DRYING OF FOODS
There are two main reasons for drying food:
to prevent (or inhibit) the growth and activity of micro-organisms and hence preserve the
to reduce the weight and bulk of food for cheaper transport and storage.
When carried out correctly, the nutritional quality, colour, flavour and texture of re-hydrated
dried foods are only slightly less than fresh food. However, if drying is carried out incorrectly
there is a greater loss of nutritional and eating qualities and, more seriously, a risk of microbial
spoilage and possibly even food poisoning.
This technical brief describes some of the requirements for proper drying and summarises
information on the various drying equipment available.
The principles of drying
In the most basic terms, drying is the removal of water from foods. Usually foods are dried using
hot air to remove the water. In some instances, such as when gari is being made from cassava, a
hot metal pan is used which comes into contact with the food and causes the moisture to
evaporate. This technical brief, concentrates on drying using hot air.
For effective drying, the air should be hot, dry and moving. These factors are inter-related and it
is important that each factor is correct (for example, cold moving air or hot, wet moving air are
both unsatisfactory). The dryness of air is referred to as the humidity - the lower the humidity,
the dryer the air. There are two ways of expressing humidity; the most useful is a ratio of the
water vapour in air to air which is fully saturated with water. This is known as the relative
humidity (RH). Air that is completely dry has a RH of 0% and air that is fully saturated with
water vapour has a RH of 100%.
The basics of drying
Drying involves removing water from the food product into the surrounding air.
For effective drying, air should be hot, dry and moving. These factors are inter-related and
it is important that each factor is correct:
- air must be dry, so it can absorb the moisture from the fruits and vegetables
- heating the air around the product causes it to dry more quickly
- if the air is not moving across the food, it cannot get rid of the water vapour that it
has collected. A fan or air blower is needed to keep the air circulating.
In summary – when food is dried, hot dry air comes into contact with the food. The hot
air absorbs water from the food and is moved away from the food. New dry air t akes its
place and the process continues until the food has lost all its water.
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