Rheological properties of protein films

Interfacial adsorption of soybean phosphatidylethanolamine in different oil phase and the stability of water-in-oil emulsion

Water-in-oil (W/O) emulsion holds great potential in designing fat-reduced foods. However, due to the lack of W/O-type surfactant, formation of all-natural W/O emulsion is challenged. This study aimed to investigate the effect of oil phase on interfacial adsorption of soybean phosphatidylethanolamine (SP) and stability of W/O emulsion. Five oils, including medium chain triglycerides oil (MO), coconut oil (CO), palm kernel oil (PKO), sunflower oil (SO) and rapeseed oil (RO), were selected. Results showed that diffusion rate of SP to the interface ranked as MO > CO > PKO > SO ≈ RO, increasing interfacial adsorption from 50.2 % to 85.3 %. Higher interfacial adsorption improved the deformation resistance of interfacial layer, causing more significant decrease in interfacial tension (3.54 mN/m). So, the largest water fraction (65 %) was stabilized by SP with MO and CO, and exhibited smaller droplet sizes and better stability. Consequently, shorter-chain oil was more suitable for preparing W/O emulsions.

Lipid oxidation in emulsions: New insights from the past two decades

Lipid oxidation constitutes the main source of degradation of lipid-rich foods, including food emulsions. The complexity of the reactions at play combined with the increased demand from consumers for less processed and more natural foods result in additional challenges in controlling this phenomenon. This review provides an overview of the insights acquired over the past two decades on the understanding of lipid oxidation in oil-in-water (O/W) emulsions. After introducing the general structure of O/W emulsions and the classical mechanisms of lipid oxidation, the contribution of less studied oxidation products and the spatiotemporal resolution of these reactions will be discussed. We then highlight the impact of emulsion formulation on the mechanisms, taking into consideration the new trends in terms of emulsifiers as well as their own sensitivity to oxidation. Finally, novel antioxidant strategies that have emerged to meet the recent consumer's demand will be detailed. In an era defined by the pursuit of healthier, more natural, and sustainable food choices, a comprehensive understanding of lipid oxidation in emulsions is not only an academic quest, but also a crucial step towards meeting the evolving expectations of consumers and ensuring the quality and stability of lipid-rich food products.

Influence of heat treatment before and/or after high-pressure homogenization on the structure and emulsification properties of soybean protein isolate

This study investigates the effects of heat treatment before high-pressure homogenization (HHPH) and heat treatment after high-pressure homogenization (HPHH) at different pressures (20, 60, and 100 MPa) on the structural and emulsification properties of soy protein isolate (SPI). The results indicate that HHPH treatment increases the surface hydrophobicity (H0) of the SPI, reduces β-fold and irregular curls, leading to the formation of soluble aggregates, increased adsorbed protein content, and subsequent improvements in emulsification activity index (EAI) and emulsion stability index (ESI). In contrast, the HPHH treatment promoted the exchange of SH/SS bonds between protein molecules and facilitated the interaction of basic peptides and β-subunits, leading to larger particle sizes of the soluble aggregates compared to the HHPH-treated samples. However, excessive aggregation in HPHH-treated aggregates leads to decreased H0 and adsorbed protein content, and increased interfacial tension, negatively affecting the emulsification properties. Compared to the HPHH treatment, HHPH treatment at homogenization pressures of 20 to 100 MPa increases EAI and ESI by 5.81-29.6 % and 5.31-25.9 %, respectively. These findings provide a fundamental basis for soybean protein manufacturers to employ appropriate processing procedures aimed at improving emulsification properties.

