Azarpazhooh, Ramaswamy - Osmotic Dehydrationprocesses. The potential of PEF during osmotic dehydration for the first time was dem-
onstrated by Rastogi et al. (1999). This finding has created more research looking into
the ability of PEF as pre-treatment during osmotic dehydration of plant foods. The
Pulsed Electric Field as a non-thermal method can cause permeable cells to whiten a
short time (μs to ms range) while keeping the product matrix unaltered, thereby posi-
tively accelerating mass transfer during osmotic dehydration (Ade-Omowaye et al.,
2001 ). Taiwo et al. (2001) studied the effect of high-intensity electric field pulses (HELP)
pretreatment on the diffusion kinetics of apple slices. They reported that HELP has a
very minimal effect on solids gain; and application of HP is advantageous when moisture
reduction and minimal alteration in product taste are desired. Moreover, firmer texture,
brighter color, and better retention of vitamin C are the advantages of applying HELP
with osmotic dehydration. Lazarides and Mavroudis (1996), Ade-Omowaye et al. (2001)
and Taiwo et al. (2001) reported that PEF pre-treatment might be a better alternative to
processing at high temperatures.
4.4.6. Application of microwave during osmotic dehydration
Microwave-osmotic dehydration is a novel technique with a good potential for more
efficient osmotic drying of fruits and vegetables. Carrying out osmotic drying in a mi-
crowave environment enhances moisture removal when moist food is immersed in a
concentrated solution of an osmotic agent (Li and Ramaswamy, 2006c). The osmotic
concentration gradient effect existing between the solution and food, which is the driv-
ing force for the removal of moisture from the food into the osmotic medium, is en-
hanced under the microwave field. This is due to selective absorption of microwave
energy by the water molecules resulting in increased moisture outflux, which also has
the tendency to limit the simultaneous transfer of solute from the solution into the food.
Li and Ramaswamy (2006a.b,c) investigated the mass transport coefficients under mi-
crowave-osmotic dehydration (MWOD, immersion medium) and compared it with the
conventional osmotic dehydration process (COD). They reported that MWOD significant-
ly increased the rate of moisture loss and decreased the rate of solids gain. They also
found that the osmotic dehydration under microwave heating made it possible to obtain
a higher diffusion rate of moisture transfer at lower solution temperatures. In their ex-
periments, they immersed the apple slices in the osmotic solution placed within the mi-
crowave field. In such an immersion medium, because the sample is surrounded by a
large volume of the solution, the absorption of microwaves by the sample itself will be
limited, thus reducing the moisture outflux effectiveness of the microwaves. This finding
has provoked new research. Microwave osmotic dehydration under continuous flow
medium spray conditions was developed and shown to provide a means of effecting
moisture loss and limiting solids gain that was far superior to three other techniques
under similar continuous-flow conditions (Azarpazhooh and Ramaswamy, 2010a). It
was clearly demonstrated that spray mode heating enhanced the efficiency of the system.
This is likely due to the direct and more efficient exposure of the sample to the micro-
wave field. As opposed to the large volume of solution that surrounds the sample in the
MWOD immersion system, the spray mode only uses a thin layer of osmotic solution that
is continuously flushed down due to the rapidly flowing medium and gravity. The spray
mode also eliminates the problem of sample floating, which can restrict the application
of immersion mode (Azarpazhooh and Ramaswamy, 2010a). Microwave drying has the