Lynne Wong's PhD thesis

Table 5.2. Some commonly used three-parameter isotherm equations with a
temperature term for the calculation of the equilibrium moisture
content (m) of agricultural products (Jayas and Mazza, 1993).
Model
Equation*
Modified Chung-Pfost (Jayas & Mazza, 1993;
Nilsson et al., 2005)
m =
−
1 ⎡
n
d
⎢
⎣
( T + c) n
( a )
−
b
w
⎤
⎥
⎦
Modified GAB (Weisser, 1986; Nilsson et al.,
( d / T )
2005) (1 − c a
w
) [ 1 − c a
w
+ c a
w
( d / T )]
m =
b c a
w
Modified Halsey (Nilsson et al., 2005)
m =
⎡ −
⎢
⎣
( b+
cT ) d
e
n H
R
⎤
⎥
⎦
1
Modified Henderson (Nilsson et al., 2005) ⎡ n
( 1 − H
R
)
m = ⎢
− b ( T + d )
Modified Oswin (Nilsson et al., 2005)
⎣
b +
H R
cT
m =
1
⎛
⎜
⎝
1
⎞
− 1
⎟
⎠
d
⎤
⎥
⎦
1
c
* where a w is the water activity, H R is the relative humidity expressed as a fraction, b, c
and d are constants in the sorption models and T is the temperature in °C.
5.3.2 Applicability of different adsorption isotherm models
Many researchers have developed equations and models to describe experimental EMC
results obtained from agricultural products. Chirife and Iglesias (1978) have compiled 23
such mathematical equations, and their use for fitting sorption isotherms of foodstuffs.
They pointed out that no one equation, however, was found to give accurate results
universally for all types of food materials and throughout the whole range of relative
humidities. Labuza (1975) attributed this to the fact that water is associated with the food
matrix by different mechanisms in different activity regions.
In 1981, Van den Berg and Bruin identified 77 isotherm models to predict the EMC of
substrates. Of these, the most widely utilised and versatile model is recognised to be the
GAB equation, particularly in the case of food and food products.
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