back to main pageMicrowave
monitoring of deep frying oils.
S.V. von
Gratowski, V.V. Meriakri, L. J. Pangonis. (Institute of Radio Engineering and
Electronics,
Fried food of a wide
variety is very popular all around the world and its consumption is markedly
increasing every year. An associated problem with the frying of food is that
frying oils undergo changes with repeated use. As oil or fat is heated in the
deep-frying process, both physical and chemical changes occur and various
decomposition products are formed in the oil as a result of frying. Some of
these substances are considered to be harmful to human health.
Due to the
high temperature experienced during the deep-frying process, fats and oils
deteriorate rapidly. The rate of degradation of fried oil and fat depends on
certain parameters such as contact with air, temperature and duration of heating,
degree of oil unsaturation, and presence of pro-oxidants or anti-oxidants. In
addition to these factors, concern also exists with the type of fat and food
being fried. Thus it is not possible to suggest a general period of safe usage
for deep frying oils. Determination of frying oil quality depends on many
factors and must be analyzed during frying sessions. Nowadays production of
healthy foods, as well as foods with improved characteristics is very
important. Control of deterioration
through monitoring of deep frying fats
and oils is essential.
Factors Influencing
Frying Oils Analysis.
The 3rd International
Symposium on Deep-Fat Frying held in 2000 at Hagen, Westphalia, Germany worked out recommendations for frying
oils analysis that were the topics for further consideration at the 4th
International Symposium on Deep-Fat Frying held during January 11 – 13, 2004 at
the same location.(netlink: www.dgfett.de/material/recomm.htm).
It was recommended that
analyses should consider:
· Principle
quality index for deep-fat frying to be sensory parameters of the food being
fried
· Analysis of
suspect frying fats and oils to utilize two tests to confirm presence of
deterioration products with upper limits of Total Polar Materials at 24%, and
Polymeric Materials at 12%
· Use of rapid
tests for monitoring of oil quality that would reveal characteristics
that would
o
correlate with internationally recognized
standard methods
o
show ease in application
o
provide for safe use in food processing
o
give quantification of oil degradation
o
have field rugged properties
o
be independent of type of food and fat
o
be applicable over a wide temperature range
o
be insensitive to water content
Conventional analytical methods for
determination of the degradation of deep-frying fats and oils are highly
time-consuming and labor-intensive. In addition the possible use of large
volumes of solvents is considered as a potential environmental problem. Several
rapid methods have been developed based on physical parameters (viscosity,
dielectric changes) and on chemical parameters (free fatty acids, polymerized triglycerides,
etc.).
One of the best known method for
rapid testing is based on dielectric
changes. Currently all dielectric tests are conducted at one very low or zero
frequency. It is known
that the dielectric constant is related to the level of polar components in a
frying oil. Frying is a dehydration process. When food is fried, water is
pumped out of the food into the surrounding oil. Water is one of the most polar substances, and
water play an important role in deep frying. The comparison between laboratory analysis
methods and dielectric quick testings pointed out that moisture and particles
in the oil samples can affect readings.
Other known disadvantages of the
dielectric method are:
These disadvantages are the reason
to search for a new generation of rapid methods for the analysis of deep frying
oils. One such possible alternative method is microwave and millimeter wave
spectroscopy.
Microwave and millimeter wave spectroscopy
Microwave
frequency range involves frequencies f approximately from 200MHz up to
30GHz, whereas millimeter wave frequency range involves frequencies f from
30 up to 300 GHz. The wavelength in air for the electromagnetic wave with the
frequency f is c/f , where c is velocity of light
in air.
As electromagnetic wave
passes through the sample it causes an alternating polarization/depolarization
of dielectric material. The polarization from one side reduces the wavelength
of electromagnetic wave in substances in comparison with its
wavelength in air and from the other side
causes loss of energy due to friction and therefore reduction of the wave
magnitude. These changes of electromagnetic wave in the media are connected
with the dielectric properties of the media and can be expressed with help of a
complex dielectric permittivity ε=ε’+iε’’. Here ε’ is a
measure of polarization and ε’’ is the measure of losses, i =
√-1 . Measurement of ε gives the possibility of
determining substances. (Dielectric permittivity is the same as dielectric
constant and is the degree to which a medium resists the flow of electric
charges).
