Stability and bubble size of BSA foams by hydrocolloids with different physical properties

Hydrocolloids are widely employed to enhance the stability of protein-based foams, yet a systematic understanding of how their molecular properties govern foam behavior remains limited. In this study, we investigate the influence of three polysaccharides—guar gum, xanthan gum, and iota-carrageenan—on foams stabilized by bovine serum albumin (BSA). These hydrocolloids were deliberately selected for their distinct polarity, charge, and chain flexibility, enabling a controlled comparison of how specific polymer attributes affect the interfacial and bulk properties of foams. The results demonstrate that these molecular features produce reproducible and quantifiable changes in foam microstructure and stability, revealing clear structure–function relationships. This work provides a mechanistic insight into hydrocolloid–protein coupling at complex protein coated interfaces and establishes a systematic framework for the rational selection and design of polymer additives for foam stabilization in soft matter systems.

Protein-stabilized foams have gained increasing interest, especially in food physics (Hailing and Walstra, 1981; Foegeding et al., 2006; Murray, 2020). In contrast to simple surfactants, such as polar or ionic lipids (Dickinson, 2006; Wang and Yoon, 2006; Ho et al., 2023), proteins offer a large variety of surface activities, depending on their structure (Vilgis, 2015b)—for example, in different states of denaturation or at different pH values (Dachmann et al., 2020; Li and Girard, 2023; Zhang et al., 2023; 2024).

However, in realistic systems, competing multiple surfactants reduce foam stability due to their distinct interactions between polar and hydrophilic parts (Hunter et al., 2009). For example, in dairy-based phospholipid foams, various proteins and (solid) fat particles are included in the foam formation (Vilgis, 2015b); in plant-based foams, complex protein mixtures contribute to interfacial activity (Murray, 2020); even in simple egg whites, a large number of different proteins (Pu et al., 2023; Chen et al., 2019) provide limited foam stability. Apart from interfacial properties, which are expressed macroscopically in surface tensions, foam stability is affected by coalescence when bubbles merge as thin liquid films between them rupture. Foam stability is also limited by the gravitational drainage of liquid from the lamellae between bubbles (Etemad et al., 2022). Therefore, many food systems additionally contain polysaccharides as thickeners, which may additionally affect foam stability by increasing viscosity to slow down drainage and capillarity effects. However, different hydrocolloids—linear water soluble polymers—induce distinct effects due to their physical properties, mainly in their intrinsic stiffness and electrostatic stiffness, their polarity, and their ionic strength (Vilgis, 2015b; Glicksman, 2022; Williams and Phillips, 2021).

In food foams, thickening agents are mainly applied to enhance the viscosity of the solution and thus improve foam stability (Chen et al., 2014). Higher viscosities of the liquid continuous phase slow down drainage and bubble mobility, which are relevant processes for the destabilization of a foam (Wilde and Clark, 1996; Tesch and Schubert, 2002; Townsend and Nakai, 1983; Cox et al., 2009; Kováts et al., 2020), regardless of their distinct interactions with the emulsifiers. However, from a naive physical perspective rather than an applications-based viewpoint, the exploration of fundamental relationships in these foams is complicated by their inherent complexity. Furthermore, the limited knowledge surrounding the components in question hinders a molecular understanding of the processes involved in foam formation and stability. However, to enhance the stability of a foam, knowledge of its fundamental relationships is essential. The primary focus of this investigation is the impact of the physical characteristics of the hydrocolloids in question. To achieve this, a well-characterized protein, bovine serum albumin (BSA), is used as surfactant, with the hydrocolloids added according to their physical properties.

BSA is a well-characterized protein (Rosenoer et al., 1977; Pereira et al., 2003; Curvale et al., 2008; Vetri et al., 2007; Gulseren et al., 2007; Zhang et al., 2013; Li et al., 2016; Glaser et al., 2007; Jahanban-Esfahlan et al., 2019; Chiang et al., 2023), which is available in high purity. It originates from the blood plasma of bovines (Duarte et al., 1999) and is also found in cow’s milk (Litwińczuk et al., 2011). Its foam stabilizing ability has been studied in the context of solution pH (Kim and Kinsella, 1985; Engelhardt et al., 2012; Zawala et al., 2010). These studies focus on very low BSA concentrations and use the bubbling (sparging) method for foam formation. Whipping is often applied for foaming food systems (Wilde and Clark, 1996; Waniska and Kinsella, 1979) but requires higher protein concentrations (Waniska and Kinsella, 1979). At such concentrations, solution viscosities have been investigated (Yadav et al., 2011), although foam formation has not been performed.

