The water content decrease of dehydrated products induces
supersaturation of their components, leading to an increase
in the cohesive forces between the molecules of water and
solutes, decreasing the molecular mobility. For this reason,
the presence of water and its interactions with other components
are important factors to consider not only in the
process control, but also on dehydrated food stability and
quality. In this work, different sugar infusion pretreatments
and two dehydration methods (air- and freeze-drying) were
applied to raspberries. After both drying processes, the final
water activity of the products ranged between 0.30 and 0.34.
Table 1 shows some parameters that characterize the raspberries
after the application of the different pretreatments and
drying processes: water content, glass transition temperature
(Tg) and t2 relaxation time. The water content of raspberries
decreased from 85% (wet basis) (reference fruit) to ≈51% (wet
basis) after infusion pretreatmentin all cases, with a final aw of
0.85 (Sette et al., 2015). After further drying, raspberries experienced
different changes in water content according to the type
of applied pretreatment (Table 1). Water content (X) of samples
without pretreatment (C) was reduced ≈97% after drying.
Pretreated samples, partially dehydrated by osmosis during
infusion, exhibited a slightly higher water loss in freeze-dried
samples (≈90%) when compared to air-dried ones (≈86%).