Adsorption of Milk Proteins (β-Casein and β-Lactoglobulin) and BSA onto Hydrophobic Surfaces

Here, we study films of proteins over planar surfaces and protein-coated microspheres obtained from the adsorption of three different proteins ( β -casein, β -lactoglobulin and bovine serum albumin (BSA)). The investigation of protein films in planar surfaces is performed by combining quartz crystal microbalance (QCM) and atomic force microscopy (AFM) measurements with all-atomic molecular dynamics (MD) simulations. We found that BSA and β -lactoglobulin form compact monolayers, almost without interstices between the proteins. However, β -casein adsorbs forming multilayers. The study of the electrokinetic mobility of protein-coated latex microspheres shows substantial condensation of ions from the buffer over the complexes, as predicted from ion condensation theories. The electrokinetic behavior of the latex-protein complexes is dominated by the charge of the proteins and the phenomenon of ion condensation, whereas the charge of the latex colloids plays only a minor role.

The self-assembly, aggregation and phase transitions of food protein systems in one, two and three dimensions

The aggregation of proteins is of fundamental relevance in a number of daily phenomena, as important and diverse as blood coagulation, medical diseases, or cooking an egg in the kitchen. Colloidal food systems, in particular, are examples that have great significance for protein aggregation, not only for their importance and implications, which touches on everyday life, but also because they allow the limits of the colloidal science analogy to be tested in a much broader window of conditions, such as pH, ionic strength, concentration and temperature. Thus, studying the aggregation and self-assembly of proteins in foods challenges our understanding of these complex systems from both the molecular and statistical physics perspectives. Last but not least, food offers a unique playground to study the aggregation of proteins in three, two and one dimensions, that is to say, in the bulk, at air/water and oil/water interfaces and in protein fibrillation phenomena. In this review we will tackle this very ambitious task in order to discuss the current understanding of protein aggregation in the framework of foods, which is possibly one of the broadest contexts, yet is of tremendous daily relevance.

Adsorption of unstructured protein β-casein to hydrophobic and charged surfaces

In this Monte Carlo simulation study we use mesoscopic modeling to show that β-casein, an unstructured milk protein, adsorbs to surfaces not only due to direct electrostatic and hydrophobic interactions but also due to structural rearrangement and charge regulation due to proton uptake and release. β-casein acts as an amphiphilic chameleon, changing properties according to the chemical environment, and binding is observed to both positively and negatively charged surfaces. The binding mechanisms, however, are fundamentally different. A detailed, per-residue-level analysis shows that the adsorption process is controlled by a few very specific regions of the protein and that these change dramatically with pH. Caseins, being the most abundant proteins in milk, are crucial for the properties of fermented dairy products, such as nutrition, texture, and viscosity, but may also influence adhesion to packaging materials. The latter leads to product losses of about 10%, leading to economical and environmental problems.

Isolation, characterization, and emulsifying properties of wattle seed (Acacia victoriae Bentham) extracts

Extracts from either ground whole wattle seeds or uncoated cotyledons were obtained using water, alkali, or ethanol. These extracts were then analyzed for their protein molecular weight and electrophoretic profiles using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and capillary electrophoresis, respectively. Water extracts and those from the cotyledons gave higher material yield and contained significantly more proteins than other extracts. Furthermore, the proteins ranged in molecular weight from 6 to 92 kDa, with the highest concentration between 27 and 61 kDa. Water extracts, even at very low protein concentrations (0.17-1.12%), formed stable emulsions, containing up to 50% canola oil, and these emulsions were affected by pH (4-9), ionic strength (0.25-1% NaCl), and retorting (115 degrees C for 30 min). The study showed that water-soluble wattle extracts have great potential as emulsifiers and stabilizers for the food industry, especially at low pH levels.

