Vacuum ultrasound assisted

The fact that the mechanism of action of ultrasound in tissues seems to be primarily mechanical, and perhaps partly thermal at the tissue level (Schossler et ol., 2011 Schossler et id., 2012c), may explain this limited effect on product quality. Yet, as long as the harsh conditions of cavitation can be avoided the product quahty can be maintained when the parameters of the ultrasound treatment are well chosen. Nonetheless, detailed investigations of product quahty must accompany the optimization of process parameters, especially in the case of critical applications, such as ultrasound-assisted osmotic dehydration, or apphcations involving special conditions, such as vacuum freeze-drying. 

The sudden collapse of a cavitation bubble in an acoustic field results in an inrush of liquid to fill the partial vacuum enclosed by the bubble. The shock wave produced by this bubble collapse can disrupt the weak inter-molecular forces which contribute to the structural characteristics of solvents. The disruption can influence chemical reactivity by altering solvation of the reactive species present. An example of this is to be found in the ultrasonically assisted reaction of 2-chloro-2-methylpropane in aqueous alcoholic media (Table 10.3) where the ratio of the first-order rate constants (in the presence of ultrasound) and knon (under conventional conditions) are compared [7] ... 

Traditionally-used techniques for the extraction of bioactive compounds are discontinuous, continuous, and hybrid approaches. The discontinuous techniques include the use of either organic solvents (sometimes assisted by ultrasound) or water, while steam distillation and vacuum distillation are continuous methods. Some methods involving both continuous and discontinuous approaches, such as distillation-extraction Soxhlet extraction has also been reported [14],... 

Ultrasonic waves can be excited into screens, which transfer the vibration to the food product, and the ultrasound emitter can be attached to the center of the screen, as well as to its frame. Direct transmission to the product allows sonication to be used in a large variety of food processes, throughout the complete drying process. Future equipment developments could enable ultrasound-mediated transfer not only to screens but also to trays or conveyor belts, and in this way airdrying in batch systems, as well as in continuous systems, could be assisted by ultrasonic vibrations. Even in vacuum freeze-drying, where air transmission is impossible due to the vacuum, the product can be placed on supports and treated with ultrasonic waves (Schbssler et al, 2012b). 

---