Ultrasonic and homogenization: An overview of the preparation of an edible protein-polysaccharide complex emulsion

Emulsion systems are extensively utilized in the food industry, including dairy products, such as ice cream and salad dressing, as well as meat products, beverages, sauces, and mayonnaise. Meanwhile, diverse advanced technologies have been developed for emulsion preparation. Compared with other techniques, high-intensity ultrasound (HIUS) and high-pressure homogenization (HPH) are two emerging emulsification methods that are cost-effective, green, and environmentally friendly and have gained significant attention. HIUS-induced acoustic cavitation helps in efficiently disrupting the oil droplets, which effectively produces a stable emulsion. HPH-induced shear stress, turbulence, and cavitation lead to droplet disruption, altering protein structure and functional aspects of food. The key distinctions among emulsification devices are covered in this review, as are the mechanisms of the HIUS and HPH emulsification processes. Furthermore, the preparation of emulsions including natural polymers (e.g., proteins-polysaccharides, and their complexes), has also been discussed in this review. Moreover, the review put forward to the future HIUS and HPH emulsification trends and challenges. HIUS and HPH can prepare much emulsifier-stable food emulsions, (e.g., proteins, polysaccharides, and protein-polysaccharide complexes). Appropriate HIUS and HPH treatment can improve emulsions' rheological and emulsifying properties and reduce the emulsions droplets' size. HIUS and HPH are suitable methods for developing protein-polysaccharide forming stable emulsions. Despite the numerous studies conducted on ultrasonic and homogenization-induced emulsifying properties available in recent literature, this review specifically focuses on summarizing the significant progress made in utilizing biopolymer-based protein-polysaccharide complex particles, which can provide valuable insights for designing new, sustainable, clean-label, and improved eco-friendly colloidal systems for food emulsion. PRACTICAL APPLICATION: Utilizing complex particle-stabilized emulsions is a promising approach towards developing safer, healthier, and more sustainable food products that meet legal requirements and industrial standards. Moreover, the is an increasing need of concentrated emulsions stabilized by biopolymer complex particles, which have been increasingly recognized for their potential health benefits in protecting against lifestyle-related diseases by the scientific community, industries, and consumers.

Study on the Stability Mechanism of Peanut OBs Extracted with the Aqueous Enzymatic Method

In this study, the internal relationships among oil bodies (OBs), the protein-phospholipid interactions in aqueous phase, oil-water interface behavior, and the stability of reconstituted OBs were analyzed from the bulk phase, interface, and macro perspectives, and the stability mechanism of OBs was discussed. OB proteins and phospholipids were combined through hydrophobic and electrostatic interactions, resulting in the stretching of protein conformation. OB proteins and phospholipids act synergistically to increase interface pressure and the rate of increase in interface pressure with relatively stable elastic behavior, which is beneficial to the formation and stability of interfacial films. When OBs were reconstituted by an OB protein-phospholipid complex system, phospholipids bound to OB proteins through hydrophobic and electrostatic interactions. OB proteins and phospholipids uniformly covered the oil droplet surface of reconstituted OBs to form a stable interfacial film, which maintained the stability of OBs. The addition of phospholipids significantly reduced the particle size of OBs prepared by OB proteins in a dose-dependent manner, and particle size decreased with the increase in phospholipid content (p < 0.05). Phospholipids increased the net surface charge, enhanced electrostatic repulsion, and improved the physicochemical stability of reconstituted OBs. The stability mechanism elucidated in this study provides a theoretical basis for the demulsification of peanut OBs.

Differentiating the effects of hydrophobic interaction and disulfide bond on the myofibrillar protein emulsion gels at the high temperature and the protein interfacial properties

The study presented the effects of modulating the hydrophobic interaction and disulfide bond on the properties of myofibrillar protein (MP) emulsion gels at high temperature (95 °C) and the differentiation on the contribution of non-covalent (hydrophobic interaction) and covalent intermolecular interactions (disulfide bond) to the properties of interfacial protein films were also determined. The hydrophobic interactions among MP were modulated by the addition of octenyl succinic anhydride (OSA), and the disulfide bonds were modulated by the SH/SS exchange reactions mediated by GSH. The results showed that the MP emulsion gel properties at 95 °C were improved by modulating the hydrophobic interaction or disulfide bonds, and the dynamic interfacial adsorption of MP and dissipation quartz crystal microbalance experiments showed the interfacial adsorption pattens of protein were also changed. In addition, the hydrophobic interactions putted emphasis on improving the gel matrix, whereas the disulfide bonds focused on increasing the stiffness of interfacial protein films.

