Mechanisms Determining Crispness and Its Retention in Foods with a Dry Crust - Food Colloids: Self-Assembly and Material Science - page 489

dissipated and W f is the fracture energy. 1,31 Fracture propagation occurs when
the value of W 0 is so high that the differential energy that is released due to
stress relaxation in the material in the vicinity of the formed crack surpasses the
differential energy required for crack growth. 31,32 Energy dissipation processes
other than that resulting from fracturing (W 00 ) directly affect the amount of
energy available for crack growth and hence the speed of crack propagation.
Crack growth speed may become high enough to get sound emission if the
released energy available for crack growth is sufficiently high, i.e., if energy
dissipation is low and the volume in which stress relaxation occurs is large
enough. Our hypothesis is that even a small increase in energy dissipation
(e.g., due to an increase in water content) will result in (i) an increase in the
amount of energy required to get fracture, implying an anti-plasticizing effect
of small amounts of water (higher fracture stress); and (ii) a delay of the
transition from crack initiation to crack propagation, implying fewer acoustic
emission events and maybe another pitch of the emitted sound.
The molecular approach has great benefits in understanding crispy behaviour
and the loss of it during storage under deteriorating conditions. However, there
are also observations that cannot be explained based solely on a molecular
approach. For instance, it is hard to explain in molecular terms why the crust of
most deep-fried snacks loses its crispy behaviour much faster than the crust of
baked bread-type products (Figure 2). Such differences can be explained,
however, by also considering mechanisms determining crispness acting on longer
length-scales, i.e., mesoscopic and macroscopic length-scales. Vincent 33,34 and
Luyten et al. 2,7 point to the importance of the structure at the mesoscopic length-
scale for determining the number and size of the force drops and therewith
crispness perception. Moreover, oil adsorption by the cellular structure after
deep frying of snacks may be an important factor in the fast decrease in crispness
of these snacks after frying. 35

34.4 Mechanism Acting at the Mesoscopic Scale

34.4.1 Estimation of the Structural Length-Scale for Crispness
As mentioned above, for a food product to be perceived as crispy its fracture
behaviour should be characterized by multiple fracture and sound events while
the work of mastication should be relatively low. To get acoustic emission during
fracture of a material, the crack speed has to accelerate from zero to the required
speed of 300-400 m s 1 . This already sets a minimum for the required length of
the crack involved. Beams and lamellae that are thinner than this minimum
length will fracture at a low force, but there will be no acoustic emission.
Fracture will start at small defects (cracks or pores) in the beams and
lamellae forming the cellular structure. Vincent 33 has reported a defect size in
crisps of 10 mm, which is comparable to the size of the smallest pores in

biscuits (Figure 3) and other crisp products. On the other hand, larger pores
may cause a growing crack to stop or to slow down temporarily; in any case a

B