The bulk density was affected differently by the drying procedures.
The low density found for FD apple slices is typical for
freeze dried material. The compact structure after MVD and AD
shows that the drying was associated with changes in the microstructure
and cellular collapse, which is a result of tissue contraction
and thus associated with high bulk density. Despite the
breakup of the compact structure with the MVD puffing process,
the porosity is increased only slightly compared to AD. For a larger
puffing-effect, it is necessary to increase energy input, which is also
accompanied by a strong temperature increase (data not shown).
High temperatures during AD can prevent shrinkage, because the