Dehydration Flashcards
Dehydration Methods
1) Evaporation
2) Drying
3) Osmotic Dehydration
Benefits of Dehydration
• Extended Shelf Life
• Reduced Weight and Volume
• Convenience
• Minimized Food Waste
• Cost Savings
Water Activity
⦁ Pathogenic microorganisms can not grow at
water activity less than 0.86.
⦁ Yeast and molds can not grow at water activity less
than 0.62.
⦁ Free water can be removed by concentration,
dehydration and freeze drying.
⦁ The lower the water activity, the longer the
storage life.
EVAPORATION
➢To remove water from liquid foods to obtain
concentrated liquid products
➢Differs from dehydration because the final product of the evaporation process remains in a liquid state
➢The evaporator has two principal functions, to exchange heat and to separate the vapor that is formed from the liquid.
➢ Example: Evaporated milk, condensed milk, dehydrated egg
EVAPORATOR SYSTEM
➢the heat exchanger (noncontact)
➢the evaporating section, where the liquid boils and
evaporates,
➢the separator in which the vapor leaves the liquid and
passes off to the condenser or to other equipment
➢The product inside the evaporation chamber is kept
under vacuum.
➢In the presence of vacuum, the product boils at relatively
low temperatures, thus minimizing heat damage.
➢The vapors produced are conveyed through a condenser
to a vacuum system.
• Vapors produced:-
➢are discarded without further utilizing their heat (to condenser)
• (Single effect evaporators)
➢are reused as a heating medium in another
evaporator chamber
• (Multi-effect evaporators)
SINGLE EFFECT EVAPORATOR
• Heat exchanger-stream is used as a heating medium for transferring heat from a low-pressure stream to the product.
• The product inside the evaporator the chamber is kept under vacuum, thus introducing temperature differences between streams and products, which
boils at low temperature (minimize heat damage)
• The vapors leaving the product are conveyed through a condenser to a vacuum system.
• Condenser - affects condense of vapor and separates it out of the system
• The concentrated product is then pumped out of the evaporator
MULTIPLE EFFECT EVAPORATOR
• Dilute liquid feed is pumped into the first evaporator chamber, while steam enters the heat exchanger and condenses, thus discharging heat to the product. The condensate is discarded.
• The vapors produced from the first effect are used as the heating medium in the second effect, where the feed is partially concentrated product from the first effect and so on.
• The desired final concentration is pumped out of the evaporator chamber of the third effect.
1) Batch-Type Pan Evaporator
➢ Simplest and oldest types of evaporator
➢ The product is heated in a steam-jacketed
spherical vessel
➢ The heating vessel may be open to the
atmosphere or connected to a condenser and
vacuum.
➢ Vacuum permits boiling the product at
temperatures lower than the boiling point at
atmospheric pressure, thus reducing the
thermal damage to products.
➢ The heat-transfer area per unit volume is small
and the residence time of the product is longer
up to several hours
2) Natural Circulation Evaporator
➢Short vertical types, 1-2 m long and 50-100mm in
diameter.
➢The heat-transfer area per unit volume is small
➢The residence time of the product is longer up to
several hours.
➢The concentrated liquid falls back to the base of the vessel through a central annular
section.
- Falling-film Evaporator
➢It has a thin liquid film moving downward under gravity on the inside of the vertical tubes.
➢This is a more complicated system than rising-film evaporator.
➢The residence time is about 20-30 seconds, compared with a residence time of 3-4 minutes in a
rising-film evaporator.
➢best suited for highly heat-sensitive products such as orange juice & other juices and high viscous
products than Rising film Evaporator
• The basic factors that affect the rate of evaporation
are the:
➢ The amount of heat rate can be transferred to the liquid.
➢ Quantity of heat required to evaporate water from product.
➢ Pressure which the evaporation takes place.
➢ Surface area that contacts between hot medium to the product.
