and front acrylic disks (diameter: 14 cm) were used to distribute a
sample ofmaximum250 g of strawberries. Individual berries were separated
1 cm from each other to allow individual drying. Trays were
completely covered by a vertical cylindrical container (diameter:
18 cm; length: 24 cm; thickness: 6 mm) made of black polyethylene,
which allows passage of microwaves. This container was adjusted to a
circular slot at the rear side of the oven for hermetical sealing during operation,
where sub-atmospheric pressure was provided by a vacuum
pump. Condenser with refrigeration liquid (−10 °C) and condensate
trap were connected in-line with the vacuum hose to remove moisture
from sub-atmospheric air before reaching the vacuum pump. Absolute
pressure was controlled by a manometer and maintained in the range
of 10 to 70 mm Hg by the vacuum pump and a vent for air inlet. Four
probes of optical fiber (model TMI4, FISO Technologies Inc., Canada)
for temperature measurement were inserted in the center of different
berries. The vacuum microwave drying system operated for 3 h at a
temperature of 50 ± 1 °C, removing the excess of microwave heat by
recirculation of cold water between two polypropylene containers outside
the vacuum chamber and by using an on–off temperature control
system for the magnetron after the first 20 min after starting each experiment.
Then the microwave control system was switched off, but
the vacuum drying conditions were maintained for one extra hour.