Azarpazhooh, Ramaswamy - Osmotic Dehydrationarea/volume ratio, but that the moisture loss was optimal for the ring shape. Van Nieu-
wenhuijzen et al. (2001) reported that moisture loss and solids gain increases as particle
size is decreased under same processing conditions.
4.3.2. Type of osmotic agent
The type, molecular weight and ionic behavior of osmotic agents are strongly af-
fected by the kinetics of water removal. The most commonly used osmotic agents are
carbohydrates (sucrose, sorbitol, corn syrup, glucose, and fructose) or salts (NaCl, CaCl 2 )
or their mixtures. In most published literature, sucrose is used for fruits and sodium
chlorides for vegetables, fish and meat. The size and molar concentration of the ionized
salts are different with unionized sugars; therefore, the smaller salt ions can more easily
diffuse through the cell membrane resulting in a gain in higher solids, and a reduction in
water loss as the osmotic potential gradient is reduced. However, larger molecules such
as sugar cannot easily diffuse through the cell membrane (Ponting et al., 1966). Al-
though sucrose is very effective, convenient and produces a desirable flavor, it promotes
a greater solids uptake. Maltose can preserve cell structure and color stability, and also
it causes lower solids gain, enhancing a positive impact on nutritional and sensory pro-
files (Forni et al., 1997; Ferrando and Spiess, 2001). Moreover, sucrose can be used in a
binary system in order to reduce the cost of osmotic agents and improving the effective-
ness of osmosis (Hawkes and Flink, 1978; Islam and Flink, 1982). Heredia and Andras
(2008) reported that the use of ternary solutions in osmotic dehydrations of tomatoes
could be more appropriate than the use of binary solutions with the aim of maximizing
water loss and minimizing solutes gain. The low molecular weight of sugars such as glu-
cose is more effective in the transfer of water than the higher molecular weight due to
limiting solids uptake of food material. Invert sugar has twice as many molecules per
unit volume, and is more effective than sucrose. During osmotic dehydration, leaching
the acid from the fruit into the syrup leads to accelerated hydrolysis of sucrose to glu-
cose and fructose, resulting in increasing water removal (Bolin et al., 1983). It is recom-
mended using osmotic dehydration less than 50% weight reduction due to the decrease
in the osmosis rate with time (Torreggiani, 1993). It is reported that water loss mainly
occurs during the first two hrs and the maximum solid gain within 30 min (Conway et al.,
1983 ). Lazarides et al. (1995) showed that under the same osmotic process conditions,
using corn syrups as osmotic agents result in lowering sugar uptake.
4.3.3. Contact time
The contact time of food with the osmotic solution is an important variable during
osmotic dehydration. During osmotic dehydration, increasing the time of the osmotic
treatment results in decreasing the rate of mass transfer while weight loss in food so
treated is increased (Fasina et al., 2002). In terms of the contact time, the rate of both
moisture loss and solids gain is the highest within the first hour of osmosis followed by
progressively lower rates for the rest of the time. On average, moisture loss rates drop
to about 20% of the initial rate during the first hour of dehydration and nearly level off
at about 10% of the initial rate within three hrs. Solid gain rates show a similar decrease
trend. Rapid loss of water in the beginning is due to the large osmotic driving force be-
tween the dilute sap of the fresh fruit and the surrounding hypertonic solution.