The turgor is due to the presence of a cell membrane containing
various transport enzymes, which can keep the molality of the cellular liquid
at a high level. This mechanism needs energy, and when the cell metabolism
stops, energy cannot be supplied any more and the turgor disappears. This is
what we observe when a leaf of lettuce wilts, which shows that the turgor
can strongly contribute to the stiffness of a tissue. But the other two factors
also contribute, to a variable extent. All three factors are generally affected
when the tissue is heat treated: the turgor disappears, the middle lamella can
become greatly weakened, and the cell walls will soften owing to partial
degradation of their structural polysaccharides. Altogether, the relative
contribution of the three factors varies widely among tissues, during storage
and upon processing.
The theory as briefly touched on in this section cannot be applied to
plant tissues. Their mechanical properties are highly specific and need to be
studied separately in each case, as is often the case with natural products.
17.6 RECAPITULATION
Soft solids are always composite materials. Their softness means that a small
stress leads to a relatively large deformation, either because the elastic
modulus is quite small, or because the material readily yields (shows lasting
deformation).
Rheology. Determination of mechanical properties can be done in
various deformation modes (shearing, compression, extension), leading to
different results. Of the parameters to be determined, the modulus or
stiffness of materials has been studied and explained best; results can give
information about structure.
In practice, large deformation properties are far more useful, and
determination of the full relation between stress and strain gives the best
information. Materials vary widely in their ‘‘linear region,’’ i.e., the strain
range over which stress and strain remain proportional. Deforming it much
farther, the material may eventually break. Relevant parameters then are
fracture stress or ‘‘strength,’’ fracture strain or ‘‘shortness,’’ and work of
fracture or ‘‘toughness.’’ The correlation between fracture parameters and
the modulus is often poor. Since many soft solids exhibit viscoelastic
behavior, the values of these parameters can depend, often markedly, on the
strain rate.
Some materials break at high strain rates but yield at small rates.
Several soft-solid foods yield under the conditions normally applied, as
during spreading on bread. The magnitude of the yield stress then correlates