Moisture diffusivity and transfer modelling in dry biscuit

Experimental moisture transfer at 20 degreesC within a biscuit was evaluated using water vapour sorption kinetics or moisture migration experiments in a two compartment 0.99 a(w) agar gel/biscuit system. In each case, moisture transfer was successfully modelled using mathematical models based on Fick's second law. The model for moisture transfer in composite food was successfully validated with a lower agar gel a(w) (0.90) and also with an acetylated monoglycerides film at the interface of biscuit and agar gel. Diffusivity estimated from sorption kinetics was found to increase till a moisture content close to the monolayer value, and then, to decrease at higher moisture content. Diffusivity estimated from moisture migration experiments in an agar gel/biscuit system varied in the same way but was found about two fold higher than diffusivity values estimated from sorption kinetic. This difference in diffusivity values could be partially explained by the significant influence of an external resistance to moisture transfer (evaluated to about 0.018 m/s) at the air/biscuit interface during sorption kinetics. (C) 2003 Elsevier Ltd. All rights reserved.