Influence of the ultrasonic power applied on freeze drying kinetics

Air drying is one of the oldest methods applied for preserving food. However this operation has as main drawbacks, energy consumption and the long processing time, therefore, is an operation highly prone for intensification. In this sense, the atmospheric freeze drying (AFD) technique allows obtaining high quality products, similar to those obtained by vacuum freeze drying, but at lower costs because the continuous production and no vacuum is needed. The effects caused by high intensity ultrasound (turbulence, structure diffusion, acoustic streaming, etc.) can enhance heat and mass transfer. However the magnitude of these effects is dependent on the product and process variables. Thus, in hot air drying processes, the air velocity, the internal structure of the material being dried and the ultrasonic power applied has a significant influence on the increase of drying kinetics induced by ultrasound application. With regard the ultrasound application in AFD processes, it has been observed that the internal structure of material do not influence on the magnitude of ultrasonic effects. At the temperatures used in these processes, the external resistance to mass transport is negligible compared to the internal one. As a consequence no significant influence of air velocity is found. The main goal of this work was to determine the influence of the ultrasonic power applied during AFD. To examine these effects drying kinetics of apples was addressed. Experimental results showed a significant (p<0.05) influence of the ultrasound application on drying kinetics. The drying time was shorter as higher the ultrasonic power applied. The effective diffusion coefficient identified was significantly higher when ultrasound was applied increasing along the ultrasonic power applied. It can be highlighted that the effective diffusion coefficient was three times higher when ultrasound was applied at the lowest power tested (10.3 kW/m3) that illustrate the high intensification potential of ultrasound application in the AFD.