The primary, secondary, and tertiary structures of BSA have been well investigated. BSA has a heart-shaped form with a molecular weight of approximately 66.5 kDa (Smilgies and Folta-Stogniew, 2015; Mylonas and Svergun, 2007; Akiyama, 2010). The primary structure contains 583 amino acids (607 amino acids including the signal peptide) (Bienert et al., 2017). The most important structural features of BSA are shown in Figure 1 for completeness. For example, Figure 1a shows the charge distribution at pH 7 of negatively and positively charged amino acids, which ensure the water solubility of BSA, while part b indicates the surface activity of hydrophilic (blue) and hydrophobic (different shades of red) amino acids, and part c shows the distribution of the amino acid cysteine. Spatially closely located cysteines indicate internal cross-linking by disulphide bridges of the protein.

The assembly of hydrophobic and hydrophilic amino acids at the surface of BSA renders it a surface-active molecule, thereby enabling it to act as an emulsifier in a liquid foam. Gräff et al. (2022) investigated protein stabilized foam films. They observed that the assemblies of BSA at the air–water interface demonstrate relative immobility.

The three different hydrocolloids chosen for investigation are xanthan gum (XG), guar gum (GG), and iota-carrageenan (IC), all non-starch polysaccharides. The reason for this choice lies in their distinct and different physical properties. XG molecules are distinguished by their high intrinsic rigidity, which is further augmented by electrostatic repulsion facilitated by negatively charged side chains. In the simplest approximation, XG solutions can therefore be regarded as an ensemble of rigid, repulsive polyelectrolytes, which may form randomly oriented, “frozen” structures in solutions (Vilgis, 2015a; Harding, 1998). IC can be regarded as a simple polyelectrolyte (Holm et al., 2004). It is less negatively charged but more flexible (Hege et al., 2020). The electrostatic repulsion of the negative charges exerts a force on the self-avoiding walk of the chain. Consequently, the polymer chain elongates and arranges into a string of independent blobs, each containing a self-avoiding sub-chain of an equal number of monomer units (de Gennes, 1979). GG chains are uncharged and interact only with dipolar interactions via their hydroxyl OH-groups. It can therefore be regarded as a self-avoiding polymer in good solvent (de Gennes, 1979; Hege et al., 2020). The large number of chain conformations provide high entropic contributions (Schreiber et al., 2020). These simple models are sketched in Figure 2.

Thus, the size, shape, and dynamics of the hydrocolloid determine the viscosity of the foamable solution (Einstein, 1906, 1911; Harding, 1998; Yadav et al., 2011). Moreover, their charges and polar groups interact in distinct ways with BSA. Consequently, changes in foam stability can be attributed to the physical properties of the hydrocolloids in use.

Furthermore, it is useful to keep the typical sizes of the different components in mind to perceive the interactions across different length scales. BSA in heart (N) form shows pH-values at approximately 7 and a radius of 3–4 nm (Varga et al., 2016). The hydrocolloids have macromolecular dimensions. XG chains show persistent lengths of approximately 150 nm up to micrometers in pure water, with contour lengths often reaching 150 µm (Camesano and Wilkinson, 2001), indicating their high stiffness. The chains from GG, guaran, have very low persistence lengths of approximately 2 nm, with coiled dimensions of approximately 20–100 µm; however, in concentrated solutions, they tend to form aggregates with sizes up to 90 µm (Richardson et al., 1998; Gittings et al., 2000; Laguna et al., 2003). Individual IC chains show typical thicknesses of 0.3 nm in dissolved form and approximately 2.7 nm in helix form. Typical chain lengths are more than 100 nm in broad distributions (Millane et al., 1988; Schefer et al., 2015).

These hydrocolloids interact with a BSA in distinct way. GG as a polymer with polar OH-containing monomers mainly interacts by short-range dipolar/hydrogen interactions. Therefore, their binding to BSA through the polar amino acids remains weak, about 2 − 5 k B T, which compares with hydrogen bonding (Frenking and Shaik, 2014). XG and IC are both negatively charged. Therefore, these polymers may interact strongly with BSA at their positively charged amino acids, which are found prominently at the protein surface (Figure 1a). Charged polyelectrolytes such as XG and IC chains are able to bind electrostatically by higher energies of the order of 50 k B T. However, entropy, and thus the stiffness and flexibility of the chains, plays a major role (Figure 3).

