The development of an isothermal drying apparatus and the evaluation of the diffusion model on hygroscopic porous material

Abstract

A drying apparatus combining microwave energy and hot air was developed to dry materials isothermally and its applicability for hygroscopic porous materials has been demonstrated. With on-line sample mass measurements, the apparatus is suitable for determining drying kinetics and investigating the mechanism(s) that limit the drying process for a variety of food materials. Drying curves have been obtained from bread samples using the isothermal drying apparatus. These drying curves were analyzed to compare effective diffusivities calculated from convective hot air conditions (2.35-4.21 × 10-9 m2/s) to effective diffusivities calculated from isothermal conditions (7.6-18.3 × 10-9m2/s), and a significant difference was shown. However, Fick's diffusion equation was shown only to predict drying time and not drying rate. The results of this research showed that proper parameter evaluation did not solve the problem of accurately predicting moisture transfer. Therefore a new model needed to be developed to predict moisture transfer. We hypothesized that during isothermal drying of hygroscopic porous material evaporation is the governing moisture loss mechanism, and Fick's second law was unable to predict moisture loss when this mechanism occurs.