3. Introduction
โซOsmotic dehydration is the process of water removal by
immersion of water containing cellular solid in a
concentrated aqueous solution.
โซOsmotic dehydration has been recognized as a good pre-
treatment prior to regular drying.
โซOsmotic dehydration has the potential to remove water at
low temperatures.
โซ It is an energy efficient method, as water does not go
through a phase change.
4. ๏ง It gives rise to three types of simultaneous
counter current flows-
โซWater diffusion into the solution
โซSolute diffusion into food
โซLeaching out of the product's own solutes
5. Why Osmotic Dehydration ??
Dehydration Osmotic dehydration
1. Sensorial properties are
affected.
2. Loss of flavor
3. Huge nutrients loss.
4. Chance of physical
contamination
1. Sensorial properties are very
good
2. Retention of fresh fruit flavors.
3. Retention of nutrients.
4. No chance of contamination
Hence, for improved quality & Ready To Eat products
osmotic dehydration is the best choice!!!
10. Because there are more freely moving water molecules on
the left, more diffuse through the pores of the membrane
from left to right than from right to left.
membrane
water sugar solution
12. Flow chart of osmotic dehydration process
Selection of raw material
Washing
Preparation of fruits
Osmatic
treatment
Draining
Pretreatment
Packaging
Dehydration of
fruits slices
Storage
Peeling
Labeling
13. Factors affecting osmotic dehydration
โซ Type of osmotic agent
โซ Concentration of the osmotic solution
โซ Time of treatment
โซ Temperature of the osmotic solution
โซ Size and geometry of the material
โซ Solution-to-material mass ratio
โซ Level of agitation of the solution
14. Osmotic agent Remark References
Calcium
chloride
Increase firmness of apple pieces,
preserve the texture during storage.
Ponting et al.
(1972)
Lactose Has much lower level of sweetness
than sucrose & Low solubility in
aqueous solution.
Hawkes and Flink
(1978)
Sodium chloride Retards oxidative and non enzymatic
browning. Sometime bleaching
effects on colored products.
Khin et al. (2007)
Singh et al. (2007)
Sucrose Dry sugar is unsuitable due to
oxidative browning. Difficulty in
disposing the sugar syrup formed.
Flink (1975)
Mixture of
sucrose and salt,
ethanol and salt
More effective than sucrose alone
due to combination of properties of
both the solutes.
Lenart and Flink
(1984a, b)
Type of osmotic agent
15. โซConcentration of the osmotic solution
โซHigh concentrations of solution and long osmotic times
increased the penetration depth of osmotic agents and
decreased the moisture content of samples.
โซTime of treatment
โซThe rate of both moisture loss and solids gain is the
highest for first 2 hour followed by progressively lower
rates for the rest of the time.
โซRapid loss of water in the beginning is due to the large
osmotic driving force between the fresh fruit and the
surrounding hypertonic solution.
16. Temperature of the osmotic solution
โซHigher process temperatures promote faster moisture
loss through better mass transfer characteristics on the
surface due to lower viscosity of the osmotic medium.
โซHowever, above 45ยฐC, enzymatic browning and flavour
deterioration begins to take place.
โซThe best processing temperature depends on type of
food:
Example, for green beans, 20ยฐC gives better
results, while 40ยฐC is too high a temperature.
17. Size and geometry of the material
โซ The surface area to volume ratio has been shown to be
the influencing factor with higher ratios favoring better
osmotic dehydration rates (Islam and Flink ,1982).
Solution-to-material mass ratio
โซ As the solution to sample ratio increased, the osmosis
rate increased up to a certain level and then level off .
โซ The weight ratio of solution to product most often used
is between 4 and 10.
Level of agitation of the solution
โซ The osmotic dehydration is faster when syrup is
agitated. This is because of reduced mass transfer
resistance at the surface.
18. Enhancement of osmotic dehydration
Application of
โซ Microwave during osmotic dehydration
โซ Blanching as a pretreatment
โซ High hydrostatic pressure as a pretreatment
19. Application of microwave during osmotic
dehydration
โซMaterial absorbing microwave energy directly and
internally and converting it into heat.
โซMoisture outflux effectiveness of the microwaves in
immersion mode is reduced due resistance of medium
itself for absorption of microwaves.
โซThe spray mode uses a thin layer of osmotic solution
that is continuously flushed down.
โซThis eliminates the problem of sample floating.
20. Application of blanching as a pretreatment
โซ Hot water or steam blanching is a pretreatment
before osmotic dehydration with the purpose of
โซEnzyme inactivation
โซTo promote gas removal from surfaces and
intercellular spaces
โซPrevention of oxidation, discoloration, and off-
flavour development and microbial growth.
โซ Water blanching (85โ100 ยฐC) usually results in loss
of nutrients such as minerals and vitamins (Akyol et
al., 2006).
21. Application of high hydrostatic pressure as a
pretreatment
โซHigh-pressure treatments can be applied to liquid and
solid foods, with or without packaging, at pressures
between 100 and 800 MPa.
โซHigh pressures cause permeabilization of the cell
structure leading to the enhancement of mass transfer
rates during osmotic dehydration.
โซ High hydrostatic pressure with the combination of
mild heat treatment can be used for blanching
purposes in carrots, green beans, and green peas.
22. Advantages
โซQuality improvement in terms of color, flavor and
texture
โซLower energy requirements
โซPackaging and distribution cost reduction
โซRetention of nutrients during storage
โซThis provides better retention of volatile flavours and
reduces tissue damage compared with conventional air
and sun drying processes.
23. Disadvantages
โข The reduction in acidity level reduces the
characteristic taste of some products. This can
overcome by adding fruit acid in the solution.
โข Sugar coating is not desirable in some products and
quick rinsing in water may be necessary after the
treatment.
โข Osmotic dehydration with other combined processes
such as vacuum drying, air drying or blanching were
found expensive
โข It is a time taking process
