Sun drying of crops is most widespread method for preservation of food .There are some drawbacks relating to traditional method of drying i.e. spreading the crops in thin layers of mats, trays or paved grounds and exposing the products to the sun and wind. These include poor quality of food caused by contamination of dust, insect attack, birds stool, enzymatic reaction and infection by microorganism.
Also the system is time and labor intensive, as crops have to be collected during rain and crops continually have to be protected from domestic animals. Insufficient and non-uniform drying also lead to deterioration of crops during storage. Serious drying problem occurs especially in humid tropical regions where some crops have to be dried during the rainy season.
How do solar dryer work?
The solar dryer is a relatively simple concept. The basic principles employed in a solar dryer are:
- May be we put icon Converting light to heat: Any black on the inside of a solar dryer will improve the effectiveness of turning light into heat.
- Trapping heat: Isolating the air inside the dryer from the air outside the dryer makes an important difference. Using a clear solid, like a plastic bag or a glass cover, will allow light to enter, but once the light is absorbed and converted to heat, a plastic bag or glass cover will trap the heat inside. This makes it possible to reach similar temperatures on cold and windy days as on hot days.
- Moving the heat to the food: Both the natural convection dryer and the forced convection dryer use the convection of the heated air to move the heat to the food.
There are a variety of solar dryer designs. Principally, solar dryers can be categorized into three groups: a) natural convection dryers, which are solar dryers that use the natural vertical convection that occurs when air is heated and b) forced convection dryers, in which the convection is forced over the food through the use of a fan and c) tunnel dryers. While several different designs of the solar dryers exist, the basic components of a solar dryer are illustrated in Figure 1. In the case of a forced convection dryer, an additional component would be the fan. Drying is an important step in the food production process. The main argument for food drying is to preserve the food for longer periods of time. However, it is important to note that the process is not just concerned with the removal of moisture content from the food. Additional quality factors are influenced by the selection of drying conditions and equipment:
- Moisture Content: It is essential that the foodstuff after drying is at a moisture content suitable for storage. The desired moisture content will depend on the type of food, duration of storage and the storage conditions available. The drying operation is also essential in minimizing the range of moisture levels in the batch of food as portions of under-dried food can lead to deterioration of the entire batch.
- Nutritive value: Food constituents can be adversely affected when excessive temperatures are reached.
- Mould growth: The rate of development of micro-organisms is dependent on the food moisture content, temperature and the degree of physical damage to the food.
- Appearance and smell of the food: For example, the colour of milled rice can be adversely affected if the paddy is dried with direct heated dryers with poorly maintained or operated burners or furnaces.
Therefore, it is essential to not only monitor the moisture content of the foodstuffs, but to also monitor temperature, mould growth, appearance and smell of food, air flow, etc. Whether a natural convection dryer, a forced convection dryer or a tunnel dryer is appropriate depends on the amount of food, the climate and the demands placed on the end-product (how long does it need to be stored, in what quantities, etc.).