Chemistry Reference
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Table 3 Number of force peaks and sound peaks larger than threshold values of
1 N and 1.5 Pa, respectively, that can be distinguished in the force and
sound versus deformation curves as a function of ageing time for deep-
fried model crust snacks, with and without wiping to remove liquid oil.
In brackets are standard deviations
Number of force peaks
Number of sound peaks
Ageing Time (min)
Standard
Wiped
Standard
Wiped
5
21.3 (3.3)
22.4 (2.7)
75 (18)
111 (24)
7
23.3 (2.0)
25.3 (1.7)
69 (21)
107 (25)
10
20.3 (2.6)
23.7 (2.3)
46 (13)
77 (15)
12
19.1 (1.6)
18.7 (2.1)
38 (16)
69 (17)
15
17.6 (2.4)
15.2 (1.6)
42 (15)
59 (19)
1.3
10
6
kg m
2
s
1
,
respectively. The reflection coefficient R for sound is given by
R
¼
ð
Z
2
=
Z
1
Þ
material;
47
values for air and oil are
4
10
2
and
B
B
p
ð
Z
2
=
Z
1
Þþ
1
ð
n
1
Þ
tan
2
a
i
p
;
ð
4
Þ
1
ð
n
1
Þ
tan
2
a
i
where n
¼
(c
2
/c
1
)
2
and
a
i
is the angle of incidence of the sound wave. Symbols
with subscript 1 refer to the material on the side from which the sound comes.
For an oil-air interface this results in a reflection coefficient of
1, implying
total reflection.
In spite of total reflection of sound at an oil-air interface, one can still hear a
sound signal on fracture of crispy solids containing oil, although it is less loud
than for the same material containing no oil or less oil. We assume that the oil
penetrating into the cellular solid only covers part of the solid-air interface,
thereby allowing part of the acoustic emission during fracture of the solid
material to pass directly into the air without passing through an oil layer.
Moreover, new surfaces formed on fracture are unlikely to be covered by oil.
It was also noticed that there was an effect of the type of oil for the rusk roll
experiments.
35
Oil that became solid at room temperature had less effect on the
sound than liquid oil. The acoustic impedance for a solid fat is likely to be
somewhat higher than for a liquid oil, and so a difference in reflection cannot
explain this observation. Our tentative explanation is that, on solidification of
the oil, small cracks are formed in the fat layer which allow passage of the
sound.
34.4.6 Change in Mode of Fracturing with Increasing Water
Content
Another point that should be considered is that, on deterioration of a crispy
product, the mode of fracturing changes on the mesoscopic scale, as depicted
schematically in Figure 6 for the case of fracture due to penetration by the tip of
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