Remove water or moisture from a product.
Drying food material in the sun is the oldest form of food preservation. The sun's energy is used to evaporate most of the moisture from the food. The simplest way of drying food material is to spread thin layers of the materials on a piece of matting in an open, sunny space. However, if the food material is unprotected, then there is a chance of it becoming contaminated by dirt, insects, rain and animals walking over it.
The two types of solar driers
are not only designed to keep the food material clean during the drying operation but also reduce the time required to dry the material.
When using solar driers, they should be placed in a sunny location. In addition, they should be placed on concrete or any surface which is not wet and which will not retain moisture (i.e. grassy areas or muddy areas are not suitable surfaces to put solar driers on). The driers should be slightly raised off the ground. A clearance space of at least 1 10 cm is recommended to allow the air to circulate freely. The clear plastic surface of the drier should be kept dry and clean. It is best to allow the solar drier to warm up in the sun before putting the material to be dried inside. Material being dried by this method should be put out in the sun as in the early morning to make full use of the sun.
Solar driers are cheap to use and simple to make. However, the disadvantages are that the drying of food material is entirely dependent on the weather. It often takes more than one day to thoroughly dry the food material and the food may spoil if the weather changes during the drying process. For commercial operations, depending on the weather for drying is not a reliable operation.
The solar drying cabinet (view image) is made from 2.5 cm thick wood, which is painted black all over. The black surface absorbs heat from the sun and helps to raise the temperature of the air inside the cabinet to facilitate a faster rate of drying the product. The frame of the lid is a made of thin, lightweight wood (also painted black) which supports a sheet of clear plastic. The clear plastic cover allows the suns energy to enter the cabinet and heat the air inside. The base and upper edges of the cabinet are perforated with 1 cm diameter holes at intervals of 1- cm. The holes allow air to circulate freely throughout the chamber of the cabinet and remove the water which is being evaporated from the food material. The food material is placed on a series of wooden framed trays within the cabinet. The frames of the trays are made of a lightweight wood which support fine mesh material (such as mosquito netting;). The trays sit on 2.5 cm square pieces of wooden rods so that they do not touch the based of the cabinet. This again facilitates the free movement of air around the product and a faster rate of drying.
The solar tent (view image) works on the same principle as the solar drying cabinet but is lighter in weight, cheaper to make and easier to carry from place to place. The tent frame is made from bamboo or 2.5 cm square pieces of wood, secured in an A frame arrangement. The clear plastic sheet is secured over the A frame. The material to be dried is placed on a "table" made out of a bamboo frame and fine mesh. The table is placed on a sheet of black plastic and covered by the tent frame.
Hot air driers which use gas to heat up the air is another means of drying food quickly and relatively cheaply. The hot air drying cabinet illustrated below is relatively simple in design and does not involve the use of electric fans to distribute the hot air over the material being dried. Two gas burners situated at the bottom of each metal tube, quickly heats up the surrounding air. The hot air rises and enters the drying cabinet chamber. It hits the deflector plate at the top of the metal tube, is dissipated within the chamber cabinet and passes over the food material, spread out on the stacked trays. As the air escapes out of the chimney' on the top of the cabinet, it carries with it the moisture which has evaporated from the food material. With hot air driers, food can be dried within a 4 - 6 hours. The temperature of the air inside the drying cabinet should be monitored as air temperatures can become too hot causing the food material to burn rather than dry. A digital thermometer and rod type probe (see quality control section for details of this type of instrument) is ideal for monitoring air temperatures during the drying process. Air temperatures should not exceed 50 - 55°C.
The hot air drying cabinet (view image) is made out of wood and metal. The main frame of the cabinet is 5cm square wood with 2cm thick wooden side panels. Ideally all the wood used should have been pre - treated to prevent the risk of fire. The sliding doors are made of 1.25cm thick plywood. The chimney pieces, deflector plates and tubes leading from the gas burners are made of 1 mm thick galvanised sheet steel. The screws used to secure the metal pieces onto the wooden frame are made of brass to prevent rusting over time. The trays are made of a thin wooden frame with metal mesh centre. The trays slide into the cabinet along rails made of wooden rods. The cabinet should be brushed with linseed oil from time to time to counteract the effects of over drying and cracking.