5 2012. Drying may also reduce weight and volume of the products that lead to substantial
savings in packaging, transport, and storage cost Sivasankar 2002. Drying is a complex process requiring precise process control. There are some factors that must be considered in
drying process, including drying temperature and drying time which depend on composition, moisture content, size, and shape of the raw materials; airflow velocity; and
relative humidity. Product quality plays a major role in food drying process. Drying causes both
desirable and undesirable physical, biochemical, and sensory changes. In many cases, improper drying may lead to irreversible change to product quality. However, it should be
noted that free water which is removed from the product should easily be rehydrated to reconstruct structure of the product Ahmed 2011.
2.2.2 Hot Air Drying
Most of drying processes are carried out in convective drying, in which air used for heating product and removing evaporated water. The heat is mainly transferred by
convection from the air to the surface of wet product and by conduction within the product. It is assumed that all heat is transferred to the product surface from the air by convection
and that process occurs from one large surface only Brennan and Grandison 2012. If the rate of moisture content change is plotted against time, as shown in Figure 2, the drying
curve for a wet product in hot air at constant temperature, relative humidity, and airflow velocity, consists of three periods as follows:
ZZZZZ
1. A-B: equilibration period. Wet surface of the product goes into equilibrium with the air. All heat provided in the air is used to heat up the product to a
predetermined drying temperature. 2. B-C: constant rate period. Surface of the product is saturated with free water.
Water on the surface evaporates and free water within the product moves to the surface that maintain the surface in a saturated state. Drying rate remains
constant during this period.
Figure 2.
Drying curve Brennan et al. 1990
6 3. C-D: falling rate period. Temperature on the product surface increases and
almost reaches dry bulb temperature of the air as drying goes to completion. Hence, it is towards end of the process that any heat damage may occur to the
product. Most of the drying takes place in falling rate period Ahmed 2011; Brennan and Grandison 2012.
Cabinet tray drier is commonly used in hot air drying of solid food piece. It is a multipurpose and batch operated drier. Cabinet drier consists of an insulated cabinet,
equipped with a fan, an air heater, and a space occupied by trays of food. Heating may be by an air flowing throughout trays, by conduction from heated trays, or by radiation from
heated surfaces. The drier can vary in size from a bench-scale unit holding one or two small trays of food to large unit taking stacks of large trays. Cabinet drier has low capital and
maintenance cost Sudheer and Indira 2007; Brennan and Grandison 2012.
2.3 PRETREATMENT 2.3.1 Blanching
