Food drying process by power ultrasound.

Drying processes, which have a great significance in the food industry, are frequently based on the use of thermal energy. Nevertheless, such methods may produce structural changes in the products. Consequently, a great emphasis is presently given to novel treatments where the quality will be preserved. Such is the case of the application of high-power ultrasound which represents an emergent and promising technology. During the last few years, we have been involved in the development of an ultrasonic dehydration process, based on the application of the ultrasonic vibration in direct contact with the product. Such a process has been the object of a detailed study at laboratory stage on the influence of the different parameters involved. This paper deals with the development and testing of a prototype system for the application and evaluation of the process at a pre-industrial stage. Such prototype is based on a high-power rectangular plate transducer, working at a frequency of 20 kHz, with a power capacity of about 100 W. In order to study mechanical and thermal effects, the system is provided with a series of sensors which permit monitoring the parameters of the process. Specific software has also been developed to facilitate data collection and analysis. The system has been tested with vegetable samples.

Recent developments in vibrating-plate macrosonic transducers.

As is known, the stepped-plate transducer [Ultrasonics 16 (6) (1978) 267] represents an optimum system for the efficient generation of high-intensity sonic and ultrasonic radiation in fluid media. Nevertheless, the design of this transducer may be difficult to adapt to some specific problems. Such is the case of the treatment of large volumes in industrial installations. A solution is the enlargement of the surface of the radiating plate. However, that means to work at high-order vibration modes which implies numerous practical problems. Another case is the application of the stepped-plate transducer for the generation at sonic frequencies where the height of the steps of the radiating plate, which has to be half a wavelength of the radiation, becomes too high and it makes the transducer construction impractical. To face these specific situations a series of new designs in transducer development have been recently carried out. This paper presents the characteristics of two new transducer devices, one for the treatment of large industrial volumes and the other for low-frequency sonic applications. Both devices are based on vibrating-plate radiators and represent a novel approach to practical existing problems.