Mechanisms Determining Crispness and Its Retention in Foods with a Dry Crust - Food Colloids: Self-Assembly and Material Science - page 498

Figure 6 Schematic representation of the response on penetration by the tip for a rounded
cylinder of a dry crispy cellular solid (left) and of the same product after storing
at high (80%) relative humidity (right)
a rounded cylinder. 8 During the transition from 'crispy' to 'non-crispy', the
fracturing mechanism of a cellular solid material will change from brittle
fracture of 'single' beams and lamellae at relatively low forces to ductile
fracture initially involving the bending of groups of beams and lamellae, which
finally may rupture together at a much larger overall force. The latter will be
the case as long as the water content does not become too high, even though the
fracture force of the individual beams and lamellae may already have decreased
due to the weakening of the solid matrix. For a toasted rusk roll, an increase in
the cutting force has been observed from o 1 N up to 6-7 N at a cutting speed
of 0.4 mm s 1 . 9
34.5 Effects at the Macroscopic Scale
At the macroscopic scale, the difference in mechanical properties between the
crust and the inner moist part of a product is of importance. Depending upon
the ratio of the stiffness of the crust to that of the moist layer below it, the crust
may behave in different ways when the product is subjected to deformation by a
wedge-like structure (e.g., a tooth). When the stiffness of the two parts of the
product is of the same order, the wedge will penetrate the crust layer; but when
the stiffness of the layer below it is clearly lower, the crust layer will deform
more in a bending mode, the extent of deformation depending upon the ratio of
the stiffnesses. In extreme cases, the transition from a weak to a stiff sub-layer
will cause the same change as going from a three-point bending test to the
wedge penetration of a crust layer situated on a solid foundation. In the latter
case the fracture process generally will take longer for the same deformation
speed. Fracture of a crust in three-point bending occurs mostly as one main
fracture event, sometimes preceded by smaller fracture events, while in wedge
penetration there are more fracture events of comparable size. Three-point
bending involves more of a snapping type of fracture, whereas wedge penetra-
tion is more of a crushing type of fracture. So, in principle, the ageing and
drying out of the crumb during bread storage may also induce a change in
crispness perception of the crust. In practice, however, other processes that can
cause a change in crust crispness, such as the uptake of water, will tend to
proceed faster.