Onion (Allium cepa L.) is one of the most popular vegetables in the world, and finds widespread usage in both fresh and dried forms. It contains vitamin B and traces of
vitamin C, iron and calcium. The most distinctive characteristic of onion is its pungency, which is due to allyl-propyl disulphide. Onions compared with other fresh vegetables have relatively high heat energy value, intermediate protein content and are rich in calcium and riboflavin. Dried onions are of considerable importance in world trade and are made in several forms: flaked, minced, chopped and powdered. It is also used as a flavour additive in a wide variety of food formulations (Sharma et al, 2005). Presently, hot air drying method is in use for onions and this is not only costly but cause considerable shrinkage due to cellular collapse following the loss of water, poor rehydration characteristics of the dehydrated product and undesirable changes in colour, texture, flavour and nutritive value. New and innovative techniques like microwave-assisted (Wang et al, 2003) and heat pump-assisted convective drying (Pal and Khan, 2007), which increase the drying rates and enhance dried onion quality, are receiving considerable attention. Microwave
drying is now gaining popularity because of its inherent advantages over conventional heating such as reduced drying time and lower energy consumption of biological
material without quality loss (Bal, 2010). Heat pump-assisted convective drying is known to be more energy efficient than traditional convective drying operations. Most of the research studies infer that the heat pump-assisted convective drying offers products of beer quality with reduced energy consumption (Sahoo, 2012). Agricultural products and foodstuffs are commonly pre-treated prior to industrial drying to speed up the drying process and preserve the quality of the product. Lewicki et al. (1998) found that both variety and pre-treatment affect the course and rate of drying. Drying time of onion
could be reduced to less than half by introducing an hour of osmotic dehydration in a salt solution, which also preserves its colour (Ndjouenkeu, 2005). The drying rates could be improved by pre-treatments like blanching and dipping the fruits in chemicals before drying (Doymaz, 2004). Pretreatments reduced the drying time and yielded a good quality dried product. Potassium metabisulphite is most commonly and commercially used for pretreatment, because it prevents enzymatic browning of fruits and increases the permeability of water (Tunde-Akintunde, 2005).
Farag et al. (2004) subjected sliced onions to various treatments in order to inhibit enzymatic browning. All applied treatments reduced ascorbic acid by 50 % due to
leaching and volatile sulphur compounds more than 30 %. Raj et al. (2005) observed significant differences in the retention of moisture, reducing sugars, non-reducing sugars, ascorbic acid, acidity, rehydration ratio and overall acceptability of dried onion rings pretreated with potassium metabisulphite (2.5 g/L of solution) during storage. Tsamo et al. (2005) stated that osmotically dehydrated onion slices in sugar (600 g/L), salt (300 g/L) or mixed salt and sugar (45:15 by mass) solutions for 15 min followed by
convective drying with air at 60 °C increased the drying rate and moisture diffusitivity. Baroni and Hubinger (1998) also studied air drying of fresh and osmotically pretreated onion slices for 60 min at 40, 50 and 60 °C and observed faster drying rate of samples soaked in 10 % NaCl solution. Recently, Bebara et al. (2014) suggested drying the 2-mm onion slices under fluidized bed at 60 °C and air velocity of 4 m/s to obtain a beer quality product with
savings in drying time.
Drying is the most accepted and probably the oldest method of preservation among others (Mazumba, 2000). Generally, the term drying refers to the removal of relatively small amount of moisture from a solid material by evaporation. The removal of moisture prevents the growth and reproduction of decay causing microorganisms and minimizes moisture induced deteriorating reaction. Drying methods include domestic drying (smoking, salting, sun drying e.t.c.) and industrial drying (solar drying, tunnel drying, vaccum oven drying and cabinet drying among others) (Baysal et al, 2003 ). In essence, large quantities of food products are dried to improve shelf life, retain original flavor, reduce packaging costs, lower weights, enhance appearance, and maintain nutritional value (Baysal et al, 2003). The drying of food materials also had advantage on quality control, achievement of hygienic conditions and reduction of product loss.
Factors that affect drying the most are related to drying air, temperature, relative humidity and velocity in addition to the product initial moisture content. However, utilization of high amount of energy in the drying industries makes drying one of the most energy- intensive operations with great industrial significance (Carsky, 2008; Dincer, 2000). Thus, reduction in the energy utilization during drying is important in other to conserve the remaining little energy for other energy required processes. Thereby maximizing the production and minimizing the cost of energy.
Kinetics of drying process have been undertaken by several researchers (Akpinar et al, 2003). Kinetics of drying is useful in design and analysis of mass and heat transfer processes during drying. Thermodynamics analysis, however has appeared to be an essential tool for system design, analysis and optimization of thermal systems (Dinner and Sahin, 2004). Exergy can be interpreted as the maximum amount of work which can be produced by a stream of matter, heat or work as it comes to equilibrium with a reference environment (Dincer, 2000). Exergy is thus the energy that is available to be used. Exergy analysis which may be considered as accounting of the use of energy and material resources provide information on how effective a process takes place towards conserving natural resources. In the drying industry, the goal is to use a minimum amount of energy for maximum moisture removal for the desired final conditions of the product.
Onions compared with other fresh vegetables have relatively high heat energy value, intermediate protein content and are rich in calcium and riboflavin. Dried onions are of considerable importance in world trade and are made in several forms: flaked, minced,
chopped and powdered. It is also used as a flavour additive in a wide variety of food formulations (Sharma et al, 2005).
A single variety of onions is to be used for this study and only three drying methods (sun,solar and cabinet drying) will be employed.
1.3 AIMS AND OBJECTIVE
- To determine the effect of drying methods on the nutritional composition of onion
- To determine how different drying methods affect the proximate composition of onion
- To determine the drying method that will give the best nutritional output.
- To determine the general acceptability of the dried onion by the consumer in relation to the method used