Dilational Rheology Research Articles

Interfacial properties of protein nanofibrils with different morphology prepared using aqueous solvent with ethanol: Part II. Effect of oil phase hydrophobicity

Self assembled β-lactoglobulin nanofibrils of different morphologies (long semi-flexible or short worm-like) were prepared at pH 2 and 85 °C in the aqueous solution containing different amount of ethanol. The effects of fibril morphology and polarity of the hydrophobic sub-phase on the interfacial adsorption kinetics, viscoelasticity, and structuration of the protein fibrils were investigated using dilatational rheology. It was found that the fibrils have three-stage adsorption at all the oil-water interfaces: diffusion-controlled adsorption in early stage, penetration adsorption near the interface and rearrangement within the interface. Diffusion adsorption rate (Kdif) decreases while penetration rate (kads) and rearrangement (kr) increase with increasing oil polarity. The strength and structuration of the interfacical fibril networks decrease with increasing oil polarity. The forces maintaining the stability of the protein fibril interface were mainly hydrophobic interaction. Kdif, kads and kr of the short fibrils are higher than those of the long fibrils due to their smaller size and higher surface hydrophobicity. The long fibril interface displayed strain softening response during both extension and compression, but form stronger interface than short fibrils, and thus demonstrate better foaming and emulsification ability. The short worm-like fibril interface displays strain softening during extension but strain hardening during compression, which is to some degree similar to the behavior of native flexible protein molecules.

The synergism of improving interfacial properties between betaine and crude oil for enhanced oil recovery

Betaine, an emerging surfactant for enhanced oil recovery (EOR), has garnered increased attention and recognition. This study focuses on linear alkylbenzene sulfonate betaine (ASB) as the research subject, and different crude oils were utilized to prepare two types of oil–water formulations with high interfacial film strengths but differed interfacial tensions. The interfacial properties were characterized and explored using interfacial dilational rheology, droplet squeeze coalescence, interfacial tension, emulsification, microscopic visualization, and other measuring methods. The actual emphasis in interfacial film properties of betaine as a standalone chemical EOR formulation was clarified. The results indicated that betaine had generally high interfacial film strength with selected crude oils, at this time, the slight difference in film strength had a negligible effect on enhancing the recovery. The main contributing factor was the difference in interfacial tension between the oil and water. Ultra-low interfacial tension could increase the stripping efficiency of oil in pore, reduce the energy required to emulsification or oil droplets initiation, and ensure that droplets were displaced and collected in smaller sizes, thereby avoiding large oil droplets trapped in the pore throat and unable to pass. Making good use of the advantages of improving sweep efficiency brought by high interfacial film strength, at the same time giving the system ultra-low oil–water interfacial tension, ensuring the start-up and migration of porous crude oil, is the core construction principle of betaine surfactant applied to EOR.

Designing Gastric-Stable Adsorption Layers by Whey Protein-Pectin Complexation at the Oil-Water Interface.

This work aims to design gastric-stable emulsions with food-grade biopolymers using a novel multiscale approach. The adsorption layer formation at the oil-water interface was based on opposite charge interactions between whey proteins and pectin (with different esterification levels) at pH 3.0 by a sequential adsorption method. The interfacial assembly and disassembly (interfacial complexation, proteolysis, lipolysis) during in vitro gastric digestion were evaluated using a quartz crystal microbalance with dissipation monitoring, ζ-potential, dynamic interfacial tension, and interfacial dilatational rheology. Besides, the evolution of the particle size and microstructure of bulk emulsions during the digestion was investigated by static light scattering and light microscopy. Compared with whey protein isolate (WPI)-stabilized emulsions, the presence of an additional pectin layer can prevent or at least largely delay gastric destabilization (giving rise to coalescence or/and oiling off). Especially, the esterification degree of the pectin used was found to largely affect the emulsion stability upon gastric digestion.

