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PRINCIPLES OF

DEHIDRATION

AND SUN DRYING

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The main advantages of dehydration

or sun drying over other methods of 

preservation include :

Light weight-between 60% and more than90% of most foods is water, and most of thisis removed by dehydration.

Compactness-most dehydration productoccupy less space than original, frozen or canned material, especially if compressedinto blocks.

 Ambient storage-no refrigeration is required,but there are limitations on the maximumstorage temperature for adequate storagelife.

convenience

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The main disadvantages of 

dehydration or sun drying, some of 

which can be overcome with newer 

techniques and pre-dehydrationtreatments, include :

Heat sensitivity, as all food products are

heat sensitive to some degree and candevelop burnt flavours under badly

controlled drying conditions.

Loss of volatile flavours.Structural changes, as the result of 

shrinkage during water removal.

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Case hardening, which reduces rates and

gives poor quality product.

Browning as the result of non-enzymicsreactions involving concentrated reactants,

and oxidation of lipid components.

Microbial deterioration if drying rates are

initially slow, or if final moisture contents are

too high.

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The main factors which

affect the rate of drying of 

a foodstuff are :The physical and chemical properties of theproduct (shape, size, composition, moisturecontent).

The geometrical arrangement of the productin relation to the heat transfer surface or medium.

The physical properties of the dryingenvironment (air temperature, humidity,velocity)

The characteristics of the drying equipment

(heat transfer efficiency).

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aw and Microbial

Stabilityaw, and not water content, determines the

lower limit of available water for microbial

growth.Most bacteria do not grow below aw 0,91.

Most yeast cease to grow below aw 0,80.

Fungi have been shown to grow in the range0,70-0,75 and cease to grow below 0.7

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Environmental factors affect the level of water 

activity required for microbial growth.

The general principle which often applies isthat the less favourable the other 

environmental factors (nutritional adequacy,

pH, oxygen pressure, temperature) the higher 

becomes the minimum water activity at whichmicroorganisms can grow.

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Methods of drying and

dehydration A. Sun drying and solar drying

B. Methods of dehydration

Some of methods of dehydration suitablefor solid foods are :

• cabinet or tray drying

• continuous belt drying

• bin drying

• fluidized bed drying

• freeze drying

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Methods of dehydration suitable for liquid,

pulpy or pureed foods includes :

• drum drying,

• spray drying,

• foam mat drying,

• tray drying,

• freeze drying.