The unit operation drying is the one of the oldest preservation technique still in practice in modern food processing industry. The dried foods have long shelf life if the post process conditions are maintained thoroughly and effectively. The principal reasons behind this are the destruction of microorganisms which cause spoilage and decay of food by their growing and multiplying and also many of the enzymes that cause for undesired changes in the food cannot function properly at low water or no water level. But during drying there are two mechanisms which take place in the food, firstly heat is transferred to the food to provide sufficient amount energy for latent heat of vaporisation and secondly mass transfer is taken place as the water goes away from the food stuff (Earle, 1983).
The Rate of drying depends on the type of food, the processing conditions and the design of the drier (Fellows, 2000). The practical of dehydration of tomatoes was carried out in a cabinet dryer that was operated at the temperature of 80°C. In the cabinet dryer the food material was spread out on the trays and heating was facilitated by an air current sweeping across the trays (Earle, 1983). Figure 4 illustrates the air flow inside the cabinet dryer, supported from a fan for efficient functionality.
Figure 4. Cabinet dryer ( Source: Earle, 1983)
Among many factors that influence the drying rate, the study was carried out to determine the size of the food pieces that has a considerable effect on the rate of drying. According to the graph Figure (1) which was drawn for the dry basis moisture content loss, there was significant different between drying of sliced tomatoes and ½ cuts. The sliced tomatoes were experienced higher rate of removal of water than the ½ cut tomatoes, this type of behaviour can be obtained because small pieces increases the surface area that available for evaporation of water as well as reduces the distance for moisture to travel through...
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