Biomedical subjects
Publications and source records attributed to Eric Dickinson.
At CO2 pressures between 40 and 100 bar, we have succeeded in creating very stable carbon dioxide-in-water emulsions using a relatively inexpensive and innocuous protein (principally beta-lactoglobulin) as the sole emulsifying agent.
The influence of the non-ionic surfactant Tween 20 on the microstructure of beta-lactoglobulin-stabilized emulsions with substantial excess free protein present was investigated via confocal microscopy. The separate distributions of oil droplets and protein were determined using two different fluorescent dyes. In the emulsion at ambient temperature the excess protein and protein-coated oil droplets were associated together in a reversibly flocculated state. The pore-size distribution of the initial flocculated emulsion was found to depend on the surfactant/protein ratio R, and at higher values of R the system became more inhomogeneous due to areas of local phase separation. Evidence for competitive displacement of protein from the oil-water interface by surfactant was obtained only on heating (from 25 to 85 degrees C) during the process of formation of a heat-set emulsion gel. By measuring fluorescence intensities of the protein dye inside and outside of the oil-droplet-rich areas, we have been able to quantify the evolving protein distribution during the thermal processing. The results are discussed in relation to previous work on the competitive adsorption of proteins and surfactants in emulsions and the effect of emulsion droplets on the rheology of heat-set protein gels.
We present Brownian dynamics simulations of the compression and expansion of monolayers adsorbed at a planar interface. The surface-active species are modelled as monodisperse spherical particles that can form particle-particle elastic bonds. The objective is to model the large compression and expansion of viscoelastic protein films investigated in Langmuir trough experiments. We determine the stress-strain response of the system and the associated microstructural changes induced by the large deformation of the interface as a function of particle adsorption energy, and bond breakability and stiffness. We also study the effect of the velocity of compression and the type of compression (uniaxial or homogeneous) on the mechanism of collapse of the adsorbed films. Furthermore, we present simulations on complex mixed systems containing both bond-forming particles (modelling protein) and nonbond-forming particles (modelling surfactant). We find that the preferential desorption of one type of particle or the other, upon compression, is sensitive to the extent of bond breakability of the bond-forming species.
A novel design of apparatus is described that allows observation of the coalescence stability of bubbles at a planar interface when the planar interface and the bubble surface both expand. Bubbles are introduced beneath the planar air-water interface contained within a square barrier made of perfluorocarbon rubber. The bubbles are then expanded by reducing the air pressure above the interface, while at the same time the rubber barrier is mechanically expanded, maintaining its square shape, to give the same rate and extent of expansion of the planar interface. The area can typically be increased by a factor of three over time scales as short as 0.2 s. This arrangement has been designed to mimic the behavior of aerated products when they exit from a pressurized aeration unit or product dispenser. Compared to results obtained via a previous technique, where it was only possible to expand the bubbles but not the planar interface, the bubbles are less stable. The apparatus has been used to compare the stabilizing effects of ovalbumin, beta-lactoglobulin, whey protein isolate, and sodium caseinate, in a model aqueous food system thickened with 40% invert sugar. Stability improved with increasing concentration of all the proteins and with a decrease in expansion rate, but considerable instability remained even at protein concentrations as high as 4 to 6 wt % and also at very low expansion rates, though the systems were stable in the absence of expansion. However, the stability was greatly improved by the replacement of the above proteins by the hydrocolloids gelatine or polypropylene glycol alginate. Detailed analysis revealed that the coalescence of individual bubbles in clusters of bubbles were not strongly correlated in distance or time, but larger bubbles and bubbles toward the outside of a cluster were found to be, on average, less stable than smaller bubbles and bubbles located more toward the interior of a cluster. The different degrees of stability are discussed in terms of local deformation, fracture behavior, and time-dependent composition of the adsorbed layers.
We investigate the effect of sucrose on the microstructure of sodium caseinate gels induced by acidification. The average pore size and the fractal properties of two-dimensional slices of the gels are studied using confocal scanning laser microscopy and image analysis. The addition of sucrose promotes stronger and more fine-stranded gels while re-arrangements of the network tend to be prevented. Whereas the fractal dimension itself is not sensitive to changes in the gel microstructure upon addition of sucrose, the maximum cut-off distance, up to which fractal scaling behaviour applies, is substantially reduced, as is the average pore size. The overall microstructural changes seem to be consistent with previous rheological and light scattering studies of the same system.
