Mechanisms Determining Crispness and Its Retention in Foods with a Dry Crust - Food Colloids: Self-Assembly and Material Science - page 496

Table 2 Number of force peaks and sound peaks per unit area of material during
a cutting experiment of toasted rusk roll with a razor blade. In brackets
are standard deviations (n ¼ 3)

Number of force peaks
per cm 2

Number of sound peaks
per cm 2 Sample treatment

Dry, no oil 79 (9) 100 (20)

Soaked with oil at room temperature 81 (6) 42 (7)

Soaked with oil at 1801C 87 (12) 66 (2)

time curves look about the same (the small differences are not significant). The
same applies to the data for the number of mechanical peaks per unit area of
cut cross section of the rusk roll (Table 2). In contrast, however, the curves of
sound pressure against time differ considerably. The average size of the peaks
after oil treatment was smaller, indicating a lower acoustic energy. Further-
more, the number of acoustic peaks was much lower for the rusk rolls that were
dipped in oil (Table 2), the effect being larger for the samples soaked in oil at
room temperature. A possible explanation for the smaller effect on sound
emission of dipping the rusk roll in hot oil is that more oil was already drained
from the sample before the cutting test because of the much lower oil viscosity
at 1801C. The average take-up after dipping toasted rusk roll in oil was 286
20 wt% for cold oil and 226 33 wt% for hot oil (n ¼ 5).
Similar results were obtained for the effect of wiping oil from the model crust
directly after frying. In Table 3 the numbers of force and sound peaks are given
that are larger than the threshold values of 1 N and 1.5 Pa, respectively. While
the number of force peaks does not differ significantly between the standard
and wiped snacks, the sound peak number is significantly higher for the wiped
snacks. Moreover, the sound pressure level of the sound signals was on average
5 dB higher for the wiped snacks (data not shown).
So the conclusion is that sucking of oil into the cellular sponge structure of a
crispy product has no significant short-term effect on its mechanical properties.
But it strongly affects the sound emitted on fracture. Uptake of oil will result in
the presence of oil in the pores or oil droplets in the larger holes. 44-46 This
means that at least part of the sound that is emitted near the crack tip will have
to pass an oil layer before it is transferred to the surrounding air. In principle,
then, there are a few acoustic mechanisms that may explain this effect. The
main ones are damping of the sound when it travels through the air or oil and
reflection and refraction of the sound at the oil-air interface. Damping of
sound in air is very weak. 47 In oil it is greater because of the higher viscosity of
the oil. But, since the oil layers involved are very thin ( r 1 mm) the attenuation
of sound during its transport through the oil should be very low. In contrast,
reflection of sound at the oil air interface will be very large because of the large
difference in acoustic impedance between air and oil. The acoustic impedance Z
is defined as the product of the density and the acoustic velocity c in the