Azarpazhooh, Ramaswamy - Osmotic Dehydrationing osmotic treatment has a significant effect on the kinetics of water loss, sugar gain,
and firmness loss, as well as on the microstructure of osmotically dehydrated different
products and processes in liquid–solid system, such as osmotic dehydration of apples
(Carcel et al., 2007). Water effective diffusivity increases with the use of ultrasound and
decreases the amount of sugar in the fruit to produce a dried low-sugar fruit (Rodrigues
and Fernandes, 2007). Gallego-Juarez et al. (1999) used high-intensity ultrasound to ac-
celerate the osmotic dehydration rate of apples. Duan et al. (2008) used ultrasound pre-
treatment to improve the freeze- drying rate.
4.4.2. Application of blanching as a pretreatment
Hot water or steam blanching is a pretreatment before osmotic dehydration with the
purpose of enzyme inactivation, and also to promote gas removal from surfaces and in-
tercellular spaces; oxidation, discoloration, and off-flavor development and microbial
growth are thereby prevented (Rahman and Perera, 1999). Blanching has been applied
prior to drying of fruits and vegetables, however blanching has some drawbacks such as
causing changes in the chemical and physical state of nutrients and vitamins as well as
having an adverse environmental impact on large water and energy usage (Rahman and
Perera, 1999 ). Water blanching (85–100 °C) usually results in loss of nutrients such as
minerals and vitamins (Akyol et al., 2006).
4.4.3. Application of high hydrostatic pressure as a pretreatment
High-pressure treatments have been tested for their effectiveness as an alternate to
thermal blanching (Eshtiaghi and Knorr, 1993) because they can be applied to liquid and
solid foods, with or without packaging, at pressures between 100 and 800 MPa
(Eshtiaghi et al., 1994). Akyol et al. (2006) showed that high hydrostatic pressure (HHP)
with the combination of mild heat treatment can be used for blanching purposes to inac-
tive peroxidase (POD) and lipoxygenase (LOX) in carrots, green beans, and green peas.
In addition, high pressures cause permeabilization of the cell structure (Eshtiaghi et al.
1994 ) leading to the enhancement of mass transfer rates during osmotic dehydration.
Rastogi and Niranjan (1998) reported that the application of HP on pineapples damaged
cell wall structure, leaving the cells more permeable with a reduction in intercellular
material. Taiwo et al. (2001) report that high pressure may be considered during osmot-
ic dehydration when product formulation through sugar uptake is desired.
4.4.4. Application of vacuum as a pretreatment and during osmotic dehy-
dration
Application of vacuum impregnation (VI) simultaneously with osmotic treatment for
a short period of time has been widely studied (Fito, 1994). Vacuum impregnation is
widely used simultaneously with osmotic treatments to enhance mass transfer and
promote more homogeneous concentration profiles in the fruits (Fito et al., 2001). The
total transport of water and solute during vacuum pulse osmotic dehydration is caused
by two mechanisms: the hydrodynamic mechanism (HDM) and pseudo-fictions mechan-
ism. HDM is promoted by pressure gradients and penetration into the pores of plants
over a short time period and the pseudo-fiction mechanism is driven by activity gra-
dients over longer time frames (Fito, 1994). During vacuum impregnation especially in
porous products, the action of hydrodynamic mechanisms (HDM) is combined by diffu-