How to use the DRYIT semi-continuous tray dryer
Practical Action
The principles of drying of foodstuffs and the tray dryer
This section has been divided into four sub-sections:
• Technology
• Science
• Maths
• Summary
Each sub-section can exist on its own and hence be read either independently of the others or
in combination with the other sub-sections depending on what the reader would like to know
and when he or she wants to know.
A. Drying as technology
In this section the tray dryer user will be introduced to the four key factors that control the
drying process and that can be controlled by the tray dryer user.
Drying involves the removal of water from a raw material to produce a dried food. Dried food is
more stable than fresh food. The most common way of drying foodstuffs is to allow the air
that surrounds the food to take up this water in the form of water vapour.
People have been drying foodstuffs and other materials for thousands of years. In those early
times the principle reason for drying was to conserve the food for use when fresh food
supplies were less. The 'tools' then available for drying were the sun, the dry air in certain
parts of the world and the heat from a fire. Today of course the conservation of foods by
drying, especially cereals and pulses, still plays a very important role and in many parts of the
world the traditional practises are still used.
However, today there is a much greater variety of dried foods and a greater variety of tools to
do the job. These tools cannot always replace the traditional tools in all situations - mainly for
economic reasons - but these tools enable the drying process to be better controlled, which is
a very important factor for many dried foods that are of high value and very high quality. The
modern tools are also relatively much more expensive than the traditional ones and this can
limit the opportunity for uptake by the small business.
What is controllable drying?
Drying is controlled by four main factors:
• The temperature of the air;
• The humidity of the air
• The quantity of the air
• The surface area of the material
One every day example which can be used to illustrate these factors is drying clothes on a line
outdoors.
• On what sorts of days do clothes dry best?
• What is the effect of hanging clothes on a line?
• Why are clothes dried outside whenever possible?
Why drying needs to be controlled
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