Structure of mixed β-lactoglobulin/pectin adsorbed layers at air/water interfaces; a spectroscopy study

From theoretical aspects to practical food Pickering emulsions: Formation, stabilization, and complexities linked to the use of colloidal food particles

We noticed that in literature, the term Pickering emulsion (PE) is used as soon as ingredients contain particles, and in this review, we ask ourselves if that is done rightfully so. The basic behavior taking place in particle-stabilized emulsions leads to the conclusion that the desorption energy of particles is generally high making particles highly suited to physically stabilize emulsions. Exceptions are particles with extreme contact angles or systems with very low interfacial tension. Particles used in food and biobased applications are soft, can deform when adsorbed, and most probably have molecules extending into both phases thus increasing desorption energy. Besides, surface-active components will be present either in the ingredients or generated by the emulsification process used, which will reduce the energy of desorption, either by reduced interfacial tension, or changes in the contact angle. In this paper, we describe the relative relevance of these aspects, and how to distinguish them in practice. Practical food emulsions may derive part of their stability from the presence of particles, but most likely have mixed interfaces, and are thus not PEs. Especially when small particles are used to stabilize (sub)micrometer droplets, emulsions may become unstable upon receiving a heat treatment. Stability can be enhanced by connecting the particles or creating network that spans the product, albeit this goes beyond classical Pickering stabilization. Through the architecture of PEs, special functionalities can be created, such as reduction of lipid oxidation, and controlled release features.

Exploring proteins at soft interfaces and in thin liquid films - From classical methods to advanced applications of reflectometry

The history of the topic of proteins at soft interfaces dates back to the 19th century, and until the present day, it has continuously attracted great scientific interest. A multitude of experimental methods and theoretical approaches have been developed to serve the research progress in this large domain of colloid and interface science, including the area of soft colloids such as foams and emulsions. From classical methods like surface tension adsorption isotherms, surface pressure-area measurements for spread layers, and surface rheology probing the dynamics of adsorption, nowadays, advanced surface-sensitive techniques based on spectroscopy, microscopy, and the reflection of light, X-rays and neutrons at liquid/fluid interfaces offers important complementary sources of information. Apart from the fundamental characteristics of protein adsorption layers, i.e., surface tension and surface excess, the nanoscale structure of such layers and the interfacial protein conformations and morphologies are of pivotal importance for extending the depth of understanding on the topic. In this review article, we provide an extensive overview of the application of three methods, namely, ellipsometry, X-ray reflectometry and neutron reflectometry, for adsorption and structural studies on proteins at water/air and water/oil interfaces. The main attention is placed on the development of experimental approaches and on a discussion of the relevant achievements in terms of notable experimental results. We have attempted to cover the whole history of protein studies with these techniques, and thus, we believe the review should serve as a valuable reference to fuel ideas for a wide spectrum of researchers in different scientific fields where proteins at soft interface may be of relevance.

Effects of Aqueous Isotopic Substitution on the Adsorption Dynamics and Dilational Rheology of β-Lactoglobulin Layers at the Water/Air Interface

The effect of the degree of isotopic substitution of the aqueous medium on the adsorption kinetics and the surface dilational rheological behavior at the water/air interface of the globular protein β-lactoglobulin was investigated. Aqueous solutions with fixed concentrations of 1 μM protein and 10 mM hydrogenous buffer with controlled pH 7 were prepared in H2O, D2O, and an isotopic mixture of 8.1% v/v D2O in H2O (called air contrast matched water, ACMW). Using a bubble shape analysis tensiometer, we obtained various experimental dependencies of the dilational viscoelasticity modulus E as a function of the dynamic surface pressure and of the frequency and amplitude of bubble surface area oscillations, either in the course of adsorption or after having reached a steady state. In general, the results revealed virtually no effect from substituting H2O by ACMW but distinct albeit relatively weak effects for intermediate adsorption times for D2O as the aqueous phase. In the final stage of adsorption, established after around 10 h, the equilibrium adsorption and the dilational rheological behavior of all protein layers under investigation are only very weakly affected by the presence of D2O. The obtained results help to design experimental protocols for protein adsorption studies, for example, by neutron reflectivity.

