Note: Descriptions are shown in the official language in which they were submitted.
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VARIABLE FREQUENCY AUTOMATED CAPACITIVE RADIO
FREQUENCY (RF) DIELECTRIC HEATING SYSTEM
Background and Summary
This invention pertains to methods and apparatuses for the capacitive
radio frequency (RF) dielectric heating of food and biological products.
A variety of different methods are available for the thermal
processing of various materials. Heat is supplied by hot water, steam,
resistive
heating elements, burners, torches, ovens, electrical conduction (ohmic
heating),
induction heating (magnetic), capacitive heating (dielectric), and
electromagnetic
radiative heating (resonant ovens, cavities or chambers) and many other
heating
methods. Applications include sterilization, pasteurization, thawing, melting,
curing, drying, bonding (e.g., laminates), welding, brazing, heating for
chemical
reactions, and many others. Heated materials include ceramics, rubber,
plastics (and
other polymers), composites, metals, soils, wood and many types of biological
materials including food.
An important application of heating technologies is in the area of the
pasteurization and sterilization of foods, particularly foods in large-
dimensioned
packages. Food safety and quality is becoming an increasingly important topic
with
the many incidents where people have become sick or died due to unkilled
microbial
populations in food. For example, alfalfa and radish seeds are raw
agricultural
commodities that can become contaminated with organic material that harbor
pathogens such as Salmonella or E. coli 0157:H7 during growing and harvest.
Seed
processing and storage procedures are aimed at reducing varietal contamination
of
seeds through the elimination of weed seeds and foreign matter. Such seed
cleaning
and certification programs insure varietal purity, but provide no means of
food
safety intervention for seeds destined for sprouting and consumption as food.
As a
result, there are increasing reports of microbial outbreaks in sprouted seed
products
such as radish and alfalfa sprouts. Human salmonellosis (due to Salmonella
bacteria) and outbreaks of E. coli 0157:H7 have been associated with the
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consumption of alfalfa and radish sprouts in several countries. Alfalfa and
radish
sprouts, a definitive highly nutritious and perceived healthy food, have been
implicated in multi-site outbreaks of food-borne illnesses. Seeds were linked
to
about 150 confirmed cases of salmonellosis in Oregon and British Columbia in
1996. Also in 1996, radish sprouts were associated with Japan's largest
recorded
outbreak of E. coli 0157:H7 infection with an estimated 11,000 cases that led
to
eleven deaths. In June and July 1997, simultaneous outbreaks of E. coli
0157:H7
infections in Michigan and Virginia were independently associated with eating
alfalfa sprouts grown from the same seed lot. A total of 60 people with E.
coli
0157:H7 infection were reported to the Michigan Department of Community Health
and 48 cases reported to the Virginia Department of Health. Recently, the
California
Department of Health Services identified six cases of E. coli 0157 :NM with
illness
onsets from June 16 through June 27, 1998, caused by eating an alfalfa-clover
sprout
mixture.
The lack of standardization in some heating time/temperature
relationships that are required to ensure food product safety is also
attracting more
focus. In addition, food quality or taste/texture issues are important in our
selective
consumer oriented society. Therefore there is a need for a heating technology
that
will achieve the desired microbial kill rates uniformly over that whole food
product
in a reasonable amount of time with a minimum altering of the overall quality
of the
food.
In the seafood industry, for example, existing heating technologies
for the pasteurization of seafoods employ either hot water or steam. These
technologies have several limitations including reliance on thermal conduction
from
the product surface (resulting in non-uniform heating), slow heating rates
(especially
in the product center), large floor space requirements, poor overall energy
efficiency, generation of large amounts of waste water and limitations on the
product
geometry (i.e., need to be thin or flat).
Capacitive radio frequency (RF) dielectric heating is used in several
industries. They include the drying of various wood and sawdust products in
the
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timber industry, preheating and final drying of paper, drying of textiles,
drying of
glass fibers and spools, drying water-based glues in the paper-cardboard
industry,
drying pharmaceutical products, welding plastics, sealing, preheating plastics
prior
to forming, firing foundry cores in casting, polymerization of fiber panels,
gluing of
woods such as laminated plywood, printing and marking in the textile,
leatherware
and shoe industries, melting honey, heating rubber prior to vulcanization,
welding
glass formed sections, bonding multi-layer glass products, drying of powders,
drying
leathers and hides, curing of epoxy, curing of plastisol, curing of brake
linings,
impregnating resins, thermosetting adhesives, curing hardboard and particle
board,
and many other applications.
