Valerie J. Pinfield

SELF-CONSISTENT-FIELD MODELLING OF ADSORBED CASEIN: INTERACTION BETWEEN TWO PROTEIN-COATED SURFACES

The equilibrium distribution of polymer segments and the interaction free energy between flat hydrophobic surfaces covered with the individual milk proteins, α s1 -casein and β-casein, have been calculated over the pH range 5.5–7.0 using Scheutjens–Fleer self-consistent-field theory. The interaction potential between the β-casein layers is predicted to be repulsive at all ionic strengths, whereas that between the α s1 -casein layers has an attractive well above a certain ionic strength (ca. 0.05 M). The strong repulsion between β-casein layers is attributed to combined steric and electrostatic interaction involving the charged dangling tail of adsorbed β-casein. The attraction between α s1 -casein layers is attributed to extensive bridging of chains between the opposite surfaces. These predictions are consistent with the poorer stability of α s1 -casein emulsions towards flocculation by salt.

A comparative study of ultrasound and laser light diffraction techniques for particle size determination in dairy beverages

The particle size distribution (PSD) of milkfat droplets of raw and homogenized milk was determined by a diagnostic ultrasound technique that was initially calibrated with colloidal silica. The quality of the fit between ultrasound theory and experiment is discussed and provides a basis for comparison with the reference laser light scattering technique. The predicted ultrasonic attenuation was determined from the ECAH (Epstein P S and Carhart R R 1953 J. Acoust. Soc. Am. 25 553–65, Allegra J R and Hawley S A 1972 J. Acoust. Soc. Am. 51 1545–64) model using thermophysical data for milkfat and milk serum. New thermophysical properties of milkfat were established. The experimental data were obtained with the Ultrasizer spectrometer. Both techniques were affected by the milk proteins and could provide only qualitative milkfat PSD for the raw and the homogenized milk samples. Therefore, they showed their limitations to characterize multiple emulsions. However, preliminary results are shown for the ultrasound technique that account for the impact of the protein on the measured data and hence obtain quantitative results for the milkfat size distributions.

Neutron Reflectivity Study of Competitive Adsorption of beta;-Lactoglobulin and Nonionic Surfactant at the Air–Water Interface

We have measured the neutron reflectivity of β-lactoglobulin films adsorbed at the air–water interface in the presence of varying concentrations of the nonionic surfactant hexaoxyethylene dodecylether (C12E6). Applying a model-independent Guinier analysis to the measurements of reflectivity as a function of the momentum transfer vector normal to the surface, we have obtained estimates of surface coverage and adsorbed layer thickness. Exploiting the differing neutron scattering powers of hydrogen and deuterium, we have used surfactant with a fully deuterated hydrocarbon chain to obtain details of surfactant adsorption in the presence of protein, mapping both protein displacement and surfactant adsorption. The results show extensive adsorption of surfactant with little protein displacement and only slight increases in layer thickness. Further adsorption of surfactant leads to displacement and an increase in layer thickness before complete protein displacement. All of the surface coverage observations are time dependent, with protein levels increasing over a period of hours of measurement at the expense of surfactant being displaced from the initially formed film.

Self-consistent-field modelling of casein adsorption: comparison of results for ?s1-casein and ?-casein.

The theoretical adsorption behaviour of the milk proteins, α s1 - and β-casein, has been studied using a self-consistent-field (SCF) model. Previously published results for β-casein on the effects of ionic strength and pH on protein conformation are compared with those for α s1 -casein. We find a lower adsorbed amount for α s1 -casein, and a more complex adsorbed conformation because of its more heterogeneous primary structure. The predominant conformation appears to involve a substantial loop for α s1 -casein, producing a thinner adsorbed layer than is predicted for β-casein, which has, predominantly, a long tail extending away from the surface into the aqueous region. The overall layer structure for both proteins is shown to consist of a combination of many coexisting protein conformations. The relative proportion of the different conformations controls the overall layer properties and their variation with pH and ionic strength.

The application of modified forms of the Urick equation to the interpretation of ultrasound velocity in scattering systems

Three methods are presented that use modified forms of the Urick equation to interpret ultrasound velocity in dispersed systems in which scattering is significant. The relationship between multiple scattering theory and the modified Urick equation is discussed in each case. These techniques are intended to be practical methods of accounting for the effects of scattering in a simple way. The methods apply to volume fraction determination in emulsions, creaming experiments and crystallization experiments in oil-in-water systems.