The time-dependent evolution of the phase-separated microstructure of a caseinate-stabilised emulsion containing xanthan gum added before emulsification has been investigated by confocal laser scanning microscopy, image analysis and rheology. Moderately low levels of xanthan addition lead to depletion flocculation and gravity-induced phase separation. Increasing the polysaccharide concentration causes immobilisation of the microstructure due to an increase in the local viscoelasticity: that is, the emulsion structure cannot easily rearrange to expel xanthan-enriched aqueous serum phase because a weak gel-like network is generated. The effect of xanthan on the evolving microstructure of phase-separated regions, which reflects indirectly the local emulsion micro-rheology, has been estimated from image analysis of time sequences of confocal micrographs. A comparison has been made between object shape analysis using four different shape descriptors. The roundness parameter has been found to be a convenient descriptor for reliably quantifying the structural change in terms of the relaxation rate of xanthan-rich aqueous drops. The Taylor parameter has been used to link the kinetics of drop relaxation to the time-dependent small-deformation rheological behaviour. The analysis of the combined experimental data reveals the difficulty of relating the evolving microstructure to bulk rheological measurements.
Heat stability has been studied in model systems of oil-in-water emulsions (3 wt.% total protein, 45 vol.% n-tetradecane, pH 6.8, ionic strength 30-50 mM) with pure beta-lactoglobulin (beta-lg) as the main emulsifier. The effect of small additions of sodium caseinate, beta-casein or alpha s1-casein prior to emulsion preparation has been investigated. Samples heated for 3 min at 90 degrees C were monitored with respect to changes in viscosity and particle-size distribution. As expected, the pure beta-lg-stabilized emulsions were susceptible to heat-induced changes. But the replacement of just 1% of the beta-lg by sodium caseinate (0.03 wt.% caseinate in the total emulsion) led to complete elimination of any heat-induced viscosity or particle size increase. These findings show that a very small proportion of casein can inhibit the susceptibility of a beta-lg-based emulsion to heat-induced destabilization. The magnitude of the effect is dependent on the type of casein, with the order of effectiveness being beta-casein>sodium caseinate>alpha s1-casein. This work has potential implications for the development of milk protein-stabilized emulsions of improved shelf life.
Air bubbles have been formed using partially hydrophobic silica nanoparticles as the stabilizer. The particles were of primary particle size 20 nm, chemically treated to different degrees with dichlorodimethylsilane to render them partially hydrophobic. Above a certain bubble size range (typically 80-microm diameter), the bubbles seemed to be almost indefinitely stable, while for any size above 20 microm their stability against disproportionation is far better than bubbles stabilized by any protein film investigated in previous studies. A possible theoretical justification for this observation is presented. Bubbles could be formed by shaking water with the particles, but a much higher volume fraction of bubbles was obtained by pressurizing the aqueous phase to 5 atm overnight followed by suddenly releasing the pressure to nucleate bubbles within the silica dispersion. Sonicating the silica dispersion before nucleation also gave more bubbles, which were also found to be more stable. There appeared to be an optimum degree of surface hydrophobicity that gave maximum foamability and foam stability, where around 20-33% of the silanol groups on the silica surface had been converted to dimethylsilane groups. However, a sharp increase in stability occurred when between 1.8 and 2 mol dm(-3) NaCl was also included in the aqueous phase. The change in stability due to inclusion of salt can be rationalized in terms of changes occurring in the value of the particle contact angle. The effects of increasing sonication and an optimum surface chemical treatment can be explained by the need to make the particles sufficiently hydrophobic so that they adsorb strongly enough, while at the same time minimizing their tendency to aggregate in the bulk aqueous phase, which hinders their adsorption. Furthermore, confocal laser scanning microscopy of the bubble dispersions suggests that a large volume fraction of stable bubbles is only formed when the particles adsorbed to the bubbles are also part of a spanning silica particle network in the bulk aqueous solution, forming a weak gel with a finite yield stress.
