Solar Drying: Fundamentals,Applications and Innovations - National ...

Visavale - Principles, Classification and Selection of Solar Dryers
Figure 1.6. Schematic model of BET theory (Belessiotis and Delyannis, 2011)
1.6.2. The Bet Equation
The BET (Brunauer et al., 1938), equation is a modification of the previous Lagmuir equation,
widely accepted for foods. The BET equation is:
M w =
M b Ba w
(1−a w )[1+(B−1)a w ]
(1.13)
where Mw is the water content (kg water/kg dry solids), Mb is the BET monolayer
water content (kg water/kg dry solids) and B is a constant related to the net heat of
sorption. The BET isotherm holds well between water activities of 0.05 and 0.45, an
adequate range for the calculation of parameters Mb and B (Rahman, 1995).
1.6.3. GAB Equation
The GAB (Guggenheim, 1966; Anderson, 1946; de Boer, 1953) equation is the most
widely accepted isotherm model as it fits isotherms of products in a range of water activities
from aw = 0 up to aw = 0.99 (Van der Berg, 1981).
The GAB equation is:
M w =
M g CKa w
[(1−Ka w )(1−Ka w +CKa w )]
(1.14)
where M g is the GAB monolayer water content (dry basis). C is a constant related to
the monolayer heat of sorption and K is a factor related to the heat of sorption of the
multilayer. BET and GAB models are the most commonly used models to fit sorption data
of food materials. The GAB isotherm equation is an extension of the BET model taking
into account the modified properties of the sorbate in the multilayer region and the bulk
liquid water properties through the introduction of a third constant K. Estimation of
three parameters in GAB using two variables (i.e. water content and water activity)
leads to a non-linear optimization. The BET monolayer value is more acceptable than
that of GAB monolayer value, although the GAB model provides accurate prediction over
the water activity range up to 0.90 (Rahman, 1995).
1.7. THIN LAYER DRYING AND DEEP BED DRYING
Drying rate is controlled by the external process factors of the process and the internal
diffusion mechanisms. Apart from the external factors, the type and size of the product
also affect the drying rate. Thus many products, i.e. fruits, vegetables, sliced fruits,
are better to be dried in thin layers, but grains, i.e. corn, beans, etc., can be dried in deep
beds, in dryers, or into bins.
1.7.1. Thin Layer Solar Drying
Thin layer drying models fall into the three categories, namely theoretical, semitheoretical
and empirical, contributing to the understanding of the drying characteris-
12 Drying of Foods, Vegetables and Fruits