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Drying of food items on a large scale is gaining importance with
globalization, and the quantitative understanding of drying operations is crucial for
process design, quality control, product handling and energy saving (McMinn, 2006).
Freshly harvested chilli contains about 300% moisture on dry basis (Hu et al., 2006). Currently, India exports only 5-8% of its production, which can be increased
only after preventing wastage (Kaleemullah and Kailappan, 2006).
In India, chilli is dried by direct exposure to sun resulting in a dull color,
which reduces its market potential, and does not avoid spoilage due to microbial
action for various reasons (Mangaraj et al., 2001). The improved solar driers such
as natural convection (Hossain and Bala, 2007) and forced convection solar
driers (Mangaraj et al., 2001) have prolonged heating which causes loss of pungency
in chilli. These driers depend on weather conditions and there are more chances
for the food items to be spoiled. Hot air drying is carried on by the convective
drier (Akpinar et al., 2003 and Kaleemullah and Kailappan, 2006), forced convective
air drier and rotary drier (Kaleemullah and Kailappan, 2005). This method has
many disadvantages, including poor quality of dried products, low energy
efficiency and a long drying time. The prolonged exposure to elevated drying
temperatures, results in a substantial deterioration of quality attributes such as color,
nutrient concentration, flavor and texture (Bondaruk et al., 2007).
Microwave Vacuum drying (MWV) is one of the emerging food
processing methods in recent years. It relies on the principle of dielectric loss properties in
a microwave field. As the microwaves directly penetrate the material, the
entire product is heated (from inside out) in a fast and uniform manner, causing
rapid evaporation of water and creating an outward flux of rapidly escaping vapor,
and vacuum reduces the boiling point of water. In MWV drying process, heat
and mass transfer are in the same direction and it helps to improve the rate of
moisture removal. Microwave Vacuum drying can therefore accomplish drying tasks in
a shorter time and at lower temperature. The drying of chilli using MWV
technique is expected to gain greater importance on a large scale due the advantages
of reduction in processing time, to preserve the quality attributes of the final
product. Thin layer models are important for the industrial scale designing of the
MWV drier. Thin layer models are categorized as theoretical, semi-theoretical
and empirical (McMinn, 2006). |
