Chemistry Reference
In-Depth Information
Table 15.9.
Representative values of the dielectric properties of milks with various
fat content, and of water.
"
0
is the permittivity and
"
00
is the dielectric loss factor
"
0
"
00
Sample
Temp (8C)
Frequency (GHz)
Source
Skim milk
25
3
*68
*18
Mudgett et al. (1974)
Skim milk
55
3
59
16
1% fat milk
20
**2.45
70.6
17.6
Kudra et al. (1992)
2% fat milk
20
2.45
69.4
17.8
3.25% fat milk
20
2.45
67.9
17.6
Water
20
2.45
80.2
13.4
Lide and Frederikse (1996)
*
"
0
and
"
00
are expressed as ratios of the actual values to the corresponding values for free space. The values
shown are thus dimensionless quantities.
**This frequency is an internationally permitted ISM (industrial, scientific and medical) frequency. It is the
frequency at which domestic microwave ovens normally operate.
Mudgett et al. (1974) found that, over the temperature range 25-558C,
experimentally determined values of the dielectric properties of skim milk
agreed to within 5% with the theoretically predicted values for a 0.1 M
solution of sodium chloride, provided that, in the case of the permittivity
(
"
0
), a correction was made for the combined depressive effect of the non-ionic
milk components casein and lactose. The required correction at 3 GHz and
258C was
4.9.
The data of Kudra et al. (1992) in Table 15.9 show that for whole milk,
at 2.45 GHz and 208C,
"
0
was inversely related to fat content, while
"
00
was
insensitive to fat content. Thus, fat, an additional non-ionic component in
whole milk, depresses the permittivity further.
Nunes et al. (2006) measured the frequency dependence of the permit-
tivity and loss factor of retail samples of UHT skim, low-fat and whole milk
over the frequency range 1-20 GHz at 208C. At given frequencies, values read
from the
"
0
versus frequency and
"
00
versus frequency spectra presented by
Nunes et al. (2006) are in accord with those shown in Table 15.9.
For all three milk types, permittivity decreased with increasing fre-
quency. The loss factor, on the other hand, exhibited a net increase with
frequency over the frequency range studied, but went through a minimum
near the lower end and a maximum near the higher end. For the whole milk,
permittivity dropped by
57%, and loss factor increased by
100%, between
1 and 20 GHz.
Nunes et al. (2006) also measured the dielectric property spectra of
mixtures of whole milk and deionized distilled water, and of deionized
distilled water itself. They modelled these spectra, and those of the three
UHT milks, using the following theoretical equations:
