Power ultrasound is a novel technology to be applied in drying processing taking energy saving aims into consideration. The main goal of this work was to address from a kinetic and microstructural point of view the influence of power ultrasound application on convective drying of a high porosity product like eggplant.
Convective drying kinetics of eggplant cylinders (height 20 mm and diameter 20.4 mm) were carried out at 40 °C and 1 m/s. Trials were also conducted at the same experimental conditions applying acoustic powers from 15 to 90W. A diffusion model was used to quantify the kinetic effects on mass transfer process induced by ultrasound. All these samples were analysed by Cryo Scanning Electron Microscopy (Cryo-SEM).
Experimental results showed the reduction of drying time with the ultrasonic power, the higher the power applied, the faster the drying kinetic. Thus, a maximum drying time reduction (70 %) was reached when the maximum acoustic power tested of 90W was applied. The ultrasonic effect was quantified by the effective moisture diffusivity, which showed a significant (p<0.05) linear relationship with the acoustic power.
The main cellular tissue in eggplant is endocarp. This tissue is formed by cells interconnected each other with large intercellular spaces occupied by air, similar to a highly porous sponge. In hot air dried samples, endocarp cells appeared highly degraded and a compacted tissue may be observed without practically intercellular spaces. However, the combined treatments with ultrasound were less drastic in degradation terms than those only with air, this fact was more evident when ultrasound was applied at moderate powers (45W). In these samples, the microstructure of the tissue appeared less modified, endocarp cells still maintained the individuality and even, intact cell walls were found. Therefore, 45W may be considered an optimal power for the application of ultrasound on eggplant drying concerning not only kinetic aspects but also quality issues.
