Slide 1

Jangam, Mujumdar - Basic Concepts and Definition

Figure 1.6. Different types of sorption isotherms

Measurement of sorption isotherm

The sorption isotherms are determined using diverse methods which can be classi-
fied as, gravimetric, manometric and hygrometric methods. The gravimetric method is
the most common and involves measurement of mass changes which is done continuous
or periodic manner. The manometric methods involve use of highly sensitive manome-
ters to measure the vapor pressure of water in equilibrium with the food product under
study at particular moisture content. However, the hygrometric method measures the
relative humidity of the air surrounding the food material of given moisture content
(Basu et al., 2006 ). There have been numerous efforts to develop more accurate me-
thods for measurement of sorption isotherms. The most commonly method uses small
containers with salt solutions at the bottom to maintain certain relative humidity in the
container where the food material is placed. The main drawback of this process is long
equilibration time and inability to maintain high relative humidity values. Mujumdar
(2000) has reported the most important features of the method to be used, which
should involve maintaining the constant temperature, accurate measurement of the ini-
tial moisture content, maintaining desired relative humidity around the sample, faster
equilibration and repeatability. See Mujumdar (2006), Handbook of Industrial Drying
for discussion of various experimental methods for measurement of equilibrium mois-
ture content

Modeling of sorption isotherm

Various researchers have discussed modeling of sorption isotherms and numerous
models have been proposed based on different theories. Some 80 correlations, ranging
from those based on theory to those that are purely empirical, have appeared in the lite-
rature. Two of the most extensive compilations are due to Wolf et al. (1985) and Iglesias
and Chirife (1982). Aside from temperature, the physical structure as well as composi-
tion of the material also affects water sorption. The pore structure and size as well as
physical and/or chemical transformations during drying can cause significant variations

Water activity (aw)

Moisture Content (%)

Type 1

Type 2

Type 3

Type 4

Type 5

aw = 0 aw = 1