1.1 Background of Study
Cocoyam (Colocasia spp and Xanthosoma spp) is grown in the tropics and sub-tropical regions of the world particularly in Africa for human nutrition, animal feed, and cash income for both farmers and traders (Onwueme and Charles, 1994). Cocoyam is vegetatively propagated using the corms and to a lesser extent the cormels. As food for human consumption, the nutritional value of the various parts of cocoyam is primarily caloric (Davies et al, 2008). The underground cormels provide easily digested starch; and the leaves are nutritious spinach-like vegetable, which give a lot of minerals, vitamins and thiamine (Tambong et al, 1997). There are two major types commonly grown in Nigeria, namely; Colocasia spp and Xanthosoma spp. In Nigeria, it is regarded as a major crop especially in female headed households. Nigeria is one of the largest producers of cocoyam in the world contributing about 40% of total annual production (Kinpscheer, 2001). Cocoyam is a tropical starchy tuberous root crop. There are many varieties of cocyam but the most common are the soft variety (Colocasia esculenta) used mainly as soup thickeners. Cocoyam is planted a bit later in the rainy season, in the months of May and June. This is because it requires more moisture to germinate. Harvest for cocoyam begins in late September and ends around January, just before the dry season becomes too hot. Cocoyam, like yam, can be stored for several months and it will still retain its taste. It is best stored in a cool, dry and well ventilated place. Most times they are stored on raised racks because the bare floor causes them to rot. Cocoyam, when in season is readily available in the open Nigerian markets. It is hard to find them in African food stores outside Nigeria. 1.1.1 Preservation
Drying is defined as a process of moisture removal due to simultaneous heat and mass transfer. It is a classical method of food preservation, which provides longer shelf-lighter weight for transportation and small space for storage (Ertenken & Yaldiz, 2004). It enhances the resistance of high humid products against degradation by decreasing their water activity (Doymaz & Pala, 2003; Hadrich et al., 2008; Simal et al., 2005), as the losses of fruits and vegetables in developing countries are estimated to be 30-40% of the production (Azharul Karim & Hawlader, 2006). Therefore, in many agricultural countries, large quantities of food products are dried to improve shelf life, reduce packaging costs, lower weights, enhance appearance, retain original flavor and maintain nutritional value (Baysal et al., 2003; Demir et al., 2007; Simal et al., 2000; Sokhansanj & Jayas, 1987). However, utilization of high amount of energy in drying industry, makes drying one of the most energy-intensive operations with great industrial significance (Carsky, 2008; Dincer, 2000; Dincer & Cengel, 2001; Dincer & Sahin, 2004; Shi et al., 2008). Conventional (air) drying is the most frequently used dehydration operation in food and chemical industry (Nicoleti et al., 2001; Singh et al., 2008), due to its controllable conditions and less dependency on climatic conditions (Lertworasirikul & Tipsuwan, 2008). Drying is therefore indispensable in order to facilitate preservation, improve palatability and product quality as well as reduce cyanogenic glucoside toxicity [Ademiluyi et al., 2007].
1.1.2 Drying Processes
The drying process takes place in two stages. The first stage happens at the surface of the drying material at constant drying rate and is similar to the vaporization of water into the ambient. The second stage takes place with decreasing (falling) drying rate. The condition of the second stage determined by the properties of the material being dried (Can, 2000). Open sun drying is the most commonly used method to preserve agricultural products like grains, fruits and vegetables in most developing...
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