Mechanisms Determining Crispness and Its Retention in Foods with a Dry Crust - Food Colloids: Self-Assembly and Material Science - page 493

Table 1 Minimum and maximum sizes of the structural elements at the meso-
scopic length-scale in a dry cellular crispy food as estimated from sound
emission properties and mechanical and physiological constraints

Structural
element

Minimum size
( m m)

Maximum size
( m m) Reason Reason

Beam 50-100 For fast fracture

B 400

To prevent too hard
texture

Pore

B 100

For
distinguishable
events

450-650 To produce overlap

of sound events

for the gas cells that allow us to get 3-6 fracture events per millisecond, taking
into account that each gas cell is surrounded by 6-8 lamellae. This rough
calculation gives a maximum pore size of 450-650 mm. Vincent 33 also

concluded that fracture events in a crispy food have to overlap in time for
the reason that otherwise the force drop on fracture would be too small to be
detected during biting. Another aspect that should be taken into account is that
the cellular structure of the product is typically heterogeneous. Depending
upon the thickness of the beam or lamella involved, the force drop and emitted
sound pressure on fracture will vary. Presumably it is only the relatively large
events that are noticed during consumption of a crispy product.
An overview is given in Table 1 of the minimum and maximum sizes of the
structural elements in a dry cellular crispy food required for an optimum crispy
product as estimated from sound emission properties and mechanical and
physiological constraints. Of course, the precise numbers will depend to some
extent on the particular product involved.

B

34.4.4 Frequency Spectrum of Emitted Sound
As mentioned above, the emitted sound signal on fracture of a single beam or
lamella in a crispy product only lasts for at most 1 ms. A common way to
analyse the frequency distribution of sound is by fast Fourier transform (FFT)
analysis. 40 In view of the constraint on the duration of a single sound event,
there can be no contribution to the signal with frequency below (1/10 3 s), i.e.,
below 1 kHz. A frequency calculation involving low frequencies is still possible
if the whole sound signal consists of many exact repetitions of the sound event,
so producing a long periodic signal, although this is not precisely the case for
the complete sound signal for crispy products. However, for a not too hetero-
geneous product, probably one can deal with the complete sound signal as a
periodic signal. Several researchers (e.g., de Belie et al. 41,42 ) have indicated that
the low frequency part contributes to crispness. The question is which product
properties determine the low frequency part of the signal.
At least part of the set of low frequencies calculated by FFT analysis for a
long periodic sound signal will be determined by the periodicity, i.e., the