Hydrophobicity Enhances the Formation of Protein-Stabilized Foams

Screening proteins for their potential use in foam applications is very laborious and time consuming. It would be beneficial if the foam properties could be predicted based on their molecular properties, but this is currently not possible. For protein-stabilized emulsions, a model was recently introduced to predict the emulsion properties from the protein molecular properties. Since the fundamental mechanisms for foam and emulsion formation are very similar, it is of interest to determine whether the link to molecular properties defined in that model is also applicable to foams. This study aims to link the exposed hydrophobicity with the foam ability and foam stability, using lysozyme variants with altered hydrophobicity, obtained from controlled heat treatment (77 °C for 0-120 min). To establish this link, the molecular characteristics, interfacial properties, and foam ability and stability (at different concentrations) were analysed. The increasing hydrophobicity resulted in an increased adsorption rate constant, and for concentrations in the protein-poor regime, the increasing hydrophobicity enhanced foam ability (i.e., interfacial area created). At higher relative exposed hydrophobicity (i.e., ~2-5 times higher than native lysozyme), the adsorption rate constant and foam ability became independent of hydrophobicity. The foam stability (i.e., foam collapse) was affected by the initial foam structure. In the protein-rich regime-with nearly identical foam structure-the hydrophobicity did not affect the foam stability. The link between exposed hydrophobicity and foam ability confirms the similarity between protein-stabilized foams and emulsions, and thereby indicates that the model proposed for emulsions can be used to predict foam properties in the future

Impact of Particle Sedimentation in Pendant Drop Tensiometry

Understanding the interface-stabilizing properties of surface-active components is key in designing stable macroscopic multiphase systems, such as emulsions and foams. When poorly soluble materials are used as an interface stabilizer, the insoluble material may sediment and interfere with the analysis of interfacial properties in pendant (or hanging) drop tensiometry. Here, the impact of sedimentation of particles on the interfacial properties determined by pendant drop tensiometry was evaluated using a model system of whey protein isolate and (non surface-active) glass beads (2.2-34.7 μm). Although the glass beads did not adsorb to the air-water interface, a 1% (w/w) glass bead solution appeared to decrease the surface tension by nearly 12 mN/m after 3 h. A similar effect was shown for a mixture of whey proteins and glass beads: the addition of 1% (w/w) of glass beads led to an apparent surface tension decrease of 31 mN/m rather than the 20 mN/m observed for pure whey proteins. These effects are attributed to the sedimentation of particles near the apex of the droplet, leading to droplet shape changes, which are interpreted as a decrease in surface tension using tensiometer software. The droplet density at the apex increases due to sedimentation, and this density increase is not accounted for when fitting the droplet shape with the Young-Laplace equation. The result is the observed apparent decrease in surface tension. In contrast to the significant impact of sedimenting material on the surface tension measurements, the impact on the results of oscillatory deformations was limited. These findings show that the impact of sedimentation should be considered when studying the interface-stabilizing properties of materials with reduced solubility, such as certain plant protein extracts. The presence of such particles should be carefully considered when conducting pendant drop tensiometry

Investigating the effect of temperature on the formation and stabilization of ovalbumin foams

The effect of temperature (below denaturation temperature) on protein foam formation and stabilization is potentially large, but has received little attention. This study aims to identify the effect of temperature (15–60 °C) on ovalbumin-stabilized foams at different concentrations (0.05–50 g L−1), and place this in a theoretical perspective. With increasing temperature the initial adsorption rate (dΠ/dt) increased logarithmically from 0.006 mN m−1 s−1 at 5 °C to 0.084 mN m−1 s−1 at 60 °C. A concentration increase resulted in a linear increase of dΠ/dt. This concentration effect was also observed in the foam ability, although the foam ability increased logarithmically rather than linearly with concentration, as expected based on theory and dΠ/dt. The foam ability was hardly affected by temperature (in contrast to theory and dΠ/dt). This was attributed to the strong decrease of foam stability with increasing temperature, which was expected based on theory. At elevated temperatures, the poor foam stability interferes with the foam ability (i.e. foam stability ≈ timescale of foam formation), a situation also happening at low concentrations. When formation was faster than destabilization, the foam ability relates to the effective adsorption rate. The effective adsorption rate includes the decrease in adsorption probability with increasing surface coverage. The observed balance between the effect of adsorption rate and foam stability on foam ability is not quantitatively predictable based on current theoretical models.</p

Relative contributions of charge and surface coverage on pH-induced flocculation of protein-stabilized emulsions

