The regular freeze-dried banana (Fig. 6a) had uniform and
small porous structure, but no damage and disruption of cellular
walls at the heated surface. On the other hand, the IR pretreated
samples with 20% WR (Fig. 6b) and 40% WR (Fig. 6c)
showed collapse of cellular tissue in the surfaces of the banana
slices, forming a crust on the surfaces of the banana slices.
This is as expected since the surfaces were exposed to heat
during drying. Crust formation is more apparent in higher
weight reduction. The formation of crust resulted in reduced
drying rate during freeze-drying. It was also observed that
large pores existed in the center region of the banana slices
with 20% weight reduction, which could be due to water vapor
created during IR drying. The water vapor forced the expansion
of the cellular tissue and pores. However, when the banana
slices were further dried to 40% weight reduction, the
pores were collapsed. The crust formation and changes in
the pore sizes could affect the quality characteristics of the
dried banana chips.
It was observed that the acid dipping treatment prior to IR
drying improved the drying rate during freeze-drying compared
to the samples without the dipping treatment (Fig. 5).
The reason could be because the acid treatment washed out
some starch, sugar and protein from the surface of banana slices,
resulting in formation of more porous surfaces. However,
the effect of improving drying rate was mainly observed during
the early drying stage. The dipped samples with 20%
weight reduction in IR predehydration required 2 h less than
regular freeze-drying to achieve 5 g moisture/100 g wet
weight in the finished products. But, the drying rate of the
samples with 40% weight reduction in predehydration was
much lower than regular freeze-drying even with the dipping
treatment. More studies need to be done to further characterize
and explain the effect of the acid treatment on the drying rate.