The arrangements between BSA and the different hydrocolloids in bulk phases are ruled by highly competing interactions. BSA shows positive and negative charges at the surface, which allows selective interactions with negative charges of XG and IC.

The electrostatic rigidity exhibited by XG chains is attributable to their helical geometry—a property that functions to maximally repel the charges of the side chains. In higher concentrated (salt-free) XG solutions, the chains can assume an ordered phase due to the high interchain repulsion. The random orientation of the stiff chains, forming an “orientation glass” by jamming the highly inflexible molecules, results in a minimized free energy of the solution (Edwards and Vilgis, 1986; Vilgis, 2012; Vilgis, 2015a).

In the presence of BSA, such randomly oriented phases become more restricted (Figure 3a). Strongly alternating charges on the surfaces of the BSA prevent the attraction of the negative charges of XG chains. Moreover, a randomly oriented jammed phase of XG chains becomes less probable. The overall entropy of the solution will be reduced, and a depletion or partial phase separation of BSA–XG solutions will become likely. A similar behavior had been observed earlier for milk-protein–XG solutions (Hege et al., 2020). In consequence, the formation of larger BSA–XG complexes is unlikely.

These restrictions by the molecular stiffness are less pronounced for the polyelectrolytes in BSA-IC solutions. IC polyelectrolytes can be at first sight viewed as chains of electrostatic blobs (De Gennes et al., 1976; de Gennes, 1979) (Figure 2c), each of which carries a typical energy of k B T. Close to the positively and negativity charged surface of BSA, IC chains are able to adjust their conformation due to the entropy of a quasi-flexible self-avoiding walk of the monomers inside the blobs (Barrat and Joanny, 1996). Therefore, BSA and IC may form stable complexes in solutions (Antonov et al., 2022).

Polar GG chains are largely compatible with BSA on the local scale. The weak polar interactions do not allow for strong complex formation. It is unlikely that GG chains bind to BSA molecules over longer time scales.

Throughout the course of the experiments, a rotor stator device was utilized for foaming. The present apparatus has demonstrated a higher shear rate than industrial foaming systems that, for example, utilize gases (Adhikari et al., 2018). However, within the laboratory setting, the utilization of the rotor-stator device is employed to ensure the reproducibility of results. Moreover, the objective of the present study was to conduct a series of systematic experiments to explore the physical influences of the various polymers, as opposed to comprehending the more intricate process mechanism. The utilization of BSA as a stand-alone component in practical applications is, in any event, limited. The decision to use this protein was primarily influenced by the existence of a substantial corpus of scientific literature on it (Anand and Mukherjee, 2013; Baler et al., 2014; Behera et al., 2023). Consequently, focus can be directed to the physical distinctions inherent within the hydrocolloids.

Sample preparation

For all measurements, BSA (A7030, heat shock fraction, protease free, fatty acid free, essentially globulin free, pH 7, ≥ 98%) from Sigma-Aldrich (Saint Louis, MO, United States) was used as received.

The polysaccharides GG (Unipektin Ingredients AG, Eschenz, Switzerland) and XG (Jungbunzlauer Suisse AG, Basel, Switzerland) were dissolved in ultra-pure water (18.2 M Ω cm) by stirring overnight. The next morning, the GG was centrifuged at 15,090 × g for 20 min at 20 °C to remove cell material. IC (Carl Roth GmbH + Co. KG, Karlsruhe, Germany) was dissolved in ultrapure water and heated to 70 °C for 20 min to complete solution. After preparing the polysaccharide solution, BSA was added and dissolved by further stirring. The final solutions contained 4 wt% BSA and 0.4% polysaccharide in ultra-pure water.

Measurement of solution viscosity

XG, GG, and IC form shear-thinning solutions when dissolved in water (Imeson, 1999; Iglauer et al., 2011; Martín-Alfonso et al., 2018). To measure solution viscosity, flow sweeps were performed in a couette geometry (DIN concentric cylinders with smooth surfaces, bob diameter 28 mm, bob length 42 mm, cup diameter 30.4 mm, gap 5921 µm, and loading gap 90 mm) at 25 °C with a DHR3 rheometer from TA Instruments (New Castle, DE, United States). After filling the measuring cup, samples were allowed to rest for 300 s. Measurements were performed in rotation, and the shear rate was varied every 30 s. The shear rate was increased from 0.01 s −1 –1000 s −1 and, subsequently, it was decreased again from 1000 s −1 to 0.01 s −1.