Interfacial Rheological Study of β-Casein/Pectin Mixtures at the Air/Water Interface

Colloidal food products, such as emulsions, foams, gels, and dispersions, are complex systems that need the presence of stabilizing agents to enable their formation and provide stability. Proteins are often used for food foams and emulsions because of their ability to lower interfacial tension and make viscoelastic interfaces. Generally, to improve the resistance against rupture, polysaccharides are used in association with the proteins. Pectin is a complex polysaccharide that can help to stabilize foams or emulsions. This work aims at studying the mechanical resistance of the interface formed by mixtures of β-casein and pectin at high and low methoxylation degrees at the air/water interface using dilatational and shear kinematics. Frequency sweep tests, in the linear region, were performed in shear at different aging times and in dilatational mode, and the rheological data were analyzed. The transient data of the surface tension were analyzed by kinetic models to obtain the characteristic rates of the interfacial phenomena. The kinetic mechanisms of the protein/pectin mixed systems are controlled by protein and show a weak gel behavior for short aging times. The interfaces obtained with both pectins in a mixture with β-casein evolved with time, gelling and showing a solid-like behavior at concentrations of 1 and 10 g/L and after 3.5 h of aging time. The interfacial shear trend obtained suggests a good stabilizing effect of the pectins from citrus with long aging times.

The Assembly Mechanism and Mesoscale Architecture of Protein-Polysaccharide Complexes Formed at the Solid-liquid Interface

Protein-polysaccharide composite materials have generated much interest due to their potential use in medical science and biotechnology. A comprehensive understanding of the assembly mechanism and the mesoscale architecture is needed for fabricating protein-polysaccharide composite materials with desired properties. In this study, complex assemblies were built on silica surfaces through a layer-by-layer (LbL) approach using bovine beta-lactoglobulin variant A (βLgA) and pectin as model protein and polysaccharide, respectively. We demonstrated the combined use of quartz crystal microbalance with dissipation monitoring (QCM-D) and neutron reflectometry (NR) for elucidating the assembly mechanism as well as the internal architecture of the protein-polysaccharide complexes formed at the solid-liquid interface. Our results show that βLgA and pectin interacted with each other and formed a cohesive matrix structure at the interface consisting of intertwined pectin chains that were cross-linked by βLgA-rich domains. Although the complexes were fabricated in an LbL fashion, the complexes appeared to be relatively homogeneous with βLgA and pectin molecules spatially distributed within the matrix structure. Our results also demonstrate that the density of βLgA-pectin complex assemblies increased with both the overall and local charge density of pectin molecules. Therefore, the physical properties of the protein-polysaccharide matrix structure, including density and level of hydration, can be tuned by using polysaccharides with varying charge patterns, thus promoting the development of composite materials with desired properties.

Complexes of β-lactoglobulin and high methyl-esterified pectin as a one-shot delivery system for reinforcing oil/water interfaces

Electrostatic complexation of negatively charged polysaccharides with β-lactoglobulin (β-lg) has been shown to bolster the protein films at oil/water interfaces thereby improving emulsion stability. However, recent sub-phase exchange experiments demonstrated that highly charged polysaccharides such as low methyl-esterified pectin are complementary only if sequentially introduced to a pre-formed interfacial β-lg film. In this study, results of transient interfacial shear rheology show that, by using high-methylesterified pectins instead, complexes can be formed in pre-mixed solutions with β-lg at pH 4 that can lead to reinforced protein films at dodecane/water interfaces. Using this one-shot adsorption of such complexes, pectins as well as short chain polysaccharides like homogalacturonan nearly doubled the steady state shear elastic moduli as compared to that of a pure β-lg film. The lag times of film formation were established to be primarily decided by the charge density and pattern on the polysaccharide. Based on the results from mixed solutions of β-lg monomers, it is proposed that the polysaccharide at pH 4 strengthens the resulting interfacial layer by concatenating adsorbed β-lg molecules thereby establishing cross-links in the aqueous phase.