Interpretation of ultrasound velocity creaming profiles

Ultrasound velocity measurements are used to observe the creaming behaviour of dispersions. A simple relationship between velocity and concentration is required in order to calculate the dispersed phase concentration profiles from velocity measurements. A method is proposed by which the concentration profiles can be calculated with a knowledge of only the continuous phase velocity and the initial dispersed phase concentration. The method is applicable whether or not the Urick equation applies, and without a knowledge of all the physical parameters required for a full scattering theory calculation. This renormalisation technique is applied to experimental measurements of creaming in a sunflower oil in water emulsion. Some restrictions on the method are presented.

Ultrasonic wave propagation in concentrated slurries--the modelling problem.

The suspended particle size distribution in slurries can, in principle, be estimated from measured ultrasonic wave attenuation across a frequency band in the 10s of MHz range. The procedure requires a computational model of wave propagation which incorporates scattering phenomena. These models fail at high particle concentrations due to hydrodynamic effects which they do not incorporate. This work seeks an effective viscosity and density for the medium surrounding the particles, which would enable the scattering model predictions to match experimental data for high solids loading. It is found that the required viscosity model has unphysical characteristics leading to the conclusion that a simple effective medium modification to the ECAH/LB is not possible. The paper confirms the successful results which can be obtained using core-shell scattering models, for smaller particles than had previously been studied, and outlines modifications to these which would permit rapid computation of sufficient stability to support fast particle sizing procedures.

A comparison of stochastic and effective medium approaches to the backscattered signal from a porous layer in a solid matrix

This paper reports a study of the backscattering behavior of a solid layer containing randomly spaced spherical cavities in the long wavelength limit. The motivation for the work arises from a need to model the responses of porous composite materials in ultrasonic NDE procedures. A comparison is made between models based on a summation over discrete scatterers, which show interesting emergent properties, and an integral formulation based on an ensemble average, and with a simple slab effective medium approximation. The similarities and differences between these three models are demonstrated. A simple quantitative criterion is established which sets the maximum frequency at which ensemble average or equivalent homogeneous medium models can represent echo signal generation in a porous layer for given interpore spacing, or equivalently, given pore size and concentration.

Acoustic scattering in dispersions: Improvements in the calculation of single particle scattering coefficients

Measurements of ultrasound speed and attenuation can be related to the properties of dispersed systems by applying a scattering model. Rayleighs method for scattering of sound by a spherical object, and its subsequent developments to include viscous, thermal, and other effects (known as the ECAH model) has been widely adopted. The ECAH method has difficulties, including numerical ill-conditioning, calculation of Bessel functions at large arguments, and inclusion of thermal effects in all cases. The present work develops techniques for improving the ECAH calculations to allow its use in instrumentation. It is shown that thermal terms can be neglected in some boundary equations up to approximately 100 GHz in water, and several simplified solutions result. An analytical solution for the zero-order coefficient is presented, with separate nonthermal and thermal parts, allowing estimation of the thermal contribution. Higher orders have been simplified by estimating the small shear contribution as the inertial limit is approached. The condition of the matrix solutions have been greatly improved by these techniques and by including appropriate scaling factors. A method is presented for calculating the required Bessel functions when the argument is large (high frequency or large particle size). The required number of partial wave orders is also considered.

Shear-mediated contributions to the effective properties of soft acoustic metamaterials including negative index.

Here we show that, for sub-wavelength particles in a fluid, viscous losses due to shear waves and their influence on neighbouring particles significantly modify the effective acoustic properties, and thereby the conditions at which negative acoustic refraction occurs. Building upon earlier single particle scattering work, we adopt a multiple scattering approach to derive the effective properties (density, bulk modulus, wavenumber). We show,through theoretical prediction, the implications for the design of “soft” (ultrasonic) metamaterials based on locally-resonant sub-wavelength porous rubber particles, through selection of particle size and concentration, and demonstrate tunability of the negative speed zones by modifying the viscosity of the suspending medium. For these lossy materials with complex effective properties, we confirm the use of phase angles to define the backward propagation condition in preference to “single-” and “double-negative” designations.