Relevance of the air-water interfacial and foaming properties of (modified) wheat proteins for food systems

A shift from animal protein- to plant protein-based foods is crucial in transitioning toward a more sustainable global food system. Among food products typically stabilized by animal proteins, food foams represent a major category. Wheat proteins are ubiquitous and structurally diverse, which offers opportunities for exploiting them for food foam and air-water interface stabilization. Notably, they are often classified into those that are soluble in aqueous systems (albumins and globulins) and those that are not (gliadins and glutenins). However, gliadins are at least to an extent water extractable and thus surface active. We here provide a comprehensive overview of studies investigating the air-water interfacial and foaming properties of the different wheat protein fractions. Characteristics in model systems are related to the functional role that wheat proteins play in gas cell stabilization in existing wheat-based foods (bread dough, cake batter, and beer foam). Still, to further extend the applicability of wheat proteins, and particularly the poorly soluble glutenins, to other food foams, their modification is required. Different physical, (bio)chemical, and other modification strategies that have been utilized to alter the solubility and therefore the air-water interfacial and foaming properties of the gluten protein fraction are critically reviewed. Such approaches may open up new opportunities for the application of (modified) gluten proteins in other food products, such as plant-based meringues, whippable drinks, or ice cream. In each section, important knowledge gaps are highlighted and perspectives for research efforts that could lead to the rational design of wheat protein systems with enhanced functionality and overall an increased applicability in food industry are proposed.

Impact of Particle Sedimentation in Pendant Drop Tensiometry

Understanding the interface-stabilizing properties of surface-active components is key in designing stable macroscopic multiphase systems, such as emulsions and foams. When poorly soluble materials are used as an interface stabilizer, the insoluble material may sediment and interfere with the analysis of interfacial properties in pendant (or hanging) drop tensiometry. Here, the impact of sedimentation of particles on the interfacial properties determined by pendant drop tensiometry was evaluated using a model system of whey protein isolate and (non surface-active) glass beads (2.2-34.7 μm). Although the glass beads did not adsorb to the air-water interface, a 1% (w/w) glass bead solution appeared to decrease the surface tension by nearly 12 mN/m after 3 h. A similar effect was shown for a mixture of whey proteins and glass beads: the addition of 1% (w/w) of glass beads led to an apparent surface tension decrease of 31 mN/m rather than the 20 mN/m observed for pure whey proteins. These effects are attributed to the sedimentation of particles near the apex of the droplet, leading to droplet shape changes, which are interpreted as a decrease in surface tension using tensiometer software. The droplet density at the apex increases due to sedimentation, and this density increase is not accounted for when fitting the droplet shape with the Young-Laplace equation. The result is the observed apparent decrease in surface tension. In contrast to the significant impact of sedimenting material on the surface tension measurements, the impact on the results of oscillatory deformations was limited. These findings show that the impact of sedimentation should be considered when studying the interface-stabilizing properties of materials with reduced solubility, such as certain plant protein extracts. The presence of such particles should be carefully considered when conducting pendant drop tensiometry.

Hyaluronic acid and human/bovine serum albumin shelled nanocapsules: Interaction with mucins and in vitro digestibility of interfacial films

Liquid lipid nanocapsules are oil droplets surrounded by a protective shell, which enable high load and allow controlled delivery of lipophilic compounds. However, their use in food formulations requires analysing their digestibility and interaction with mucin. Here, serum albumins and hyaluronic acid shelled olive oil nanocapsules are analysed to discern differences between human and bovine variants, the latter usually used as model system. Interfacial interaction of albumins and hyaluronic acid reveals that human albumin presents limited conformational changes upon adsorption, which increase by complexation with the polysaccharide present at the interface. The latter also promotes hydrophobic interactions with mucin, especially at pH 3 and protects albumin interfacial layer under in vitro gastric digestion. The interfacial unfolding induced in human albumin by hyaluronic acid facilitates in vitro lipolysis while its limited conformational changes provide the largest protection against in vitro lipolysis.

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.

