It has been known for a long time that both the crystal structure and kinetics of crystallisation can be affected by ultrasound. In the past systems used have relied on high power ultrasonic probes to produce crystals. The majority of these probes produce cavitation in the system and it has been difficult to differentiate between effects caused by the ultrasound alone or by the cavitation produced by ultrasound on the crystal structure. Some materials, such as fats, are very susceptible to the production of free radicals that lead to "off-flavours" being obtained. These "off-flavours" are easily produced when the standard high power probes are used. This has meant that, although the crystal structure of the final product might be improved, the presence of 'off' flavours has prevented ultrasound being considered as a commercial technique for the crystallisation of edible fats. At Unilever R&D a system has been developed which can investigate the effect of ultrasound on the crystallisation of fats under controlled conditions covering a range of intensities and cooling rates. The intensity levels used were both below and above the cavitational threshold. By keeping the cooling regime constant it has been possible to show that the structure of the final product can vary from a material looking similar to cottage cheese through to a fine cream simply by varying the ultrasonic intensity. This paper describes the effect of ultrasound on both the crystal structure and kinetics of palm oil crystallisation at intensities below and above the cavitational threshold.
This work reports dynamic video images of the influence of ultrasonic cavitation on the sonocrystallisation of ice at a microscopic level. This has been achieved through the construction of a unique ultrasonic system for an optical microscope. The system consists of (1). an ultrasonic cold stage, (2). a temperature control system, and (3). a microscope and imaging setup. This allows the temperature of a sample to be systematically controlled while it is subjected to simultaneous excitation with alternating pressures in the ultrasonic frequency range. Both the amplitude of excitation and the frequency can be varied. Experiments on ice crystals in pure water and sucrose solutions were conducted. Three distinct phenomena were observed. Firstly, there is a tendency for cavitation bubbles to form at the grain boundaries between ice crystals. Secondly, there is a progressive melting of ice by cavitation bubbles which appear to eat their way into the ice phase. Thirdly, the dendritic ice structures may fragment under the influence of ultrasound, thus increasing the number of nuclei which may subsequently grow (secondary nucleation). These observations form the basis of a significantly enhanced understanding and exploitation of the sonocrystallisation of ice.
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