Vegetables selected for dehydration experiments were
potatoes, apples, carrots and courgettes, characterized by the
same maturity level. Initial water content, shape and size of
the samples are as indicated in Table 1.
The combined process analyzed consisted of two steps: a
partial dehydration of the sample in a controlled temperature
and humidity forced convection oven, FCD, at a
temperature of 50°C and a relative humidity of 15%down to
a predetermined moisture content. The moisture content at
which the drying pretreatment was stopped was indicated as
change-over moisture content. In the second step the
complete dehydration of the product was achieved in a
freeze-drier, FD, operating at a residual pressure of 0.2 mbar
and with a shelf temperature of 35°C.
To characterize the complete dehydration process, drying
rates, bulk density and rehydration kinetics were measuredon samples dehydrated with combined processes as well as in
single stage FCD and FD processes. The results are an average
of triplicate experiments and their spread is within 10%.
To determine the bulk density of samples a glycerol
displacement technique, based on the Archimedes principle,
was utilized. An analytical balance measures, through an
anchored pivot, the force (the weight) necessary to
submerge the sample in the liquid completely. The bulk
volume is thus evaluated as the ratio between the weight,
de® ned above, and the density of glycerol. Before each
measurement, samples were coated with a thin ® lm of liquid
metacrilic glue to prevent liquid absorption on the
surface3 ,6 .