For hot air cabinet dryer followed by microwave oven drying
method, moisture ratio-time diagram of grapes along the drying
period is shown in Fig. 2 when grapes dried achieved by
starting drying in hot air cabinet dryer for 30 min, 60 min
and 120 min and finishing drying in microwave oven for
1 min at different power levels. As seen in Fig. 2, an increase in drying time occurred with the increasing drying time in the
hot air cabinet dryer from 30 to 120 min at any microwave
power level. The drying rate curves indicated the absence of
a constant-rate drying period. After reaching its maximum value
the drying rate initially dropped rapidly, signifying a falling-
rate period. The drying behavior when the grapes dried
by starting drying in hot air cabinet dryer for 30 min and finishing
by microwave oven for 1 min at different power levels
were not different. Within a certain microwave power level
range (75–900 W in this study), increasing microwave power
speeds up the drying process, thus shortening the drying time.
Also, when grapes drying started in hot air cabinet dryer,
increasing drying time from 30 to 120 min until the moisture
ratio was near to 0.02, the drying time increased.
For microwave oven followed by hot air cabinet drying
method, moisture ratio-time diagram of grapes along the drying
period when grapes dried by starting drying in microwave oven for 1 min at different power levels and finishing drying in
hot air cabinet dryer for 30 min, 60 min and 120 min is shown
in Fig. 3. A decline in drying time occurred with the increasing
microwave power levels. The drying period of grapes had big
difference when the grapes were dried by starting drying in
microwave oven for 1 min at 900 W power level compared to
other microwave power level of 75W and finishing drying in
hot air cabinet dryer for 120 min. Within a certain microwave
power level range (75–900 W in this study), increasing microwave
power speeds up the drying process, thus shortening
the drying time. Also, increasing drying time from 30 to
120 min when grapes drying finished in hot air cabinet dryer
until the moisture ratio was near to 0.02, the drying time increased.
So, no benefits in drying time were seen when increasing
drying time from 30 to 120 min when grapes drying started
in microwave oven for 1 min at any power level and finished in
hot air cabinet dryer.