3.6. Effects of combined treatments on the coating layers
The effect of the combined treatments on coated green bean
samples was also investigated with the help of a lipophilic
fluorescent dye added to the mandarin oil emulsion.
Fig. 3 reports micrographs of histological sections of green
beans, uncoated, coated and treated by HHP, under brightfield and
fluorescence, in order to highlight the location of the mandarin oil
emulsion droplets, stained with a
fluorescent dye. In particular,
Fig. 3a and b shows the control green beans, with no coating: some
fluorescence is evident in the inner cells, which can be attributed to
chlorophyll content. The coating deposition results in the
appearance of a layer on the surface of the green beans
(Fig. 3c). Remarkably, an intense
fluorescence is concentrated in
the coating layer, resulting from the lipophilic Nile red mixed in the
mandarin oil emulsion droplets (Fig. 3d).
Fig. 3 also reports the micrographs of the HHP-treated samples,
in order to make evident the strong impact on the cell structure,
previously discussed. It is clear that the cell organization observed
for the control samples does not exist anymore after a HHP
treatment at 300 MPa for 5 min, but a thick layer appears, which
probably contributes to increase the
firmness of the sample
(Fig. 3e). In addition, as highlighted by the distributed
fluorescence
in such thick layer, HHP treatment also caused the coating layer to
penetrate into the surface of the samples, as shown by the
fluorescence appearing also far from the green bean surface.
The impact of PL on the coating layer is instead negligible. Fig. 4
shows coated green beans before (Fig/ 4a and b) and after PL
treatment (Fig. 4c and d).
No evident difference can be observed in coating structure and
intensity of nile red
fluorescence before and after PL application