Mechanisms Determining Crispness and Its Retention in Foods with a Dry Crust - Food Colloids: Self-Assembly and Material Science - page 487

34.3 Mechanism Acting at the Molecular Scale
In the literature the subject of 'crispness' is often exclusively linked to processes
acting at the molecular scale. It is clear that, without the right material
properties at the molecular scale, a product cannot be crispy. The main
requirement for the material properties is related to the observation that the
material must exhibit brittle fracture accompanied by acoustic emission. This
means solid-like behaviour and high crack-growth speeds. Figure 1 shows
results obtained from the slow cutting of a rusk roll. 9
To generate sound emission the crack growth speed should be above 1/4 to
1/2 of the maximum speed of stress waves in a material, the Rayleigh speed, as
defined by 10

s E

r

v Rayleigh ¼ ; ð 1 Þ

where E is Young's modulus and r the density of the solid material. For most
dry crispy foods, the modulus of the solid material is 1-2 10 9 Nm 2 , 11,12

B

1-1.5 kg m 3 . This implies a Rayleigh speed of 1000-1500
ms 1 and a crack growth speed of 300-400 m s 1 . 10 The crack growth speed in
the crispy biscuit has been found to be too high to measure by a high-speed
camera (>100 m s 1 ). 2
Another requirement for a product to be perceived as crispy is that the work
of mastication should be relatively low. The product should not be too hard.
This implies a relatively low fracture force. Since the modulus (stiffness) of
solids is high, a low fracture force can only be achieved if the fracture strain is

and the density is

B

Force (N)

4

2

0

Time (s)

Sound emission (Pa)

1

0

-1

Time (s)

Figure 1 Measured force and accompanying sound emission as a function of time during
cutting of a toasted rusk roll at a speed of 0.4 mm s 1