Effects of preheat treatment and polyphenol grafting on the structural, emulsifying and rheological properties of protein isolate from Cinnamomum camphora seed kernel

In this study, protein isolate (PI) and purified polyphenol extract (PPE) were prepared from Cinnamomum camphora seed kernel (CCSK). The effects of preheat treatment (50-90 °C) combined with polyphenol grafting (5 % PPE, w/w) on the structural, emulsifying and rheological properties of PI were investigated. Results demonstrated the preheat treatments at 80 and 90 °C significantly increased the extent of protein aggregation of PI. Fluorescence spectra and thermal behavior analysis revealed that preheat-treated PI exhibited more compact structure and higher thermal stability. Moreover, the emulsifying stability and apparent viscosity of PI were enhanced after preheat treatments at 50, 60 and 70 °C. After modification by PPE, the secondary structural changes of preheat-treated PI were confirmed by FTIR. PPE modification improved the thermal stability and antioxidant activities of preheat-treated PI. These results provide a novel way to combine the advantages of preheat treatment and polyphenol grafting in developing a novel protein ingredient.

Bioprinted hASC ‐laden collagen/ HA constructs with meringue‐like macro/micropores

Extrusion‐based bioprinting is one of the most effective methods for fabricating cell‐laden mesh structures. However, insufficient cellular activities within the printed cylindrical cell‐matrix blocks, inducing low cell‐to‐cell interactions due to the disturbance of the matrix hydrogel, remain to be addressed. Hence, various sacrificial materials or void‐forming methods have been used; however, most of them cannot solve the problem completely or require complicated fabricating procedures. Herein, we suggest a bioprinted cell‐laden collagen/hydroxyapatite (HA) construct comprising meringue‐like porous cell‐laden structures to enhance osteogenic activity. A porous bioink is generated using a culinary process, i.e., the whipping method, and the whipping conditions, such as the material concentration, time, and speed, are selected appropriately. The constructs fabricated using the meringue‐like bioink with MG63 cells and human adipose stem cells exhibit outstanding metabolic and osteogenic activities owing to the synergistic effects of the efficient cell‐to‐cell interactions and HA stimulation released from the porous structure. The in vitro cellular responses indicate that the meringue‐like collagen bioink for achieving an extremely porous cell‐laden construct can be a highly promising cell‐laden material for various tissue regeneration applications.

Impact of Phenolic Acid Derivatives on β-Lactoglobulin Stabilized Oil-Water-Interfaces

The physical stability of protein-based emulsions depends on intra- and intermolecular interactions of the interfacial protein-film. As studied in aqueous systems before, phenolic acid derivatives (PADs) non-covalently or covalently crosslink proteins depending on pH-value and thus, may impact interfacial protein-films. Whether these interactions occur in the same manner at the interface as in water and how they vary the properties of the interfacial protein-film has not been clarified. The present study aimed to investigate the interfacial protein-film viscoelasticity and physical emulsion-stability after non-covalently (pH 6.0) and covalently (pH 9.0) crosslinking depending on PAD-structure. For this purpose, we studied an interfacial β-lactoglobulin film with dilatational rheology after crosslinking with PADs, varying in number of π-electrons and polar substituents. Then, we analyzed the physical emulsion-stability by visual evaluation and particle size distribution. The results indicate that PADs with a high number of π-electrons (rosmarinic acid and chicoric acid) weaken the protein-film due to competing of phenol-protein interactions with protein-protein interactions. This is reflected in a decrease in interfacial elasticity. PADs with an additional polar substituent (verbascoside and cynarine) seem to further weaken the protein film, since the affinity of the PADs to the interface increases, PADs preferentially adsorb and sterically hinder protein-protein interactions. In emulsions at pH 6.0 and thus low electrostatic repulsion, PADs promote bridging-flocculation. Due to higher electrostatic repulsion at pH 9.0, the PADs are sterically hindered to form bridges, even though they are polymeric. Hence, our research enables the control of protein-film viscoelasticity and emulsion-stability depending on the PAD-structure.