To predict the stability of protein-stabilized emulsions against flocculation under different conditions (pH and concentration), a quantitative description of the effect of the relevant factors is essential. Typically, pH is considered to affect the protein charge (viz. zeta potential) and thereby the interactions between the emulsion droplets. In this study, it is shown that emulsion flocculation is not only determined by the interactions between the droplets (pH/charge), but also by the surface coverage (pH and protein concentration). Two distinct regimes of flocculation were identified. At zeta potentials of |9-27|mV, emulsions were stable against flocculation if the protein concentration was sufficiently high to fully cover the interface (C>Ccr). At a lower zeta potential (i.e. below a critical zeta potential ζcr of |9|mV), flocculation occurred even at high concentrations. In this regime, flocculation below ζcr was reversible at C>Ccr, while it was partly irreversible at Ccr, indicating a type of bridging flocculation at Ccr. This shows that emulsion flocculation can be estimated based on the relevant parameters (ζ and protein radius including the association behaviour of the protein).</p

Acid-induced gels from soy and whey protein thermally-induced mixed aggregates : Rheology and microstructure

In this study, we explored how substituting whey protein isolate (WPI) with soy protein isolate (SPI) affects the linear and non-linear rheological behavior of acid-induced gels, and their microstructures. Commercial SPI and WPI dispersions (pH 7.0, 3.0 mS/cm) were preheated (95 °C, 30 min) at different protein concentrations (2%, 4%, 6%, and 8% w/w) and SPI: WPI ratios (0: 4, 1: 3, 2: 2, 3: 1 and 4: 0). The resultant thermally-induced aggregates were characterized before gelation was induced by glucono-δ-lactone (GDL). Small and large amplitude oscillatory shear (SAOS and LAOS) tests showed that replacing WPI with SPI decreased the strength (lower G′) and stretchability (lower γc) of acid-induced gels in the linear viscoelastic (LVE) regime. Gels containing SPI behaved more similar to pure SPI gels in the non-linear viscoelastic (NLVE) regime: displaying a relatively elastic response at large strain and a gradual transition to plastic behavior. The changes in rheological properties were explained by the differences in the gel microstructures, via fractal scaling theory, multiphoton laser scanning microscopy (MLSM) and scanning electron microscopy (SEM). WPI gels formed denser and homogenous gel networks with very strong inter-floc links, while hybrid gels and pure SPI gels formed coarser and more porous networks with intermediate inter-floc links. The constituent flocs in the latter were larger, with rougher, more elongated and branched structures. The present results provide useful information for future attempts to replace WPI with SPI in food products based on acid-induced gelation

Identification of critical concentrations determining foam ability and stability of β-lactoglobulin

To understand the properties of protein stabilized foam, quantitative parameters, such as the concentration dependence of the foam properties need to be determined. Recently, a concept was proposed that predicts the emulsifying ability (i.e. the droplet size in emulsions) based on different parameters, including the protein concentration. The aim of the present study is to investigate whether a similar concept can be applied to describe the foam ability and stability of protein stabilized foams. To achieve this, the foam, thin film and molecular properties of β-lactoglobulin (BLG) were determined at different concentrations and different pH values (pH 3-7). At each pH, a certain critical concentration for foam ability CFA, could be identified above which the set foam volume was reached, while below that value the set volume was not reached. Furthermore, for all pH another critical concentration (Ccrr32) at C > CFA was identified as the point where the bubble radius (measured at the end of foam formation) reached a minimal value. The foam ability increased with increasing pH (pH 3-7). The difference in foam ability as a function of pH was reflected in the adsorption rate (slope Π/t0.5 curve) of BLG. The foam stability increased with increasing concentration at each pH value but even in the protein rich regime where C > Ccrr32 different foam stabilities were observed, which were highest at pH 7.</p

Immunomodulatory properties of oat and barley β-glucan populations on bone marrow derived dendritic cells

Specific structures of oat and barley β(1,3)(1,4)-glucans induced different in vitro immunomodulatory effects in bone marrow derived dendritic cells (BMDC) from TLR2/4 knock out mice. All barley β-glucan fractions induced larger amounts of cytokines in BMDCs than their oat equivalents. The particulate fractions of both glucans induced high amounts of cytokines, especially after sample homogenisation. The small particulate barley β-glucans induced more cytokines than the equivalent oat fraction, hence there are more features influencing the immunomodulatory properties of β-glucans than only the particle size. The soluble glucan fraction and heated suspension induced only low amounts of cytokines, but were different for the two β-glucans, indicating that molecular specificity matters for immunomodulation. Immunomodulatory activity is influenced by the insolubility of β-glucans, to which characteristics as particle size, granule conformation and particulate homogeneity are related. Consequently, sample preparation influences the immunomodulatory activity of β-glucans.</p