Foam formation

For foam formation, a T 25 easy clean control ULTRA-TURRAX (IKA-Werke GmbH & Co. KG, Staufen, Germany) equipped with a dispersion tool S25N-10G was used. 2 g of protein solution were dispersed at a speed of 20,000 rpm for 1 min in a custom-built glass cylinder with overflow protection. Foaming time and speed were consistent across all experiments to prevent the samples from overheating. No alterations in temperature were recorded.

Imaging of the foam

The temporal evolution of the samples was observed using an Axioscope. An A1 light microscope from Carl Zeiss (Oberkochen, Germany) with a 10 × magnification was used, simultaneously with a Canon EOS 7D camera equipped with an objective Canon Macro Lens EF 100 mm 1:2.8 L IS USM and a Canon TC-80N36D remote release with timer function (Canon Inc. Headquarters, Tokyo, Japan), for 30 min or 60 min, respectively. The camera was mounted on a repro stand RS 2 XA from Kaiser Fototechnik (Kaiser Fototechnik GmbH & Co. KG, Buchen, Germany), and samples monitored by the camera were placed atop a luminous plate LED Slimlite plano (Kaiser Fototechnik GmbH & Co. KG, Buchen, Germany). Directly after foam formation, a sample of it was transferred to a microscope slide and covered with a cover slip. For microscope images, 0.005 g foam was transferred using a microspoon; for camera images, 0.01 g was used.

Analysis of the mean bubble size

For more reliable statistics, camera images were taken for bubble size analysis. The image was cropped to a size of 3000 × 3000 pixels to show only the bubbles of the foam, not the area surrounding the sample. Subsequently, the image contrast was enhanced twofold. The image was then converted into a binary image. A Python script counted all connected same-color pixels as one bubble. The number of pixels of one bubble was converted into micrometers to provide the area of the bubble. The equivalent diameter was calculated from the area, which was the diameter of a circle of the same area as the measured bubble. Having computed the equivalent bubble diameter of each bubble in the image, the mean equivalent bubble diameter of all bubbles in this image was calculated. All measurements were repeated for three identical samples, and means and standard deviations were calculated. All bubbles in the camera photos were thoroughly examined (Figure 4b). As the bubbles were smaller at the beginning, there are more bubbles in the image than at the end of the measurement period, when the bubbles were larger. Between 500 and 11,000 bubbles were examined, which is a significant quantity.

Foam images

Figure 4 shows microscope and camera images of a BSA-stabilized foam. The camera images were used to analyze the temporal evolution of the mean bubble size because these show a larger number of bubbles and therefore provide more reliable statistics. To visualize the properties of foams with different polysaccharide compositions, microscope images of BSA-stabilized foams without and with GG, IC, or XG are shown in Figure 5.

All foams containing polysaccharide appear wetter than those stabilized only by BSA. The higher liquid content of the foam arises from the enhanced water-holding capacity of the hydrocolloids (Milani and Maleki, 2012), which retains larger amounts of water in the lamellae between the bubbles. According to the microscope images, the foams containing XG or IC retain a higher liquid fraction longer than those containing GG. The increased viscosity slows down-drainage, which contributes to the retention of a higher liquid fraction. A high liquid fraction reduces the rate of coalescence because bubble surfaces are separated by sufficient liquid. Lower coalescence rates contribute, in turn, to a slower increase of the mean bubble size and a more stable foam (Wilde and Clark, 1996; Tesch and Schubert, 2002; Cox et al., 2009). As is evident from the microscope images, the ripening of the wet foams containing IC or XG is dominated by Ostwald-ripening.

Solution viscosity

Figure 6 shows the results of the flow sweeps performed for solutions of BSA with added polysaccharides: XG, IC, and GG, respectively. The dynamic viscosity η is displayed in dependence of the shear rate γ ̇.