The interfacial thermodynamics and structure of ternary mixtures of the type A+B+solvent are investigated. According to the Gibbs phase rule, the coupling between the bulk phase and the interfacial region-which is related to the reversibility of the adsorption of the corresponding species-is a determinant as to whether phase separation can be observed at the interface. For an n-component adsorbing solution, at least one of the species has to adsorb irreversibly over the experimental time scales in order not to fix more intensive variables than those required to observe surface phase separation. We present results for a lattice model planar interface consisting of the ternary mixture A+B+solvent. The solvent molecules and the type A molecules have fixed chemical potentials at the interface since they are equilibrated with a bulk solution. In contrast, the type B molecules are irreversibly adsorbed at the interface and do not equilibrate with the bulk. Mean-field theory is compared with Monte Carlo simulation. Interestingly, the spinodal line in the interaction-composition plane shows a reentrant on the B-rich phase side. We discuss the implications of these results for surface phase separation of adsorbing mixtures of proteins and low-molecular-weight surfactants.
The uniaxial compression of a monolayer of nanosized monodisperse spherical particles adsorbed at a planar interface is simulated using the Brownian dynamics technique. Initially, the particles spread at the interface form crystalline loosely interconnected clusters. As the interface is compressed, the gaps between the clusters are removed and a close-packed monolayer is formed. At this stage, the structure of the interface consists of two-dimensional crystalline grains separated by defect boundaries. Further interfacial compression induces desorption of nanoparticles at these boundaries and creates striplike patterns of a secondary adsorbed layer. The structural features observed show remarkable agreement with recent experimental studies of the compression of gold nanoparticles in a Langmuir trough.
We have investigated the combined effect of ionic calcium and ethanol on the visual creaming behavior and rheology of sodium caseinate-stabilized emulsions (4 wt% protein, 30 vol% oil, pH 6.8, mean droplet diameter 0.4 microm). A range of ionic calcium concentrations, expressed as a calcium/caseinate molar ratio R, was adjusted prior to homogenization and varying concentrations of ethanol were added shortly after homogenization. A stability map was produced on the basis of visual creaming behavior over a minimum period of 8 h for different calcium/caseinate/ethanol emulsion compositions. A single narrow stable (noncreaming) region was identified, indicating limited cooperation between calcium ions and ethanol. The shear-thinning behavior of the caseinate-stabilized emulsions is typical of systems undergoing depletion flocculation. Addition of calcium ions and/or ethanol was found to lead to a pronounced reduction in viscosity and the onset of Newtonian flow. The state of aggregation was correlated with emulsion microstructure from confocal laser scanning microscopy. Time-dependent rheology (18 h) with a density-matched oil phase (1-bromohexadecane) revealed that the visually stable emulsions were time-independent low-viscosity fluids. Surface coverage data showed that increasing amounts of caseinate were associated with the oil-water interface with increasing R and ethanol content. A decrease in free calcium ions in the aqueous phase with moderate increases in R and ethanol content was observed, which is consistent with greater calcium-caseinate binding (aggregation). Ostwald ripening occurred at the high-ethanol emulsion compositions that were stable to depletion flocculation. While the coarsening rate was low, this can account for the cream plug formation observed during gravity creaming experiments. The caseinate emulsion with no ionic calcium or ethanol exhibits depletion flocculation from excess nonadsorbed caseinate submicelles. Addition of calcium ions reduces the submicelle number density via specific calcium-binding in the aqueous phase (fewer, larger calcium-caseinate aggregates) and at the droplet surface (increased surface coverage). Nonspecific ethanol-induced (calcium-dependent) caseinate submicelle aggregation in the bulk phase and on the droplet surface (increased surface coverage) culminates in a reduction in the number density of caseinate submicelles. A narrow window of inhibition of depletion flocculation occurs in systems containing both calcium ions and ethanol, both species combining to aggregate the protein and so reduce the density of free submicelles.