Competitive Surface Activity of Monoclonal Antibodies and Nonionic Surfactants at the Air-Water Interface Determined by Interfacial Rheology and Neutron Reflectometry

Interfacial stresses can destabilize therapeutic formulations containing monoclonal antibodies (mAbs), which is proposed to be a result of adsorption and aggregation at the air-water interface. To increase protein stability, pharmaceutical industries add surfactants, such as Polysorbate 20 (PS20), into protein formulations to minimize mAb adsorption at the interface but rarely quantify this process. We determine that mAb adsorption in surfactant-free solutions creates a monolayer with significant viscoelasticity, which can influence measurements of bulk mAb solution viscosity. In contrast, PS20 absorption leads to an interface with negligible interfacial viscosity that protects the air-water interface from mAb adsorption. These studies were performed through a combined study of surface tensiometry, interfacial rheology, capillary viscometry, and neutron reflectometry to determine the surface activity of a model surfactant, PS20, and mAb system, which can be useful for the successful formulation developments of biotherapeutics.

β-Lactoglobulin Adsorption Layers at the Water/Air Surface: 3. Neutron Reflectometry Study on the Effect of pH

Several characteristics of β-lactoglobulin (BLG) layers adsorbed at the air/water interface exhibit a strong pH dependence, but our knowledge on the underlying structure-property relations is still fragmental. Here, we therefore extend our recent studies by neutron reflectometry (NR) and provide a comprehensive overview through direct measurements of the surface excess Γ and the layers' molecular structure. This enables comparison with available literature data to draw general conclusions. The NR experiments were performed at various pH values and within a wide range of protein concentrations, C_BLG. Adsorption kinetics measurements in air-contrast-matched-water and over a narrow Q_z range enabled direct quantification of the dynamic surface excess Γ(t) and are found to be consistent with ellipsometry data. Near the isoelectric point, pI, the rates of adsorption and Γ are maximal but only at sufficiently high C_BLG. NR data collected over a wider Q_z range and in two aqueous isotopic contrasts revealed the structure of adsorbed BLG layers at a steady state close to equilibrium. Independent of the pH, BLG was found to form dense monolayers with average thicknesses of 1.1 nm, suggesting flattening of the BLG globules upon adsorption as compared with their bulk dimensions (≈3.5 nm). Near pI and at sufficiently high C_BLG, a thick (≈5.5 nm) but looser secondary sublayer is additionally formed adjacent to the dense primary monolayer. The thickness of this sublayer can be interpreted in terms of disordered BLG dimers. The results obtained and notably the specific interfacial structuring of BLG near pI complement previous observations relating the impact of solution pH and C_BLG on other interfacial characteristics such as surface pressure and surface dilational viscoelasticity modulus.

Effect of protein type, concentration and oil droplet size on the formation of repulsively jammed elastic nanoemulsion gels

Rheology of sodium caseinate (SC) and whey protein isolate (WPI)-stabilized nanoemulsions (NEs) was investigated as a function of protein (1-5 wt%) and oil (30 and 40 wt%) concentration and storage time. For SC NEs, gel strength increased with an increase in protein and oil concentration and a decrease in droplet size and below a critical size transformed into a strong elastic gel that did not flow under gravity. Surprisingly, WPI NEs, although stable and had similar droplet size to SC NEs, did not form elastic gels. The stability of the NEs was studied for 3 months, and no significant change was observed. Considerable higher storage modulus (G') of SC NEs compared to WPI NEs was attributed to an increased effective droplet volume fraction (φeff) due to a thicker steric barrier of SC compared to WPI. The DLVO interdroplet potential was used to calculate the thickness of the charge cloud at an overall repulsive interaction of 1 kBT, which was added to the steric barrier to calculate the effective droplet size and φeff. At the highest φeff (0.79) for 5% SC NEs with 40% oil, the nanodroplets and associated repulsive barrier randomly jammed, leading to the formation of a strong elastic gel. For WPI NEs, maximum φeff was 0.57, leading to a lack of jamming and viscous fluid-like behaviour. Re-plotting G' with φeff for SC NEs with different protein concentration showed a linear trend followed by a rapid increase in G' at a critical φeff, confirming the transition from weak glassy region to strong randomly jammed structure. SC-stabilized repulsively jammed NE-gels could be used as a novel soft material where a lower oil volume fraction and long-term stability is required.