Using Self-Consistent-Field Theory to Understand Enhanced Steric Stabilization by Casein-Like Copolymers at Low Surface Coverage in Mixed Protein Layers

We present a statistical mechanical approach to predicting the properties of mixed copolymer layers using the Scheutjens-Fleer self-consistent-field theory. Our model copolymers are based on the primary structures of the major bovine casein monomers, alpha(s1)-casein and beta-casein. Numerical calculations have been carried out to determine the polymer segment density profiles at an isolated hydrophobic surface and the interaction forces as a pair of polymer-coated surfaces is brought to close interlayer separation. For a copolymer model containing hydrophilic and hydrophobic segments, we show how the steric stabilizing capacity of a casein-like macromolecule at very low surface coverage is enhanced in the presence of a thin dense layer of shorter tethered amphiphilic chains. Using a more refined protein model, which also distinguishes between the charged and uncharged hydrophilic segments along the chain, we clearly demonstrate that the enhanced steric repulsion from beta-casein exceeds that from alpha(s1)-casein. These calculations explain how the replacement of just a few percent of beta-lactoglobulin by casein can inhibit the heat-induced thickening and flocculation behavior observed experimentally with some whey protein-stabilized oil-in-water emulsions.

Beta-casein adsorption at the silicon oxide-aqueous solution interface: calcium ion effects

Neutron reflectometry was used to investigate effects of calcium ions on the interfacial behavior of beta-casein at the silicon oxide-aqueous solution interface. The structural characteristics of the adsorbed layer were determined from reflectivity curves fitted to three- and two-layer optical models. The results showed that the presence of divalent calcium ions decreased the specific electrostatic adsorption affinity of the protein to silica compared with the calcium-free buffer system studied in an earlier work. In addition, it speeded up the adsorption suggesting that the slow kinetics seen in the calcium-free system are related to conformational adjustments of the beta-casein structure driven by the maximization of the number of positive charges on the polypeptide interacting with negative surface charges. In the calcium-free system, a dense inner layer resulted from this process, with cationic segments firmly bound to the negative surface, whereas in the presence of calcium, a less dense inner layer was formed. The difference in binding is also mirrored by the effects on the interfacial layer of a specific proteolytic enzyme, i.e., endoproteinase Asp-N. In the calcium-free case, an inner dense layer remained at the surface after the proteolytic cleavage of the polypeptide, whereas virtually nothing was left after enzymatic action in the presence of calcium ions.

Beta-casein adsorption at the hydrophobized silicon oxide-aqueous solution interface and the effect of added electrolyte

The effect of the presence of NaCl, CaCl(2), or MgCl(2) at the same ionic strength on the structure of beta-casein layers adsorbed on hydrophobic surfaces has been investigated by neutron reflectivity measurements. The data were fitted to a four-layer model. The volume fraction versus distance profiles have a similar shape whether beta-casein is adsorbed from NaCl, CaCl(2), and MgCl(2) of the same ionic strength or whether the protein concentration is lowered 10 times. In particular at larger distances from the surface, the volume fraction values are low and similar. However, close to the hydrophobic surface the volume fraction of protein decreases in the order CaCl(2) > MgCl(2) > NaCl. We have also used a specific proteolytic enzyme, endoproteinase Asp-N, which cleaves off the hydrophilic part of beta-casein, as a tool to reveal the interfacial structure of the protein. For all the different types of added electrolytes, endoproteinase Asp N only affects the outermost beta-casein layer. Subsequent addition of beta-casein in all cases led to large increases in amounts adsorbed and in the thickness of the outer layers.

beta-Casein adsorption at the silicon oxide--aqueous solution interface

Neutron reflectometry was used to investigate the time-dependent beta-casein adsorption at the silica-aqueous solution interface. The transient and steady-state structural characteristics of the adsorbed layer were determined from reflectivity curves, fitted to three-layer and two-layer models. The results show that the beta-casein adsorption to silica is very slow. The adsorption process involves the formation of an inner dense protein layer with a mean thickness of about 30 A onto which a more hydrated outer layer is self-associated. The surface excess and the total layer thickness of the asymmetric bilayer were, after 5 h adsorption time, estimated to be about 6.5 mg/m2 and 105 A, respectively. The adsorption behavior observed on silica contrasts with that previously reported for hydrophobic substrates, where beta-casein adsorption is much more rapid and the final surface excess is less than half of that observed for silica. Rinsing the silica surface with protein-free buffer resulted in a substantial desorption; much more pronounced than observed for hydrophobic substrates. This behavior suggests a weak adsorption affinity for a fraction of the adsorbed casein molecules; most likely the outer self-associated casein molecules in the adsorbed bilayer. The comparative desorption from hydrophobic surfaces was shown to be marginal. The difference between the layer structures adopted on hydrophobic and hydrophilic surfaces is also mirrored in the effects that the addition of a specific proteolytic enzyme (endoproteinase Asp-N) has on the adsorbed layer properties. The rinsing and endoproteinase cleavage processes result together in more than 80% reduction of the originally adsorbed mass at the silica surface. Only a thin but dense adsorbed layer remains after these treatments. The corresponding reduction reported for the hydrophobic adsorbent system was only about 20%. It is concluded that beta-casein adsorption on silica results in the formation of an asymmetric surface bound bilayer that stands in strong contrast to the monolayer structure formed at hydrophobic surfaces. This finding support the previous results obtained by using ellipsometry. The study also shows that neutron reflection, despite its limitations in time resolution, can be used for studying dynamic interfacial phenomena in protein 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.