The use of capacitive (RF) dielectric heating methods for the
pasteurization and sterilization of foods offer several advantages over non-
electromagnetic heating methods. These include rapid heating, near
independence
of the thermal conductivity of the medium (i.e., heat internal portions of
medium
directly), high energy efficiency, good heating even in the absence of DC
electrical
conductivity, high energy densities, reduced production floor space, and easy
adaptation to automated production batch and/or continuous flow processing.
Because capacitive (RF) dielectric heating is rapid, the food product being
heated
loses less moisture than in conventional heating processes, which is
advantageous.
Another application of this technology is in the thawing of frozen
foods. Common thawing applications again rely on the thermal conduction of
heat
from the surface to the interior to provide thawing. Due to freshness and
product
quality constraints thawing often is done by immersion in water baths that are
only
slightly above freezing themselves or in refrigerators set to slightly above
freezing
(e.g., 35-40 F). Thawing times are often very long. With capacitive heating
technologies that heat over the entire volume uniformly, thawing can be
performed
much more rapidly.
Capacitive (RF) dielectric heating differs from higher frequency
electromagnetic radiative dielectric heating (e.g., microwave ovens) in that
with
capacitive heating the wavelength of the chosen frequency is large compared to
the
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dimensions of the sample being heated whereas with electromagnetic radiative
heating the wavelength is comparable or even small compared to the dimensions
of
the sample being heated. An example of capacitive heating is two large
parallel
electrodes placed on opposite sides of a wood sample with an AC displacement
current flowing through it to heat and dry the wood. An example of
electromagnetic
radiative heating is a metal chamber with resonant electromagnetic standing
wave
modes such as a microwave oven. Capacitive heating also differs from lower
frequency ohmic heating in that capacitive heating depends on dielectric
losses and
ohmic heating relies on direct ohmic conduction losses in a medium and
requires the
electrodes to contact the medium directly (i.e., cannot penetrate a plastic
package or
air gap). (In some applications, capacitive and ohmic heating are used
together.)
Capacitive (RF) dielectric heating methods offer advantages over
other electromagnetic heating methods. For example, capacitive (RF) dielectric
heating methods offer more uniform heating over the sample geometry than
higher
frequency radiative dielectric heating methods (e.g., microwave ovens) due to
superior or deeper wave penetration into the sample as well as simple uniform
field
patterns (as opposed to the complex non-uniform standing wave patterns in a
microwave oven). In addition capacitive (RF) dielectric heating methods
operate at
frequencies low enough to use standard power grid tubes that are both lower
cost
(for a given power level) as well as allow for generally much higher power
generation levels than microwave tubes.
Capacitive (RF) dielectric heating methods also offer advantages over
low frequency ohmic heating. These include the ability to heat a medium that
is
enclosed inside an insulating plastic package and perhaps surrounded by an air
or
de-ionized water barrier (i.e., the electrodes do not have to contact the
media
directly). The performance of capacitive heating is therefore also less
dependent on
the product making a smooth contact with the electrodes. Capacitive (RF)
dielectric
heating methods are not dependent on the presence of DC electrical
conductivity and
can heat insulators as long as they contain polar dielectric molecules that
can
partially rotate and create dielectric losses. A typical existing design for a
capacitive
CA 02720494 2013-03-20
dielectric heating system is described in Orfeuil, M. 1987. Electric Process
Heating:
Technologies/Equipment/Applications. Columbus: Battelle Press.
Capacitive (RF) heating devices have been used in the food industry,
but the reported energy efficiency has been low and heating has not always
been
5 uniform. Proctor Strayfield has developed a magnatube pasteurization
system
(Koral, A.L., 1990. Proctor-Strayfield Magnatube Radio Frequency Tube Heating
System. Proctor Strayfield, A Division of Proctor & Schwartz, Inc.) that has
been
demonstrated to be successful in the cooking/sterilization of scrambled eggs
as well
as in the creation of a "skinless" meatloaf from a pumped slurry using a
vertical tube
system. Houben et. al of the Netherlands (Houben, J., Schoenmakers, L., van
Putten, E., van Roon, P. and Krol, B. 1991. Radio-frequency pasteurization of
sausage emulsions as a continuous process. J. Microwave Power &
Electromagnetic
Energy. 26(4): 202-205.) in 1991 showed that sausage emulsions could be
successfully pasteurized using RF heating. Bengtsson et al of Sweden
(Bengtsson,
N.E., and W. Green. 1970. Radio-Frequency Pasteurization of Cured Hams.
Journal of Food Science. V35: 681-687) in 1970 demonstrated that cured hams
could be pasteurized successfully by RF heating. RF heating feasibility
experiments
were conducted on packaged and unpackaged surimi seafood samples at a test
facility of PSC, Inc. of Cleveland, Ohio. The test system was a high power
single-
frequency capacitive heater set at 18 MHz. Tests on samples placed between
parallel electrodes showed that when properly oriented, surimi seafoods could
be
heated to pasteurization temperatures (85 C) in less than 10 minutes. The
results
also showed, however, that packaging can be a complicating factor. For
example,
small amounts of food trapped in the packaging seams can cause rapid local
heating
and burning.