Transient measurement and structure analysis of protein-polysaccharide multilayers at fluid interfaces

The formation of electrostatic protein-polysaccharide multilayers has attracted attention for the design of fluid interfaces with enhanced stability and functionality. However, current techniques are often limited to measuring final multilayer properties. We present an interfacial shear rheology setup with simultaneous subphase exchange, allowing the transient measurement of biopolymer multilayers by their viscoelasticity. The successive and simultaneous adsorption of β-lactoglobulin (β-lg) and low-methoxyl pectin were investigated at the n-dodecane/water interface at pH 4. The successive injection of pectin increased the viscoelasticity of an adsorbed β-lg layer by electrostatic complexation. On the other hand, simultaneous adsorption impeded adsorption kinetics and interfacial layer strength due to complexation in the bulk phase prior to adsorption. Neutron reflectometry at the air-water interface confirmed the formation of an initial β-lg layer and electrostatic complexation of a secondary pectin layer, which desorbed upon pH-induced charge inversion. The layer formed by simultaneous adsorption mainly consisted of β-lg. We conclude that protein-polysaccharide complexes show limited surface activity and result in a lower effective protein concentration available for adsorption.

Routine single particle CryoEM sample and grid characterization by tomography

Single particle cryo-electron microscopy (cryoEM) is often performed under the assumption that particles are not adsorbed to the air-water interfaces and in thin, vitreous ice. In this study, we performed fiducial-less tomography on over 50 different cryoEM grid/sample preparations to determine the particle distribution within the ice and the overall geometry of the ice in grid holes. Surprisingly, by studying particles in holes in 3D from over 1000 tomograms, we have determined that the vast majority of particles (approximately 90%) are adsorbed to an air-water interface. The implications of this observation are wide-ranging, with potential ramifications regarding protein denaturation, conformational change, and preferred orientation. We also show that fiducial-less cryo-electron tomography on single particle grids may be used to determine ice thickness, optimal single particle collection areas and strategies, particle heterogeneity, and de novo models for template picking and single particle alignment.

Subphase exchange experiments with the pendant drop technique

INTRODUCTION

The development of the coaxial double capillary 15 years ago opened up the possibility to undertake accurate desorption and penetration studies of interfacial layers in the pendant drop technique. Drop and bubble methods offer several advantages with respect to other interfacial techniques. They allow a more stringent control of the environmental conditions, use smaller amounts of material and provide a much higher interface/volume ratio than in conventional Langmuir Troughs.

EXPERIMENTAL

The coaxial capillary was developed 15 years ago at the University of Granada as an accessory for the pendant drop surface film balance. It allows exchanging the subphase of the drop without disturbing the surface film and preserving the drop volume throughout the subphase exchange. Hence, this methodology enables one to carry out a great variety of interfacial studies well beyond the usual adsorption profiles. Penetration studies, sequential adsorption measurements, desorption kinetics, reversibility of adsorption and testing of enzymatic treatments on interfacial layers are amongst the principal applications. The coaxial capillary has been recently upgraded to a multi-exchange device which has boosted its applicability. It can be now used to address multilayer formation, create soft interfacial nano-composites such as membranes, polyelectrolyte assemblies and simulate in vitro digestion in a single droplet.

