DEHYDRATION Chapter 5 INTRODUCTION Water Activity is related to the amount of water contained in the food material and defines the amount of water that can be used for the growth and reproduction of micro-organisms, Water is essential solvent that is needed in biochemical reaction in living organism. FREE WATER VS BOUND WATER The term water activity defines the degree in which such water molecules are “bound”. A portion of the total water content present in a food material is strongly bound to specific sites on the molecules that comprise the food material. FOOD PRESERVATION BY CONTROLLING WATER ACTIVITY Preservation of many food materials can be achieved by lowering the amount of water available for the growth of micro-organisms. CONTROLLING BROWNING REACTIONS Food products are subject to browning reactions. Dehydrated products are probably more so than others. FOOD DRYING Food is dried when water is removed from it. The oldest and simplest way of water removal is sun drying. The use of drying equipment heated by gas, electricity or steam is called artificial drying or dehydration. Another method of drying food is by frying or oil drying. ADVANTAGES OF DRIED AND DEHYDRATED FOODS Decrease in weight and bulk Convenience Shelf stable MECHANISM OF DRYING/DEHYDRATION Two processes: Heat transfer Mass transfer The rate of drying is affected by the following factors: Humidity Surface Area Temperature Air velocity Ventilation/ moist air outlet Atmospheric pressure and vacuum STAGES OF DRYING Constant rate period Falling rate period CONSTANT RATE PERIOD At the start of the drying process, the food material is still completely wet. FALLING RATE PERIOD As drying proceeds the moisture content falls and the access of water form the interior of the food to the surface. TYPES OF FOOD TO DRY Seeds, leaf vegetables and herbs are probably the easiest to dry. Lettuce, melons, and cucumbers are a few foods that do not dry well by conventional methods. CASE HARDENING The higher the temperature and the lower the humidity, the more rapid the rate of dehydration will be, but if drying takes place too fast, “case hardening” will occur. UNIFORM DRYING Simple drying equipment do not dry food uniformly.

1. DEHYDRATION
Chapter 5

2. INTRODUCTION
• Water Activity is related to the amount
of water contained in the food material
and defines the amount of water that
can be used for the growth and
reproduction of micro-organisms,
• Water is essential solvent that is needed
in biochemical reaction in living
organism.

3. FREE WATER VS BOUND WATER
• The term water activity defines the degree in which such water
molecules are “bound”.
• A portion of the total water content present in a food material is
strongly bound to specific sites on the molecules that comprise the
food material.

4. FOOD PRESERVATION BY
CONTROLLING WATER ACTIVITY
• Preservation of many food materials can
be achieved by lowering the amount of
water available for the growth of micro-
organisms.

5. CONTROLLING BROWNING
REACTIONS
• Food products are subject to browning reactions.
• Dehydrated products are probably more so than others.

6. FOOD DRYING
• Food is dried when water is removed from
it.
• The oldest and simplest way of water
removal is sun drying.
• The use of drying equipment heated by
gas, electricity or steam is called artificial
drying or dehydration.
• Another method of drying food is by frying
or oil drying.

7. ADVANTAGES OF DRIED AND
DEHYDRATED FOODS
1. Decrease in weight and bulk
2. Convenience
3. Shelf stable

8. MECHANISM OF
DRYING/DEHYDRATION
Two processes:
1. Heat transfer
2. Mass transfer

9. The rate of drying is affected by the following factors:
1. Humidity
2. Surface Area
3. Temperature
4. Air velocity
5. Ventilation/ moist air outlet
6. Atmospheric pressure and vacuum

10. STAGES OF DRYING
1. Constant rate period
2. Falling rate period

11. CONSTANT RATE PERIOD
• At the start of the drying process, the food material is still
completely wet.

12. FALLING RATE PERIOD
• As drying proceeds the moisture content falls and the access
of water form the interior of the food to the surface.

13. TYPES OF FOOD TO DRY
• Seeds, leaf vegetables and herbs are probably the easiest to
dry.
• Lettuce, melons, and cucumbers are a few foods that do not
dry well by conventional methods.

14. CASE HARDENING
• The higher the temperature and the lower the humidity, the
more rapid the rate of dehydration will be, but if drying takes
place too fast, “case hardening” will occur.

15. UNIFORM DRYING
• Simple drying equipment do not dry food uniformly.