Graphical abstract

Selective Detection of Liquid Viscosity Using Acoustic Plate Waves with In-Plane Polarization

Using plates of weak piezoeletcric crystal (quartz) loaded with various liquids, it is shown that along with common modes, whose sensitivity towards different liquid parameters comparable with each other, there are some uncommon modes, whose amplitude responses towards viscosity η are much larger than towards temperature T and electric conductivity σ. The search of the modes with the selective properties is accomplished by varying plate thickness h, crystal orientation, wave length λ, and mode order n. It is found that all modes possessing the property are characterized by small surface-normal displacement, avoiding wave radiation into adjacent liquid, large in-plane displacements, enhancing viscous coupling the modes and liquids, and small electro-mechanical constant, reducing electro-acoustic interaction. Basing on the modes, the sensor prototypes with selective operation are developed and tested for η from 1 to 1500 cP, σ from 0 to 1.2 S/m, and t from 0 to 55 °C. Because of operation at ultrasonic frequency (tens MHz) the prototypes have different sensitivities in various η-ranges: 0.3 dB/cP for 1−20 cP, 0.12 dB/cP for 20−100 cP, and 0.015 dB/cP for 100−1500 cP. Viscosity responses of the prototypes become comparable with their electric outputs only for η < 2 cP. Temperature responses are almost zero in air, but when plate is coated with liquid they increase depending on liquid properties, allowing measurements of the temperature dependence of the liquid viscosity.

Interfacial Polyelectrolyte-Surfactant Complexes Regulate Escape of Microdroplets Elastically Trapped in Thermotropic Liquid Crystals

Polyelectrolytes adsorbed at soft interfaces are used in contexts such as materials synthesis, stabilization of emulsions, and control of rheology. Here, we explore how polyelectrolyte adsorption to aqueous interfaces of thermotropic liquid crystals (LCs) influences surfactant-stabilized aqueous microdroplets that are elastically trapped within the LCs. We find that adsorption of poly(diallyldimethylammonium chloride) (PDDA) to the interface of a nematic phase of 4-cyano-4'-pentylbiphenyl (5CB) triggers the ejection of microdroplets decorated with sodium dodecylsulfate (SDS), consistent with an attractive electrical double layer interaction between the microdroplets and LC interface. The concentration of PDDA that triggers release of the microdroplets (millimolar), however, is three orders of magnitude higher than that which saturates the LC interfacial charge (micromolar). Observation of a transient reorientation of the LC during escape of microdroplets leads us to conclude that complexes of PDDA and SDS form at the LC interface and thereby regulate interfacial charge and microdroplet escape. Poly(sodium 4-styrenesulfonate) (PSS) also triggers escape of dodecyltrimethylammonium bromide (DTAB)-decorated aqueous microdroplets from 5CB with dynamics consistent with the formation of interfacial polyelectrolyte-surfactant complexes. In contrast to PDDA-SDS, however, we do not observe a transient reorientation of the LC when using PSS-DTAB, reflecting weak association of DTAB and PSS and slow kinetics of formation of PSS-DTAB complexes. Our results reveal the central role of polyelectrolyte-surfactant dynamics in regulating the escape of the microdroplets and, more broadly, that LCs offer the basis of a novel probe of the structure and properties of polyelectrolyte-surfactant complexes at interfaces. We demonstrate the utility of these new insights by triggering the ejection of microdroplets from LCs using peptide-polymer amphiphiles that switch their net charge upon being processed by enzymes. Overall, our results provide fresh insight into the formation of polyelectrolyte-surfactant complexes at aqueous-LC interfaces and new principles for the design of responsive soft matter.

Interfacial properties of milk proteins: A review

The interfacial properties of dairy proteins are of great interest to the food industry. Food manufacturing involves various environmental conditions and multiple processes that significantly alter the structure and colloidal stability of food materials. The effects of concentration, pH, heat treatment, addition of salts etc., have considerable influence on the surface activity of proteins and the mechanical properties of the interfacial protein films. Studies to date have established some understanding of the links between environmental and processing related parameters and their impacts on interfacial behavior. Improvement in knowledge may allow better design of interfacial protein structures for different food applications. This review examines the effects of environmental and processing conditions on the interfacial properties of dairy proteins with emphasis on interfacial tension dynamics, dilatational and surface shear rheological properties. The most commonly used surface analytical techniques along with relevant methods are also addressed.

