Incompatibility and Phase Separation in a Bovine Serum Albumin/beta-Casein/Water Ternary Film at the Air-Water Interface

Physical and culinary analysis of long gluten-free extruded pasta based on high protein quinoa flour

The consumption of foods with high protein content from pseudocereals is of great industrial interest. Pasta has a high gluten content; consequently, obtaining these gluten-free products is a technological challenge. The products obtained from quinoa show excellent results in protein and fiber with low glycemic index. This work focused on studying the effect of quinoa fat on the production of long pasta by extrusion in different mixtures of hyperprotein quinoa (HHP). It was observed that formulations with high percentages of starch showed a higher expansion rate, due to a higher fat content. Likewise, extruded pastes showed higher values of brightness than those containing lower percentages of starch and crude fat. The fracturability results were associated with the resistance of the paste to the pressure exerted for its deformation, which does not exceed 3.73 mm. The formulations with lower fat content presented high values in fracture stress due to the low diffusion of water and lipids. It was shown that fat has an indirect influence with a strong correlation with the expansion index and fracture stress and a moderate correlation (p > 0.05) with Young's modulus, indicating that increasing the added fat content increases the percentage of mass loss by baking. The melting and cohesiveness of all components improved during extrusion due to the pregelatinization of cassava starch, the addition of defatted HHP and the availability of quinoa starch granules.

Interfacial protein-protein displacement at fluid interfaces

Protein blends are used to stabilise many traditional and emerging emulsion products, resulting in complex, non-equilibrated interfacial structures. The interface composition just after emulsification is dependent on the competitive adsorption between proteins. Over time, non-adsorbed proteins are capable of displacing the initially adsorbed ones. Such rearrangements are important to consider, since the integrity of the interfacial film could be compromised after partial displacement, which may result in the physical destabilisation of emulsions. In the present review, we critically describe various experimental techniques to assess the interfacial composition, properties and mechanisms of protein displacement. The type of information that can be obtained from the different techniques is described, from which we comment on their suitability for displacement studies. Comparative studies between model interfaces and emulsions allow for evaluating the impact of minor components and the different fluid dynamics during interface formation. We extensively discuss available mechanistic physical models that describe interfacial properties and the dynamics of complex mixed systems, with a focus on protein in-plane and bulk-interface interactions. The potential of Brownian dynamic simulations to describe the parameters that govern interfacial displacement is also addressed. This review thus provides ample information for characterising the interfacial properties over time in protein blend-stabilised emulsions, based on both experimental and modelling approaches.

Adsorption of Hydrophobin-Protein Mixtures at the Air-Water Interface: The Impact of pH and Electrolyte

The adsorption of the proteins β-casein, β-lactoglobulin, and hydrophobin, and the protein mixtures of β-casein/hydrophobin and β-lactoglobulin/hydrophobin have been studied at the air-water interface by neutron reflectivity, NR. Changing the solution pH from 7 to 2.6 has relatively little impact on the adsorption of hydrophobin or β-lactoglobulin, but results in a substantial change in the structure of the adsorbed layer of β-casein. In β-lactoglobulin/hydrophobin mixtures, the adsorption is dominated by the hydrophobin adsorption, and is independent of the hydrophobin or β-lactoglobulin concentration and solution pH. At pH 2.6, the adsorption of the β-casein/hydrophobin mixtures is dominated by the hydrophobin adsorption over the range of β-casein concentrations studied. At pH 4 and 7, the adsorption of β-casein/hydrophobin mixtures is dominated by the hydrophobin adsorption at low β-casein concentrations. At higher β-casein concentrations, β-casein is adsorbed onto the surface monolayer of hydrophobin, and some interpenetration between the two proteins occurs. These results illustrate the importance of pH on the intermolecular interactions between the two proteins at the interface. This is further confirmed by the impact of PBS, phosphate buffered saline, buffer and CaCl2 on the coadsorption and surface structure. The results provide an important insight into the adsorption properties of protein mixtures and their application in foam and emulsion stabilization.