Orthokinetic flocculation of caseinate-stabilized emulsions: influence of calcium concentration, shear rate, and protein content

Calcium-induced flocculation of caseinate-stabilized soybean oil-in-water emulsions in conditions of Couette flow was studied. A concentrated emulsion (20% oil, 0.5-2.0% sodium caseinate in 20 mM imidazole, pH 7) was diluted 20 times in buffer containing concentrations of CaCl(2) between 9 and 17 mM and sheared at rates between 335 and 1340 s(-)(1). The average particle size (d(43)), measured by integrated light scattering, increased in a sigmoidal manner with shearing time. An increased shear rate resulted in an increased flocculation rate, because of the increased number of collisions between particles, but a decreased value of the maximum d(43), because higher shear rates increasingly disrupted the flocs. The flocculation rate was increased by increasing the calcium concentration, indicating an increased collision efficiency. The orthokinetic stability of the emulsions was increased with increased protein content, and it is postulated that the increased surface coverage and hydrodynamic thickness of the adsorbed protein layer increased steric repulsion between droplets, so that higher calcium concentrations were necessary to induce sufficient conformational change of the proteins to allow flocculation. At high caseinate concentrations, calcium may also induce precipitation of unadsorbed caseins from the serum to the oil-water interface, thereby increasing steric repulsion and hence increasing orthokinetic stability.

Electrostatic Interactions in Adsorbed Protein Layers Probed by a Sedimentation Technique

A simple centrifugation technique has been used to determine the thicknesses of layers of alphaS1-casein and beta-casein adsorbed onto monodisperse polystyrene latex particles. The influence of electrostatic interactions within the layers on their thickness and stabilizing ability is investigated by varying the ionic strength of the suspension or by including calcium ions, known to bind specifically to the caseins. Copyright 1998 Academic Press.

A Comparison of the Functional and Interfacial Properties of beta-Casein and Dephosphorylated beta-Casein

The functional and interfacial properties of beta-casein and dephosphorylated beta-casein (DeP beta-casein) were studied at pH 7.0 in 10 mM phosphate buffer. A decrease in emulsion stability and an increase in foamability was observed. Results from a variety of interfacial techniques including electrophoretic mobility, thin film thickness, surface and interfacial tension, surface rheology, adsorbed layer thickness, and adsorption isotherms of dephosphorylated beta-casein and beta-casein are reported. The results demonstrate that the phosphorylated groups of the N-terminal region of beta-casein are important for stabilizing emulsions. This is either as a direct result of charge repulsion between beta-casein N-terminal regions or more probably as an indirect result of the reduced N-terminal charge permitting DeP beta-casein to adopt a different interfacial conformation resulting in a loss or reduction of a steric barrier. Copyright 1997 Academic Press. Copyright 1997Academic Press

Neutron reflection study of bovine beta-casein adsorbed on OTS self-assembled monolayers

Specular neutron reflection has been used to determine the structure and composition of bovine beta-casein adsorbed on a solid surface from an aqueous phosphate-buffered solution at pH 7. The protein was adsorbed on a hydrophobic monolayer self-assembled from deuterated octadecyltrichlorosilane solution on a silicon (111) surface. A two-layer structure formed consisting of one dense layer of thickness 23 +/- 1 angstroms and a surface coverage of 1.9 milligrams per square meter adjacent to the surface and an external layer protruding into the solution of thickness 35 +/- 1 angstroms and 12 percent protein volume fraction. The structure of the (beta-casein) layer is explained in terms of the charge distribution in the protein.