The relatively short drying period and the resulting low water
content of the MVD apple slices are due to the pulsed energy entry
combined with the continuous vacuum. Consequently, the product
quality can be improved while pulsed drying shortens the duration
time of the drying process (Mousa & Farid, 2002). The process
regime leads to a puffing effect of the compact parenchyma structure
of the apple slice, thereby enhancing the porosity. In turn, the
increased porosity will enhance the release of the water vapor.
Furthermore, such short power-off and power-on times can increase
energy efficiency compared to continuous MVD
(Yongsawatdigul & Gunasekaran, 1996). Unpublished data of our
own research showed that the effect of the microwave pulse
(1000 W, 60 s) on the parenchyma puffing is dependent on the
water content of the apple slices. At water contents above 70%, the
puffed structure will collapse again, whereas at markedly lower
water contents, the structure of the parenchyma is too firm to allow
a puffing effect. Similar to the AD process, the drying rate dropped
clearly when the product weight decreased below 30% related to
the start weight (100%). Also the product temperature rose from
35 C to 50 C, indicating slow water diffusion from the inner parts
of the parenchyma to the surface.