Characterization of food emulsions using ultrasonic spectroscopy
Analytical instruments based on ultrasonic spectroscopy have been recently developed to measure the size distribution and droplet concentration of emulsions. Nevertheless, there are a number of problems that currently limit the practical application of these instruments to food emulsions: (i) ultrasonic measurements often have to be made at different temperatures; (ii) many food emulsions are concentrated; and (iii) many food emulsions are flocculated. The objective of this study was therefore to investigate the influence of temperature, droplet concentration and droplet flocculation on the ultrasonic properties of emulsions, hence developing ultrasonic techniques that could be applied to food emulsions. In addition, this study also aimed at testing the efficiency of the newly developed ultrasonic scattering theories (UST) for describing ultrasonic properties of flocculated and concentrated emulsions.
The influence of temperature was investigated by measuring ultrasonic velocity and attenuation coefficient of emulsions of different compositions and droplet sizes at 5 to 50°C. The ultrasonic properties of emulsions were sensitive to temperature as well as to composition and droplet size, and could be described by UST.
The effect of droplet concentration (0–50 wt%) and diameter (0.2–0.58 μm) on ultrasonic properties was determined in non-flocculated oil-in-water emulsions. The traditional UST gave poor predictions of ultrasonic properties of emulsions at high droplet concentration, small droplet size and low ultrasonic frequency, while an extended UST, which incorporated the thermal overlap effects, gave much better predictions under the same conditions.
The effect of flocculation on ultrasonic properties of emulsions was studied over a range of frequency (1–120 MHz) using 5% oil-in-water emulsions, flocculated by either depletion or an electrostatic mechanism. A new theory that took into account the thermal overlap effects could well explain the ultrasonic properties of flocculated emulsions. As flocculation increased, the attenuation coefficient decreased at low frequency due to the thermal overlap effect, while at high frequency the attenuation coefficient increased due to the scattering effect.
The usefulness of ultrasonic techniques in characterizing actual foods was tested using salad dressing and chicken. The ultrasonic techniques could provide valuable and accurate information about the composition of these foods.