Ultrasonic characterization of crystal dispersions and frozen foods

Ultrasonic wave propagation is sensitive to the physical properties and structure of materials and so ultrasonic sensors are used in the non-destructive evaluation of materials. Since ultrasonics is a non-invasive, relatively inexpensive technology that can be adapted to hygienic requirements and processing conditions that occur in the food industry, it is possible to use ultrasonic sensors in the analysis of foods, in particular online during production. The relationships between the ultrasonic properties of foods and many physical and structural parameters have been well-established (Coupland, 2004) and the motivation of this work is to establish similar relationships for frozen foods. Phase transitions (i.e., freezing) are characterized by rapid changes in physical properties of materials, including ultrasonic properties so it should be possible to relating the changes in ultrasonic properties of food systems to other physical properties such as ice content. In order to do that, however, the influence of food composition and microstructure on the ultrasonic properties must be well understood.;The first objective of this study was to investigate the effect of ice content on the ultrasonic properties of model frozen solutions. The speed of sound was measured in sucrose, glycerol and orange juice solutions as a function of temperature (10°C--10°C). Ultrasonic velocity rapidly increased starting at the onset of freezing as the samples were frozen. For samples below -8°C (i.e., closer to thermal equilibrium), the ice content in the samples was approximately a linear function of the change in ultrasonic velocity. The ultrasonic attenuation in the frozen model systems was unexpectedly high.;The ice-solution system is difficult to precisely characterize experimentally so to investigate the mechanisms for the large ultrasonic losses seen a simpler model system was iv considered. The ultrasonic attenuation from solid or liquid emulsion can be well-described by scattering theory, except at their melting point when there are large excess losses (McClements et al. 1993). The solid-liquid equilibrium in emulsion droplets is similar to the ice-solution equilibrium in frozen samples and has the advantage of being readily controlled and described. The second objective was to investigate the excess attenuation for melting lipids in emulsion droplets and suggest a mechanism for the losses that may be responsible for the very high attenuation. Large losses were seen at the melting point but these could be described by an extended version of scattering theory that incorporated effective values of the physical properties of the droplets accounting for the rapid changes in solids content at melting point (i.e., the effective specific heat and thermal expansion coefficient were much larger over the melting range). The influence of oil and emulsifier type and particle size on the excess losses on melting was investigated in detail to show the validity of extended theory.;The final objective was to measure the attenuation in model frozen foods and interpreting relevance of the losses in terms of the mechanism developed for the emulsion droplet model. Ultrasonic velocity and attenuation was measured as a function of temperature in partially frozen sucrose solutions that had been prepared either with or without prior degassing. Ultrasonic velocity increased approximately linearly with ice content in all samples; however the rate of change in velocity was dependent on initial sugar concentration and to a lesser extent on degassing. Attenuation increased rapidly on freezing but the final value was more a function of the degassing step and hence the presence of air bubbles than on the final ice content. This finding suggests that contrary to my initial hypothesis, air bubbles rather than ice crystals are most significant in determining the ultrasonic attenuation of partially frozen model foods.;References. 1. Coupland, J.N. 2004. Low-intensity ultrasound. Food Research International, 37(6), 537-543. 2. McClements, D.J., Povey, M.J.W. & Dickinson, E. 1993. Absorption and velocity dispersion due to crystallization and melting of emulsion droplets. Ultrasonics, 31(6), 433-437.