APPLICATIONS

This review aims to compile the experimental work done, using the pendant drop subphase exchange in the last decade, and how its use has provided new insights into the surface/interfacial properties of many different materials. Special emphasis is placed on recent work regarding simulation of in vitro digestion in order to address issues relating to metabolism degradation profiles. The use of this methodology when dealing with interfacial studies allows setting the foundations of interfacial engineering technology. Based on subphase exchange experiments, we aim to develop models for competitive adsorption of different compounds at the interface and build up layer-by-layer interfacial structures. Future challenges comprise the design of finely adjusted nanoengineering systems, based on multilayer assemblies with tailored functionalities, to match the application demand.

Application of X-ray and neutron small angle scattering techniques to study the hierarchical structure of plant cell walls: a review

Plant cell walls present an extremely complex structure of hierarchically assembled cellulose microfibrils embedded in a multi-component matrix. The biosynthesis process determines the mechanism of cellulose crystallisation and assembly, as well as the interaction of cellulose with other cell wall components. Thus, a knowledge of cellulose microfibril and bundle architecture, and the structural role of matrix components, is crucial for understanding cell wall functional and technological roles. Small angle scattering techniques, combined with complementary methods, provide an efficient approach to characterise plant cell walls, covering a broad and relevant size range while minimising experimental artefacts derived from sample treatment. Given the system complexity, approaches such as component extraction and the use of plant cell wall analogues are typically employed to enable the interpretation of experimental results. This review summarises the current research status on the characterisation of the hierarchical structure of plant cell walls using small angle scattering techniques.

Synergetic effect of sodium polystyrene sulfonate and guanidine hydrochloride on the surface properties of lysozyme solutions

A study of the dilational surface viscoelastic properties of mixed solutions of lysozyme and denaturants allows us to characterize the changes of protein tertiary structure in the surface layer upon adsorption at the liquid–gas interface.

Protein/polysaccharide complexes and coacervates in food systems

Since the pioneering work of Bungenberg de Jong and co-workers on gelatin-acacia gum complex coacervation in the 1920-40s, protein/polysaccharide complexes and coacervates have received increasing research interest in order to broaden the possible food applications. This review focuses on the main research streams followed in this field during the last 12 years regarding: i) the parameters influencing the formation of complexes and coacervates in protein-polysaccharide systems; ii) the characterization of the kinetics of phase separation and multi-scale structure of the complexes and coacervates; and iii) the investigation of the functional properties of complexes and coacervates in food applications. This latter section encompasses various technological aspects, namely: the viscosifying and gelling ability, the foaming and emulsifying ability and finally, the stabilization and release of bioactives or sensitive compounds.

Interfacial assembly of proteins and peptides: recent examples studied by neutron reflection

Through reviewing a number of recent neutron reflection studies of interfacial adsorption of peptides and proteins, this paper aims to demonstrate the significance of this technique in studying interfacial biomolecular processes by illustrating the typical structural details that can be derived. The review will start with the introduction of relevant theoretical background, followed by an outline of representative biomolecular systems that have recently been studied to indicate the technical strengths of neutron reflection.

Mixed layers of sodium caseinate + dextran sulfate: influence of order of addition to oil-water interface

We report on the interfacial properties of electrostatic complexes of protein (sodium caseinate) with a highly sulfated polysaccharide (dextran sulfate). Two routes were investigated for preparation of adsorbed layers at the n-tetradecane-water interface at pH = 6. Bilayers were made by the layer-by-layer deposition technique whereby polysaccharide was added to a previously established protein-stabilized interface. Mixed layers were made by the conventional one-step method in which soluble protein-polysaccharide complexes were adsorbed directly at the interface. Protein + polysaccharide systems gave a slower decay of interfacial tension and stronger dilatational viscoelastic properties than the protein alone, but there was no significant difference in dilatational properties between mixed layers and bilayers. Conversely, shear rheology experiments exhibited significant differences between the two kinds of interfacial layers, with the mixed system giving much stronger interfacial films than the bilayer system, i.e., shear viscosities and moduli at least an order of magnitude higher. The film shear viscoelasticity was further enhanced by acidification of the biopolymer mixture to pH = 2 prior to interface formation. Taken together, these measurements provide insight into the origin of previously reported differences in stability properties of oil-in-water emulsions made by the bilayer and mixed layer approaches. Addition of a proteolytic enzyme (trypsin) to both types of interfaces led to a significant increase in the elastic modulus of the film, suggesting that the enzyme was adsorbed at the interface via complexation with dextran sulfate. Overall, this study has confirmed the potential of shear rheology as a highly sensitive probe of associative electrostatic interactions and interfacial structure in mixed biopolymer layers.

