Azarpazhooh, Ramaswamy - Osmotic Dehydration
4 .3.4. Temperature of the solution
The temperature of the osmotic treatment is the most significant factor that influ-
ences the process of osmotic dehydration. The positive effect of temperature on the re-
moval of water from the food during osmotic treatment has been shown by several re-
searchers (Raoult-Wack et al., 1994; Lazarides and Mavroudis, 1996). Higher process
temperatures generally promote faster moisture loss through better mass transfer cha-
racteristics on the surface due to lower viscosity of the osmotic medium. High tempera-
tures, i.e. over 60°C, modify the tissue characteristics so favoring impregnation pheno-
mena and thus the solid gain; however, above 45°C, enzymatic browning and flavor de-
terioration begins to take place (Lenart and Flink, 1984a). The best processing tempera-
ture depends on the food: for example, for green beans, 20°C gives better results, while
40°C is too high a temperature (Biswal et al., 1991).
4.3.5. Agitation and food/ solution ratio
Agitation of the osmotic solution is an important aspect of the osmotic treatment.
The agitation ensures that the concentrated solutions are restored around the particle
surface and that a concentration difference favorable to mass transfer is recreated. The
ratio of osmotic solution to fruit is an important consideration and often influences the
production logistics, since it dictates the mass transfer momentum and the equilibrium
concentrations. High solution/fruit ratios maintain constant solution concentration, and
prevent dilution. On an industrial scale, the ratio needs to be as low as possible to re-
strict plant size and costs of solution regeneration. On the other hand, use of a low ratio
leads to significant transient changes in the solution composition. Most development
studies are carried out with a large excess of osmotic solution to ensure minimal
changes in solution concentration during test runs. The weight ratio of solution to prod-
uct most often used is between 4 and 10.
4. 4. ENHANCEMENT OF OSMOTIC DEHYDRATION
Osmotic dehydration is relatively slow so acceleration of mass transfer would be advan-
tageous. There are various methods to increase the mass transfer, such as application of
ultrasound, high hydrostatic pressure, high electrical field pulses, vacuum and centrifug-
al force and microwave
4.4.1. Application of ultrasound during osmotic dehydration
Ultrasound in the food industry is relatively new and it has not been explored in-
depth until recently (De Gennaro et al., 1999). Ultrasound has been applied in the food
industry to determine food properties due to low-frequency, high-energy ultrasound. It
can travel through a solid medium; therefore, it can influence mass transfer. A pheno-
menon known as acoustic cavitation is generated during the application as ultrasonic
waves can generate minute vapor-filled bubbles that collapse rapidly or generate voids
in liquids. Consequently, rapid pressure fluctuations are induced within the wet material
by the ultrasonic waves. Ultrasound can be carried out at ambient temperature as no
heating is required for reducing the potential of thermal degradation (Rodrigues and
Fernandes, 2007). It can influence mass transfer through structural changes, such as
“sponge effect”, and microscopic channels (Carcel et al., 2007). Applying ultrasound dur-