Azarpazhooh, Ramaswamy - Osmotic Dehydration
specific advantage of rapid and uniform heating due to the penetration of microwaves
into the body of the product. The most important characteristic of microwave heating is
volumetric heating, which refers to the material absorbing microwave energy directly
and internally and converting it into heat. Heat is generated throughout the material,
leading to faster heating rates (compared to conventional heating, where heat is usually
transferred from the surface to the interior) and producing rapid and uniform heating
(Gowen et al., 2006). Microwave heating, causing a positive outflux of moisture from the
product, not only results in greater moisture loss but also a higher solids gain. Immer-
sion of the fruits in syrup in the MWODI mode limits the exposure of fruits to the MW
field because of the surrounding syrup. However, in the MWODS mode, the same treat-
ment provides a more direct exposure of the fruit to MW because as the continuous
spray trickles down the fruit bed, it only retains a thin layer of the syrup at the fruit sur-
face. It is interesting to note that applying spray can also overcome one of the problems
with osmotic dehydration- the floating of the fruit in the solution.
4.5. MODELING OF THE OSMOTIC DEHYDRATION
Although considerable efforts have been made to improve the understanding of
mass transfer in osmotic dehydration, fundamental knowledge about predicting mass
transport is still a gray area (Raoult-Wack et al., 1991). Modeling of the osmotic dehy-
dration process is necessary for optimizing the osmotic dehydration and subsequent
drying processes, in order to achieve the highest possible quality at minimum energy
costs (Saguy et al., 2005). The unusual features come from the interaction between the
solution and materials of biological origin. Mass transfer in osmotic dehydration of cellu-
lar plant foods, such as fruit and vegetables, involves several physical effects due to the
complex morphology of plant tissues. These can be described, in an ideal way, as osmo-
sis, diffusion and hydrodynamic mechanism (HDM) penetration (Fito and Pastor, 1994).
Two basic approaches can be used to model osmotic processes (Ramaswamy, 1982; Sal-
vatori, 1998). The first one, the macroscopic approach, assumes that the tissue is homo-
geneous and the modeling is carried out on the cumulated properties of cell walls, cell
membranes and cell vacuoles. The second one, the microscopic approach, identifies the
heterogeneous properties of the tissue and is based on cell microstructure (Fito et al.,
1996 ).
4.5.1. Macroscopic approaches
Macroscopic analysis has been carried out on pseudo-diffusion, square root of time,
irreversible thermodynamic and other approaches (Fito et al., 1996) Very little work has
been developed from the microscopic point of view (LeMaguer, 1996). The analysis of
the concentration profiles developed throughout mass transfer processes, using a ma-
croscopic approach, can be useful to clarify the mass transfer mechanisms and their
coupling, especially if data are correlated with micro-structural features (shape, size and
geometry changes in cell and intercellular spaces, cell wall deformation and relaxation
changes, etc.) observed by a microscopic technique (Alzamora et al., 1996). However,
concentration profiles allow us to calculate mass transfer kinetics (Lenart and Flink,
1984b). Mathematical modeling may provide a useful insight into the underlying me-
chanisms and several mathematical models have been proposed based on a cellular