Some prior work in the area of dielectric heating has been conducted
on seed germination enhancement effects. The possibility of utilizing
dielectric
energy for stimulating or improving the germination and growth of seeds and
for
controlling insects has been variously considered for the last forty years
(Nelson,
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S.O. and Walker, E.R. 1961, "Effects of radio-frequency electrical seed
treatment,"
Agricul. Eng. 42(12): 688-691; Nelson, S.O. 1976, "Use of microwave and lower
power frequency RF energy for improving alfalfa seed germination," J.
Microwave
Power 12(1):67-72; Nelson, S.O. 1996, "Review and assessment of radio-
frequency
and microwave energy for stored-grain insect control," Transactions of the
ASAE.
39(4):1475-1484.). Reported effects ranged from accelerated germination and
early
growth and the killing of fusarium spores to early flowering and high yields
of
plants grown from treated seeds. Nelson and Walker (1961) reported that brief
exposure of alfalfa containing considerable hard seed shells to electrical
fields has
been highly successful in reducing the hard-seed percentages and producing a
corresponding increase in normal seedling germination. Also, benefits from
electric
treatment have been shown in alfalfa seeds for up to 21 years in storage with
no
evidence of any short or long term detrimental effects (Nelson, 1961, 1976).
Nelson
(1976) found that the moisture content of seeds at the time of treatment
influenced
the degree of response. Generally seeds of lower moisture content responded
more
favorably to treatment than high moisture content seeds. The final temperature
of
seeds treated at any given moisture content seemed to be a good indicator of
the
degree of favorable response. Some work has been done using higher frequency
microwave heating for the treatment of seeds. Cavalcante et al. (Cavalcante,
M.J.B.
and Muchovej, J.J. 1993, "Microwave irradiation of seeds and selected fungal
spores," Seed Sci. & Technol. 21:247-253) investigated the use of microwave
irradiation on seeds and its effects on the control of selected fungal spores.
Some work has been done to characterize the dielectric properties of
food and packaging materials. There is preliminary data at lower frequencies
for
polymers to show temperature-dependent Debye resonance effects (Malik, T.M.,
R.E. Prud'Homme. 1984. Dielectric Properties of Poly(a-Methyl-a-N-Propyl-P-
Propiolactone)/Poly(Vinyl Chloride) Blends. Polymer Engineering and Science.
v24, n2 p144-152; Scarpa, P.C.N., Svatik, A. and Das-Gupta, D.K. 1996.
Dielectric
spectroscopy of polyethylene in the frequency range of 10-5 Hz to 106 Hz.
Polymer
Eng. & Sci. 36(8): 1072-1080). And, for food in the medium frequency ranges,
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limited tabulated data exists (Von Hippel, A.R., 1954. Dielectric Materials
and
Applications. MIT Press; Kent, M. 1987. Electrical and Dielectric properties
of food
materials. Science and Technology Publishers, England; Mudgett, R.E. 1985.
Electrical Properties of Foods. In Microwaves in the Food Processing Industry,
R.V. Decareau (Ed.). New York: Academic Press; Pethig, R. 1979. Dielectric and
Electronic Properties of Biological Materials. Chichester: John Wiley & Sons,
Inc.;
Tinga, W.R. and S.O. Nelson. 1973. Dielectric Properties of Materials for
Microwave Processing-Tabulated. I of Microwave Power. 8:1-65; Tran, V.N. and
Stuchly, S.S. 1987. Dielectric properties of beef, beef liver, chicken and
salmon at
frequencies from 100 to 2500 MHz. J. Microwave Power. 29-33). Most data for
food has been collected at higher frequencies (> 100 MHz) and tied closely to
the
dielectric behavior of the water in the medium, for applications toward
microwave
ovens.
A specific disadvantage of capacitive (RF) dielectric heating methods
is the potential for thermal runaway or hot spots in a heterogeneous medium
since
the dielectric losses are often strong functions of temperature (e.g., small
pockets of
a lossy dielectric food material, for example a small thermal mass trapped in
the
seams of a package, may heat rapidly and could bum itself and the melt the
package). Another disadvantage of capacitive heating is the potential for
dielectric
breakdown (arcing) if the electric field strengths are too high across the
sample
(making sample thicker and reducing air gaps allows operation at a lower
voltage).
