Joint International Conference on Long-term Experiments ...

Tension: For bread prepared under laboratory baking conditions from the same wheats, but
loaded in tension parallel to the vertical axis of the loaf, values for Young’s modulus were
8.0 and 7.0 kN m_2 Considerably larger values have been obtained when very small strain
(0.02%) dynamic compressive/ tensile tests have been used to determine the mechanical
properties of bread crumb: a value of approximately 50 kN m_2 having been obtained for
the elastic part of Young’s modulus. The ability to propagate a crack in bread crumb
specimens with a long notch means that it is possible to derive a fracture toughness value
for bread crumb.
3.2. Measuring mechanical properties of cell walls
Although the mechanical properties of the bread are a function of the crumb cell wall
materials and the structure created by processing, changes in bread crumb physical texture
with shelf life (staling) are ascribed solely to cell wall property changes. A number of
factors account for this. Of the two phases in bread crumb, only the physical properties of
the solid phase will change. Most studies indicate that changes in the starch polymers in the
crumb walls are responsible for the firming of bread crumb over time (Zobel & Kulp, 1996),
although loss of moisture will evidently contribute to a change in the properties of the cell
walls which would be manifest as a firming of the crumb (Piazza & Masi, 1995). Changes
in bread crumb structure are not usually discussed in relation to staling (Zobel & Kulp,
1996), even though image analysis studies (Zghal et al., 1999) indicated substantial
structural changes arising from crumb moisture losses, with the magnitude of these changes
dependent on flour type used to make the crumb. Therefore, to eliminate structural
rearrangements within the crumb from affecting studies of firming mechanisms within
crumb wall components, direct measurement of the properties of the solid phase is desirable.
The most common method of measuring the mechanical properties of the solid phase is to
perform mechanical measurements on the compressed crumb (Scanlon et al., 2000).
RESULTS AND DISCUSSION
4. Relationship between structure and physical texture of cellular solids
In order to predict mechanical parameters that can be used to characterize bread physical
texture, physical models must be employed to relate the mechanical properties to the
structure quantified by imaging techniques. The degree of sophistication of the model is to a
certain extent determined by the amount and type of information provided by the imaging
system. Therefore, a number of factors that influence the mechanical properties of cellular
solids will be described along with descriptions of specific models and/or experiments that
have linked these structural properties to mechanical properties.
4.1. Measurements of structure related to volume fraction
Bulk density: Bulk density, like density, is defined as mass per unit volume (kg m_3). Bulk
density has been used to describe the density of the cellular solid. However, bulk density is
usually used to describe the packing density of commodities such as grains, where the space
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