3.4. The temperature changes and distribution during
MVD process
Table 4 presents the temperature distribution of paste mixture
sheets at different drying stage. The paste mixture was
obtained according to the optimal ratio and drying conditions
discussed above. It can be seen that the temperature of paste
mixture sheets increased with drying time increase. At the
beginning of drying, materials absorbed microwave energy
quickly and reach the 33 ◦C (evaporation temperature at 5 kPa).
But the measured temperatures were lower than 33 ◦C due to
evaporation absorbing heat from materials and the cooling
when materials were taken out. It can also be seen that materials
temperature below 40 ◦C after drying for 12 min, which
means that the temperature inflation was low. This is because
the microwave energy absorbed by materials was totally or
mostly used to evaporate water when the materials contained
higher water content. The highest temperature of materials
exceeded 60 ◦C at the last minute during MVD and this can be
explained by the fact that materials absorb microwave energy
which is not only used to evaporate water but also used to
heat up themselves, when water content is low. The increase
in materials temperature would results in the increasing of
ε values (Ryynanen, 1995; Venkatesh and Raghavan, 2004),
which induced the temperature of materials to increase further.
Although higher temperature (>60 ◦C) would damage the
quality aspects of fruits and vegetables, especially flavor, the
quality of dried chips was well maintained due to relatively
very short drying time