Azarpazhooh, Ramaswamy - Osmotic Dehydration
4.1. INTRODUCTION
Dehydration is a versatile, widespread technique in the food industry; it is the oldest
and most frequently used method of food preservation. The main objective of drying is
the removal of moisture so as to reduce the water activity and hence the associated mi-
crobial and enzymatic activity and product quality deterioration. Drying methods have
been applied to extend the shelf life of the product; however, they often affect the quality
of the final product. The most common quality defects associated dehydrated products
are poor reconstitution, loss in texture, loss in the nutritive and sensory properties such
as flavor and color. These are mainly due to the exposure of the product to high temper-
atures and long drying times mostly in the presence of air (Lenart, 1996; Lin et al., 1998).
A new interest has recently arisen in finding new ways to improve the quality of dried
food products. Many alternatives have been recognized such as the use of vacuum so
that lower temperatures could be used, use of freeze drying which is done under condi-
tions below the triple point of water facilitating sublimation thereby protecting the
product texture and other quality factors, use of rapid drying techniques which would
reduce the drying time, use of novel heating sources like microwave and radio frequency
heating (significant reducing in drying time), use of various treatments which promote
better mass transport phenomena etc. Osmotic dehydration has been lately recognized
as a good pretreatment prior to regular drying to promote better quality and reduce
energy needs (Torreggiani, 1993). Osmotic dehydration has the potential to remove wa-
ter at low temperatures; in addition, it is an energy efficient method, as water does not
go through a phase change (Bolin et al., 1983).
Osmotic dehydration is gaining popularity as a complementary processing step in
the chain of integrated food processing in the food industry due to its quality and energy
related advantages. It has been shown that osmotic pre-treatment improves the quality
of dried products including: reduced discoloration of the fruit by enzymatic browning
(Ponting et al., 1966; Contreras and Smyrl, 1981), reduced heat damage to texture, color
(Torreggiani, 1993), increased retention of volatiles (Flink, 1975; Dixon and Jen, 1977),
increased sugar to acid ratio which improves the textural quality (Raoult-Wack, 1994)
and low operating costs (Bolin et al., 1983). Osmotic dehydration is acknowledged to be
an excellent energy saving method as moisture is efficiently removed from a food prod-
uct without a phase change (Bolin et al., 1983). In addition, the product is processed in a
liquid phase, generally giving good heat- and mass-transfer coefficients (Raoult-Wack,
1994 ). The cost of shipping, packing and storing is also reduced due to the lower mois-
ture content of the product (Rao, 1977; Biswal and Bozorgmehr, 1992). Since the water
activity of the product is decreased, microbial growth is largely inhibited. However, the
product is not shelf-stable since relatively large proportion of moisture still exists (up to
50%). Additionally, complementary treatments such as freezing (Tregunno and Goff,
1996 ), freeze drying (Donsì et al., 2001), vacuum drying (Rahman and Mujumdar, 2007),
air drying, osmo-convective drying (Islam and Flink, 1982; Corzo et al., 2008) and mi-
crowave drying (Orsat et al., 2007) are necessary in order to provide shelf stability to
the product. Osmotic dehydration is a time-consuming process; therefore, supplementa-
ry methods are needed to increase the mass transfer without affecting the product quali-
ty (Rastogi et al., 2002). One of the distinctive aspects of osmotic dehydration in compar-