The use of edible films to extend shelf life of food products and
protect them from harmful environmental effects has been emphasized in the
recent
years. Interests and research activities in edible films have been especially
intense
over the past ten years. Edible films are very promising systems for the
future
improvement of food quality and preservation during processes and storage.
Indeed,
edible films can be used where plastic packaging cannot be applied. For
example,
they can separate several compartments within a food. Although edible films
are not
meant to totally replace synthetic films, they do have the potential to reduce
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packaging and to limit moisture, aroma, and lipid migration between food
components where traditional packaging cannot function.
An edible film is defined as a thin layer of one or more edible
materials formed on a food as a coating or placed (pre-formed) on or between
food
components. Most edible films are natural polymers obtained from agricultural
products such as animal and vegetable proteins, gums, and lipid and are
perfectly
biodegradable and usually water soluble. The general materials that are used
to
manufacture edible films are cellulose ethers, starch, corn zein, wheat
gluten, soy
proteins and milk protein. Examples include methyl cellulose (MC),
hydroxypropyl
cellulose (HPC), sodium and calcium caseinates (SC or CC), and whey protein
concentrates (WPC).
The performance of edible packaging is comparable to that of
traditional synthetic polymer films with respect to mechanical strength,
barrier
properties, and compatibility. Applications of edible packaging include its
use in
inhibiting migration of moisture, oxygen, carbon dioxide, aromas, and lipids,
etc.
within composite foods; carrying food ingredients (e.g., antioxidants,
antimicrobials,
flavor); and/or improving mechanical integrity or handling characteristics of
the
foods.
Moisture transport through polymer films is influenced by several
polymer properties including chemical structure, method of polymer
preparation,
polymer processing condition, free volume, density, crystallinity, polarity,
tacticity,
crosslinking and grafting, orientation, presence of additives, and use of
polymer
blends. An increase in crystallinity, density, orientation, molecular weight
or
crosslinking results in decreased permeability of edible films.
Although capacitive (RF) dielectric heating systems have been used
for heating foods in the past, there remains a need for improved methods and
apparatuses to rapidly, efficiently and uniformly heat food products or parts
of food
products.
It has now been discovered that certain capacitive (RF) dielectric
heating devices and/or methods can be used to rapidly, efficiently and/or
uniformly
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heat food products, including conventional foods and seeds, as well as any
related
packaging, for safe pasteurization, sterilization and/or thawing.
Brief Description of the Drawings
In the drawings:
FIG. 1 is a schematic diagram of an existing capacitive (RF)
dielectric heating system;
FIGS. 2A, 2B and 2C are equivalent circuit diagrams of the dielectric
heating system of FIG. 1 for different types of food products;
FIG. 3 is a block diagram of the dielectric heating system of FIG. 1;
FIG. 4 is a block diagram showing the high power RF signal
generation section of the dielectric heating system of FIG. 3 in greater
detail; and
FIG. 5 is a block diagram of a capacitive (RF) dielectric heating
system;
FIG. 6 is a top plan view of a grid electrode, which may be used in
the systems of FIGS. 5 and 9;
FIG. 7 is a sectional view taken along line 7-7 of FIG. 6;
FIGS. 8A-8D are block diagrams of four manufacturing process
flows which benefit from use of a dielectric heating system;
FIG. 9 is a block diagram similar to FIG. 5, except showing an
alternative embodiment of a capacitive (RF) dielectric heating system; and
FIGS. 10 and 11 are flow charts illustrating steps of impedance
matching methods for use in capacitive (RF) dielectric heating systems.
FIGS. 12 and 13 are tables showing heating and germination rates for
the seeds of first and second tests, respectively, described in Example 6.
FIG. 14 is a table showing microbial test results for the seeds of
Example 6.
FIGS. 15A and 15B are schematic diagrams showing the container
configuration and relative electrode position for the first and second tests,
respectively, of Example 6.
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FIG. 16 is a temperature vs. time profile showing results of a
capacitive dielectric (RF) heating process described in Example 5.
FIG. 17 is a plot of temperature vs. time showing heating uniformity
for radish seeds in the first tests of Example 6.
5 FIG. 18 is a plot of temperature vs. time showing the heating rate
for
alfalfa seeds in the second tests of Example 6.
FIG. 19 is a plot of temperature vs. time showing the temperature
distribution for alfalfa seeds in the second tests of Example 6 relative to
their
position within the container.
10 FIG. 20 is a temperature vs. time plot for surimi gels as
described in
Example 7.
FIG. 21 is a heating rate vs. time plot for surimi gels as described in
Example 7.
FIG. 22 is a plot of heating rate vs. time for surimi gels similar to
FIG. 21 but showing improved uniformity in heating rates due to better
impedance
match.
Detailed Description
FIGS. 1-4 show an example of a known capacitive (RF) dielectric
heating system. A high voltage RF frequency sinusoidal AC signal is applied to
a
set of parallel electrodes 20, 22 on opposite sides of a dielectric medium 24
as
shown in FIG. 1. The medium 24 to be heated is sandwiched or placed between
the
electrodes 20, 22, in an area defined as the product treatment zone so that an
AC
displacement current flows through the medium 24 as a result of polar
molecules in
the medium aligning and rotating in opposite fashion to the applied AC
electric
field. Direct conduction does not occur but instead an effective AC current
flows
through the capacitor due to polar molecules with effective charges rotating
back
and forth. Heating occurs because these polar molecules encounter interactions
with
neighboring molecules resulting in lattice and frictional losses as they
rotate.
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The resultant electrical equivalent circuit of the device of FIG. 1 is
therefore a capacitor in parallel with a resistor as shown in FIG. 2A. There
is an in-
phase IR component and an out-of-phase lc component of the current relative to
the
applied RF voltage, and the in-phase component IR corresponds to the resistive
voltage loss. These losses get higher as the frequency of the applied signal
is
increased for a fixed electric field intensity or voltage gradient due to
higher speed
interactions with the neighboring molecules. The higher the frequency of the
alternating field, the greater the energy imparted into the medium 24 until
the
frequency is so high that the rotating molecules can no longer keep up with
the
external field due to lattice limitations.
This frequency, which is referred to as a "Debye resonance
frequency" after the mathematician who modelled it, represents the frequency
at
which lattice limitations occur, and is the frequency at which the maximum
energy
can be imparted into a medium for a given electric field strength (and
therefore the
maximum heating). This high frequency limitation is inversely proportional to
the
complexity of the polar molecule. For example, proteins with amino acid polar
side
groups or chains have a slower rotation limitation, and thus lower Debye
resonance,
than simple polar water molecules. These Debye resonance frequencies also
shift
with temperature as the medium is heated.
Some media may be represented by different resultant electrical
equivalent circuits than the circuit shown in FIG. 2A. Media of interest in
this
application are food products, which are defined herein to include
conventional
foods, agricultural products from which foods are derived (e.g., seeds for
sprouts), as
well as other edible substances (e.g., edible films used to package seeds).
For
example, with surimi seafoods, which generally have a high moisture content
(approximately 74-84%) and a high salt content (approximately 1-4%), the
resultant
electrical circuit is simply a resistor (FIG. 2B), because the ohmic
properties
dominate. For seeds, however, which have a much lower moisture content
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(approximately 3-6%), the resultant electrical circuit is a capactitor in
series with a
resistor (FIG. 2C).
Various other foods and food-type products may have different
electrical circuit analogs. More complex models having serial and parallel
aspects in
combination to address second order effects are possible. Any of the
components in
any of the models may have temperature and frequency dependence.
An example of a conventional RF heating system is shown in FIGS. 3
and 4. In this system, a high voltage transformer/rectifier combination
provides a
high rectified positive voltage (5kV to 15kV) to the anode of a standard
triode power
oscillator tube. A tuned circuit (parallel inductor and capacitor tank
circuit) is
connected between the anode and grounded cathode of such tube as shown in FIG.
4, and also is part of a positive feedback circuit inductively coupled from
the
cathode to the grid of the tube to enable oscillation thereby generating the
RF signal.
This RF signal generator circuit output then goes to the combined capacitive
dielectric and resistive/ohmic heating load through an adapter network
consisting of
a coupling circuit and a matching system to match the impedance of the load
and
maximize heating power delivery to the load, as shown in FIG. 3. An applicator
includes an electrode system that delivers the RF energy to the medium 24 to
be
heated, as shown in FIG 1.
The known system of FIGS. 1-4 can only operate over a narrow band
and only at a fixed frequency, typically as specified by existing ISM
(Industrial,
Scientific, Medical) bands. Such a narrow operating band does not allow for
tuning
of the impedance. Any adjustment to the system parameters must be made
manually
and while the system is not operating. Also, the selected frequency can drift.
Therefore, to the extent that the known system provides any control, such
control is
not precise, robust, real time or automatic.
New capacitive (RF) dielectric heating methods and systems
described below provide improved overall performance and allow for more
precise
and robust control of the heating processes. With the new methods and systems,
specific dielectric properties of food products are determined and/or used in
the
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process, either directly as process control parameters or indirectly as by
reference to
a model used in the process that includes relationships based on the
properties. New
ways of using capacitive (RF) dielectric heating in the various phases of
processing,
packaging and preparing foods are disclosed. Two exemplary approaches are
described in detail.
