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Drying of Fruits and Vegetables
 Dehydration means to get rid of ALL the water in a given
compound by a controlled method of removing moisture
(controlled temperature and humidity) , commonly using
sophisticated equipment.
 A drying process requires simple method of moisture removal
like sun drying..
Drying is New Again
 One of the oldest methods of preserving food
 Alternative to canning & freezing
 Simple, safe and easy to learn
 No refrigeration needed
 Takes little space
How Drying Preserves
 Removes moisture from food so bacteria, yeasts & molds
can’t grow and spoil food
 Slows down action of enzymes but doesn’t inactivate them
Factors that Affect Heat and Liquid Transfer in Food Products
 Surface area
 The greater the faster the product dries
 Temperature
 The greater the difference between the product and drying
medium, the greater the rate of drying
 Humidity
 The higher, the slower the drying
 Atmospheric pressure
 The lower, the lower the temperature required to remove
water
Solute Concentration
 Foods high in sugar and other solutes dry more slowly
 As drying progresses the concentration of solutes becomes
greater in the water remains causing the drying rate to slow
Binding of Water
 As a product dries, its free
 water is removed
 This water evaporates first
 Water in colloidal gels, such as starch, pectin or other gums is
more difficult to remove
 Water that is most difficult to remove is that chemically bound
in the form of hydrates
Chemical Changes
 Caramelization
 Occurs if the temperature is too high
 Enzymatic browning
 Caused by enzymes
 Prevented by inactivating the enzymes before drying
 Nonenzymatic browning
 Controlled by drying the foods rapidly
 Loss of ease of rehydration
 Loss of flavor
Oxidation
 Enzymatic browning occurs in foods high in polyphenols like
apples, grapes, and tea
 Many foods are pretreated to inactivate the enzymes and
prevent browning
Controlling enzymatic activity
 Heat Treatment: blanching (vegetables)
 Sulfuring: fruits exposed to fumes from burning sulfur,
sulfur combines with oxygen  sulfur dioxide
 Advantage: shortened dry time, repels insects, fumes
inhibit mold growth
 Sulfiting: food soaked in sodium bisulfite solution, drained,
and dehydrated
 Disadvantages: increased drying time by 15-20%, food
allergies to chemical
Both: heat
sensitive
nutrients (vit C)
not affected,
food retains
original color
 Fresh material soaked in a heavy (thick liquid sugar solution)
and /or strong salt solution and then the material is sun/solar
dried.
 Protective effect on color, flavor, and texture.
 High quality.
 Little use of sulfur dioxide.
Osmotic dehydration
Sun drying
 Dry, warm climates
 Slow drying method (several days)
 Inexpensive
 Invasion by insects, birds, rodents,
microorganisms
Spray drying
Tray (air) drying
Heated air ( 140-176o
F) at set velocity and with a low % relative humidity
Poor/ moderate rehydration properties
Food shrinkage, dense product, shape not held
Quick, inexpensive
Small scale
10-20 hrs neede
Ex) Pasta, vegetables, fruit, herbs and spices
Tunnel drying
Hot air is blown over the product, such as vegetables. The
concurrent system dries the food rapidly with little shrinkage, but
leaves a relatively high moisture content. The counter-current
system is slower, but produces a product with a low moisture
level.
A disadvantage of this process is that the product tends to shrink
and is less easy to rehydrate.
Fluidised bed drying
Warm air is blown upwards directly underneath the food,
causing it to flow and remain separated. This procedure is
suitable for small items such as peas and coffee.
Roller drying
The food product, in a liquid or paste form, is uniformly spread
over heated rollers or drums which rotate slowly.
The heat causes the moisture to evaporate leaving a dried product
behind.
A scraper then removes this for use. This method is suitable for
instant mashed potato and baby foods.
Drum drying
248o
F248o
F
Food is more likely to be
flakey
Ex) purees, pastas and
mashed foods
Mashed potatoes, tomato
pastes
Size of drum and speed of
rotation is determined by
drying time needed
Freeze Drying (lyophilization)
 Food must be in frozen state
 Vacuum chamber
 Heat from radiant heaters- energy
 Water will sublime:
 Water ( solid) water (vapor)
 No transition through a liquid state (no evaporation)
Freeze drying (characteristics)
Voids from ice crystals when water sublimes ( act as channels)
Excellent rehydration
No translocation of water soluble constituents to the surface
Shape of food is retained (no shrinkage)
food closely resembles starting material
very expensive– used for high value foods
Instant soups, high quality coffee, vegetables, military rations,
space food
Packaging requirements for dehydrated foods
 Protect against moisture absorption
 Dehydrated foods- hygroscopic
 Prevent transmission of water vapor
 Physical protection
 Prevents crushing
 Freeze dried foods- porous structures with spaces;
easily crushed
 Protection from oxygen and light
 photo- oxidation
Drying of Fruits and Vegetables

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Drying of Fruits and Vegetables

  • 1. Drying of Fruits and Vegetables
  • 2.  Dehydration means to get rid of ALL the water in a given compound by a controlled method of removing moisture (controlled temperature and humidity) , commonly using sophisticated equipment.  A drying process requires simple method of moisture removal like sun drying..
