Protein-Polysaccharide Interactions for Stabilization of Food Emulsions

Proteins, polysaccharides and their blends, as examples of natural biopolymers, are surface active materials. Biopolymers may be considered as amphiphilic macromolecules that play an essential role in stabilizing food formulations (foams, emulsions and dispersions). Under specific conditions (such as protein-to-polysaccharide ratio, pH, ionic strength, temperature, mixing processing), it has been stated that proteins and polysaccharides form hybrids (complexes) with enhanced functional properties in comparison to the proteins and polysaccharides alone. Different protein-polysaccharide pairs are reviewed with particular attention to the emulsification capability of their mixtures. In the case of uncomplexed blends of biopolymers, competitive adsorption onto hydrophobic surfaces is generally reported. Conversely, electrostatic complexation between oppositely charged proteins and polysaccharides allows better anchoring of the new-formed macro-molecular amphiphile onto oil-water interfaces. Moreover, improved thermal stability and increased resistance to external treatment (high pressure) involved in food processing are obtained. This review presents basic and applied knowledge on protein-polysaccharide interactions in aqueous medium and at the oil-water interface in food emulsion systems. Electrostatic interactions and thermodynamic incompatibility in mixed biopolymer solutions are correlated to the functional properties (rheology, surface hydrophobiciry, emulsification power) of these interesting blends. Basic and industrial selected systems of different families of hydrocolloids (as gum Arabic, galactomannans, pectins) and protein (caseins, whey, soya, gelatin) mixtures are reviewed.     

Surface adsorption behaviour of milk whey protein and pectin mixtures under conditions of air-water interface saturation

Milk whey proteins (MWP) and pectins (Ps) are biopolymer ingredients commonly used in the manufacture of colloidal food products. Therefore, knowledge of the interfacial characteristics of these biopolymers and their mixtures is very important for the design of food dispersion formulations (foams and/or emulsions). In this paper, we examine the adsorption and surface dilatational behaviour of MWP/Ps systems under conditions in which biopolymers can saturate the air-water interface on their own. Experiments were performed at constant temperature (20 °C), pH 7 and ionic strength 0.05 M. Two MWP samples, β-lactoglobulin (β-LG) and whey protein concentrate (WPC), and two Ps samples, low-methoxyl pectin (LMP) and high-methoxyl pectin (HMP) were evaluated. The contribution of biopolymers (MWP and Ps) to the interfacial properties of mixed systems was evaluated on the basis of their individual surface molecular characteristics. Biopolymer bulk concentration capable of saturating the air-water interface was estimated from surface pressure isotherms. Under conditions of interfacial saturation, dynamic adsorption behaviour (surface pressure and dilatational rheological characteristics) of MWP/Ps systems was discussed from a kinetic point of view, in terms of molecular diffusion, penetration and configurational rearrangement at the air-water interface. The main adsorption mechanism in MWP/LMP mixtures might be the MWP interfacial segregation due to the thermodynamic incompatibility between MWP and LMP (synergistic mechanism); while the interfacial adsorption in MWP/HMP mixtures could be characterized by a competitive mechanism between MWP and HMP at the air-water interface (antagonistic mechanism). The magnitude of these phenomena could be closely related to differences in molecular composition and/or aggregation state of MWP (β-LG and WPC).     

Interaction between components of plant-based biopolymer systems

Proteins and polysaccharides are key elements in formulated foods, cosmetics, and pharmaceuticals. Their interaction behavior mainly determines the organoleptic, optical, textural, and rheological properties of foods. Traditionally, animal-based biopolymers have been widely used because of their excellent techno-functionality; however, plant-based alternatives gained enormous interest among scientists and manufacturers because of sustainable, religious, ethical, and nutritional reasons. The directed complexation of mixed biopolymers entirely originated from plants might be used to stabilize food colloids, modulate interfacial and bulk properties, control the release of bioactives, and mask bitter components. As such, this review highlights the general separation mechanism of mixed biopolymers systems entirely composed of plant-based biopolymers to be used as functional food ingredients. Particularly, ‘traditional’ and ‘novel’ proteins and polysaccharides obtained from different plant sources (e.g. soy, wheat, pea, potato, apple, citrus) are introduced to be assembled to modulate interfacial and bulk properties of food colloids.     

