Drying of food
Practical Action
Artificial (mechanical) dryer
Artificial dryers use fuel to increase the air temperature, and reduce the RH and fans to increase
air speed. They give close control over the drying conditions and hence produce high quality
products. They operate independently of the weather and have low labour costs. However, they
are more expensive to buy and operate than other types of dryers. In some applications, where
consistent product quality is essential, it is necessary to use mechanical dryers.
Light bulb dryer
This consists of an electric light bulb inside a wooden box. If electricity is available this is a
simple, low cost dryer which may be suitable for home preservation. The capacity is very small
and it is not likely to be useful for income generation. The bottom of a box is painted black, or
covered in soot or black cloth. The sides are covered in shiny material (for example aluminium
paint) to reflect the heat onto the black surface. Air circulates by natural convection in a similar
way to the solar cabinet dryer, but in this case the dryer can operate all night as well as all day.
Cabinet dryer
The design is similar to the solar type but in this case the heat is supplied by burning fuel or
electricity. If electricity is available, a fan can be used to increase the speed of air moving over
the food and therefore increase the rate of drying. To be economical it is likely that this type of
dryer should be relatively large (1-5 tonnes). These are successfully used for drying herbs, tea
and vegetables.
Practical Action has developed a range of drying systems including a small, low-cost industrial
type which can be fabricated in countries of intended use. Its small size makes it suitable for
decentralised use in crop-growing areas. The price at about US$ 3,000, is substantially lower
than for standard, commercially available units.
The small unit is a semi-continuous drying cabinet with hot air supplied by an indirect heater-
blower unit (see the Technical Brief on Tray Dryers). Intended for round-the-clock operation, the
semi-continuous tray dryer is designed for maximum fuel efficiency. It takes about four hours
for the first (bottom) tray to dry: after that, it can be removed, the remaining trays lowered,
leaving a space at the top for a tray of fresh material. Trays can then be removed every twenty
minutes.
Silo dryer
These dryers consist of trays or tanks containing a deeper layer of food than those found in
cabinet dryers.
They have a larger capacity and are often used for grain drying, where the amount of water to be
removed is smaller than for example fruits and vegetables, but the quantities involved are larger.
There is considerable scope for the use of small dryers that combine the low cost of solar heating
with better control of mechanical dryers.
References and further reading
Drying selection of Practical Action Technical Briefs:
Drying, Food Cycle Source Book 6 UNIFEM and Practical Action Publishing 1995
Preservation of Fruit and Vegetables: Agrodok 3, Agromisa 1997
Try Drying It! Case Studies in the Dissemination of Tray Drying Technology, B Axtell,Practical
Action Publishing 1991
Producing Solar Dried Fruit and Vegetables for Micro and Small scale Rural Enterprise
Development, A Series of Practical Guides' written by the Natural Resources Institute.
Setting up a food drying business P Thuillier, Practical Action Publishing, 2002
Drying Food for Profit B Axtell, Practical Action Publishing, 2002
Drying Foodstuffs: Techniques, Processes, Equipment, Jean-Franois Rozis, Backhuys
Publishers, 1997
11