Effects of high-pressure treatment (HPT) on the rheological parameters and gelation of iota-carrageenan (iota-Car) and mixtures of micellar casein (MC) and iota-Car have been investigated under neutral pH conditions. It was established that HPT showed no significant effect, in the presence or absence of ionic calcium, with or without initial thermal processing, on the rheology or gelation/melting temperatures of the pure iota-Car solution. However, in mixed systems containing varying concentrations of iota-Car (up to 1 wt %) and MC (up to 8 wt %), considerable changes were detected. At the higher molar ratios of MC to iota-Car, and especially at the higher pressures, the dispersions were not thermoreversible in gelation, presumably due to the strong interactions of disrupted casein micelles with iota-Car molecules, as well as due to the formation of a dominant proteinaceous network at higher concentrations of MC. The associative protein-polysaccharide interactions in these systems are highly dependent on the ionic calcium content.
Proteins and low-molecular-weight (LMW) surfactants are used in the food industry as emulsifying (and foaming) ingredients and as stabilizers. These attributes are related to their ability to adsorb at fluid-fluid (and gas-fluid) interfaces lowering the interfacial (and surface) tension of liquids. Hence, the study of the properties of adsorbed layers of these molecules can be expected to lead to a better understanding of their effect on food products. Direct proof of the validity of mesoscopic models of systems of proteins and LMW surfactants can only be achieved by quantitative theoretical predictions being tested against both macroscopic and mesoscopic experiments. Computer simulation constitutes one of the few available tools to predict mathematically the behaviour of models of realistic complexity. Furthermore, experimental techniques such as atomic force microscopy (AFM) now allow high resolution imaging of these systems, providing the mesoscopic scale measurements to compare with the simulations. In this review, we bring together a number of related findings that have been generated at this mesoscopic level over the past few years. A useful simple model consisting of spherical particles interacting via bonded and unbonded forces is described, and the derived computer simulation results are compared against those from the imaging experiments. Special attention is paid to the adsorption of binary mixtures of proteins, mixtures of LMW surfactants, and also protein+surfactant mixed systems. We believe that further development of these mathematically well-defined physical models is necessary in order to achieve a proper understanding of the key physico-chemical processes involved.
The rate of shrinkage of air bubbles, of initial radius from 50 to 200 microm, injected beneath a planar air-water interface has been measured. Bubbles were stabilized in solutions of 0.05 wt% gelatin or pure beta-lactoglobulin. It has been observed that small size differences between two closely spaced or touching bubbles result in markedly divergent rates of shrinkage for the two. By studying a number of different initial bubble configurations, it is demonstrated that the overall change in bubble size distribution is strongly dependent on local, interbubble gas diffusion. In this respect, the strong tendency for the gelatin-stabilized bubbles to aggregate and shrink, while remaining in contact, produced patterns of disproportionation significantly different from those observed with beta-lactoglobulin. In beta-lactoglobulin solutions, it was usually found that bubbles initially in contact shrank away from each other with time, becoming increasingly isolated as a result. A theoretical approach that can exactly incorporate the perturbation of local diffusion fluxes due to the proximity of two bubbles is presented. This enables one to map a "stability diagram" that delineates regions where the larger bubble of a pair will either shrink or grow, according to the relative size of the bubbles and their separation. Theoretical calculations show that it is possible for a bubble to exhibit more complex shrinkage behavior than is predicted by a mean field approach or the Lifshitz, Slyozov, and Wagner (LSW) theory of Ostwald ripening for dilute systems. The inclusion of dilatational elasticity in the theoretical model introduces additional complications, which are also briefly discussed.