The influence of pressure on the lower critical solution temperature miscibility behavior of aqueous solutions of poly(vinyl methyl ether) and the relation to the compositional curvature of the volume of mixing

In mixtures of PVME and water, the influence of pressure on the LCST miscibility gap is determined covering the whole composition range and pressures from atmospheric pressure up to 900 MPa. The cloud point curve at atmospheric pressure has the characteristic bimodal shape in agreement with literature data. Upon increasing pressure the cloud point curve at the low concentration side decreases with pressure, whereas at the high concentrations the cloud point curve increases with pressure. The overall influence of pressure results in a less pronounced bimodality and ultimately the bimodal shape disappears. In addition to the pressure dependence of the miscibility behavior, the density of mixtures of water and PVME are determined at atmospheric pressure. The experimental excess specific volumes are negative for all measured compositions, but the compositional curvature varies with composition. The curvature of the excess specific volume is positive for the higher concentrations but it is negative in the lower composition range. The density measurements are linked to the pressure dependence of the LCST miscibility behavior using exact thermodynamic relationships. The excess specific volume and miscibility results are shown to be in good agreement. Moreover, it is shown that the Clapeyron equation, which is exact for pure components and also frequently assumed to apply to mixtures, is not valid in the system PVME/water. The system PVME/water is an example where the usual approximation of one-to-one correspondence between curvature and excess volume does not apply. Finally, the molecular origins for the observed excess volume and miscibility behavior are briefly discussed from theoretical and molecular simulation points of view.     

Cooperative conformational transitions of linear biopolymers

Conformational transitions in proteins, nucleic acids, and other biopolymers evidently play a decisive role in many biological processes, particularly in control processes. They often proceed cooperatively, i.e. the elementary process of the transition of an individual segment of these macromolecules in influenced by the state of other segments via intramolecular interactions. In general, the segments favor the same state as their neighbours. The resulting equilibrium properties of cooperative systems, e.g. the sharpness of the transitions and their dependence on the chain length, can be quantitatively explained for linear systems by the linear Ising model. The molecular causes of the cooperativity can be explained for simple model polymers.     

Assessment of the potential models of acetone/CO2 and ethanol/CO2 mixtures by computer simulation and thermodynamic integration in liquid and supercritical states

Binary mixtures of CO(2) with ethanol and with acetone are studied by computer simulation, including extensive free energy calculations done by the method of thermodynamic integration, at 313 K, i.e., above the critical point of CO(2) in the entire composition range. The calculations are repeated with three different models of acetone and ethanol, and two models of CO(2). Comparisons of the molar volume of the different systems as well as of the change of their molar volume accompanying the mixing of the two components with experimental data reveal that, among the model pairs tested, the best results are obtained if both components are described by the Transferable Potentials for Phase Equilibria (TraPPE) force field. Around the ethanol/acetone mole fraction of 0.05 all ethanol/CO(2) and almost all acetone/CO(2) model pairs considered predict the existence of a sharp maximum of the molar volume. Due to the lack of experimental data in this composition range, however, these predictions cannot be verified/falsified yet. Most of the model pairs considered also predict limited miscibility of these compounds, as seen from the positive values of the free energy change accompanying their mixing, and the miscibility gap is located at the same composition range as the aforementioned molar volume maximum.     

Hierarchical assembly of model cell surfaces: synthesis of mucin mimetic polymers and their display on supported bilayers

Molecular level analysis of cell-surface phenomena could benefit from model systems comprising structurally defined components. Here we present the first step toward bottom-up assembly of model cell surfaces-the synthesis of mucin mimetics and their incorporation into artificial membranes. Natural mucins are densely glycosylated O-linked glycoproteins that serve numerous functions on cell surfaces. Their large size and extensive glycosylation makes the synthesis of these biopolymers impractical. We designed synthetically tractable glycosylated polymers that possess rodlike extended conformations similar to natural mucins. The glycosylated polymers were end-functionalized with lipid groups and embedded into supported lipid bilayers where they interact with protein receptors in a structure-dependent manner. Furthermore, their dynamic behavior in synthetic membranes mirrored that of natural biomolecules. This system provides a unique framework with which to study the behavior of mucin-like macromolecules in a controlled, cell surface-mimetic environment.     

