Malcolm J. W. Povey

Ultrasound techniques for characterizing colloidal dispersions

Interest in the interaction of acoustic waves with particulate mixtures has a long history—dating back to the work of Rayleigh in the 19th century. This interest has intensified over the last fifteen years as advances in electronics and instrumentation science have brought the possibility of using ultrasound to characterize colloidal mixtures both in the laboratory and in-process, and in both of these contexts a small number of instruments are currently in use. The characterization of colloidal mixtures by ultrasound requires a formal theoretical basis which relates the properties of the mixture, particularly the dispersed phase particle size distribution (PSD), to the complex wavenumber governing propagation. The number of theoretical treatments is vast, having evolved over more than a century. This paper is intended to provide a review of these developments in a form which will enable new researchers in the field to climb a very steep learning curve in a relatively short time. We discuss definitions and production techniques for colloidal mixtures and the basic physical phenomena underlying wave propagation through them. We identify two approaches to the propagation problem—scattering and coupled-phase; these are treated both separately and comparatively, particularly in relation to limitations that arise when the concentration of particles is high and the basic theories break down. We introduce the basic method for the measurement of PSD and show how dynamic effects such as flocculation and crystallization can be observed and modelled. The core of all ultrasonic characterization procedures is the physical measurement of the ultrasonic wave attenuation coefficient and phase velocity as functions of frequency; here we discuss these techniques on the basis that what is observable or measurable about a colloid depends on both its physical properties and the frequency bandwidth available for measurement. This paper concludes with our view on future developments of measurement technique and theoretical treatments.

Ultrasonics in food engineering. Part I: Introduction and experimental methods

Abstract The basic ideas underlying the use of ultrasound in non-destructive testing are reviewed with a special emphasis on their relevance to food engineering. Sound velocity is a valuable engineering tool because of its relative ease of measurement, ease of interpretation of the consequent data and its greater accuracy than attenuation measurements. It is a non-destructive, non-invasive, non-intrusive technique. Low-intensity applications are reviewed and their potential in the measurement of physical properties is emphasised. Such measurements include the determination of adiabatic compressibility, rigidity and, in two-phase systems, particle size and dispersed-phase volume fraction. Experimental techniques which the authors have found useful for measurements in food systems are described and the accuracy of available techniques is compared.

Scattering of ultrasound by emulsions

Ultrasonic scattering theory is used to relate the ultrasonic velocity and attenuation of emulsions to their physical properties (e.g. droplet size and volume fraction). This theory includes visco-inertial and thermal scattering mechanisms and corrections due to multiple scattering. The physical significance of the scattering mechanisms is discussed so as to highlight the factors which influence ultrasonic measurements in these systems. Theoretical predictions were compared with experimental measurements of velocity and attenuation in sunflower oil in water emulsions with varying droplet radii (0.14-0.74 mu m), mass fraction (0-0.5) and frequency (1.25-10.0 MHz) at 293.1 K. Appreciable excess attenuation and velocity dispersion were observed which could be attributed to thermal scattering. The relationship between the measurable ultrasonic parameters and the physical properties of emulsions means ultrasonic scattering should prove a useful means of investigating these systems.

Ultrasonics in food engineering Part II: Applications

Abstract Applications of ultrasonic techniques in the food industry are reviewed, relevant applications in other industries are considered and the future of ultrasonics in food engineering examined. Data on sound velocity and attenuation in food and food-related materials are tabulated, together with acoustically measurable properties such as adiabatic compressibility, rigidity and acoustic impedance. Measurement of sound velocity in vegetables, doughs and chocolate is described. Only low-power applications of ultrasonics of relevance to the measurement of food properties are considered.

Ultrasonic analysis of edible fats and oils

Low intensity ultrasound is a powerful analytical technique for investigating the physico-chemical properties of many biological and non-biological materials. In this article its application for the characterization of edible fats and oils is assessed. Ultrasound can be used to determine the dynamic rheology and composition of oils, the oil content and droplet size of emulsions and the solid fat content of partially crystalline emulsions. It is capable of rapid and precise measurements, is non-destructive and non-invasive, can be used on-line or off-line and is relatively inexpensive. Ultrasonic techniques will therefore prove a useful addition to the existing analytical techniques used to characterize fats and oils.