Implications of interfacial characteristics of food foaming agents in foam formulations

The manufacture of food dispersions (emulsions and foams) with specific quality attributes depends on the selection of the most appropriate raw materials and processing conditions. These dispersions being thermodynamically unstable require the use of emulsifiers (proteins, lipids, phospholipids, surfactants etc.). Emulsifiers typically coexist in the interfacial layer with specific functions in the processing and properties of the final product. The optimum use of emulsifiers depends on our knowledge of their interfacial physico-chemical characteristics - such as surface activity, amount adsorbed, structure, thickness, topography, ability to desorb (stability), lateral mobility, interactions between adsorbed molecules, ability to change conformation, interfacial rheological properties, etc. -, the kinetics of film formation and other associated physico-chemical properties at fluid interfaces. These monolayers constitute well defined systems for the analysis of food colloids at the micro- and nano-scale level, with several advantages for fundamental studies. In the present review we are concerned with the analysis of physico-chemical properties of emulsifier films at fluid interfaces in relation to foaming. Information about the above properties would be very helpful in the prediction of optimised formulations for food foams. We concluded that at surface pressures lower than that of monolayer saturation the foaming capacity is low, or even zero. A close relationship was observed between foaming capacity and the rate of diffusion of the foaming agent to the air-water interface. However, the foam stability correlates with the properties of the film at long-term adsorption.

Microscopy, microstructure and displacement of proteins from interfaces: implications for food quality and digestion

The use of atomic force microscopy to visualise the molecular structure of protein-stabilised oil-water and air-water interfaces has provided new information on the microstructure of the interfaces. The images have shown the formation of heterogeneous protein networks and demonstrated how this heterogeneity affects the competitive displacement of proteins from interfaces by surfactants. The data collected have led to the discovery of a new 'orogenic' mechanism of protein displacement. This mechanism is generic and provides new insights into understanding and controlling the stability of food foams and emulsions. The present article describes the practical aspects of the orogenic model for enhancing food quality, discusses the potential implications for understanding the influence of interfacial structures in food emulsions on fat metabolism and also generic aspects of the displacement mechanism outside the food area.

Rheology of mixed beta-casein/beta-lactoglobulin films at the air-water interface

The adsorption of dilute mixtures of beta-casein/beta-lactoglobulin to the air-water interface was investigated using surface dilatation and surface shear rheology. The data were fitted to simple rheological models to try to gain further information regarding the composition and nature of the interface. The dilatational measurements suggested that the composition of the interface could be determined using these models and that the surface concentration was dominated by the beta-casein in the early stages of adsorption but that high levels of beta-lactoglobulin were present in the final stages. Surface shear rheological measurements showed a similar trend. However, the shear measurements appeared to be more sensitive to the strength of the network than to the composition of the interface. Fluorescence microscopy supported the findings and demonstrated that any "phase separation" capable of affecting the surface rheological measurements occurred at the sub-micrometer scale. The results also demonstrated that the heterogeneity of the interface, once formed, is kinetically trapped, and no further phase separation occurs over the time span of the experiments.

Competitive adsorption of proteins and low-molecular-weight surfactants: computer simulation and microscopic imaging

Proteins and low-molecular-weight (LMW) surfactants are used in the food industry as emulsifying (and foaming) ingredients and as stabilizers. These attributes are related to their ability to adsorb at fluid-fluid (and gas-fluid) interfaces lowering the interfacial (and surface) tension of liquids. Hence, the study of the properties of adsorbed layers of these molecules can be expected to lead to a better understanding of their effect on food products. Direct proof of the validity of mesoscopic models of systems of proteins and LMW surfactants can only be achieved by quantitative theoretical predictions being tested against both macroscopic and mesoscopic experiments. Computer simulation constitutes one of the few available tools to predict mathematically the behaviour of models of realistic complexity. Furthermore, experimental techniques such as atomic force microscopy (AFM) now allow high resolution imaging of these systems, providing the mesoscopic scale measurements to compare with the simulations. In this review, we bring together a number of related findings that have been generated at this mesoscopic level over the past few years. A useful simple model consisting of spherical particles interacting via bonded and unbonded forces is described, and the derived computer simulation results are compared against those from the imaging experiments. Special attention is paid to the adsorption of binary mixtures of proteins, mixtures of LMW surfactants, and also protein+surfactant mixed systems. We believe that further development of these mathematically well-defined physical models is necessary in order to achieve a proper understanding of the key physico-chemical processes involved.

