Model-based investigation into atmospheric freeze drying assisted by power ultrasound

Abstract

Atmospheric freeze drying consists of a convective drying process using air at a temperature below the freezing point of the processed product, and with a very low relative humidity content. This paper focuses on the use of a simple one-dimensional model considering moving boundary vapor diffusion to describe the ultrasonic assisted atmospheric freeze-drying of foodstuffs. The case study is the drying of apple cubes (8.8 mm) at different air velocities (1, 2, 4 and 6 m/s), temperatures ( 5, 10 and 15 C), without and with (25, 50 and 75 W) power ultrasound application. By fitting the proposed diffusion model to the experimental drying kinetics, the effective diffusivity of water vapor in the dried product was estimated. The model was successfully validated by drying apple samples of different size and geometry (cubes and cylinders). Finally, a 23 factorial design of experiments revealed that the most relevant operating parameter affecting the drying time was the applied ultrasound power level.