Joint International Conference on Long-term Experiments ...

in the image. In examining the structure of other cellular products, a number of image
acquisition methods have been used including:
1. transmission of light through thin sections, where the solid phase appears at the low end
of the grey level scale;
2. reflection of light from the surface of the specimen , where the shadows cast by cut cell
walls attenuate the light intensity that is reflected from the recessed cell walls, so that
the solid phase appears at the high end of the grey level scale;
3. x-ray tomography, where differences in absorption between the two phases permits
nondestructive evaluation of the internal structure.
3. Mechanical properties of bread crumb
3.1. Measuring crumb mechanicalp roperties
Compression: The most commonly used method to measure crumb physical texture is
indentation (AACC, 1983), a compressive loading test from which ‘‘modulus’’ values can
be obtained. Deformation of a crumb specimen between parallel plates in a uniaxial
compression test can also be used to measure the mechanical properties of bread crumb.
Fig. 2 shows a typical stress–strain curve for bread crumb under compression, to
indicate two important parameters used for characterizing the properties of the bread crumb:
Young’s modulus and critical stress. Bread crumb stressed parallel to the loaf’s longest axis
had values for Young’s modulus 2–3 times those of specimens compressed in the other two
directions, and values for critical stress 2–2.5 times those in the other directions. Uniaxial
compression of wheat starch bread (also tested fresh) gave a value of 18 kN m_2 for the
compressive modulus of crumb specimens compressed parallel to the long axis. Fresh
specimens of a soft wheat flour bread, compressed parallel to the long axis of the loaf,
yielded rather low values for the compressive modulus at 300 N m_2 (Piazza & Masi,
1995).
Fig. 2. Typical stress strain curves for bread crumb under tensile and
compressive loading. Young’s modulus (E), critical stress (_c), and failure stress (_f)
denoted.
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