Forced air drying assisted by air-borne ultrasound has been developed by applying stepped-plate ultrasound generators. Another procedure has been developed and tested in which the ultrasonic vibration is applied in direct contact with the vegetable samples and together with a static pressure. The good acoustic impedance matching between the vibrating plate of the transducer and the food material favours the deep penetration of acoustic energy and increases the effectiveness of the process. The vegetable is subjected to high ultrasonic stresses which, due to the rapid series of contractions and expansions, produce a kind of "sponge effect" and the quick migration of moisture through natural channels or other channels created by the wave propagation which result in moisture release from the product. In addition, the production of ultrasonic cavitation inside the liquid may help to the separation of the moisture strongly attached.
The drying effect remarkably improves with respect to the forced-air drying (with and without air-borne ultrasound). In fact, the dehydration process is not only quicker and less energy consuming but it is more powerful: the final moisture content could be less than 1%. In addition, due to the processing time and low temperature of the air flow, the product qualities are very well preserved.