Understanding bubble dynamics within sonicated edible lipids to enhance their physicochemical properties

Abstract

High-intensity ultrasound (HIU) has been identified as a key processing tool to induce the onset of crystallisation and enhance the physical properties of edible lipids. However, the underlying mechanism behind its action remains unclear. This thesis details an investigation into the bubble dynamics generated at a piston-like emitter ultrasonic source, which have never been fully explored within lipid systems. A better understanding of the HIU mechanism could help to tailor the crystallisation behaviour of healthier edible lipid sources in future. Initial studies were carried out within liquid soybean oil, in the absence of lipid crystals, to characterise the bubble dynamics under different sonication conditions. A sequence of distinct cavitation clusters were identified beneath the piston-like emitter employing complimentary high-speed imaging and hydrophone pressure measurements. Each cluster was generated by a specific HIU power and showed a growth-collapse cycle which occurred over different multiples of the tip oscillation. This led to the discovery of a unique bi-cluster event, consisting of two clusters which operate with a 180` phase shift with respect to one another. The observation of the bi-cluster event during the ring-up to higher cluster orders requires a pre-existing bubble population and its lifetime is highly temperature dependent. The results indicated that the dynamics of the cluster are driven by the HIU power levels and this could impact the lipid sonocrystallisation process. The crystallisation kinetics and the physicochemical properties of an all-purpose shortening are shown to be sensitive to the presence of the different cavitation clusters. Only samples crystallised at low supercooling display significant differences in induction time between each of the HIU conditions. Better energy efficiencies for the induction of crystallisation were reported for the bi-cluster cavitation regime compared to single clusters present at higher power levels. The physical properties measured include crystal microstructure, melting profile, viscoelasticity, solid fat content and hardness. Results showed that APS samples with both greater elasticity and hardness were obtained in the presence of the highest order cluster regime. A greater acoustic attenuation effect during the 10 second sonication period was also recorded under these HIU conditions. Several smaller scale studies were conducted to interrogate the key elements of the HIU mechanism, including the impact of pressure shock wave emission, induced liquid flow and the importance of pre-existing crystals during the sonication process. A bespoke optical scattering sensor for the in-situ detection of ultrasonically generated gas bubbles within oil media is reported. This system was employed in the presence of each of the distinct clusters both in the presence and absence of lipid crystals. Bubble detection was shown to be highly sensitive to the location of the sensor with respect to the ultrasound source. The bubble populations were then followed for extended time periods after PLE operation was terminated. The greatest number of gas bubbles was identified for the highest order cluster and these species remained within the liquid for the longest time. The presence of bubble oscillation was identified during translocation events and the oscillation frequency was dependent upon the nature of the cavitation cluster collapse. The detection of lipid crystal clusters in the absence of HIU is also demonstrated under low supercooling conditions only

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ORCID for Peter Birkin: orcid.org/0000-0002-6656-4074

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