Every substance perturbed by electromagnetic wave approaches equilibrium. Dielectric relaxation of polarization/depolarization is a mechanism by which a substance approaches equilibrium. To reach the equilibrium state also takes time that is named dielectric relaxation time. All substances can be divided in two large groups: polar and nonpolar substances. Fresh oils are nonpolar; but decomposition products of deteriorated oils are polar. All polar substances and polar groups have their own relaxation times. Complex dielectric permittivity depends on frequency. For many polar substances the frequency dependences of e ¢¢ have one or more maxima. Near such maxima the frequency dependences of e¢ and e¢¢ are the most definite. These frequency ranges are named “ranges of dispersion of dielectric relaxation”. Within such frequency ranges the sensitivity of microwave measurements to polar substances content is maximal.
Ranges of dispersion of dielectric relaxation for some media are shown in Figure 1. The distinction between a polar substance, that has dispersion of dielectric relaxation, and all other substances, is also maximal in such frequency ranges. Measurements of complex dielectric permittivity in ranges of dispersion of dielectric relaxation allow the determinations of polar substances contents very accurately.
In contrast to polar substances nonpolar substances
such as fresh oils have no such ranges with sharp frequency dependence of complex dielectric
permittivity. Thus
influence of nonpolar fresh oils can be excluded from experimental results.
Millimeter wave spectroscopy also
has particular features. The polarity of many nonpolar materials reaches their
asymptotic values in the millimeter wave frequency range. These asymptotic
values are practically the same for all nonpolar substances. That is why in the
millimeter wave frequency range the changes in dielectric properties don’t depend on
the type of fat or oil. Thus calibration with reference substances is not necessary. In the millimeter
wave range dielectric properties practically don’t depend on the presence of
salt, metals and other conducting impurities. So millimeter wave spectroscopy
provides the possibility for analysis without the need for filtration to provide accurate
results that are not dependent on small content of salt, metals and other conducting
impurities present in used oil. Water content, however, has a great influence on dielectric properties
in the millimeter wave range. But this effect is very well known, and in the
proposed method the water content must be measured. (?by what method) Following
its quantification the influence of water can be eliminated from the measured
results.
All these features of microwave and
millimeter wave spectroscopy have allowed us to develop the method for
selective determination of Total Polar Materials in oils. This method is
The
devices, based on this method are compact, can work on line, in situ, in flow, without any chemical
reagents. Thus it is not necessary
to cool the frying oil before measuring. With the usage of additional
measurement it is also possible to determinate content of polymers. This quick
method meets all requirements and has none of the disadvantages of other
methods presently being used. A typical laboratory set up is shown in Figure 2.
One of important advantages of
elaborated method against current dielectric methods is possibility to
eliminate water, salt, metals and other conducting impurities. Water in used oils exists as a
water-in-oil emulsion. Evaporation temperature of such emulsions is higher as
100°C, that has pure water. During deep frying oils have temperature
130÷170°C, and some amount of water stays in oils. Dielectric permitivity of water is
much more as one of Total Polar Materials. Thus small amount of water gives considerable
contribution in common low frequency’s dielectric permitivity. For
example, 1% of water can contribute about 1,5% to the low frequency’s dielectric permitivity. Percentage
contribution of conducting
impurities in low
frequency’s dielectric permitivity is 80%÷10% for frequencies
1MHz÷8GHz. Thus 0,1% of conducting impurities can increase dielectric permitivity up to
2,2%. Then both disturbance factors can contribute up to 4% in low frequency’s dielectric
permitivity, although neither water nor conducting impurities are not harmful factors of used oils. In this case real harmful factors
of oil’s quality will
be about 20% and the used oil can be further exploited during up
to 10 hours. Thus new measurement method can diminish economical losses up to
15%. Some parts of elaborated sensors
are the same that are used for telecommunication. One part must be produced separately.
The serial production price (in
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