Dynamics of competitive adsorption of alphas-casein and beta-casein at planar triolein-water interface: evidence for incompatibility of mixing in the interfacial film

Competitive adsorption of alpha(s)-casein and beta-casein from a bulk solution mixture to the triolein-water interface has been studied. Although the binding affinity of alpha(s)-casein to the triolein-water interface was lower than that of beta-casein in single-component systems, in a 1:1 mixture of alpha(s)-casein and beta-casein in the bulk solution the ratio of interfacial concentrations of alpha(s)-casein to beta-casein at equilibrium was about 2:1, indicating that alpha(s)-casein was preferentially adsorbed to the triolein-water interface. Furthermore, the equilibrium composition of alpha(s)-casein and beta-casein in the interfacial film at various bulk concentration ratios did not follow a simple Langmuir adsorption model. This deviation from ideal behavior was mainly due to thermodynamic incompatibility of mixing of these caseins in the interfacial region. The value of the incompatibility parameter, X(12), for these caseins at the triolein-water interface was much greater than that at the air-water interface. Displacement experiments showed that while alpha(s)-casein could dynamically displace beta-casein when the latter was in an unsaturated monolayer state at the interface, it could not do so when beta-casein was in a saturated monolayer film state. It is hypothesized that, because of thermodynamic incompatibility of mixing, the alpha(s)-casein and beta-casein mixed film at the oil-water interface may undergo two-dimensional phase separation.

Lateral phase separation in adsorbed binary protein films at the air-water interface

Lateral phase separation in two-dimensional mixed films of soy 11S/beta-casein, acidic subunits of soy 11 (AS11S)/beta-casein, and alpha-lactalbumin/beta-casein adsorbed at the air-water interface has been studied using an epifluorescence microscopy method. No distinct lateral phase separation was observed in the mixed protein films when they were examined after 24 h of adsorption from the bulk phase. However, when the soy 11S/beta-casein and AS11S/beta-casein films were aged at the air-water interface for 96 h, phase-separated regions of the constituent proteins were evident, indicating that the phase separation process was kinetically limited by a viscosity barrier against lateral diffusion. In these films, beta-casein always formed the continuous phase and the other globular protein the dispersed phase. The morphology of the dispersed patches was affected by the protein composition in the film. In contrast with soy 11S/beta-casein and AS11S/beta-casein films, no lateral phase separation was observed in the alpha-lactalbumin/beta-casein film at both low and high concentration ratios in the film. The results of these studies proved that proteins in adsorbed binary films exhibit limited miscibility, and the deviation of competitive adsorption behavior of proteins at the air-water interface from that predicted by the ideal Langmuir model (Razumovsky, L.; Damodaran, S. J. Agric. Food Chem. 2001, 49, 3080-3086) is in fact due to thermodynamic incompatibility of mixing of the proteins in the binary film. It is hypothesized that phase separation in adsorbed mixed protein films at the air-water and possibly oil-water interfaces of foams and emulsions might be a source of instability in these dispersed systems.

Milk protein interfacial layers and the relationship to emulsion stability and rheology

The properties of milk protein-stabilised, oil-in-water emulsions are determined by the structure and surface rheology of the adsorbed layer at the oil-water interface. Analysis of the segment density profiles normal to the surface show differences in the structure between adsorbed layers of disordered casein and globular whey protein. Systematic studies of stability and rheology of model oil-in-water emulsion systems made with milk proteins as sole emulsifiers give insight into the relation between adsorbed layer properties and bulk emulsion stability. Of particular importance are effects of pH, temperature, calcium ions and protein content. Colloidal interactions between adsorbed layers on different surfaces can be inferred from an analysis of dynamic collisions of protein-coated emulsion droplets in shear flow using the colloidal particle scattering technique. The role of competitive adsorption on emulsion properties can be derived from experiments on systems containing mixtures of milk proteins and small-molecule surfactants. Shear-induced destabilisation is especially influenced by the presence of fat crystals in the emulsion droplets. Aggregated gel network properties are dependent on the balance of weak and strong interparticle interactions. In heat-set whey protein emulsion gels, the rheological behaviour is especially sensitive to surfactant type and concentration. Rearrangements of transient caseinate-based emulsion gels can have a profound influence on the quiesent stability behaviour. Computer simulation provides a general link between particle interactions, microstructure and rheological properties.