The results presented here, demonstrate that the X-ray darkfield
radiography is sensitive to the raw, frozen or defrosted state
of fruits which shows the complementarity of the contrast modality
of X-ray dark-field to conventional X-ray radiography. Whereas
almost no contrast is obtained with conventional X-ray radiography
when comparing the raw state to either frozen or defrosted, CNR
values of around 2.5 are obtained with X-ray dark-field
radiography.
However, while the dark-field signal is clearly sensitive to
whether a piece of fruit is in a raw, frozen or defrosted state, it is
still unknown what kind of change in microstructure is responsible
for the improved image contrast. One possible explanation might
be that it originates from ultra small-angle scattering from the cell
walls in the fruit, and this scattering is enhanced if the cells are
frozen e keeping the cell walls more rigid and ordered e while
defrosting the fruit might break the cell walls. Further research
experiments are needed to say anything conclusive.
A point not addressed by this study is the influence of the
temporal development of the freezing transition on the dark-field
signal. In this study only rapid freezing has been used, and it is
left to future work to investigate whether the dark-field signal will
change for a different phase transition dynamic.
The proof-of-principle experiments in this work indicate the
potential for applying X-ray dark-field radiography in further
studies of freeze injuries in fruits. However, an experiment on a
larger scale of a single type of fruit with a focus on the defrosted
state would be needed to fully evaluate the strength of the proposed
method.
Further work also is needed before the technique can be
implemented in an automatic detection system, e.g. as part of a
conveyor belt line, but a proposal has already been made for a
grating-based interferometry scanning system (Kottler, Pfeiffer,
Bunk, Grüntzweig, & David, 2007) using a number of line detectors.