Drying intensification combining ultrasound pre-treatment and ultrasound-assisted air drying

The water effective diffusivity can be increased in air drying of fruits and vegetables by applying an ultrasonic pre-treatment in ultrasonic baths or by directly applying ultrasound during air-drying. This work has examined the influence of the combination of both techniques on the dehydration of apple cubes (Malus domestica L. var Royal Gala). Experiments were carried out drying apples using conventional convective air-drying, ultrasound-assisted air-drying, ultrasound pre-treatment followed by conventional air-drying and ultrasound pre-treatment followed by ultrasound-assisted air-drying. The drying kinetics was modeled assuming internal and external resistances to mass transfer. The results showed that the effects of both ultrasonic processes are additive and that the use of the combined technique can reduce air drying time. The combination of the ultrasonic pre-treatment and ultrasound-assisted air-drying has intensified the drying process imparting in an increase of the water effective diffusivity by up to 93%, an increase of the external mass transfer by up to 30% and a reduction of up to 58% in the total drying time when compared to the conventional air-drying process.