Jangam, Mujumdar - Basic Concepts and Definition
Figure 1. 5. Various types of moisture contentA plot of ln(Ψ) against 1/T is linear with a slope of Hw/Rg where Rg is the universal gas
constant (Rg = 8.314× 103 kg kgmol-1 K-1). Note that the total heat required to evaporate
bound water is the sum of the latent heat of vaporization and the heat of wetting; the latter
is a function of the moisture content X. The heat of wetting is zero for unbound water and
increases with decreasing X. Since Hw is responsible for lowering the vapor pressure of
bound water, at the same relative humidity, Hw is almost the same for all materials (Keey,
1978 ).
For most materials, the moisture binding energy is positive; generally it is a mono-
tonically decreasing function of the moisture content, with a value of zero for unbound
moisture. For hydrophobic food materials (e.g., peanut oil, starches at lower tempera-
tures) the binding energy can, however, be negative.
Figure 1.6 shows schematically possible shapes of different sorption isotherms.
Type 1 is the well known Langmuir isotherm obtained by assuming monomolecular ad-
sorption of gas by the porous solids in a finite volume of voids. Type 2 is sigmoid iso-
therm generally found for soluble products. Type 3 is known as Flory-Higgins isotherm,
accounts for a solvent or plasticizer such as glycerol above the glass transition tempera-
ture. The type 4 represents adsorption for swellable solids until maximum of hydration
site are reached. Type 5 is the BET multilayer adsorption isotherm observed for adsorp-
tion of water on charcoal. Two isotherms commonly found in food products are types 2
and 4 (Basu et al., 2006).
RH 50%EMCBound moistureFree moisture contentX, Moisture Content (dry basis)XeUnbound moisture100%