experimental data (Table 2). Effective moisture diffusivity calculated for blueberries dried by HACD, MWVD and HACD
+ MWVD ranged from 1.73 ± 0.07 1010 to 3.11 ± 0.06 109 m2 s1 (Table 2) and were within the range of 1011–109 for
different food products (Zielinska et al., 2013). The lowest value of De was found for sample dried by HACD at 60 C, while the highest for sample dried by HACD at 90 C + MWVD. Moisture diffusivity of blueberries subjected to microwave-assisted drying processes was up to one order of magnitude higher than that noted for samples subjected to HACD (Table 2).
No constant drying rate period was observed during HACD of blueberries (Fig. 2b) suggesting that non-bound water present on the surface of frozen/thawed fruits was removed after a short period of HACD, which resulted in insignificant constant rate period. The transfer of moisture from the inside to the outer layer of berries dehydrated by HACD was insufficient to maintain the surface of fruits in a saturated state and diffusion was identified as a main mechanism esponsible for a moisture transfer. During HACD at 60 C, the initial drying rate was 3.4 103 min1
. The increase in hot air drying temperature from 60 to 90 C allowed to obtain significantly higher values of initial drying rate (5.1 103 min1). HACD at 60 and 90 C featured two distinctive falling rate periods characterized by significantly different changes in drying rate of blueberries (Fig. 2b). With a fall in moisture content, drying rate of blueberries dehydrated using HACD at 60 C immediately decreased (down to 70%) to the values close to 1.1 103 min1
. The highest drop was observed during the first 100 min of drying, when the moisture content decreased from
6.22 ± 0.02 kg H2O kg1 DM to 4.85 ± 0.02 kg H2O kg1 DM. In case of waxy skinned blueberries, such low temperature did not allow to maintain high moisture gradient between inner and outer parts of blueberries resulting in significantly lower drying rates than in other drying processes used in the study. Compared to HACD at 60 C, the changes in drying rate of blueberries dehydrated by HACD at 90 C were more visible in the final than in early stage of drying. Fig. 2b shows that the drying rate of blueberries dehydrated by HACD at 90 C remained at relatively high level (between
5.1 103 min1 and 3.2 103 min1) in the range of moisture content between 6.22 ± 0.02 kg H2O kg1 DM and 1.11 ± 0.02 kg H2O kg1 DM, and significantly dropped down in the final stage of drying. As reported by Zielinska et al. (2016), the changes in the drying rate may be attributed to the physical changes, mainly due to surface cracking. A high moisture gradient between inner and outer parts of blueberries dehydrated by HACD at 90 C induced unbalanced mechanical stresses in dried particles and promoted the surface structure collapse in the final stage of drying. As a result, residual moisture trapped inside fruit slowly migrated to the ambient air. No constant drying rate was observed during
HACD + MWVD of blueberries, and the drying rate curve can be easily divided into two distinctive falling rate periods (Fig. 2b). During the HACD stage, the drying rate decreased gradually after a rapid initial rise. The drying rate increased significantly