The rate of shrinkage of air bubbles of initial radii, r, from 50 to 150 microm injected beneath a planar air-water interface has been measured. Bubbles were stabilized by 0.05 wt% protein in approximately 0.1 mol dm(-3) ionic strength buffer at pH 7.0 and at room temperature. Four proteins were studied: commercial whey protein isolate (WPI), sodium caseinate, gelatin, and pure beta-lactoglobulin. Bubbles in all systems showed shrinkage due to diffusion of gas from the bubbles, which accelerated as the bubbles got smaller. Within approximately 1 h all bubbles had disappeared, having shrunk to below approximately 1 microm, so that in no cases was there evidence of stabilization via a surface rheological mechanism. The rates of shrinkage with the different proteins were not significantly different except in the case of gelatin, which at any given bubble size appeared to give a slightly higher rate, probably because the surface tension is higher for this system. A new theoretical analysis of the dissolution kinetics for the case of a bubble close to a planar interface has been developed. For caseinate and WPI a simple model incorporating a constant surface tension and a constant bubble-interface separation appears to account for the kinetics. Interestingly, the model predicts a linear dependence of r(n) versus time when n is closest to 3, in contrast to n = 2 expected from previous work. For gelatin and pure beta-lactoglobulin, the introduction of modest dilatational elasticities of approximately 2.3 and 7 mN m(-1), respectively, gives good agreement between theory and experiment. This is particularly the case for beta-lactoglobulin, where there is a noticeable slowing, but not cessation, of the shrinkage as the bubbles get smaller. In the light of these findings the practical significance of surface rheology with respect to stability to disproportionation is discussed. Finally, we present experimental evidence that a bubble stabilized by beta-lactoglobulin shrinks to a nonspherical protein particle consisting of the completely collapsed protein film.
The influence of high-pressure treatment (HPT) (200-800 MPa, 5 or 20 min, at 20 degrees C) on the rheological properties of solutions of amidated low-methoxyl pectin (LMP) and its mixtures with micellar casein (MC) has been investigated in the presence and absence of sucrose. The storage modulus G' of LMP gels containing 0-55 wt % sucrose and 0.1-1 wt % LMP was found to increase significantly following HPT at >or=400 MPa. Various concentrations of LMP in the presence of different amounts of MC (0.5-12 wt %) showed contrasting types of rheological behavior. In the presence of a low concentration of LMP (<0.3 wt %), HPT was found to induce a sol-gel transformation at relatively high LMP/MC molar ratios (<4 wt % MC), to reduce values of G' and the loss modulus G' ' at intermediate LMP/MC ratios (4-10 wt % MC), and to increase the values of G' and G' ' at low LMP/MC ratios (>10 wt % MC). In contrast, in the presence of a higher amount of LMP (>0.5 wt %), it was observed that HPT enhances the values of both the storage and the loss moduli over the whole range of MC concentrations.
We describe the quantitative interrelation between the thermodynamic parameters of caseinate submicelles in the presence of calcium ions (0-14 mM) in aqueous medium and the capacity of the protein to induce depletion flocculation in oil-in-water emulsions at pH 7.0 and ionic strength 0.05 mol dm(-3). Measurements have been made by static and dynamic multiangle laser light scattering of the weight-average molecular weight, the radius of gyration, the hydrodynamic radius, and the second virial coefficient of caseinate submicelles in aqueous solution. Successive thermodynamic approximations with and without consideration of correlations between caseinate submicelles have been used to calculate the osmotic pressure in caseinate aqueous solutions and the free energy of the depletion interaction between droplets in oil-in-water emulsions stabilized by caseinate. Numerical results from both thermodynamic approximations are in reasonably good agreement with experiment, predicting a pronounced decrease in the strength of the depletion attraction at concentrations of Ca(2+) in the range 4-8 mM (with a minimum value at 8 mM). This correlates well with the great enhancement of stability of these emulsions with respect to flocculation in comparison with systems having no added ionic calcium and emulsions with lower (2 mM) or higher (10 mM) Ca(2+) contents. Nevertheless, the allowance for interactive correlations between caseinate submicelles seems to lead to a better prediction of emulsion flocculation on a qualitative level over the whole range of Ca(2+) concentrations studied (2-14 mM). The calculated pronounced decrease in depletion interaction strength is attributable to marked changes in weight-average molecular weight and mean size of aggregates, and to more positive values of the second virial coefficient of caseinate submicelles with increasing Ca(2+) content. Finally, we discuss the part played by the electrical charge on the protein in determining the overall strength of the flocculation-inducing attractive interactions between droplets. Copyright 2001 Academic Press.
Colloidal particle scattering is a recently developed method for the determination of surface forces between micrometer-sized particles. In this paper we extend earlier simulation studies to interactions of the DLVO type including purely attractive potentials. We examine the criteria for capture and compare simulated results for a range of interaction parameters. We find that the scattering patterns can be represented in a simple way once a correction for angular distortion has been applied. This representation should aid the analysis and interpretation of experimental data. Copyright 2000 Academic Press.