Chemistry Reference
In-Depth Information
Martin and Sanz (1985) found that the following equation modelled the
relationship between conductivity and viscosity for the range of milks and
milk concentrates they investigated:
ð
1
0
:
011TS
Þ
¼
2
:
038
0
:
230TS
þ
0
:
0127
ð
TS
Þ
2
ð
Sm
1
Þ
(75)
where
¼
coefficient of viscosity ((Pa s)
10
3
);
TS
¼
percentage total solids (w/w).
The equation allowed EC to be calculated at a given temperature (in the range
15-608C) when the viscosity at that temperature was known, for TS of up to
about 30%. Presumably, the effect of fat content on conductivity was
accounted for by its effect on viscosity. Fern´ ndez-Martin and Sanz (1985)
discuss earlier attempts to relate EC to milk viscosity.
15.10.2. Dielectric Properties
The dielectric properties of a material, the permittivity (
"
0
) and the
dielectric loss factor (
"
00
), are the real and imaginary parts, respectively, of
the complex permittivity (
"
) (Nunes et al., 2006):
"ð!Þ¼"
0
ð!Þ
j
"
00
ð!Þ
(76)
where
!¼
frequency (Hz);
j
¼
(
1)
1/2
.
As Equation 76 indicates, these properties are frequency dependent.
"
0
is
a measure of the ability of the material to store electromagnetic energy.
"
00
is a
measure of the material's ability to dissipate electromagnetic energy as heat.
The latter phenomenon is exploited in microwave and radio frequency heating.
Representative values of the dielectric properties of milks with various fat
contents are shown in Table 15.9, which includes values for pure water. At
microwave-heating frequencies, the permittivity of skim milk is lower than that
of water while the loss factor of skim milk is higher. Both
"
0
and
"
00
decrease
with increasing temperature at these frequencies (Mudgett et al.,1974).
Nunes et al. (2006) showed that at room temperature (17-208C) the
ratio of the permittivity of UHT skim milk to water permittivity dropped
from
0.93 at 1 GHz to
0.83 at 20 GHz, while the corresponding values for
loss factor were
2.8 and
0.8. For UHT whole milk, the decreases were
from 0.91 to 0.77 for permittivity and from 3.5 to 0.8 for loss factor.
