3.4. Effects of the different processes on the characteristics of MVD
3.4.1. Effects of blanching on MVD
The MVD rate curves of A. bisporus slices pre-treated with different
blanching methods are shown in Fig. 7. As is seen from Fig. 7, for
MWB sample, the drying curve belongs to accelerated drying stage
when the XW is higher than 2.27 d.b., but enters into decelerated drying
stagewhen the XWis lower than 2.27 d.b. Therefore, the value of XW2.27
d.b. is a turning point, which is consistent with the observed ε″ value of
MWB sample before. The drying rate (slope of the curve) is the same for
HWB and MWB samples in the beginning, possibly because at this
drying stage, the removed water was mainly free water on the surface
of the sample and not internally bound water. Blanched A. bisporus
slices had shorter drying times. The drying time for the non-blanched
samples was ~30 min, whereas the required drying time for the HWB
and MWB samples was ~22.5 and ~17.5 min, respectively. A 25% and
41% reduction in drying time was observed in the samples treated
with HWB and MWB, respectively. As described above, the dielectric
properties of the samples were dependent on the blanching method.
The MWB-mediated enhancement of the drying rate resulted from a
reduction of dissolved air, which increased the ε″ of the A. bisporus
slices, thereby improving their microwave absorption. However, part
of the water in the sample was lost during blanching, especially for
MWB. This may be another factor responsible for the shorter drying
time of the MWB sample.