Adsorption and Desorption of Bile Salts at Air–Water and Oil–Water Interfaces

Bile Salts (BS) adsorb onto emulsified oil droplets to promote lipolysis and then desorb, solubilizing lipolytic products, a process which plays a crucial role in lipid digestion. Hence, investigating the mechanism of adsorption and desorption of BS onto the oil–water interface is of major importance to understand and control BS functionality. This can have implications in the rational design of products with tailored digestibility. This study shows the adsorption and desorption curves of BS at air–water and oil–water interfaces obtained by pendant drop tensiometry. Three BS have been chosen with different conjugation and hydroxyl groups: Sodium Taurocholate (NaTC), Glycodeoxycholate (NaGDC) and Sodium Glycochenodeoxycholate (NaGCDC). Experimental results show important differences between the type of BS and the nature of the interface (air/oil–water). At the air–water interface, Glycine conjugates (NaGDC and NaGCDC) are more surface active than Taurine (NaTC), and they also display lower surface tension of saturated films. The position of hydroxyl groups in Glycine conjugates, possibly favors a more vertical orientation of BS at the surface and an improved lateral packing. These differences diminish at the oil–water interface owing to hydrophobic interactions of BS with the oil, preventing intermolecular associations. Desorption studies reveal the presence of irreversibly adsorbed layers at the oil–water interface in all cases, while at the air–water interface, the reversibility of adsorption depends strongly on the type of BS. Finally, dilatational rheology shows that the dilatational response of BS is again influenced by hydrophobic interactions of BS with the oil; thus, adsorbed films of different BS at the oil–water interface are very similar, while larger differences arise between BS adsorbed at the air–water interface. Results presented here highlight new features of the characteristics of adsorption layers of BS on the oil–water interface, which are more relevant to lipid digestion than characteristics of BS adsorbed at air–water interfaces.

Surfactant Adsorption Layers: Experiments and Modeling.

During recent years, great progress has been made in understanding the adsorption of surfactants at liquid interfaces. In addition to tensiometry, new efficient methodologies have been developed, in particular interfacial selective optical methods which allow direct access to the adsorbed amounts and interfacial layer compositions. In addition to these new experimental tools, the thermodynamic description by equations of state now allows one to provide a quantitative picture of surfactant interfacial layers. This is most notable for surfactant layers at water/oil interfaces. Additional knowledge about the structure of interfacial layers was gained through different types of molecular modeling. Improved interrelationships between these three aspects are the challenges for current and future work. Particular attention must be paid to dilational interfacial rheology studies, as these mechanical quantities are much more sensitive to small changes in the interfacial composition and structure.

Interfacial adsorption behavior of the Aspergillus oryzae lipase-chitosan complex and stability evaluation of the resultant Pickering emulsion

To prompt the application of the chitosan (CS)-Aspergillus oryzae lipase (AOL) complex in the construction of novel biphasic catalysis medium, its Pickering emulsion stabilization ability as well as adsorption behavior in the oil-water interface were investigated and the stability of resultant emulsion was evaluated. The results indicated that the CS-AOL complex assembled in mass ratio 1:5 was an effective Pickering stabilizer and up to 90 % AOL could be retained in the emulsion interface. Quartz crystal microbalance with dissipation monitoring suggested that the CS-AOL complex spontaneously absorbed to oil-water interface; absorption dynamics analysis revealed that the adsorption was driven by diffusion accompanied by rapid structural rearrangement; while interfacial dilatational rheology demonstrated the formation of an elastic film in the oil-water interface. The Pickering emulsions were pseudoplastic and that in oil fraction 0.6 exhibited the elastic behavior in contrast to the viscous behavior in oil fractions 0.2 and 0.4. The Pickering emulsion exhibited excellent stability against storage for up to 28 d, pHs 2.0–12.0, heating at 25–90 °C, and up to 500 mmol/L NaCl, and the corresponding interfacial AOL retentions exceeded 80 % during exposure to these conditions. Hence, the CS-AOL complex could be used as a stabilizer to construct Pickering emulsion-based biphasic catalysis systems.

How to Regulate the Migration Ability of Emulsions in Micro-Scale Pores: Droplet Size or Membrane Strength?