Competitive adsorption of mixed anionic polysaccharides at the surfaces of protein-coated lipid droplets

Charged polysaccharides can improve the stability of protein-coated lipid droplets by forming a protective coating around them. Potentially, the interfacial characteristics of these coatings can be controlled by assembling them from mixed polysaccharides with different molecular characteristics. The purpose of this study was to examine the competitive adsorption of two anionic polysaccharides (carrageenan and pectin) to beta-lactoglobulin coated-lipid droplets. Carrageenan has a higher charge density than pectin, and carrageenan has a linear backbone whereas pectin has a linear backbone with branches. Emulsions (phi = 1 wt % oil, d43 = 0.40 +/- 0.03 microm) were mixed with polysaccharide solutions (0 or 0.04 wt%) at pH 7, then the pH was decreased to promote polysaccharide adsorption. The adsorption of the polysaccharide molecules to the droplet surfaces occurred at a higher pH for carrageenan (pH approximately 5.85) than for pectin (pH approximately 5.45). When polysaccharide mixtures were added at pH 7, the carrageenan molecules preferentially adsorbed to the droplet surfaces when the pH was reduced. At pH 3.5, carrageenan coated droplets had a higher negative charge (zeta = - 38.5 +/- 3.1 mV) than pectin-coated droplets (zeta = - 17.9 +/- 2.0 mV). Carrageenan was much more effective at displacing pectin from the surfaces of pectin-coated droplets, than pectin was at displacing carrageenan from carrageenan-coated droplets. The stability of pectin-coated droplets was better than carrageenan-coated droplets, which was attributed to steric hindrance effects. These results have important implications for the design of delivery systems based on polysaccharide/protein-coated droplets.

Nanostructured materials in the food industry

Nanotechnology involves the application, production, and processing of materials at the nanometer scale. Biological- and physical-inspired approaches, using both conventional and innovative food processing technologies to manipulate matter at this scale, provide the food industry with materials with new functionalities. Understanding the assembly behavior of native and modified food components is essential in developing nanostructured materials. Functionalized nanostructured materials are finding applications in many sectors of the food industry, including novel nanosensors, new packaging materials with improved mechanical and barrier properties, and efficient and targeted nutrient delivery systems. An improved understanding of the benefits and the risks of the technology based on sound scientific data will help gain the acceptance of nanotechnology by the food industry. New horizons for nanotechnology in food science may be achieved by further research on nanoscale structures and methods to control interactions between single molecules.

Interfacial structure and stability of food emulsions as affected by protein-polysaccharide interactions

The exploitation of protein-polysaccharide interactions offers opportunities for the design of new ingredients and interfacial structures with applications in the food and pharmaceutical industries. Association of protein and polysaccharide molecules may occur chemically through covalent bonds or physically through electrostatic interactions. Theoretical and experimental studies indicate that various molecular and thermodynamic factors can be adjusted to optimize the effectiveness of covalent conjugates and electrostatic complexes in the stabilization of interfaces, gels and emulsions. Maillard-type protein-polysaccharide conjugates have excellent emulsifying and steric stabilizing properties, especially under conditions where the protein alone is poorly soluble. Charged polysaccharides form soluble complexes or coacervates with proteins depending on pH, ionic strength, and biopolymer charge distribution. The structure and stabilizing properties of the mixed protein + polysaccharide layer depends on the sequence of adsorption of the biopolymers to the interface. There is good potential for use of interfacial protein-polysaccharide complexes in the nanoscale engineering of delivery vehicles for nutrient encapsulation and in the protection of adsorbed proteins and emulsified lipids against enzymatic breakdown during digestion.