Particle-Stabilizing Effects of Flavonoids at the Oil―Water Interface

It has been shown that some common food flavonoids can act as excellent stabilizers of oil-in-water emulsions through their adsorption as water-insoluble particles to the surface of the oil droplets, i.e., Pickering emulsions are formed. Flavonoids covering a wide range of octanol-water partition coefficients (P) were screened for emulsification behavior by low shear mixing of flavonoid+n-tetradecane in a vortex mixer. Most flavonoids with very high or very low P values were not good emulsifiers, although there were exceptions, such as tiliroside, which is very insoluble in water. When a high shear jet homogenizer was used with 20 vol% oil in the presence of 1 mM tiliroside, rutin, or naringin, much finer emulsions were produced: the average droplet sizes (d32) were 16, 6, and 5 μm, respectively. These results may be highly significant with respect to the delivery of such insoluble compounds to the gut, as well as their digestion and absorption.

Ultrasonic investigation of the particle size dependence of crystallization in n-hexadecane-in-water emulsions

Abstract Ultrasonics has been used to monitor solid—liquid phase transitions in 20 vol% n -hexadecane-in-water emulsions. The velocity and attenuation coefficients of ultrasound at 1.2 MHz were measured as a function of temperature (0–30°C) and time (at 3°C) for two emulsions with the same composition but different mean droplet diameters (0.36 and 3.3 μm). The degree of supercooling (≈14°C) and the crystallization rate at 3°C were found to depend on the droplet size in a manner consistent with homogeneous nucleation in the emulsions.

Creaming of concentrated oil-in-water emulsions containing xanthan

Abstract Time-dependent creaming profiles are reported for mineral oil-in-water emulsions (18 vol% oil, 2 wt% Tween 20) containing various amounts of the microbial polysaccharide xanthan. Data were obtained by the ultrasound velocity scanning technique at 30°C in samples of height 250 mm. In emulsions without added salt, the major instability at low xanthan concentrations (~0.02 wt%) is the rapid development of a serum layer near the bottom of the sample, whereas at higher concentrations (0.05–0.2 wt%) destabilization is associated with the development of a cream layer (60–70 vol% oil) near the top. Emulsions containing 0.05 mol/dm 3 NaCl do not exhibit such clear serum separation at low xanthan concentrations, but are much less stable to creaming at higher concentrations (~0.5 wt%). The use of the Urick equation to relate ultrasound velocity to oil volume fraction is found not to be valid for these emulsions.

Effects of pH on the ability of flavonoids to act as Pickering emulsion stabilizers

The flavonoids tiliroside, rutin and naringin have been investigated as stabilizers of Pickering oil-in-water (O/W) emulsions. The mean droplet size of tetradecane emulsions was considerably smaller at higher pH, especially for rutin. The solubility of flavonoids in the aqueous phase was 4-6 times higher at pH 8 compared to pH 2 for tiliroside and rutin, although all absolute solubilities remained low (<1 mM). This agreed with a slight increase in surface activity of tiliroside and rutin at the O-W interface at pH 8 compared to pH 2. However, improved emulsion stabilization at higher pH is better explained by the significant increase in ζ-potential of the flavonoid particles to more negative values at pH 8, which will improve particle dispersion and increase the charge on the droplets stabilized by them. A buckwheat tea extract, rich in rutin, was also shown to be an effective stabilizer of sunflower O/W emulsions.

Temperature dependence of bulk viscosity in water using acoustic spectroscopy

Despite its fundamental role in the dynamics of compressible fluids, bulk viscosity has received little experimental attention and there remains a paucity of measured data. Acoustic spectroscopy provides a robust and accurate approach to measuring this parameter. Working from the Navier-Stokes model of a compressible fluid one can show that the bulk viscosity makes a significant and measurable contribution to the frequency-squared acoustic attenuation. Here we employ this methodology to determine the bulk viscosity of Millipore water over a temperature range of 7 to 50?C. The measured attenuation spectra are consistent with the theoretical predictions, while the bulk viscosity of water is found to be approximately three times larger than its shear counterpart, reinforcing its significance in acoustic propagation. Moreover, our results demonstrate that this technique can be readily and generally applied to fluids to accurately determine their temperature dependent bulk viscosities.