In general, the application of ultrasound during drying increases
the kinetics of dehydration, affecting both the internal and the
external resistance. As can be observed in Fig. 4, the application
of ultrasound in the drying of carrot and lemon peel increased
the effective diffusivity and the mass transfer coefficient. The influence
on the effective diffusivity could be attributed to the ‘‘sponge
effect’’ or the creation of internal microchannels that make it easier
for the water to be released from the solid samples. In cryo-SEM observations, Ortuño et al. (2010) found that ultrasonically dried
orange peel albedo showed a more compressed cellular structure
with larger intercellular air spaces than conventionally dried samples.
The alternating expansions and compressions produced by
ultrasound created a highly porous material that facilitated the
water movement. These authors also found that ultrasound affected
the flavedo structure. The conventional air drying process
scattered the waxy components, closing the pores and creating a
waterproof barrier. Nevertheless, the original ring-shaped waxy
accumulations in the pores continued to be well defined. On the
contrary, in the samples dried using ultrasound application, these
ring-shaped accumulations disappeared revealing the very intense
effect that ultrasound had in the interface. The influence of ultrasound
on the external resistance to mass transfer could be linked
to the generation of differential pressures and the microstirring
at the interfaces and these effects should also affect the surface
of the treated solid.