Joint International Conference on Long-term Experiments ...

etween discrete particles contributes to a reduction in density of the bulk collection
compared to the density of the individual grains. Given the discussion in Section 2.1 on the
nature of connectivity of bread crumb, bulk density will not be used to describe the density
of bread and other cellular solids. Where the term appears to have been used to describe the
density of the cellular solid, it will be discussed simply as density (_), as distinct from solid
or cell wall density (_s) which is used to define the density of the materials comprising the
solid phase.
Density: As for other foamed polymeric materials, density (or its inverse, specific volume,
m3 kg_1) has an enormous effect on the mechanical behaviour of bread crumb (Fig. 3).
Because of the large contrast in densities between the two macroscopic phases of bread
crumb, a simple measurement of bread density permits the volume fractions to be calculated
without recourse to image analysis for determining the distribution of the two phases in the
bread. Strong relationships between density and various mechanical properties have been
reported for bread and other cellular food products. For bread crumb differing in
composition (flours of 100% rye, 50% blend of wheat and rye, and
Fig. 3. Typical compressive stress strain curves for bread crumb created by proofing for
various times to generate different densities
100% wheat), crumb firmness linearly increased while crumb elasticity linearly decreased
with increasing density.
The rate of change in mechanical properties of bread crumb as a function of density was
dependent on bread composition, with rye bread having the highest slope.
Relative density: Because there is variation in cell wall density with changes in formulation
or processing conditions, then the density of the cellular solid would be expected to change,
and therefore the effect of the structure of bread on its physical texture would be partially
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