The effect of mass load density can be observed in Fig. 5 for drying
experiments performed on carrot cubes. In conventional air
drying processes, it appears that, the increase of mass load density,
in the range tested, does not affect the effective moisture diffusivity
although it does produce a reduction of the mass transfer coefficient.
This could be linked to perturbations in the air flow through
the drying chamber, thus creating preferential pathways and, as a
consequence, increasing external mass transfer resistance (Cárcel
et al., 2011). When high intensity ultrasound was applied, the mass
transfer coefficient and the effective moisture diffusivity increased
when the mass load density used was moderate, although the
acoustic influence on mass transfer coefficient became negligible
at high mass load densities.
Therefore, from previous results, for a specific ultrasonic application
it appears to be important to carry out a study into the influence
of different process variables in order to find the optimum
drying conditions. The innovation of the process could fail if not
properly addressed.