Micro visualization has become an important means of solving colloid and interface scientific problems in enhanced oil recovery. It can establish a relationship between a series of performance evaluations of an oil-water interface under macroscopic dimensions and the actual application effect in confined space, and more truly and reliably reflect the starting and migration behavior of crude oil or emulsion in rock pores. In this article, zwitterionic surfactant alkyl sulfobetaine (ASB) and anionic extended surfactant alkyl polyoxypropylene sulfate (A145) were employed as flooding surfactants. The macroscopic properties of the surfactant solutions, such as the oil-water interfacial tension (IFT), the interfacial dilational rheology and the viscosity of crude oil emulsions, have been measured. At the same time, we link these parameters with the oil displacement effect in several visual glass models and confirm the main factors affecting the migration ability of emulsions in micro-scale pores. The experimental results show that ASB reduces the IFT through mixed adsorption with crude oil fractions. The flat arrangement of the large hydrophilic group of ASB molecules enhances the interactions between the surfactant molecules on the oil-water interface. Compared with sulfate, betaine has higher interfacial membrane strength and emulsion viscosity. A145 has a strong ability to reduce the IFT against crude oil because of the larger size effect of the PO chains at the oil side of the interface. However, the membrane strength of A145 is moderate and the emulsion does not show a viscosity-increasing effect. During the displacement process, the deformation ability of the front emulsions or oil banks is the main controlling factor of the displacement efficiency, which is determined by the membrane strength and emulsion viscosity. The strong interfacial membrane strength and the high emulsion viscosity are not conducive to the migration of droplets in pore throats and may result in low displacement efficiency.

Persimmon tannin can enhance the emulsifying properties of persimmon pectin via promoting the network and forming a honeycomb-structure

In the present study, multi-scale methods, including emulsifying capability evaluation and emulsion stability determination, were used to explore the effect of persimmon tannin (PT) on the emulsification performance of persimmon pectin (PP). A series of PP-PT solutions containing 10 mg/mL of PP and 0.05–2 mg/mL of PT were prepared and used to fabricate oil-in-water emulsions. Further, the potential influencing mechanism of PT on the emulsifying characteristics of PP was exploited by the Cryo-SEM morphology, interfacial tension, and interfacial dilatational rheology measurements. The results indicated that the emulsifying and emulsion-stability potential of PP was enhanced with the addition of high-level PT (1 and 2 mg/mL), reflected by the improvement in emulsifying capacity, droplet sizes (D 32 , D 43 , and particle size), TSI value, creaming thickness, and the strengthened tolerance to salt ions (100–1000 mmol/L NaCl and 100–500 mmol/L CaCl 2 ), heating treatment (50–90 °C for 60 min), and accelerated destruction process (thermal cycling and accelerated centrifugation). However, PT in low concentrations (0.05, 0.1, and 0.2 mg/mL) displayed emulsion-damaging effects. In addition, high concentrations of PT (1 and 2 mg/mL) promoted the 3D-network structure, formed the honeycomb structure of emulsions and enhanced the viscoelasticity of the interfacial film to resist unfavorable factors. In contrast, the network structure and interfacial viscoelasticity deteriorated when low levels of PT (0.05, 0.1, and 0.2 mg/mL) were added. Overall, high levels of PT can be a cooperative agent for enhancing the emulsification properties of PP. Our data might provide a scientific basis for broadening the application of PP and PT as functional ingredients in the food industry. • PT significantly affected the emulsification properties of PP. • High concentrations of PT (1 and 2 mg/mL) enhanced PP's (10 mg/mL) emulsifying performance; by contrast, the low levels (0.05, 0.1, and 0.2 mg/mL) damaged it. • High concentrations of PT (1 and 2 mg/mL) promoted the 3D-network structure, formed the honeycomb structure of emulsions and enhanced the viscoelasticity of the interfacial film to resist unfavorable factors.

Cerebrosides lipids and their physical effect on multiple sclerosis membranes.

