The solution of classical diffusion equation based on the assumption o
f average moisture diffusion coefficient did not adequately represent
natural convection drying of rough rice in thin vertical columns expos
ed on both sides to hot air. Instantaneous moisture diffusivity coeffi
cients determined from experimental drying curves decreased continuous
ly with an increase in exposure duration and were linearly related to
moisture ratio. The proportionality constant which was called apparent
moisture diffusion coefficient was distinctly related to air temperat
ure, relative humidity, and initial moisture content of rough rice. Th
e modified moisture diffusion model using the instantaneous moisture d
iffusion coefficient was found to best represent the moisture removal
from bulk rough rice.