Drying of food
Practical Action
Solar dryers
Solar drying is popular with agencies and research stations. However, there are no small-scale
solar dryers that are yet operating economically. There are a number of reasons for this:
• The amount of food lost in traditional drying is often over estimated (people report the
worst case and the average amount).
• The loss of quality is not necessarily reflected in lower prices. People are willing to pay
nearly the same amount for discoloured or damaged foods and there is therefore no
incentive for producers to risk higher amounts of money in a dryer when there is not a
great return.
• Different quality standards are applied by agencies and rural people. It is not necessary
to achieve export quality for sale in rural areas.
• Dryers are only needed in villages if the weather is unsuitable for traditional methods. If
these conditions are not very common, the dryer will not be needed. Even short periods
of sunshine are enough to prevent serious crop losses. Some producers wait for
sunshine rather than risk the expense of using a dryer. The food is then either spoiled or
the dryer is not big enough to handle the amounts involved.
• Other methods are available to preserve the food if it rains during harvest, for example
the harvest can be delayed, food can be stacked in a way which prevents it from getting
wet, or small amounts can be dried over a kitchen fire, or mixed with dry crop.
• Some benefits of proper drying (for example absence of mould, and better milling
characteristics of grains) cannot be seen and there is therefore no increase in value of
the food.
Other disadvantages of both solar and mechanical dryers include greater space and labour
requirements than traditional methods (for example loading, unloading of trays). These costs are
given lower value by agencies than by villagers.
Solar dryers operate by raising the temperature of the air to between 10-30ºC above room
temperature. This makes the air move through the dryer and also reduces its humidity.
There are advantages to solar drying as follows:
The combination of higher temperature, movement of the air and lower humidity
increases the rate of drying.
Food is enclosed in the dryer and therefore protected from dust, insects, birds and
animals.
The higher temperature deters insects and the faster drying rate reduces the risk of
spoilage by micro-organisms.
The higher drying rate also gives a higher throughput of food and hence a smaller drying
area.
The dryers are water proof and the food does not therefore need to be moved when it
rains.
Dryers can be constructed from locally available materials and are relatively low cost.
Types of solar dryer
Designs of solar dryer vary from very simple direct dryers (for example a box covered with plastic
to trap the sun's heat) to more complex indirect designs which have separate collectors and
drying chambers. As the name suggests, the collector collects and concentrates the suns rays
and heats up the air within the dryer. The hot air is then forced ointo the drying chamber where
it removes moisture form the food. The most common type of collector is a bare galvanised iron
plate which is painted matt black. These give a temperature increase of 10ºC and increase the
air speed to about 5m/s.
8