At the macroscopic level, the properties of the sample beds depend on
the size and shape of the flakes which are all taken into account by their
apparent, or bulk, density. The increase in apparent density of the flakes bed
during the first compression stage is inversely reflected by the ratio r* = ri/rc,
ri being the initial apparent density and rc being the density after the first
compression stage (Table 3). r* is correlated to S, the surface area under the
complete compression curve, as shown in Fig. 2, which reflects the energy
required for the crushing of the bed of samples (Fig. 14, r2 = 0.75,
P = 0.0001), although physical mechanisms of dissipation and storage cannot
be accurately established in the case of the Kramer cell. Most crispy flakes
are characterized by important densification – low value of r* – and low value
of S, whereas the opposite result is obtained for less crispy ones, as in samples
E and J. Dense materials have to be investigated more deeply for a better
understanding of their lower crispness, which can be attributed to an increase
of stiffness in the glassy domain, likely due to the anti-plasticization at a larger
moisture content, but it may also be due to its larger apparent bulk density.
The full physical interpretation of the measurements performed on this
cell, based on the various structural levels of the tested material, is beyond
the scope of this present work, but it has distinguished the commercial samples
for their crispness in a way close to the sensory panel.
CONCLUSION
A Kramer shear cell has been selected by comparison of different testing
devices as the most suited one to evidence mechanical and acoustic behaviors
and to highlight differences between flakes samples from commercial brands.
The advantages of studying the flakes in bulk rather than alone have been
shown in terms of measurement reliability. Relationships were found between
acoustic, mechanical measurements and sensory results. Crispness is linked
to mechanical properties measured during compression, but this trend is not
sufficient to classify samples and take into account all sensory attributes.
Acoustic signals in the temporal domain have been shown to be more efficient
for discriminating flakes in a way similar to sensory analysis.
This measuring device can now be employed to test model cornflakes,
processed at laboratory scale, for assessing their sensory properties