No ordinary proteins: Adsorption and molecular orientation of monoclonal antibodies

The interaction of monoclonal antibodies (mAbs) with air/water interfaces plays a crucial role in their overall stability in solution. We aim to understand this behavior using pendant bubble measurements to track the dynamic tension reduction and x-ray reflectivity to obtain the electron density profiles (EDPs) at the surface. Native immunoglobulin G mAb is a rigid molecule with a flat, "Y" shape, and simulated EDPs are obtained by rotating a homology construct at the surface. Comparing simulations with experimental EDPs, we obtain surface orientation probability maps showing mAbs transition from flat-on Y-shape configurations to side-on or end-on configurations with increasing concentration. The modeling also shows the presence of β sheets at the surface. Overall, the experiments and the homology modeling elucidate the orientational phase space during different stages of adsorption of mAbs at the air/water interface. These finding will help define new strategies for the manufacture and storage of antibody-based therapeutics.

Conformational Changes of Whey and Pea Proteins upon Emulsification Approached by Front-Surface Fluorescence

Proteins are widely used to stabilize emulsions, and plant proteins have raised increasing interest for this purpose. The interfacial and emulsifying properties of proteins depend largely on their molecular properties. We used fluorescence spectroscopy to characterize the conformation of food proteins from different biological origins (dairy or pea) and transformation processes (commercial or lab-made isolates) in solution and at the oil-water interface. The fourth derivative of fluorescence spectra provided insights in the local environment of tryptophan (Trp) residues and thus in the protein structure. In emulsions, whey proteins adsorbed with their Trp-rich region at the oil-water interface. Proteins in the commercial pea isolate were present as soluble aggregates, and no changes in the local environment of the Trp residues were detected upon emulsification, suggesting that these structures adsorb without conformational changes. The lab-purified pea proteins were less aggregated and a Trp-free region of the vicilin adsorbed at the oil-water interface.

Applications of serum albumins in delivery systems: Differences in interfacial behaviour and interacting abilities with polysaccharides

One of the major applications of Serum Albumins is their use as delivery systems for lipophilic compounds in biomedicine. Their biomedical application is based on the similarity with Human Serum Albumin (HSA), as a fully biocompatible protein. In general, Bovine Serum Albumin (BSA) is treated as comparable to its human homologue and used as a model protein for fundamental studies since it is available in high amounts and well understood. This protein can act as a carrier for lipophilic compounds or as protective shell in an emulsion-based vehicle. Polysaccharides are generally included in these formulations in order to increase the stability and/or applicability of the carrier. In this review, the main biomedical applications of Albumins as drug delivery systems are first presented. Secondly, the differences between BSA and HSA are highlighted, exploring the similarities and differences between these proteins and their interaction with polysaccharides, both in solution and adsorbed at interfaces. Finally, the use of Albumins as emulsifiers for emulsion-based delivery systems, concretely as Liquid Lipid Nanocapsules (LLNs), is revised and discussed in terms of the differences encountered in the molecular structure and in the interfacial properties. The specific case of Hyaluronic Acid is considered as a promising additive with important applications in biomedicine. The literature works are thoroughly discussed highlighting similarities and differences between BSA and HSA and their interaction with polysaccharides encountered at different structural levels, hence providing routes to control the optimal design of delivery systems.

Sequential adsorption and interfacial displacement in emulsions stabilized with plant-dairy protein blends

HYPOTHESIS

Many traditional or emergent emulsion products contain mixtures of proteins, resulting in complex, non-equilibrated interfacial structures. It is expected that protein displacement at oil-water interfaces depends on the sequence in which proteins are introduced during emulsion preparation, and on its initial interfacial composition.

EXPERIMENTS

We produced emulsions with whey, pea or a whey-pea protein blend and added extra protein post-emulsification. The surface load was measured indirectly via the continuous phase, or directly via the creamed phase. The interfacial composition was monitored over a three-day period using SDS-PAGE densitometry. We compared these findings with results obtained using an automated drop tensiometer with bulk-phase exchange to highlight the effect of sequential protein adsorption on interfacial tension and dilatational rheology.