Alexander G. Ogston’s Centennial: Researcher of hyaluronic acid and other biopolymers

Alexander G. Ogston (1911–1996) investigated hyaluronic acid and other biopolymers with physico-chemical techniques. He investigated the behavior of macromolecules in solution in the presence of other macromolecules and described the exclusion effect, known also as molecular crowding. He determined that the molecular weight and many physico-chemical properties of biopolymers depend on their origins and the conditions of their preparation.     

Thermoanalytical studies on protein-polysaccharide complexes of connective tissues

The Derivatograph has been found to be suitable for the investigation of biopolymers. consisting of polysaccharides and proteins. It could be demonstrated that interactions between the macromolecules of the collagen-proteoglycan-glycoprotein complexes significantly influence the thermal stability of the individual components.     

Viscometric study on the compatibility of some water-soluble polymer-polymer mixtures

Dilute solution viscosity behavior of three water-soluble polymer mixtures has been studied at 20 °C. The ternary systems assayed are distilled water/sodium carboxymethylcellulose (CMC)/polyacrylamide (PAM), distilled water/methylcellulose (MC)/CMC, and distilled water/polyvinylpyrrolidone (PVP)/MC. The intrinsic viscosity and the viscometric interaction parameters have been determined for the binary (distilled water/polymer) and ternary (distilled water/polymer1/polymer2) systems. Degree of compatibility of these polymer systems was estimated on the basis of five criteria: (i) the sign of Δb_m, (ii) the sign of Δb_m^′, (iii) the sign of Δ[η]_m, (iv) sign of thermodynamic parameter α, and (v) the sign of modified thermodynamic parameter β. Based on the sign convention involved in these criteria, compatibility/miscibility was observed in CMC/PAM and MC/CMC systems and incompatibility/immiscibility in PVP/MC system. The FTIR analyses also support the obtained results. The miscibility/compatibility of all these systems is in accordance with the interactions between the unlike polymer chains rather than the polymer-solvent interactions.     

Towards chemical analysis of nanostructures in biofilms II: tip-enhanced Raman spectroscopy of alginates

This study examines the feasibility of using tip-enhanced Raman spectroscopy (TERS) for label-free chemical characterization of nanostructures in biological systems. For this purpose, a well-defined model system consisting of calcium alginate fibers is studied. In a companion paper, calcium alginate fibers and their network structures were shown to be a good model for the extracellular polysaccharides of biofilms at the nanoscale. TERS analysis of biological macromolecules, such as alginates, is complicated by heterogeneity in their sequence, molecular weight, and conformations, their small Raman cross-section, and the large number of functional groups, which can chemically interact with the silver surface of the tip and cause significant band shifts. Due to these effects, Raman frequencies in TERS spectra of biopolymers do not necessarily resemble band positions in the normal Raman spectrum of the bulk material, as is the case for less complex samples (e.g., dye molecules) studied so far. Additionally, analyte decomposition due to laser heating can have a significant influence, and carbon contamination signals can sometimes even overwhelm the weak analyte signals. Based on the investigation of alginates, strategies for spectra correction, choice of appropriate reference samples, and data interpretation are presented. With this approach, characteristic frequency ranges and specific marker bands can be found for biological macromolecules that can be employed for their identification in complex environments. Figure TERS spectrum of a calcium alginate fiber bundle     

Effect of molecular dimensions of components on phase transitions in solutions of oligomeric polyethers

The molecular masses and intrinsic viscosities of a series of oligomeric poly(propylene glycols) have been studied by light scattering, analysis of chain ends, viscometry, and the cloud-point method. Phase diagrams are plotted and the Flory-Huggins thermodynamic interaction parameters and the second virial coefficients are calculated for oligomeric poly(propylene glycol)-n-alkane systems. The effects of the molecular dimensions of components on their mutual solubility and positions of boundary curves are determined. UCST decreases with an increase in the size of poly(propylene glycol) macromolecules and increases with an increase in the size of n-alkane molecules.     

New model describing adsorption from liquid binary mixtures of nonelectrolytes with limited and unlimited miscibility of components

Using the thermodynamic idea of complementary systems, and based on fundamental concepts of the theory of volume filling of micropores, we derived a new universal model describing adsorption from solutions with limited and unlimited miscibility of components. The model takes into account the differences in collision diameters of adsorbed molecules as well as the competitive nature of adsorption from solutions. The applicability of this new approach is tested against experimental data.     