All three solutions containing added hydrocolloids show shear thinning behavior, which is most pronounced for the solutions containing XG and least pronounced for the solutions containing GG. At low shear rates, the solutions containing XG show the highest viscosity, followed by IC, and the lowest viscosity is observed for solutions containing GG. However, since XG shows the steepest slope (most pronounced shear thinning behavior), the graphs cross. At shear rates above approximately 20 s −1, the viscosity of the XG-containing solutions becomes lower than that of the solutions containing IC. At shear rates above approximately 200 s −1, the viscosity of the solutions containing XG even slightly undercut the viscosity of those containing GG.

The viscosity of a dispersion is determined by the shape and charge of its components (Einstein, 1906, 1911; Harding, 1998; Yadav et al., 2011). Of the selected polysaccharides, XG is the most negatively charged and also exhibits the highest stiffness. Molecular shapes, charges, and the elongated, stiff molecules increase the viscosity of the solution—see, for example, the empirical Mark–Houwink–Kuhn–Sakurada equation [ η ] = K M α, where M is the molecular weight, and the phenomenological exponent α takes the values 0 for a compact sphere, 0.5–0.8 for random coils, and 1.8 for rigid rods.

This results in the highest viscosity observed at low shear rates. Nevertheless, at elevated shear rates, the rods adopt an orientation parallel to the flow direction, facilitating sliding between them (Calabrese et al., 2021; Detert et al., 2023), explaining the pronounced shear-thinning behavior. In contrast, GG is uncharged and flexible. Therefore, there is no viscosity increase due to electro-viscous effects. Moreover, the molecule, which behaves as a self-avoiding coil, is less extended, which leads to a lower exponent α in the Mark–Houwink–Kuhn–Sakurada equation and a smaller viscosity increase. IC is negatively charged but is flexible at scales inside the blobs (Figure 2). Its exponent α in the Mark–Houwink–Kuhn–Sakurada equation is expected to be between that for a self-avoiding coil and for a rigid rod. Due to its weaker negative charge, the electro-viscous effect is less pronounced than for XG. It can thus be concluded that the viscosity of IC is intermediate between those of XG and GG, as was measured for shear rates below approximately 20 s −1.

Temporal evolution of the mean bubble size

The temporal evolution of the mean equivalent bubble diameters d equ for BSA-stabilized foams without additives and with XG, IC, and GG added, respectively, is shown in Figure 7.

The initial mean bubble size is decreased for samples containing a polysaccharide and deviates slightly among samples containing XG, IC, or GG. During foam formation, very high shear rates are applied by the ULTRA TURRAX T25. At such shear rates, the viscosity of XG is lower than that of IC and GG due to its stronger shear thinning behavior. The viscosity during foam formation influences the incorporation of air bubbles into the foam and their initial size: the initial bubble size decreases with increasing viscosity (De Preval et al., 2014; Politova et al., 2018). These effects explain the larger initial mean equivalent bubble diameter of the pure BSA-stabilized foams ((101.8 ± 11.4) µm) compared to systems with added polysaccharide and the slightly smaller initial bubble size of the foams with added IC ((76.9 ± 3.3) µm) compared to the foams containing XG ((82.3 ± 2.0) µm) or GG ((83.6 ± 9.1) µm).

Over the entire measurement time, the mean bubble size was significantly lower for samples containing a polysaccharide than for those without a stabilizer. This observation indicates that foam stability is significantly increased by the addition of polysaccharides. By adding these, the viscosity of the sample solution is significantly enhanced, which prevents fast drainage as well as the creaming of bubbles (Cox et al., 2009; Wilde and Clark, 1996; Kováts et al., 2020). Both these effects play a pivotal role in foam stability. They impact the thickness of the lamellae, which in turn affects the rate of coalescence (Wilde and Clark, 1996; Tesch and Schubert, 2002). This explains the increased foam stability observed for foams containing polysaccharides.

At t = 0, the mean bubble sizes of the samples containing a polysaccharide deviate by only a few µm. Later, the samples containing GG develop a more significantly increased mean bubble size than those containing XG or IC. This observation indicates that the foams containing XG or IC are more stable than those with GG. While monitoring the foam under the microscope, the foam rests, its coarsening is considered sufficiently slow, and it can be considered close to mechanical equilibrium (Cantat et al., 2013). It is therefore evident that the viscosities pertinent to drainage are those measured for exceedingly low shear rates. Due to the relatively high viscosities of XG and IC solutions, these foams retain a higher liquid fraction (cf. microscope images in Figure 5) and show a slow increase in mean bubble size. The lower viscosity of GG solutions yields a faster increase in the mean equivalent bubble diameter. Although XG shows a higher solution viscosity at low shear rates than IC, the evolution of their mean bubble sizes exhibit similar courses. To explain this similarity, it is thus insufficient to only discuss the viscosity differences. The bubble size results indicate that, in addition, a destabilizing mechanism is present in the foams containing XG. For this reason, the structural differences between the stiff, negatively charged XG rods and the IC strings of independent blobs must be taken into account.