FINDINGS

Addition of a second protein increased the surface load; especially pea proteins adsorbed to pre-adsorbed whey proteins, leading to thick interfacial layers. The addition of whey proteins to a pea protein- or whey-pea protein blend-stabilized emulsion led to significant displacement of the pea proteins by β-lactoglobulin. We determined that protein-protein interactions were the driving force for this displacement, rather than a decrease in interfacial tension. These outcomes could be instrumental in defining new strategies for plant-animal protein hybrid products.

A miniaturized radial Langmuir trough for simultaneous dilatational deformation and interfacial microscopy

INNOVATION

Interfacial rheological properties of complex fluid-fluid interfaces are strongly influenced by the film microstructure. Experimental investigations for correlating interfacial morphology and rheology are notoriously challenging. A miniaturized radial Langmuir trough was developed to study complex fluid-fluid interfaces under purely dilatational deformations that operates in tandem with a conventional inverted microscope for simultaneous interfacial visualization.

EXPERIMENTS

Two materials were investigated at an air-water interface: poly(tert-butyl methacrylate) (PtBMA) and dipalmitoylphosphatidylcholine (DPPC). Surface pressure measurements made in the radial Langmuir trough were compared with a commercial rectangular Langmuir trough. Interfacial in situ visualization for each material was performed during the compression cycle in the radial trough. Challenges associated with the small size of the radial Langmuir trough, such as the influence of capillary deformation on the measured surface pressure, are also quantified.

FINDINGS

Measured surface pressures between the newly developed radial trough and the rectangular Langmuir trough compare well. Micrographs obtained in the radial Langmuir trough were used to obtain film properties such as Young's modulus. The new advance in colloid and interface science is the ability to capture structure-property relationships of planar interfaces using microscopy and purely dilatational deformation. This will advance the development of constitutive modeling of complex fluid-fluid interfaces.

The preparation of modified nano-starch and its application in food industry

Starch, which is a carbohydrate polymer with a semicrystalline granular structure, has been the subject of academic research for decades due to its renewable and biodegradable property as well as various applications in food, pharmaceutical and other industries. Nano-starch (NS) is a novel type of starch material with unique physiochemical properties due to its small size. However, the nano-size nature of NS determines its tendency to agglomeration as a natural process to approach a thermodynamically steady state, and the single hydroxyl functional group is also not favorable to its applications in hydrophobic environments. Thus, modified-NS with improved dispersion property, hydrophobicity, and stability is emerging as a new research direction. However, information about modified-NS is sporadic in literature, and a systematic review from its preparation, application, the problem and challenge as well as related health concerns is carried out to further the understanding of modified-NS. It is expected that the theoretical basis and new insight into the development of modified-NS will be improved.

Interfacial viscoelasticity and jamming of colloidal particles at fluid-fluid interfaces: a review

Colloidal particles can be adsorbed at fluid-fluid interfaces, a phenomenon frequently observed in particle-stabilized foams, Pickering emulsions, and bijels. Particles adsorbed at interfaces exhibit unique physical and chemical behaviors, which affect the mechanical properties of the interface. Therefore, interfacial colloidal particles are of interest in terms of both fundamental and applied research. In this paper, we review studies on the adsorption of colloidal particles at fluid-fluid interfaces, from both thermodynamic and mechanical points of view, and discuss the differences as compared with surfactants and polymers. The unique particle interactions induced by the interfaces as well as the particle dynamics including lateral diffusion and contact line relaxation will be presented. We focus on the rearrangement of the particles and the resultant interfacial viscoelasticity. Particular emphasis will be given to the effects of particle shape, size, and surface hydrophobicity on the interfacial particle assembly and the mechanical properties of the obtained particle layer. We will also summarize recent advances in interfacial jamming behavior caused by adsorption of particles at interfaces. The buckling and cracking behavior of particle layers will be discussed from a mechanical perspective. Finally, we suggest several potential directions for future research in this area.