Guidelines for buckwheat enriched bread

Thermal analysis was used to check the role of the main components of buckwheat flour (polysaccharides and proteins) to assess guidelines for novel recipes for bread from wheat and buckwheat flour blends with improved nutritional properties. The structure-related poor protein quality, namely, the lack of network-forming links, severely limits the use of buckwheat flours in bread-making. Data from TG and DSC analysis indicate that the introduction of a de-hulling step in the buckwheat milling diagram and the addition of some buckwheat polysaccharide fractions, isolated from the buckwheat husk, that contribute to the formation of the crumb structure thanks to their effect on the phase separation driven by the thermodynamic incompatibility with wheat gluten proteins, allows one to tune opposite effects and obtain bread from de-hulled buckweat/wheat flour blends with alveolar distribution much close that of the wheat bread.     

Feeling Inter‐ or Intramolecular Interactions with the Polymer Chain as Probe: Recent Progress in SMFS Studies on Macromolecular Interactions

Single‐molecule force spectroscopy (SMFS) opens new avenues for elucidating the structures and functions of large coiled molecules such as synthetic and biopolymers at the single‐molecule level. In addition, some of the features in the force–extension curves (i.e. force spectra) are closely related to primary/secondary structures of the molecules being stretched. For example, the long force plateau in the DNA stretching curve is related to the double‐helix structure. These features can be regarded as the force fingerprints of individual macromolecules. These force fingerprints can therefore be used as indicators/criteria of single‐molecule manipulation during the measurement of some unknown intra‐ or intermolecular interactions. By comparing the force spectra of a single polymer chain before and after interaction with other molecules, the mode/strength of such molecular interactions can be derived. This Review focuses on recent advances in AFM‐based SMFS studies on molecular interactions in both synthetic and biopolymer systems using a single macromolecular chain as probe, including interactions between nucleic acids and proteins, mechanochemistry of covalent bonds, conformation‐regulated enzymatic reactions, adsorption and desorption of biopolymers on a flat surface or from the nanopore of a carbon nanotube, and polymer interactions in the condensed state.     

Mixing behavior of binary insoluble phospholipid monolayers. Analysis of the mixing properties of binary lecithin and cephalin systems by application of several surface and spreading techniques

The detailed miscibility analysis of binary phospholipid monolayers requires the application of a variety of spreading and surface techniques which often yield complementary results. Testing the equilibrium state of the binary monolayer by long-time experiments is also of great importance. Studies of the compression and spreading behavior of binary monomolecular systems form a basis for the determination of binary monomolecular phase diagrams. Within these plots different phase regions occur which permit clear statements regarding the miscibility state. Additional knowledge of the miscibility properties (phase diagrams) of the binary bulk systems is required. From the analogy of the properties of the bulk systems, the miscibility state of the monolayers is also determined by the temperature, and we can classify the monolayers of binary lecithin and cephalin systems into systems of complete miscibility, partial miscibility and complete immiscibility. In addition to the differences in the chemical structure of the mixing components, the film states in the monolayer and the miscibility behavior of the bulk systems are also influencing factors. If one of the components does not produce a spreading pressure, miscibility gaps occur in the phase diagram of the phospholipid monolayer. The miscibility gap, expressed by a constant spreading pressure, indicates complete immiscibility within this concentration range. If both components produce spreading pressures, and condensed and liquid-expanded film states within the considered temperature range, partial miscibility of the components becomes probable. The most effective parameter is then the difference in the chemical structure of the components. When both components produce spreading pressures and condensed films, the chemical structure of the mixing phospholipid compounds within their hydrophilic and hydrophobic parts is of essential importance. Depending on the differences in the chemical structures of their chains and their head groups in the case of binary phospholipid monolayers, the following possibilities result: complete miscibility, partial miscibility and complete immiscibility of the lecithins and the cephalins. Complete miscibility within the binary phospholipid monolayer takes place in the case of identical head-group structure and where there are only small differences in the chain length of the fatty acid groups. With increasing hydrocarbon chain length differences, partial miscibility or even complete immiscibility can occur within the monolayer. Chemical differences in the head-group structure of the mixing components have a similar influence. In the case of binary lecithin/cephalin mixtures, the differences in the head-group structure affect the miscibility behavior more than the chain length differences do in the case of lecithin/lecithin and cephalin/cephalin mixtures.(ABSTRACT TRUNCATED AT 400 WORDS)     