According to the first approach described in connection with the
system shown in FIG. 5, a variable frequency RF waveform is generated, and the
waveform is output to an amplifier and an impedance matching network to
generate
an electric field to heat the food. Based on at least the measured temperature
of the
food and one or more specific dielectric or ohmic properties of the food, the
system
is controlled to provide optimum heating. Multiple frequency power waveforms
can
be applied simultaneously.
According to the second approach, which is described primarily in
connection with the system of FIG. 9, enhanced feedback provides for automatic
impedance matching. By matching the impedance, maximum power is supplied to
the load, and the maximum heating rate is achieved. In general, achieving the
highest possible heating rate is desirable because higher heating rates tend
to
damage the food less (e.g., prolonged heating reduces moisture content in
food). If
impedance mismatch occurs, the rate at which the food product is heated
decreases.
Specific implementations of each approach are discussed below,
following sections on the characterization and monitoring of dielectric
properties
and impedance matching.
Characterization, Monitoring and Modeling
Characterization of dielectric properties vs. frequency and
temperature assists in the design of a capacitive (RF) dielectric heating
system to
pasteurize, sterilize or thaw various foods by some methods of the present
invention.
It is usually desired to heat food, which may include pre-cooked food, without
any
objectionable degradation in quality, texture or taste. Thus, to aid in the
selection of
appropriate operating conditions, food samples are studied to assess the
effects of
RF energy on key properties of the food samples at various frequencies and
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temperatures. The results of these studies influence the design of capacitive
dielectric heating systems.
In addition, a capacitive dielectric heater may be called on to heat a
food product contained in packaging materials. Some packaging materials will
degrade if overheated, whereas other packaging materials could be heated
intentionally to transfer heat energy to a food product contained in the
packaging.
Therefore, the characterization of the dielectric properties of packaging
materials
and the effects of RF energy on those materials may be important in choosing
the
proper packaging materials and/or operating frequencies.
An electromagnetic/heat transfer mathematical model can be used to
predict the dielectric heating characteristics of various foods and packaging
materials. Such a model may involve 2-D and/or 3-D mathematical modeling
programs as well as finite element methodologies to model composite materials.
Best results are achieved with a model that integrates both electromagnetic
and heat
transfer principles.
To supply the AC displacement current at a needed frequency,
variable components of the tunable RF signal generator circuit and associated
matching networks are actively tuned to change frequency, or tuned
automatically,
or switched with a control system. Therefore, a software control system is
also
provided to set up the frequency profile. A variable frequency synthesizer or
generator and a broadband power amplifier and associated matching systems and
electrodes are useful components of such a capacitive dielectric heating
system. In
some implementations, temperature monitoring of the heated medium using
thermal
sensors or infrared scanners is conducted and data fed back into the control
system
and the generator's frequency groups are swept accordingly to track a
parameter of
interest, such as Debye resonances (explained below) or other dielectric
property, or
other temperature dependent parameters.
The key electromagnetic parameters of a medium to be tested are
defined as follows:
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a = Electrical Conductivity (S/m) E = RMS Electric Field Intensity
(V/m)
E = Electric Permittivity (F/m) H = RMS Magnetic Field Intensity
(A/m)
5 t = Magnetic Permeability (H/m) B = Magnetic
Flux Density (W/m2)
The Permittivity and permeability can be divided into loss terms as follows:
Et j611
(1)
10 (2)
where
c'= Energy Storage Term of the Permittivity
Loss Term of the Permittivity
15 Energy Storage Term of the Permeability
Loss Term of the Permeability
When analyzing the experimental data, the magnetic losses can be
assumed equal to zero and for the most part frequency can be assumed high
enough
that the dielectric loss factor E" dominates over losses due to electrical
conductivity
a (i.e., where o">> a, with angular frequency co = 27cf, f being the frequency
measured in Hz). (For surimi seafoods, however, testing has shown that
electrical
conductivity a dominates over dielectric loss in the 100 kHz to 300 MHz
range.)
The electrical conductivity a is measured and accounted for where needed
(mainly
at the lower end of the frequency range). With those assumptions in mind, the
expressions for equivalent capacitance and equivalent resistance in FIG. 2
reduce to
the following:
(3)
R =
(4)
where S is the exposed area of the plates and d is the plate separation
between
electrodes.
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As mentioned above, capacitive heating systems according to the
present invention operate at frequencies in the Medium Frequency (MF: 300 kHz -
3
MHz) and/or High Frequency (HF: 3 MHz - 30 MHz) bands, and sometimes stretch
into the lower portions of the Very High Frequency (VHF: 30 MHz - 300 MHz)
band. The frequency is generally low enough that the assumption can be made
that
the wavelength of operation is much larger than the dimensions of the food
medium,
thus resulting in highly uniform parallel electric field lines of force across
the food
medium.
