ฉัisolated mass of liquid, where surface driving forces
related to heat and mass transfer are constant, leading
to the constant rate period. When the internal resistance
to the transport of liquid water to the surface
increases, thereby lowering the drying rate, the falling
rate period occurs. Hence, internal diffusion may be
assumed to be the mechanism responsible for water
loss during the drying process. At lower drying temperatures
a more uniform moisture distribution exists,
that allow the sample to continue to shrink until the
last stages of drying with a low porosity. On the contrary,
at higher air temperatures, case hardening of the
surface may occur and the volume of the sample
becomes fixed at an earlier stage, inducing less shrinkage
and a more intensive pore formation (Bonazzi &
Dumoulin, 2011).