at the top of the microwave oven. A small hole drilled on the back wall of the
oven and fitted with a metal wire mesh as described earlier served as air outlet.
Air was introduced into the chamber using a O-25 kW fan from below the oven.
Heating elements (2 kW) were positioned along the air supply pipe length to
heat the incoming air, the temperature being controlled using a power regulator.
A mercury-in-glass thermometer fixed at the bottom of the oven was used to
monitor the inlet air temperature. A hot wire anemometer (Dwyer 5 106,
Therm0 System Inc., St Paul, MN) was used to measure the air velocity. The air
velocity was set to 1.7 m/s k 3%. Higher air flow rates were not feasible since
they resulted in fluidization of samples towards the end of the drying period.
Test samples were weighed at predetermined time intervals using a digital
electronic balance (Acculab Model 121, Canadawide Scientific, Ottawa, ON).
The moisture content of the carrots was determined by drying in a vacuum oven
at 70°C for 24 h. Some preliminary runs were carried out to find out the appro-
priate power level for drying carrots. A duty cycle of 50% (power level 5;
magnetron on for 50% of the total microwave on time) caused burning of the
product. Hence, duty cycles of 20% (power level 2) and 40% (power level 4)
were used for the study with the power off situation (power level 0) representing
the convective air drying conditions (i.e. the control).
Carrots procured from the local market were used in the studies. They were
trimmed both at top and bottom prior to cutting into uniform cubes of
12 x 12 x 12 mm using a kitchen potato slicer. These cubes were spread in the
sample holder in a single layer. The sample size was kept constant at 60 + O-5 g
for all runs. The initial moisture content of carrots was about 87% on wet basis
(or 670% dry basis). Moisture loss was recorded at 15 min intervals during
drying at power level (PL) 0 and at 5 min intervals at PL 2 and 4. Two inlet air
temperatures of 45 and 60°C were used for the experiment. At the end of each
run, the moisture content of the sample was determined by the vacuum oven
method (70°C for 24 h). Drying tests were replicated three times at each
combination of power level and inlet air temperature.