A typical MVD curve is shown in Fig. 2, representing product
parameters during the 130 min drying time. During the first drying
period, the product weight decreased rapidly and the majority of
the moisture content evaporated. Parallel to weight reduction, the
temperature of the apple slices rose from 20 2 C at the beginning
to 48 C at the end. The period is characterized by a high drying rate
and resulted in a weight decrease of w70%. At the beginning of
period 2 the power was set to zero to decrease the product temperature
before starting the puffing process, i.e. the product temperature
fell to 28 C, in order to avoid product burning during the
pulsed puffing period, which is based on a combination of high
energy input for a short time with a spontaneous temperature rise.
The third MVD period is characterized by a continuous product
weight decrease with a moderate drying rate and increased temperature
of 37 C. During the last period (period 4) the energy input
was low, the product weight decreased only slightly, while the
product temperature rose continuously to 50 C (maximum). During
the whole MVD process the slices lost 87.3% of the start weight.