Azarpazhooh, Ramaswamy - Osmotic Dehydration
dition. Correlative models have been proposed, either to compute the time required for
a given weight reduction as function of the processing temperature and of the solution
concentration or to estimate the dehydration parameters. Nsonzi and Ramaswamy
(1998b) studied osmotic dehydration kinetics of the blueberry and further modeled
moisture diffusivity and soluble solids diffusivity with quadratic functions of tempera-
ture and concentration. Azuara's model has the advantage of allowing the calculation of
the equilibrium values (MLe and SGe) (Ochoa-Martinez et al., 2007).
4 .5.2. Microscopic approach
The mass transfer phenomena occurring in plant tissues during osmosis involves
complex mechanisms, most of them controlled by the plant cells. During osmotic dehy-
dration of cellular material, mass transfer inside the cellular material depends on both
processing variables and micro-structural properties of the biological tissue. There is a
naturally wide variation in the physical nature of raw food material. When biological
cellular material undergoes osmotic dehydration, mass fluxes in the system imply
changes in structural and transport properties (volume, dimension, viscosity, density,
porosity, etc.). As a result, these changes affect the mass transfer fluxes. The changes of
material tissue volume and porosity promote the action of non-diffusional driving forces,
such as a pressure gradient associated with the relaxation of a deformed cell network to
release the structural stress (Mayor and Sereno, 2004), and changes in mechanical
properties and color changes (Krokida et al., 2000). Knowledge of and predictions about
these changes are important because they are related to quality factors and some as-
pects of food processing, such as food classification, process modeling and design of
equipment. Most of these changes, although observed at a macroscopic level, are caused
by changes occurring at the micro-structural/cellular level. In this way, the study of the
micro-structural changes during dehydration is important because it can allow us to un-
derstand and predict the changes occurring in the physical–chemical properties at high-
er levels of structure. Mass transfer (and eventually heat transfer) phenomena result in
changes at microscopic and macroscopic levels and consequently variations in the phys-
ical properties of the food system. These changes also produce alterations in mechan-
isms and kinetics in the transport phenomena (Fito and Chiralt, 2003).
4.6. COMPLEMENTARY DRYING METHODS
Osmotic dehydration is a pretreatment which can improve nutritional, sensorial and
functional properties of food without changing its integrity (Torreggiani, 1993). Drying
is a major part of osmotic dehydration, and the impact of it on complementary air drying
requires special attention.
Osmotic dehydration is generally used as a preliminary step for further processing
such as freezing (Ponting et al., 1966), freeze drying (Hawkes and Flink, 1978), vacuum
drying (Dixon and Jen, 1977), microwave heating and processing applications (Nelson
and Datta, 2001), and air drying (Mandala et al., 2005). Abundant information is availa-
ble on the application of an osmotic treatment before a conventional drying (Lemus-
Mondaca et al., 2009). Sharma et al. (1998) studied the influence of some pretreatment
parameters such as steam blanching and sulfur dioxide treatment on product quality
after osmo-air dehydration processing of apples. They found greater retention of ascor-