In addition to the influence of the polysaccharide structure, their interaction with BSA may also influence foam stability (Tesch and Schubert, 2002). To visualize the inter-polysaccharide interactions determined by their stiffness and electrical charge as well as their interaction with interfacially adsorbed BSA in thinning lamellae, models are shown in Figures 8 and 9. Each individual picture shows a bubble surface with adsorbed BSA on both the left and right side defining the edges of the lamella of varying thickness. The cases without added polysaccharides are shown in Figures 8a–d. Over time, the lamella thins due to the drainage of liquid from the foam, and the bubbles eventually coalesce (8c and 8d).

The flexible GG polymers (Figures 8e–g) show weak mutual interaction and form a random distribution within the lamella to maximize entropy. Over time, some GG polymers drain with the liquid. Due to its polarity, GG interacts more weakly with BSA than do the charged polymers XG or IC. To maximize entropy, GG will, therefore, mostly be dissolved in the water and have a minor influence on the interfacial stability of wet foams (other than enhancing solution viscosity). Due to short-distance weak dipolar, van der Waals interactions and hydrogen bonding GG chains interact with each other or BSA when in close proximity. The stability of thin lamellae in dry foams may be increased by bridging the two interfaces by adsorbed flexible GG chains. However, GG is more likely to be expelled from the lamellae in favor of translation entropy gain in wedges of spherically shaped foam bubbles.

The flocculation and creaming of oil-in-water emulsions and model food systems provoked by XG were observed by Gunning et al. (1988) and Koczo et al. (1998). They explain their observation by a depletion mechanism causing an isotropic/anisotropic phase separation according to Flory’s theory of rod-like particles (Flory, 1956, 1978; Flory and Abe, 1978).

However, the XG rods are strongly negatively charged, which makes a purely entropy-driven parallel (nematic-like) alignment unlikely under these conditions (Vilgis, 2012). To minimize the electrostatic energy, these negatively charged rods take random orientations. Such arrangements consume much space due to the associated large excluded volume effects. As the lamellae thin, the space available to such configurations is constricted. More configurations for the minimization of the electrostatic energy are available in the liquid phase of already drained liquid. For this reason, the rigid nature and strong negative charge of XG might contribute to phase separation into a bubble-rich phase at the top and a liquid phase at the bottom. Such effects destabilize the foam as they contribute to drainage and, for this reason, counteract the stabilizing effects caused by a higher solution viscosity. In addition, the electrostatic interaction of XG with BSA is relevant because its negative net charge interacts with local surface charges of BSA (Figure 9a). Thus, electrostatic attraction between BSA and XG can lead to an increased steric interaction within the lamellae (Figures 9b, c). The steric interaction of adsorbed polymers increases the stability of lamellae against coalescence and therefore enhances bubble stability (Denkov et al., 2020; Damodaran, 2005). For XG molecules attracted by interfacially adsorbed BSAs, the combination of their high stiffness and strong negative charge leads to large excluded volumes and a strong steric repulsion between approaching air–water interfaces. Such interactions counteract lamella thinning and therefore support the retention of a high liquid fraction in the foam, thereby stabilizing the lamellae against coalescence. Consequently, such steric interactions within the lamellae may increase foam stability in addition to the high viscosity of XG solutions. However, as described above, the strong mutual repulsion and high stiffness of XG molecules also cause a destabilizing mechanism, because more orientations are available to such highly charged rigid rods in the liquid phase of drained liquid. At decreasing liquid content due to drainage, free XG molecules such as those shown in the middle of Figure 9a are pushed out of the foam and drain with the liquid. Since XG is highly negatively charged and BSA shows only local surface charges, the electrostatic repulsion between XG molecules is stronger than their electrostatic interaction with BSA. For this reason, the steric stabilization by XG breaks at the further decreasing liquid content of the foam (Figures 9c, d).