The myelin sheath is a critical component of the central nervous system (CNS) and peripheral nervous system (PNS). It is a multilamellar membrane consisting of a coiled bilayer that acts as an electrical insulator for nerve impulses traveling along axons between nerve cells. Compared to other membranes in the body, the myelin sheath contains a large lipid fraction in the range of 75-80% of the dry mass, compared to about 50% in other membranes, consisting of many different lipids. The lipids in the myelin sheath play a critical role in membrane stability and compaction. While previous investigations have used model lipid bilayers and monolayers to study their interactions with important proteins including myelin basic protein (MBP) and proteolipid protein (PLP), our understanding of the role of lipid composition on membrane formation thermodynamics and rheology, particularly the effects produced by lipid cerebrosides (Hydroxylated cerebrosides, Non Hydroxylated cerebrosides and Cerebroside sulfatide). In this presentation we study the formation and interfacial dilatational rheology of lipid monolayers. We use a Langmuir trough for both surface pressure versus area isotherms and oscillating area interfacial rheology measurements, all performed at approximately body temperature (37°C). From isotherm data, we calculate differences in the Gibbs free energy of mixing between the four monolayers to show differences in intermolecular interactions and miscibility. These results are used to support observations in interfacial rheology results, where. We also analyze the nonlinear rheological behavior of the monolayers, for example observing in general film softening during compression and expansion. These first oscillatory interfacial rheology measurements of model myelin monolayers, coupled with the thermodynamic analysis of film formation, improve our understanding of the lipid the film properties, and provide clues to how the changes in lipid composition may lead to demyelination.

Dynamic interfacial properties and foam behavior of licorice root extract solutions

Licorice (Glycyrrhiza glabra) is a useful plant of the family Fabaceae, with sweet-tasting roots. The root extract of this plant is rich in saponins, so it can be considered a source of natural surfactants. This research provides some applicable information about the dynamic surface tension and foam behavior of aqueous solutions of licorice root extract (LRE). The pendant drop shape analysis was utilized to study the surface tension and dilational surface rheology of LRE at the water/air interface. The Bikerman type experiment was used to measure foamability and foam stability of aqueous LRE solutions. The equilibrium surface tensions reveal that the LRE contains surface-active components and is capable of reducing the surface tension by 25 mN/m at the critical aggregation concentration (CAC). The surface dilational visco-elasticity measurements proved that the adsorption layers are predominantly of elastic nature. Also the foamability and foam stability show a meaningful correlation with the dynamic surface properties. This study aims to contribute to the development of appropriate utilization of the benefits provided by a biosurfactant source in foam-related commercial applications.

Adsorption kinetics and dilatational rheology of plant protein concentrates at the air- and oil-water interfaces

Adsorption kinetics and dilatational rheology of plant protein concentrates at the air- and oil-water interfaces were investigated at pH 7.0 in 100 mM NaCl. Three interfaces (air-water, triglyceride-water and terpene-water) and four protein concentrates (soy, pea, mung bean and rice) were examined. The dynamic interfacial properties were monitored by axisymmetric drop shape analysis. Kinetic modelling of the early and advanced stages of protein adsorption was carried out using the Ward-Tordai and Graham-Philips thermodynamic approaches. Construction of surface pressure master curves revealed a pseudo equilibrium plateau for legume proteins of ∼20, 12, and 22 mN/m at the air, triglyceride and terpene interfaces, respectively. In contrast, rice proteins have a lower capacity to increase the surface pressure at the oil interfaces (<15 mN/m). Data modelling revealed that diffusion is mostly independent of the protein composition, but protein rearrangement at the interfaces was faster at the oil than at the air interfaces. Dilatational rheological measurements revealed more elastic films at the air than at oil interfaces, with the dilatational storage modulus reaching values up to 37 mN/m. The least elastic films were formed at the terpene interfaces, with storage moduli being <25 mN/m for all isolates investigated. Lissajous plot construction revealed a strain-hardening behaviour of films upon compression and strain-softening on extension, the magnitude of which follows the order air > terpene > triglyceride. Overall, results show that botanical source and subphase composition are critical in selecting the optimum stabilisation strategy in multiphasic foods using plant proteins.

