Change of Moisture Content of Banana Slices
During Puffing
The initial moisture content of fresh bananas was
approximately 284.60.7% (db). After citric acid or sodium
metabisulfite pretreatment for 5 min, the sample moisture
content was increased to 333.60.9% db for 2.5-mm-thick
slices and 373.50.4% (db) for 3.5-mm-thick slices. Then
the samples were blanched with hot water and the sample
moisture content was increased to 351.50.7% (db) and
396.70.9% (db) for 2.5 and 3.5mm thicknesses, respectively.
The increase in moisture content during blanching is
due to partial disruption of sample cells and swelling of
banana starch.[23] Figure 3 shows the evolution of banana
moisture content during processing steps. Blanching provided
a longer drying time in the first stage and the drying
time for the blanched sample was 5–10 min longer than that
of the unblanched sample. The drying times for each processing
step are shown in Table 1.
During the puffing step (B) shown in Fig. 3, the moisture
content of 2.5-mm-thick samples for the unblanched case
decreased faster than that of the 3.5-mm-thick sample. As
determined from the experiment after the puffing step,
the moisture contents were decreased to 14.2 and 10.3%
(db) for the samples with 3.5 and 2.5mm thicknesses,
respectively. The lower moisture content in the thinner
sample is due to the shorter distance for moisture to travel
to the exterior surface.
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.