Electrospray Mass Spectrometry of Biomacromolecular Complexes with Noncovalent Interactions—New Analytical Perspectives for Supramolecular Chemistry and Molecular Recognition Processes

The development of “soft” ionization methods in recent years has enabled substantial progress in the mass spectrometric characterization of macromolecules, in particular important biopolymers such as proteins and nucleic acids. In contrast to the still existing limitations for the determination of molecular weights by other ionization methods such as fast atom bombardment and plasma desorption, electrospray ionization (ESI) and matrix-assisted laser desorption have provided a breakthrough to macromolecules larger than 100 kDa. Whereas these methods have been successfully applied to determine the molecular weight and primary structure of biopolymers, the recently discovered direct characterization by ESI-MS of complexes containing noncovalent interactions (“noncovalent complexes”) opens new perspectives for supramolecular chemistry and analytical biochemistry. Unlike other ionization methods ESI-MS can be performed in homogeneous solution and under nearly physiological conditions of pH, concentration, and temperature. ESI mass spectra of biopolymers, particularly proteins, exhibit series of multiply charged macromolecular ions with charge states and distributions (“charge structures”) characteristic of structural states in solution, which enable a differentiation between native and denatured tertiary structures. In the first part of this article, fundamental principles, the present knowledge about ion formation mechanism(s) of ESI-MS, the relations between tertiary structures in solution and charge structures of macro-ions in the gas phase, and experimental preconditions for the identification of noncovalent complexes are described. The hitherto successful applications to the identification of enzyme–substrate and –inhibitor complexes, supramolecular protein–and protein–nucleotide complexes, double-stranded polynucleotides, as well as synthetic self-assembled complexes demonstrate broad potential for the direct analysis of specific noncovalent interactions. The present results suggest new applications for the characterization of supramolecular structures and molecular recognition processes that previously have not been amenable to mass spectrometry; for example, the sequence-specific oligomerization of polypeptides, antigen–antibody complexes, enzyme–and receptor–ligand interactions, and the evaluation of molecular specificity in combinatorial syntheses and self-assembled systems.     

Molecular hysteresis and its cybernetic significance

The general foundations for a thermodynamic analysis of hysteresis phenomena in solutions and suspensions of polyelectrolyte systems are presented using examples of molecular hysteresis in biopolymers and membranes. The fundamental cybernetic significance of metastable states and molecular hysteresis for a physical interpretation of phenomena of life such as memory recording and biological rhythms is discussed.     

Conformations of molecular brushes based on polyimide and poly(methyl methacrylate) in selective solvents: Experiment and computer simulation

The effect of solvent quality with respect to main and side chains on the conformations of molecular brushes is investigated by the methods of molecular hydrodynamics and optics as well as by computer simulation. Copolymers with a polyimide backbone and poly(methyl methacrylate) side chains are studied in solvents featuring strongly different thermodynamic qualities (chloroform, ethyl acetate, and 3-heptanone). The studied samples have close total molecular masses and backbone lengths but differ in grafting densities and lengths of side chains: a brush with densely grafted and relatively short side chains versus a brush with loosely grafted but very long side chains. For both types of brushes, similar changes in hydrodynamic behavior with improvement in the solvent quality are found experimentally. Computer simulation shows that these changes have different origins. In the former case, macromolecules are elongated, and their volumes grow simultaneously, while in the latter case, the shape of the macromolecules remains close to spherical and the changes in hydrodynamic parameters are mainly due to an increase in the volume of macromolecules.     

Polymer blends containing liquid crystal polymers

Polymer blends can be either composed of mixtures of flexible components, of a stiff chain and a flexible macromolecule, or of two stiff-chain polymers. All three cases may be dealt with in terms of the Flory lattice model. Special attention is paid to the influence of liquid crystalline order on the miscibility of the two polymers. For isotropic mixtures all three cases may be described in terms of the usual Flory–Huggins approximation. If a nematic phase is formed the miscibility of blends of rigid rods with flexible macromolecules (molecular composites) is strongly reduced because of entropic reasons. Highly ordered mixture of two stiff-chain polymers in melt can be described in terms of the regular solution theory leading to the same miscibility criterion as is valid for two flexible polymers. All deductions are compared to recent experimental work.