Impedance Matching
Electrical "impedance" is a measure of the total opposition that a
circuit or a part of a circuit presents to electric current for a given
applied electrical
voltage, and includes both resistance and reactance. The resistance component
arises from collisions of the current-carrying charged particles with the
internal
structure of a conductor. The reactance component is an additional opposition
to the
movement of electric charge that arises from the changing electric and
magnetic
fields in circuits carrying alternating current. With a steady direct current,
impedance reduces to resistance.
As used herein, "input" impedance is defined as the impedance .
"looking into" the input of a particular component or components, whereas
"output"
impedance is defined as the impedance "looking back into" the output of the
component or components.
The heating load, or, more formally, the "actual load," is the
combination of the medium (i.e., the food product and any packaging) and the
surrounding structure, e.g., the capacitive electrodes and any electrode
enclosure that
may be present. Thus, as used herein, the "actual load impedance" is the input
impedance looking into the actual load. The impedance of the medium is
influenced
by its ohmic and dielectric properties, which may be temperature dependent.
Thus,
the actual load impedance typically changes over time during the heating
process
because the impedance of the medium varies as the temperature changes.
CA 02720494 2010-11-05
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The "effective adjusted- load impedance, which is also an input
impedance, is the actual load impedance modified by any "impedance
adjustments."
In specific implementations, "impedance adjustments" include the input
impedance
of a tunable impedance matching network coupled to the load and/or the input
impedance of a coupling network coupled to the structure surrounding the load
(e.g.,
the electrodes and/or enclosure, if present). In these implementations, the
"effective
load" includes the impedance load of any impedance adjusting structures and
the
actual load. Other impedance adjustments that may assist in matching the
effective
adjusted load impedance to the signal generating unit output impedance may
also be
possible. The effective load impedance is the parameter of interest in the
present
impedance matching approach.
The signal generating unit, as used herein, refers to the component or
components that generate the power waveform, amplify it (if necessary) and
supply
it to the load. In specific implementations, the signal generating unit
includes a
signal generator, an amplifier that amplifies the signal generator output and
conductors, e.g. a coaxial cable, through which the amplified signal generator
output
is provided to the load.
The signal generating unit's impedance that is of interest is its output
impedance. In specific implementations, the signal generating unit output
impedance is substantially constant within the operating frequency range and
is not
controlled. Both the input impedance and the output impedance of the power
amplifier, as well as the signal generator out impedance and the conductor
characteristic impedance are substantially close to 50 ohms. As a result,
output
impedance of the signal generating unit is also substantially close to 50
ohms.
Thus, in specific implementations, matching the effective adjusted
load impedance to the signal generating unit output impedance reduces to
adjusting
the effective adjusted load impedance such that it "matches" 50 ohms.
Depending
upon the circumstances, a suitable impedance match is achieved where the
effective
adjusted load impedance can be controlled to be within 25 to 100 ohms, which
translates to nearly 90% or more of the power reaching the actual load.
CA 02720494 2010-11-05
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Impedance matching is carried out substantially real-time, with
control of the process taking place based on measurements made during the
process.
Impedance matching can be accomplished according to several different methods.
These methods may be used individually, but more typically are used in
combination
to provide different degrees of impedance adjustment in the overall impedance
matching algorithm:
1. The frequency of the signal generator may be controlled.
In
an automated approach, the signal generator frequency is automatically changed
based on feedback of a measured parameter.
For example, the signal generator frequency may be changed based
on the actual load temperature and predetermined relationships of frequency
vs.
temperature. The frequency may be changed to track Debye resonances as
described
above and/or to maintain an approximate impedance match. Typically, this
serves
as a relatively coarse control algorithm.
For more precise control, aspects of the power waveform supplied to
the effective load can be measured, fed back and used to control the
frequency. For
example, the forward power supplied to the effective load and the reverse
power
reflected from the effective load can be measured, and used in conjunction
with
measurements of the actual voltage and current at the load to control the
frequency.
2. A tunable matching network can be automatically tuned to
adjust the effective load impedance to match the signal generating unit output
impedance. In a first step, series inductance is used in the output portion of
the
impedance matching network to tune out the series capacitive component of the
actual load impedance. The series inductance is set by measuring the initial
capacitive component, which is determined by measuring the voltage and current
at
the actual load and determining their phase difference. It is also possible to
measure
the voltage and current within the matching network and control for a zero
phase
shift.
CA 02720494 2010-11-05
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For more complex models of the load, other models will be
necessary. An alternative approach would be to use a shunt inductor to tune
out a
shunt capacitive load.
