Slide 1

Ong, Law - Hygrothermal Properties of FVFs

Figure 2.9. Variation of moisture diffusivity with moisture content during drying

Extensive reviews with regard to the methods of experimental determination and
compilation of moisture diffusivity data for various foodstuffs have been published
elsewhere (Zogas et al., 1994; Zogas et al., 1996a). Generally, the diffusivity values fall
between 10-13 and 10-6 m^2 s-1 while majority (about 92%) falls within 10-12 and 10-8 m^2 s-

(^1) (Zogzas et al., 1996b). Various empirical models relating effective diffusivity as a func-
tion of temperature and moisture content have been compiled and reported by the au-
thors. The followings are some selected examples of effective diffusivity model reported
from recent published literatures (Figure 2.10. and Figure 2. 11 ). Overall, it can be seen
from the figures that effective diffusivity increases with temperature but varying trends
are observed with respect to moisture content. At high temperature the water molecules
are loosely bound to the food matrix, thus requiring less energy to remove than at lower
temperature (Xiong et al., 1992). In contrast, the dependency of moisture content de-
pends greatly on the structure of the food product and void fraction has been known to
affect diffusivity significantly. It has been reported that for low porosity materials the
value of Deff is very close to liquid diffusivity while for granular and porous materials
moisture is transported mainly by vapour diffusion through the void space (Karathanos
et al., 1990).
Moisture content (kg-1 water kg-1 dry matter)
Liquid phase diffusivity
Vapour phase diffusivity
Total diffusivity
Diffusivity (m
2 s
-1)
Liquid phase diffusivity
Vapour phase diffusivity
Total diffusivity