Influence of material structure on air-borne ultrasonic application in drying

• The efficiency of the application of air-borne ultrasound (SDUP) during drying was assessed for different products.
• SDPU exhibited a linear relationship with product porosity and hardness.
• The more porous and softer the product, the higher the SDUP.
• SDUP was greatly affected by the acoustic impedance of the product.
• Porous and soft products (low impedance) showed a better acoustic transmission and a higher SDUP.

This work aims to contribute to the understanding of how the properties of the material being dried affect air-borne ultrasonic application. To this end, the experimental drying kinetics (40 °C and 1 m/s) of cassava (Manihot esculenta) and apple (Malus domestica var. Granny Smith) were carried out applying different ultrasonic powers (0, 6, 12, 19, 25 and 31 kW/m3). Furthermore, the power ultrasound-assisted drying kinetics of different fruits and vegetables (potato, eggplant, carrot, orange and lemon peel) already reported in previous studies were also analyzed. The structural, textural and acoustic properties of all these products were assessed, and the drying kinetics modeled by means of the diffusion theory.A significant linear correlation (r > 0.95) was established between the identified effective diffusivity (DW) and the applied ultrasonic power for the different products. The slope of this relationship (SDUP) was used as an index of the effectiveness of the ultrasonic application; thus the higher the SDUP, the more effective the ultrasound application. SDUP was well correlated (r ⩾ 0.95) with the porosity and hardness. In addition, SDUP was largely affected by the acoustic impedance of the material being dried, showing a similar pattern with the impedance than the transmission coefficient of the acoustic energy on the interface. Thus, soft and open-porous product structures exhibited a better transmission of acoustic energy and were more prone to the mechanical effects of ultrasound. However, materials with a hard and closed-compact structure were less affected by acoustic energy due to the fact that the significant impedance differences between the product and the air cause high energy losses on the interface.