Crystallization of soft materials such as foods, cosmetics, pharmaceuticals, has been of high interest. These soft materials are often crystallized in encapsulated systems: emulsions, vesicles, micelles etc. It is expected that the crystallization processes in the encapsulated systems, in particular the nucleation processes, are somehow different from those occurring in the bulk systems. Therefore, it is important to study the crystallization behavior of the soft materials in the encapsulated systems. For this purpose, it is prerequisite to develop unique techniques which are sensitive to monitor the crystallization in the encapsulated systems, to which conventional techniques employed for the bulk systems are not easily applied. This paper describes recent experimental results of the ultrasonic velocity measurements of the fat crystallization in oil-in-water (O/W) emulsions, which were developed to in-situ monitor the crystallization processes of the liquid oil droplets dispersed in the aqueous phase. It was observed that the nucleation of the oil phase is remarkably delayed due to the encapsulation, as revealed in the decrease in crystallization temperature (T_C). However, the addition of highly lipophilic sucrose fatty acid esters (SE) to the oil phase caused considerable increase in T_c values, because the SE additives accelerated the heterogeneous nucleation at the oil/water interface. By contrast, the SE additives retarded the rate of crystal growth processes, as examined in the crystal growth rate measurements. It was clarified that the acceleration effect of the interface nucleation in the O/W emulsions occurred through two-stages with increasing concentration of the SE additives. These results indicate the possibility of developing a new method to control the crystallization in the encapsulated systems, by using the additives which specifically catalyze the heterogeneous nucleation at the interfaces in the encapsulated systems.
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