Transcript PRINCIPLES OF DEHIDRATION AND SUN DRYING

PRINCIPLES OF
DEHIDRATION
AND SUN DRYING
The main advantages of dehydration
or sun drying over other methods of
preservation include :
Light weight-between 60% and more than
90% of most foods is water, and most of this
is removed by dehydration.
Compactness-most dehydration product
occupy less space than original, frozen or
canned material, especially if compressed
into blocks.
Ambient storage-no refrigeration is required,
but there are limitations on the maximum
storage temperature for adequate storage
life.
convenience
The main disadvantages of
dehydration or sun drying, some of
which can be overcome with newer
techniques and pre-dehydration
treatments, include :
Heat sensitivity, as all food products are
heat sensitive to some degree and can
develop burnt flavours under badly
controlled drying conditions.
Loss of volatile flavours.
Structural changes, as the result of
shrinkage during water removal.
Case hardening, which reduces rates and
gives poor quality product.
Browning as the result of non-enzymics
reactions involving concentrated reactants,
and oxidation of lipid components.
Microbial deterioration if drying rates are
initially slow, or if final moisture contents are
too high.
The main factors which
affect the rate of drying of
a foodstuff are :
The physical and chemical properties of the
product (shape, size, composition, moisture
content).
The geometrical arrangement of the product
in relation to the heat transfer surface or
medium.
The physical properties of the drying
environment (air temperature, humidity,
velocity)
The characteristics of the drying equipment
(heat transfer efficiency).
aw and Microbial
Stability
aw, and not water content, determines the
lower limit of available water for microbial
growth.
Most bacteria do not grow below aw 0,91.
Most moulds cease to grow below aw 0,70.
Fungi have been shown to grow in the range
0,70-0,75.
Environmental factors affect the level of water
activity required for microbial growth.
The general principle which often applies is
that the less favourable the other
environmental factors (nutritional adequacy,
pH, oxygen pressure, temperature) the higher
becomes the minimum water activity at which
microorganisms can grow.
Methods of drying and
dehydration
Sun drying and solar drying
B. Methods of dehydration
Some of methods of dehydration suitable
for solid foods are :
•
cabinet or tray drying
•
continuous belt drying
•
bin drying
•
fluidized bed drying
•
freeze drying
A.
Methods of dehydration suitable for liquid,
pulpy or pureed foods includes :
• drum drying,
• spray drying,
• foam mat drying,
• tray drying,
• freeze drying.