Dynamic surface properties of carboxyfullerene solutions

Application of conventional methods of dilatational surface rheology in combination with optical techniques and atomic force microscopy allowed determination of the surface properties of a water-soluble fullerene derivative, carboxyfullerene C60(C(COOH)2)3. It has been shown that the surface activity of this derivative is higher than that of other soluble fullerene derivatives, for example, of fullerenols. The dynamic surface elasticity of carboxyfullerene solutions reaches up to values of about 180 mN/m. Brewster angle microscopy indicates a liquid-like nature of carboxyfullerene adsorption layers. The application of atomic force microscopy shows that these adsorption layers consist of numerous interconnected surface aggregates. The size and number of aggregates increase during adsorption. The compression of carboxyfullerene spread layers results in its partial dissolution. At the same time, the compression isotherms indicate a high stability of the layers of carboxyfullerene aggregates.

Competitive interfacial behavior in sodium caseinates-hydroxypropyl cellulose mixed systems

In this work, interactions between hydroxypropyl cellulose (HPC) and sodium caseinates (Na-cas) at the air-water interface were studied. The surface tension, surface rheology, and film compatibility of the mixed protein/polysaccharide system were investigated. Tensiometry experiments showed that HPC is more surface-active than proteins, and is preferentially adsorbed at the interface. This molecule is able to migrate to the interface faster than Na-cas despite its larger size. At low HPC concentration, proteins adsorb first but HPC being more surface-active will eventually substitute them at the interface. The use of the pendant drop tensiometer allowed the real-time monitoring of protein displacement; an immediate decrease in surface tension was observed when HPC was added to a Na-cas solution. The analysis of the compression isotherms of Na-cas/HPC mixed films revealed partial miscibility between the two species within the monolayer; the films were less expanded than expected from the ideal behavior of binary mixtures. The dilatational rheology experiments showed that the presence of a very small amount of HPC had a significant effect on the viscoelastic properties of the interface. This work reported for the first time the competitive adsorption between Na-cas and HPC at the A/W interface.

Dynamic stability of a crude oil/brine interface: Effect of anion type

The chemical composition of the aqueous phase plays an essential role in water-based enhanced-oil recovery processes. The aqueous phase ionic profile affects the snap-off response of a crude oil-brine system by linking it to the brine-crude oil interfacial rheology. In this work, we focus on the effect of the anion on the snap-off response. To this end, a sample of crude oil from the Minnelusa Sandstone formation in Wyoming was selected, and several aqueous solutions were prepared with salts containing sodium cation and different anions (chlorides, sulfates, and phosphates) at two ionic strength values, 0.6 M and 6 mM, as cases of high- and low-salinity water. Interfacial tension, interfacial viscoelasticity and interface stability were analyzed as functions of aging time using pendant drop, dilatational rheology, and liquid bridge techniques, respectively. Results show that solutions with higher ionic strength led to the formation of more rigid interfaces, i.e. higher elastic modulus, and faster formation kinetics. In the low-salinity regime, solutions with polyvalent anions produce more viscoelastic interfaces at faster rates than solutions with monovalent anions. Finally, an excellent correlation (R2 greater than 0.97) between the relative stability of the interfacial film, predicted in liquid bridge tests, and the ratio of interfacial tension and interfacial elastic modulus was found.

Non-linear dilational rheology of liquid-liquid interfaces stabilized by dipeptide hydrogels

We investigate the effects of salt concentration on the rheological properties of dipeptide hydrogel fibres at liquid-liquid interfaces. The interfaces were subjected to large amplitude oscillatory dilation (LAOD) experiments across a range of oscillation strains and frequencies. Lissajous plots of pressure-strain were used for characterizing the viscoelastic properties and for identifying apparent yielding. We show that key aspects of the rheological response of the interfaces vary significantly with salt concentration. At low strain, independent of salt concentration, Lissajous curves show an almost elliptical shape. As the strain is increased, asymmetry in Lissajous curves evidences a non-linear response. The departure from an ellipse is most obvious at negative strain (at moderate to high salt concentrations) and is suggestive of strain-hardening on compression. The Lissajous curves tilt towards the diagonal at elevated salt concentration demonstrating that the interfaces are becoming increasingly elastic. However, increasing the frequency of the oscillation has little systematic effect. We infer that the addition of salt leads to the development of structure on the interfaces from our observations strain-hardening and of the increasingly elastic response. To fully capture the range of behaviour, we suggest a modification of the analysis to calculate the strain-hardening ratio S used to quantify the degree of non-linearities from Lissajous figures, so as to better reveal the presence of instant strain-softening and strain-hardening responses.

