Compared to the sample with the same initial thickness,
the blanched sample had a lower moisture reduction rate
during puffing than the unblanched sample. The lower
drying rate of the blanched sample was related to the cell
structure collapse, which results in less porosity before
puffing, and the gelatinization of banana starch during
blanching, which retards the movement of moisture.
For the final stage, the samples were further dried at the
same drying temperature as the first step. The drying time in
the third stage drying was the longest, more than two times
that in the first stage drying. The blanched sample required
10–20% longer drying times than the unblanched sample.
Moisture Diffusivity
Table 2 illustrates the effective moisture diffusion coefficient
obtained at different banana thicknesses and blanching
times. Blanching and material thickness strongly affected
the values of effective moisture diffusion coefficient under
the same puffing and drying temperatures. Blanching caused
a lower void area fraction of banana, as seen in Fig. 4,
which makes moisture movement during the drying process
difficult, resulting in the low moisture diffusivity in the
blanched sample. In addition, the gelatinization of banana
starch during blanching limits the speed of moisture movement
within banana during drying. Marousis et al.[24]
found that gelatinized corn starch had a lower moisture
diffusion coefficient than nongelatinized corn starch.