ABSTRACT
Moisture exists in porous food material in the forms of free water, bound water, and vapor. The migration of moisture through the food can be caused by the individual or combined contributions of moisture, temperature, and pressure gradients. In microwave drying, free water transport has been attributed to diffusion (Torringa et al., 1996; Adu and Otten, 1996) or capillary flow (Wei et al., 1985; Constant et al., 1996; Lian et al., 1997; Turner et al., 1998; Ni et al., 1999). For vapor migration, diffusion (Lian et al., 1997), capillary flow (Turner et al., 1998; Ni et al., 1999) and a combination of capillary flow and diffusion (Chen and Schmidt, 1990; Constant et al., 1996) have been considered as transport mechanisms. Few studies have considered the migration of bound water. In studies regarding the mechanism of bound water transfer, Turner et al. (1998) assumed that bound water transfer was caused by diffusion, while Chen and Pei (1989) employed a capillary mechanism to characterize bound water flow.