Effect of deacetylation degree and molecular weight on surface properties of chitosan obtained from biowastes

A surface characterization of two types of unmodified chitosan properties with different molecular weight (MW) and deacetylation degree (DD) was carried out in terms of polymer concentration (a concentration lower than the saturation, the saturation concentration and a supersaturated concentration). The modification of the production conditions allowed to obtain chitosan samples with different properties (MW and DD), which eventually determined its surface activity. Thus, a kind of tailor-made chitosan may be designed to be adapted to different applications. Surface activity has been analysed by mean of measurements of surface tension, dilatational and shear rheology. The results have demonstrated that chitosan possessed a good surface activity at high ionic strength. Also, the increase in the concentration of chitosan in the solution, and the increase in molecular weight (and, therefore, a decrease in the degree of deacetylation) favoured the formation of aggregates, which improved the adsorption kinetics. The latter allowed adequate elasticity values to be reached more quickly. These properties are reached faster for the higher molecular weight chitosan (lower degree of deacetylation), due to the more hydrophobic character that facilitates the interactions between the adsorbed aggregates. Surface rheology revealed a predominantly elastic behaviour of chitosan film. All of these results strengthen the idea that chitosan behaves as a surface-active hydrophilic Pickering agent.

The Biomimetic System of Oleanolic Acid and Oleic Acid at the Air-Water Interface-Interactions in Terms of Nanotechnology-Based Drug Delivery Systems.

Oleanolic acid (OLA) and oleic acid (OA) are ubiquitous in the plant kingdom, exhibiting a therapeutic effect on human health, and are components of novel pharmaceutical formulations. Since OLA has limited solubility, the utilization of nanotechnology-based drug delivery systems enhancing bioavailability is highly advantageous. We report on the interfacial behavior of the OLA:OA system at various molar ratios, using the Langmuir technique to assess the dependence of the molar composition on miscibility and rheological properties affecting film stability. Specifically, we evaluate the interfacial properties (morphology, thermodynamics, miscibility, and viscoelasticity) of the OLA:OA binary system in various molar ratios, and indicate how the OLA:OA system exhibits the most favorable molecular interactions. We apply the Langmuir monolayer technique along with the complementary techniques of Brewster angle microscopy, dilatational interfacial rheology, and excess free energy calculations. Results demonstrate that the properties of mixed monolayers depend on OLA:OA molar ratio. Most of the systems (OLA:OA 2:1, 1:1, 1:5) are assumed to be immiscible at surface pressures &gt;10 mN/m. Moreover, the OLA:OA 1:2 is immiscible over the entire surface pressure range. However, the existence of miscibility between molecules of OLA and OA in the 5:1 for every surface pressure tested suggests that OA molecules incorporate into the OLA lattice structure, improving the stability of the mixed film. The results are discussed in terms of providing physicochemical insights into the behavior of the OLA:OA systems at the interface, which is of high interest in pharmaceutical design.

