Mechanisms Determining Crispness and Its Retention in Foods with a Dry Crust - Food Colloids: Self-Assembly and Material Science - page 488

low, but this is not enough. The modulus of solids is typically 10 9 -10 10 N

m 2 . Even for a fracture strain below 1%, this implies a fracture stress that is
about equal to that of human teeth, which is around 2 10 8 Nm 2 . To have a
fracture stress clearly below that of the teeth, the solid material of the food
must contain rather large defects, 13 or it must be built of rather thin beams,
struts and films, with a sponge- or foam-like structure. Indeed, most dry crispy
products have a sponge-like structure.
The main reason for loss of crispness is a change in properties of the solid
matrix as a result of water sorption from the environment or water transport
from other parts of the product. 14-16 This process results in an increase in the free
volume of the product, allowing a greater mobility of the molecules in the system.
The original idea was that loss of crispness, as a result of water acting as a
plasticizer, is governed by its effect on the glass transition. 17,18 At a water content
below the glass transition, the molecular mobility of the (macromolecular)
components responsible for material stiffness is close to zero. Around the glass
transition the segments of macromolecules become mobile and start moving
randomly. At a still higher water content the (whole) molecules become mobile
resulting in the gel-like behaviour of cross-linked polymer systems. The water
content at which the glass transition occurs depends strongly upon the temper-
ature. A similar transition as described above for increasing water content may
occur with increasing temperature. This transition is normally characterized by
its mid-point and is denoted as the glass transition temperature T g .
Loss of crispness occurs over a water content range and a plot of it against
the 'equilibrium' water activity has a sigmoid shape. 19 Relating loss of crispness
to the mid-point of the glass transition is not very useful for several reasons.
Most food products are composed of several ingredients, each characterized by
its own glass transition range. Moreover, it has often been observed that
crispness is already affected at water contents below T g . 20-25 Mobility
changes below T g have also been observed for various enzymes. 26 Another
complication is that other components such as small solutes like sugar mole-
cules may act as plasticizers. Moreover, at low concentrations, an increase in
the content of water 25,27 or fructose 28 may lead to an increase in the stress
required for deformation. These various observations do not fit with the
concept that T g is the only determining factor for a product being crispy or not.
The above-mentioned aspects can be understood quite well by recognizing
that fracture consists of two processes: 29,30 fracture initiation and fracture
propagation. Fracture initiation occurs when the local stress in the material
exceeds the breaking stress of the bonds between the structural elements, giving
the solid-like properties to the material. Subsequently, the small crack so
formed may grow spontaneously if certain conditions are fulfilled. This so-
called fracture propagation condition is governed by the net energy balance:

B

W ¼ W 0 + W 00 + W f

(2)

where W is the amount of energy supplied to the material during deformation,
W 0

the part of the deformation that is elastically stored, W 00

the part that is