The principle that lies behind the design of solar dryers is as follows: in drying relative and
absolute humidity are of great importance. Air can take up moisture, but only up to a limit.
This limit is the absolute (maximum) humidity, and it is temperature dependent. When air
passes over a moist food it will take up moisture until it is virtually fully saturated, that is
until absolute humidity has been reached. But, the capacity of the air for taking up this
moisture is dependent on its temperature. The higher the temperature, the higher the absolute
humidity, and the larger the uptake of moisture. If air is warmed, the amount of moisture in it
remains the same, but the relative humidity falls; and the air is therefore enabled to take up
more moisture from it’s surrounding.
To produce a high-quality product economically, it must be dried fast, but without using
excessive heat, which could cause product degradation. Drying time can be shortened by two
main procedures: one is to raise the product temperature so that the moisture can be readily
vaporized, while at the same time the humid air is constantly being removed. The second is to
treat the product to be dried so that the moisture barriers, such as dense hydrophobic skin
layers or long water migration paths, will be minimized (2).