Foaming properties of the complex of chitooligosaccharides and bovine serum albumin and its application in angel cake

The complex of chitooligosaccharides (COS) and protein is worthy of investigation, as COS is an important functional ingredient and the functional properties of protein might be improved after complexation. Using bovine serum albumin (BSA) as the model protein, this work explored the foaming properties and interfacial behavior of COS-BSA complex, as well as its application as a substitute for egg whites in angel cake. The effects of pH and mixing ratio ( r ) on the foam properties and interfacial behavior of COS-BSA complex were studied using dilute solution. And the effects of the COS-BSA substitution rate on the physical properties and microbial storability of the cake were studied. Compared with BSA, the foaming capacity and foam stability of COS-BSA complexes were significantly improved in the pH range of 3.0–10.0, and both the foaming capacity and foam stability of COS-BSA complexes at pH 7.0 were in the order of r 0.5 ≥ r 0.25 ≥ r 1.0 > r 2.0 > BSA > COS. Angel cake with the COS-BSA substitution rate of 25% had the best physical properties, in which COS and BSA had a synergistic effect on the specific volume and texture of the cake. The substitute of COS-BSA complex for egg white in angel cake significantly inhibited microbial growth during storage. This work provided COS-BSA complex as a substitute for egg white in cake making, and COS-BSA complex improved the physical properties of cake and inhibited microbial growth. • Chitooligosaccharides improved foaming properties of bovine serum albumin. • Good foaming of complexes was related with their surface dilatational rheology. • Cake with complex substitution rate of 25% exhibited the best physical properties. • Complex addition in cake significantly inhibited microbial growth during storage.

Structure and rheological properties of oil-water and air-water interfaces stabilized with micellar casein isolate and whey protein isolate mixtures

The mechanical properties of oil-water (O–W) and air-water (A-W) interfaces stabilized with mixtures of micellar casein isolate and whey protein isolate at different ratios were investigated by large-amplitude oscillatory dilatational rheology (LAOD). The composition of the layer of proteins adsorbed at the O–W interfaces was investigated by multiphoton excitation microscopy. The structure of A-W interfaces was visualized by atomic force microscopy. The results obtained for O–W interfaces showed that casein preferentially adsorbs and dominates the interface even when present in mixtures in a very low proportion. The LAOD results show that the interfacial layer formed by casein is weaker and more brittle compared with the one formed by whey proteins. In case of A-W interfaces, casein is more surface active and displays faster diffusion, and leads to a decrease of surface tension to lower values compared with whey protein. Casein will co-adsorb with whey protein at the A-W interface, and the dilatational response of the viscoelastic solid-like network normally formed by pure whey proteins at the interface is significantly affected by casein, particularly at large deformation amplitudes. The interfacial layer appears to become more brittle with an increasing proportion of caseins in the mixture.

Cruciferin versus napin – Air-water interface and foam stabilizing properties of rapeseed storage proteins

Rapeseed protein extract has high nutritional value and good techno-functionalities, e.g. in foam systems. Yet, its exact interface and foam stabilizing mechanisms are not well understood. Rapeseed proteins comprise mainly of cruciferin and napin. Our aim was to systematically investigate the interface stabilization behaviors of cruciferin, napin and combinations of both at air-water interfaces. We used surface (dilatational and shear) rheology and microstructure imaging (AFM) and linked their behavior to their foaming properties. We observed that napin adsorbed faster at the air-water interface than cruciferin due to its smaller size, leading to 90% higher foam overrun than cruciferin (320%). The interfaces showed distinct differences in structure and mechanical properties, as cruciferin formed stiff solid-like interfaces with Ed’ = 72.5 mN/m and Gi’ = 9.0·10−3 Pa m, leading to high foam stability (half-life time 220 min). Napin formed weaker less stretchable interfaces (Ed’ = 61.8 mN/m; Gi’ = 6.7·10−3 Pa m), leading to substantially lower foam stability (half-life time 23 min). Cruciferin and napin were also mixed at 3:1, 1:1 and 1:3 (w/w) ratios. Napin increased the foamability of all mixtures with foam overrun between 400 and 420%. The mixture at 3:1 cruciferin-to-napin ratio had comparable foam stability with pure cruciferin since cruciferin dominated the mechanical properties of the air-water interface. Higher napin contents largely decreased foam stability with half-life time decreasing to 80 min. These findings provide a comprehensive understanding of the behaviors of rapeseed proteins at air-water interfaces and their link to foaming properties, which can be used to tailor the properties of aerated products stabilized by rapeseed proteins.