Ong, Law - Hygrothermal Properties of FVFs
dough
Bread Amylases 30 0.36
Casein Trypsin 30 0.50
Starch Amylases 37 0.40/0.75
Galactose Galactosidase 30 0.40-0.60
Olive oil Lipase 5 - 40 0.025
Triolein, trilaurin Phospholipases 30 0.45
Glucose Glucose oxidase 30 0.40
Linoleic acid Lipoxygenase 25 0.50/0.70
2.3. HYGROTHERMAL EFFECTS ON PHYSICAL PROPERTIES
2.3.1. Effective Moisture Diffusivity
The moisture migration process during drying is complex and often involves one or
more transport mechanisms such as liquid diffusion, vapour diffusion, Knudsen diffu-
sion, surface diffusion and hydrostatic pressure differences (Mujumdar and Devahastin,
2008 ). The term effective diffusivity (Deff) is defined to describe the rate of moisture
movement, no matter which mechanism is involved. From Fick’s second law of diffusion
(Equation 2.5),
=∇.(Deff∇X)
∂∂
tX
(2.5)X is the moisture content (kg-^1 water kg-^1 dry matter), t is the drying time and Deff is the
effective diffusivity (m^2 s-^1 ).
The general solution of Equation 2.5 can be derived for various standard geometries
(slab, cylinder and sphere) using appropriate boundary conditions (Crank, 1975). In
many cases the effective diffusivity is estimated by using only the first term of the gen-
eral solution (Zogzas et al., 1996b). However, this method only assumes a constant diffu-
sion coefficient throughout the whole drying process and it can be related to product
temperature via the Arrhenius equation. In reality, the diffusion coefficient is rarely con-
stant but varies with moisture content, temperature and spatial coordinate (Puyate and
Lawrence, 2006). Karathanos et al. (1990) provided detail step in determining the effec-
tive diffusivity of the material. In most cases, the total diffusivity is the sum of the va-
pour phase and liquid phase diffusivities as shown in Figure 2.9. At high moisture con-
tent liquid diffusion is the dominant means of transport mechanism and vice versa at
lower moisture content (De Vries, 1958).