More configurations are possible for IC in the confined space of the lamellae because these molecules are less negatively charged and the strings of independent blobs are more flexible, which leads to smaller excluded volumes. For this reason, phase-separating effects caused by molecular charge and stiffness are much less pronounced.

Since IC is less negatively charged and more flexible than XG, the chains take a larger number of configurations, resulting in higher entropy. Due to its negative charge, it is still attracted by positive surface charges of BSA, but repulsion by negative surfaces charges is less strong. In addition, their overall mutual repulsion appears less strong than for stiff XG molecules. Therefore, more configurations are available to IC molecules to arrange within the thinning lamellae. As the lamellae thin, the strings of thermodynamically independent blobs bend due to the electrostatic repulsion and still provide for a steric stabilization of the lamellae. The higher polymer flexibility also allows attachment to both BSA-coated interfaces and bridging approaching bubble surfaces (Denkov et al., 2020) (Figure 9g). For this reason, thin lamellae are also well stabilized by IC. As a consequence of the strong mutual XG repulsion and large excluded volume effects, the foams containing XG show similar stabilities as those containing IC, although a lower viscosity was measured for IC solutions at low shear rates.

The microscopy results shown in Figure 5 support these interpretations. First, attention is focused on GG-BSA foams. Although the growth in droplet size is significantly faster in pure BSA foams than in BSA-GG foams (Figure 7), GG-supported foams visibly form dry foams with polyhedral bubble shapes (Figures 5b, d). This is in accordance with the weak interaction between GG and BSA. Moreover, it favors the assumption that a large amount of GG is expelled from the lamellae during drainage-induced thinning. The charged hydrocolloids do not allow dry foams during the observation time for up to 60 min. This is clearly due to their charges. However, clearly visible differences between XG and IC can be recognized in Figures 5e–j during foam development, despite the similar growth rates shown in Figure 7. The charges enhance water binding between the lamellae, and the negative charges also interact strongly with the positive charges of the surface active BSA (compare amino acids colored blue in Figure 1b). The difference between the stiff and more flexible polyelectrolyte is apparent in the appearance of the bubbles after longer foam development—here, 60 min in 5g and 5j. While IC-supported foams show a tendency to form polyhedral shapes for the larger bubbles, XG foams remain much wetter, and the bubbles remain quasi-spherical, which is in accordance with the different charges and chain stiffnesses. This suggests that a certain (minimum) amount of XG remains between the lamellae which may bind electrostatically to the BSA.

The addition of polysaccharides to BSA-stabilized solutions increases their dynamic viscosity and enhances foam stability. It could be concluded that higher viscosities lead to smaller bubble sizes and more stable foams. Foams containing XG or IC appear macroscopically more stable than GG-stabilized foams. Although solutions containing XG show higher viscosity at low shear rates, the evolution of the mean bubble size of their foams is comparable to that of foams containing IC, indicating that the molecular properties of the hydrocolloids have higher impact than viscosity alone.

The polar flexible polymer chains of GG have the lowest impact on viscosity and interact only weakly with the surfactant BSA. Thus, GG-chains are increasingly expelled into the liquid phase during the gravity-induced drying of the foams, when the lamellae shrink and the space becomes confined. This also explains why the bubbles in GG-stabilized foams roughly have a similar polyhedral structure to those in BSA foams without additional stabilizers.

The charges of the polyelectrolytes XG and IC enhance water binding in hydrate shells around their contour, and viscosity increases. The foams remain wetter for both polyelectrolytes. The differences between XG and IC foams observed during microscopic foam evolution can be explained by their molecular architecture. The stiff and highly negatively charged XG rods orient randomly due to their strong mutual electrostatic repulsion and large excluded volume effects in bulk solutions; however, in the spatial limitations in the lamellae, this becomes impossible. The strong negative charge and high stiffness of XG cause the steric stabilization by XG chains to collapse under the confinement of the lamellae space. Only a certain concentration of stiff XG chains remains in the lamellae. These chains bind water, and the foam remains wet and stable. The higher flexibility (and lower) negative charge of IC allows for the steric stabilization of approaching bubble surfaces and thin lamellae and the bridging stabilization of even thinner lamellae. IC chains partially coat the BSA interface of the bubbles. Water binding increases, and the IC-stabilized foams remain wetter than pure BSA foams but less wet than XG-stabilized foams during later periods of foam evolution. This enables the bubbles to approximate the structure of the polyhedral structures of dry foams, albeit only to a limited extent.