied in this study to pure emulsifiers (proteins and monoglycerides) spread at the air-water interface. To study the shear characteristics of spread films, a homemade canal viscometer was used. The experiments have demonstrated the sensitivity of the surface shear viscosity (eta(s)) of protein and monoglyceride films at the air-water interface, as a function of surface pressure (or surface density). The surface shear viscosity was higher for proteins than for monoglycerides. In addition, eta(s) was higher for the globular WPI than for disordered beta-casein and caseinate due to the strong forces acting on spread globular proteins. This technique makes it possible to distinguish between beta-casein and caseinate spread films, with the higher eta(s) values for the later due to the presence of kappa-casein. The eta(s) value varies greatly with the surface pressure (or surface density). In general, the greater the surface pressure, the greater the values of eta(s). Finally, the eta(s) value is also sensitive to the monolayer structure, as was observed for monoglycerides with a rich structural polymorphism (i.e., monopalmitin).