DRYING temperature and microwave power are the two
most important factors in microwave drying of industury
products. These two factors significantly influence the
drying parameters such as drying time, drying curve, drying
speed, drying efficiency, and the final product quality. To
improve microwave drying, a number of studies have been
conducted to investigate the effects of different microwave
power levels and drying temperatures, and different
prediction models have been established. In a typical
microwave drying application, a fixed microwave power
level is applied throughout the entire drying process, but a
temperature control is usually not included. With this
arrangement, a microwave drying process can be divided into
three periods according to temperature variations: a
warming-up period in which sample temperature increases
with little moisture removal; a constant temperature period in
which most of the drying takes place; and a heating-up period
when the drying rate decreases and sample temperature
increases rapidly. While the drying effects in the first and
second periods are acceptable, product charring often occurs
in the last period when the temperature reaches an
undesirably high value. The problem becomes complex when
microwave drying is combined with other drying methods,
such as hot air drying, vacuum drying, freeze drying, etc.,
where the temperature variations are either enhanced or
weakened. To achieve an ideal drying effect over the entire
microwave drying process, sample temperature must be
controlled and microwave power must be adjusted, especially
in the last drying stage. Unfortunately, such study is seldom