Thinner banana slices had lower moisture diffusivity
than thicker slices. The lower moisture diffusivity in the
thinner banana slices is due to the fact that the small
expansion during puffing as well as the low void area
fraction, as seen in Fig. 4. Hence, lower moisture diffusivity
was obtained for thinner banana slices.
Influences of Blanching, Chemical Pretreatment,
Thickness, and Puffing Temperature on Shrinkage
Figures 5a and 5b show the effects of citric acid and
sodium metabisulfite pretreatment, blanching, thickness,
and puffing temperature on the degree of shrinkage of
banana slices. When the 3.5-mm-thick samples were
blanched for 30 s or 1 min, the shrinkage of the blanched
banana samples was significantly higher than that of
unblanched samples; the shrinkage of the blanched samples
at the end of processing was in the range 50–65%, depending
on the blanching time and puffing temperature. A
longer blanching time and lower puffing temperature were
responsible for the higher shrinkage. The high shrinkage
for the blanched sample can be explained considering that
some cell components are solubilized and degraded during
blanching[25] and the starch is gelatinized. These effects
cause higher shrinkage of blanched banana during the first
stage of drying and the formation of dense layer with a
rigid structure. When puffed, the expansion of product
volume is rather more difficult. As determined from experiments,
the volume ratio of the sample before puffing
compared to that after puffing at a puffing temperature
of 160C was approximately 121 and 175% for the
blanched and unblanched cases, respectively.
Considering the shrinkage at different sample thicknesses,
it was found that the shrinkage of 3.5-mm- and
2.5-mm-thick samples was very different between the
blanched and unblanched samples. In the case of
unblanched samples, shrinkage of 2.5-mm-thick samples
was significantly higher than that of the 3.5-mm-thick
samples. This is due to the shorter distance for moisture
to travel from the interior to the surface, resulting in faster
loss of moisture content. The loss of moisture content during
early puffing time at which the temperature of banana
sample did not reach the boiling point temperature may be
in the form of liquid diffusion. However, when banana
reaches the boiling temperature, the moisture inside
banana boils, vaporizes, and transports in vapor form,
but the remaining moisture content might not be enough
to create vapor pressure to expand the sample volume,
leading to greater shrinkage for 2.5-mm-thick samples.
Rakesh and Datta[10] reported that a decrease in sample
thickness provided a small expansion of product volume.
On the other hand, when the banana was blanched
before processing, it was found that the 3.5-mm-thick
sample shrank to a significantly higher degree than the
2.5-mm-thick sample. Moisture is not a main factor in
the higher shrinkage of the blanched 3.5-mm-thick sample,
unlike in the unblanched case. The higher shrinkage