  • 3. Drying is New Again  One of the oldest methods of preserving food  Alternative to canning & freezing  Simple, safe and easy to learn  No refrigeration needed  Takes little space
  • 4. How Drying Preserves  Removes moisture from food so bacteria, yeasts & molds can’t grow and spoil food  Slows down action of enzymes but doesn’t inactivate them
  • 5. Factors that Affect Heat and Liquid Transfer in Food Products  Surface area  The greater the faster the product dries  Temperature  The greater the difference between the product and drying medium, the greater the rate of drying  Humidity  The higher, the slower the drying  Atmospheric pressure  The lower, the lower the temperature required to remove water
  • 6. Solute Concentration  Foods high in sugar and other solutes dry more slowly  As drying progresses the concentration of solutes becomes greater in the water remains causing the drying rate to slow
  • 7. Binding of Water  As a product dries, its free  water is removed  This water evaporates first  Water in colloidal gels, such as starch, pectin or other gums is more difficult to remove  Water that is most difficult to remove is that chemically bound in the form of hydrates
  • 8. Chemical Changes  Caramelization  Occurs if the temperature is too high  Enzymatic browning  Caused by enzymes  Prevented by inactivating the enzymes before drying  Nonenzymatic browning  Controlled by drying the foods rapidly  Loss of ease of rehydration  Loss of flavor
  • 9. Oxidation  Enzymatic browning occurs in foods high in polyphenols like apples, grapes, and tea  Many foods are pretreated to inactivate the enzymes and prevent browning
  • 10. Controlling enzymatic activity  Heat Treatment: blanching (vegetables)  Sulfuring: fruits exposed to fumes from burning sulfur, sulfur combines with oxygen  sulfur dioxide  Advantage: shortened dry time, repels insects, fumes inhibit mold growth  Sulfiting: food soaked in sodium bisulfite solution, drained, and dehydrated  Disadvantages: increased drying time by 15-20%, food allergies to chemical Both: heat sensitive nutrients (vit C) not affected, food retains original color
  • 11.  Fresh material soaked in a heavy (thick liquid sugar solution) and /or strong salt solution and then the material is sun/solar dried.  Protective effect on color, flavor, and texture.  High quality.  Little use of sulfur dioxide. Osmotic dehydration
  • 12. Sun drying  Dry, warm climates  Slow drying method (several days)  Inexpensive  Invasion by insects, birds, rodents, microorganisms
  • 14. Tray (air) drying Heated air ( 140-176o F) at set velocity and with a low % relative humidity Poor/ moderate rehydration properties Food shrinkage, dense product, shape not held Quick, inexpensive Small scale 10-20 hrs neede Ex) Pasta, vegetables, fruit, herbs and spices
  • 15. Tunnel drying Hot air is blown over the product, such as vegetables. The concurrent system dries the food rapidly with little shrinkage, but leaves a relatively high moisture content. The counter-current system is slower, but produces a product with a low moisture level. A disadvantage of this process is that the product tends to shrink and is less easy to rehydrate.
  • 16. Fluidised bed drying Warm air is blown upwards directly underneath the food, causing it to flow and remain separated. This procedure is suitable for small items such as peas and coffee.
  • 17. Roller drying The food product, in a liquid or paste form, is uniformly spread over heated rollers or drums which rotate slowly. The heat causes the moisture to evaporate leaving a dried product behind. A scraper then removes this for use. This method is suitable for instant mashed potato and baby foods.
  • 18. Drum drying 248o F248o F Food is more likely to be flakey Ex) purees, pastas and mashed foods Mashed potatoes, tomato pastes Size of drum and speed of rotation is determined by drying time needed
  • 19. Freeze Drying (lyophilization)  Food must be in frozen state  Vacuum chamber  Heat from radiant heaters- energy  Water will sublime:  Water ( solid) water (vapor)  No transition through a liquid state (no evaporation)
  • 20. Freeze drying (characteristics) Voids from ice crystals when water sublimes ( act as channels) Excellent rehydration No translocation of water soluble constituents to the surface Shape of food is retained (no shrinkage) food closely resembles starting material very expensive– used for high value foods Instant soups, high quality coffee, vegetables, military rations, space food
  • 21. Packaging requirements for dehydrated foods  Protect against moisture absorption  Dehydrated foods- hygroscopic  Prevent transmission of water vapor  Physical protection  Prevents crushing  Freeze dried foods- porous structures with spaces; easily crushed  Protection from oxygen and light  photo- oxidation