Surfactant Interactions with Protein-Coated Surfaces: Comparison between Colloidal and Macroscopically Flat Surfaces

Surface interactions with polymers or proteins are extensively studied in a range of industrial and biomedical applications to control surface modification, cleaning, or biofilm formation. In this study we compare surfactant interactions with protein-coated silica surfaces differing in the degree of curvature (macroscopically flat and colloidal nanometric spheres). The interaction with a flat surface was probed by means of surface plasmon resonance (SPR) while dynamic light scattering (DLS) was used to study the interaction with colloidal SiO₂ (radius 15 nm). First, the adsorption of bovine serum albumin (BSA) with both SiO₂ surfaces to create a monolayer of coating protein was studied. Subsequently, the interaction of these BSA-coated surfaces with a non-ionic surfactant (a decanol ethoxylated with an average number of eight ethoxy groups) was investigated. A fair comparison between the results obtained by these two techniques on different geometries required the correction of SPR data for bound water and DLS results for particle curvature. Thus, the treated data have excellent quantitative agreement independently of the geometry of the surface suggesting the formation of multilayers of C₁₀PEG over the protein coating. The results also show a marked different affinity of the surfactant towards BSA when the protein is deposited on a flat surface or individually dissolved in solution.

Behavior of plant-dairy protein blends at air-water and oil-water interfaces

Recent work suggests that using blends of dairy and plant proteins could be a promising way to mitigate sustainability and functionality concerns. Many proteins form viscoelastic layers at fluid interfaces and provide physical stabilization to emulsion droplets; yet, the interfacial behavior of animal-plant protein blends is greatly underexplored. In the present work, we considered pea protein isolate (PPI) as a model legume protein, which was blended with well-studied dairy proteins (whey protein isolate (WPI) or sodium caseinate (SC)). We performed dilatational rheology at the air-water and oil-water interface using an automated drop tensiometer to chart the behavior and structure of the interfacial films, and to highlight differences between films made with either blends, or their constituting components only. The rheological response of the blend-stabilized interfaces deviated from what could be expected from averaging those of the individual proteins and depended on the proteins used; e.g. at the air-water interface, the response of the caseinate-pea protein blend was similar to that of PPI only. At the oil-water interface, the PPI and WPI-PPI interfaces gave comparable responses upon deformation and formed less elastic layers compared to the WPI-stabilized interface. Blending SC with PPI gave stronger interfacial layers compared to SC alone, but the layers were less stiff compared to the layers formed with WPI, PPI and WPI-PPI. In general, higher elastic moduli and more rigid interfacial layers were formed at the air-water interface, compared to the oil-water interface, except for PPI.

Structural-rheological characteristics of Chaplin E peptide at the air/water interface; a comparison with β-lactoglobulin and β-casein

The Chaplin E peptide is a surface-active agent that can adsorb to the air/water interface and form interfacial films that display distinct interfacial properties as a function of pH. The ~2 nm thick homogeneous Chaplin E film formed under acidic conditions contains ordered structures that give a high dilatational elasticity. In contrast, the heterogeneous film formed under basic conditions contained fibrils resulting in a rough ~17 nm thick film with predominantly viscoelastic properties, probably due to the reduced intermolecular interactions. These fibrils were also susceptible to breakage, fragmenting into shorter fibrils, which gave a greater elasticity. The fibrils also lead to a greater shear viscosity compared to the ordered structures aligned within the Chaplin E film at pH 3.0. A higher stability was observed for the foam formed by the Chaplin E compared to the foam formed by β-lactoglobulin, consistent with the greater rheological properties observed for the Chaplin E film at the interface. Our findings suggest that Chaplin E has potential to provide long time stability to dispersions in food, consumer goods or pharmaceutical applications, forming films with greater rheological properties and at least similar thickness to those formed by other surface-active proteins such as β-casein and β-lactoglobulin.

Adsorption properties of plant based bio-surfactants: Insights from neutron scattering techniques

There is an increasing interest in biosustainable surfactants and surface active proteins for a range of applications, in home and personal care products, cosmetics, pharmaceuticals, and food and drink formulations. This review focuses on two plant derived biosurfactants, the surface active glycoside, saponin, and the surface active globular protein, hydrophobin. A particular emphasis in the review is on the role of neutron reflectivity in probing the adsorption, structure of the adsorbed layer, and their mixing at the interface with a range of more conventional surfactants and proteins.