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< prev - next > Crop processing Drying KnO 100295_Small scale Drying technologies (Printable PDF)
Globally, drying is the most widely used method for preserving foods for use in the home or for
sale. The most common method involves simply laying the product in the sun on mats, roofs or
drying floors. This is known as sun drying. As shown below simple sun drying has advantages and
Almost without cost
Open to contamination by dust
Ideal for products where little or no
value is added
Food usually dried close to the
Totally dependent on good weather
Very slow drying rates with danger of
mould growth
It may not be possible to dry to a sufficiently
low level of moisture to prevent mould growth
Sun drying remains, however, the only economic choice for poor people when drying foods such as
grains in large quantities and where little or no value is added by drying.
When drying foods such as fruits, vegetables, spices, herbs and nuts value is added by drying and
in such cases an investment in improved drying technologies may be economically viable. This
technical brief describes improved systems suitable for use by small scale producers.
Solar dryers
Solar dryers use a simple construction to more efficiently make use of the suns heat. Under the
correct climatic conditions they can provide many advantages over sun drying. These include:
higher drying temperatures which results in shorter drying times and the ability to dry to a lower
final moisture content.
protection from contamination by dust and from rain showers.
they are low cost and simple to construct in local workshops
Solar dryers consist of a transparent panel above a chamber or collector that is pained black to
absorb the suns heat. Polythene, which is very cheap, is commonly used to glaze the panel but it
turns yellow and opaque after a few months and needs to be replaced. Plastic films that are not
damaged by sunlight are now increasing available and should be used if possible. While more
expensive they have a life of 5 years or more. It is very important to angle the collector at the
correct angle to the sun:
the angle should be greater than 150 to allow rain water to run off
the collector should be angled at 900 to the mid-day sun
the collector should face south in the Northern hemisphere and north in the Southern
the collector should be sited away from shadows from trees or buildings
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