3.6. Effect of different processes on microstructure
Fresh stem lettuce cells are spherical and intercellular spaces
are arranged uniformly between cells (Fig. 10a). After blanching,
the shape of the cells in the stem lettuces changes; they are close
to an ellipse form but their volumes seem to remain unchanged
(Fig. 10b and c). Most of the cell walls are broken and soluble materials
flow from the interior to the exterior of the cells. Consequently,
the cell gap is enlarged, especially for those cells
blanched by microwave heating. There are cracks in the cell surfaces
of the HWB samples but no cracks on the surfaces of cells
in the MWB samples (Fig. 10d and e). Cell gaps in both frozen samples
are somewhat enlarged; they are larger in the HWB sample
after freezing. This could be caused by increased volume of ice
crystals resulting from freezing. The dark areas shown in the figures
above are caused by the melting ice in the frozen sample.
There is a clear distinct difference between the FD and MFD stem
lettuce cubes in terms of their microstructure. The cell walls in
the HWB samples were broken by water removal from the cell wall
surfaces due to small flow pathways for water vapor which affects
the drying rates significantly in the initial stages of MFD. On the
contrary, the pathways of flow of the water vapor in the intercellular
spaces in MWB sample were the largest among the three samples
over the entire MFD run; it is notable that the cell shape of
MFD was similar to that of FD sample.