Figure 1 shows the variation of moisture content
with respect to the power level. As the microwave
output power was increased, the drying time of
samples was significantly decreased. The microwave
drying process which reduced the moisture content
of Pandanus leaves from 441.126% (dry basis) to
5.042% (dry basis) took 4-14 min, depending on
microwave output power applied. By working at 720
W instead of 180 W, the drying time was shortened
by 70%. For comparison of obtained results, no
literature is available on drying of Pandanus leaves.
However, the drying times obtained in the present
study were extremely low as compared to the results
obtained using different drying methods in the
previous studies for different leaves (Akpinar, 2006).
The results obtained in the present work showed that
as compared to shade drying (2850 min), the dryingtime could be shortened by (1/200) fold by working at
microwave output power of 180 W. Since, the initial
moisture contents of Pandanus leaves used in drying
experiments were constant (441.126% dry basis), the
difference in drying time requirements was considered
to be mainly due to the difference in the drying rates
which are given in Figure 2. After the initial heating,
the drying rate of sample was increased to very high
values of 282 g/100 g.min at 720 W to 136 g/100
g.min at 180 W. As the microwave output power was
increased, the drying rates also increased causing a
noticeable reduction in total drying time. This was
followed by a falling rate period in which moisture
content decreased to about 5.0% dry basis for all
microwave output powers. However, these results are
in agreement with the study of parsley leaves dried
in a domestic microwave oven as reported by Soysal
(2004), who claims that after a short heating period,
a long constant rate period and a falling rate period
were observed. In the present work the initial heating
period decreased with increase in output power and
the constant rate period is very short in each case.
However, careful observation indicates that there is
no significant difference in drying rate achieved at
720 W and 540 W.