Initially, the resulting effective load impedance will be purely
resistive, but will likely differ from the desired 50 ohms level. In a second
step,
additional elements within the matching network are tuned to make the input
impedance of the matching network, which is defined as the effective adjusted
load
impedance for a described implementation, match the desired 50 ohm target. The
second step tuning is controlled based on the measured forward and reflected
power
levels.
3. It is possible to adjust the gap in a capacitive coupling
network positioned at the load. Such adjustments could be made automatically
during the heating process with a servo a motor.
4. It is possible to physically adjust the capacitive electrodes that
are included as a part of the actual load to make minor adjustments to the
actual load
impedance. (Other adjustments are likely more easily controlled.)
Specific implementations that incorporate impedance matching are
discussed in the following two sections.
First Approach
One exemplary system suitable for the first approach, in which at
least the measured temperature of the food being heated is monitored, is shown
in
FIG. 5. The system of FIG. 5 includes a variable RF frequency signal generator
30
with output voltage level control, a broadband linear power amplifier 32, and
a
tunable impedance-matching network 34 (for fixed or variable frequency
operation)
to match the power amplifier output impedance to the load impedance of the
capacitive load 20, 22, 24, that includes the medium 24 being heated.
The system is constructed to provide an AC RF signal displacement
current 36 at an RF frequency in the range of 300 kHz to 300 MHz. This range
includes the MF (300 kHz to 3 MHz), HF (3 MHz to 30 MHz), and VHF (30 MHz
to 300 MHz) frequencies in the lower regions of the radio frequency (RF)
range.
CA 02720494 2010-11-05
In the specific implementation shown in FIG. 5, the variable RF
frequency signal generator 30 is a multi-RF frequency signal generator capable
of
simultaneously generating multiple different frequencies. Although a single
frequency signal generator may be used, the multi-frequency signal generator
is
5 useful for methods in which frequency-dependent dielectric properties of
the food or
foods being heated are monitored and used in controlling the heating process,
such
as is explained in the following section.
Debye Resonance Frequency Implementations
As one example, the energy efficiency and/or heating rate are
10 maximized at or near the location in frequency of a "Debye resonance" of
the
medium. In other specific implementations, dielectric properties other than
Debye
resonances are tracked and used in controlling capacitive (RF) dielectric
heating,
e.g., when Debye resonances are not present or are not pronounced. These other
dielectric properties may be dependent upon frequency and/or temperature,
similar
15 to Debye resonances, but may vary at different rates and to different
extents.
Examples of such other dielectric properties are electrical conductivity and
electrical
permitivity.
In this example, the RF signal frequency is tuned to the optimal
Debye frequency or frequencies of a component or components of the food
product
20 to be heated. Multiple Debye resonances may occur in a composite
material. So,
multiple composite frequency groups can be applied to handle the several Debye
resonances. Also, the RF signal frequencies can be varied with temperature to
track
Debye frequency shifts with changes in temperature.
The RF frequency or composite signal of several RF frequencies is
selected to correlate with the dominant Debye resonance frequency groups of
the
medium 24 that is being heated. These Debye resonances are dependent on the
polar
molecular makeup of the medium 24 and thus are researched for different types
of
food to appropriately program the heating system. The generation system, in
this
case the variable RF frequency signal generator 30, is capable of generating
more
than one frequency simultaneously. The control system for this heating system
is
CA 02720494 2010-11-05
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capable of being set up or calibrated to be optimum for different types of
food or
other media.
The frequency or composite frequency groups of the RF signal used
in the heating system will track with and change with temperature to account
for the
fact that the Debye resonance frequencies of the polar molecular constituents
of the
food or other medium 24 also shift with temperature.
With the most preferred apparatuses, the RF signal power level and
resulting electric field strength can be adjusted automatically by a computer
control
system which changes the load current to control heating rates and account for
different food geometries and packaging types. The power level is controlled
by:
(1) measuring the current and field strength across the actual load with
voltage and
current measurement equipment as indicated at 35 in FIG. 5; and (2) adjusting
the
voltage (AC field strength), which in turn varies the current, until
measurements of
the current and field strength indicate that the desired power level has been
achieved. As shown in FIG. 5, the computer also controls the multi-frequency
RF
signal synthesizer 30 to change its frequency and to adjust the tunable
impedance
matching network 34.
Fig. 10 is a flowchart showing another exemplary heating process
according to the first approach in more detail.
In step 170, the signal generator 30 is set to an initial frequency or
frequencies. For expository convenience, it is assumed in this example that a
single
frequency is set, but the description that follows applies equally to cases
where
multiple frequencies are set.
The set frequency may be selected with reference to a predetermined
frequency or frequency range based on a known relationship between frequency
and
temperature. For example, the set frequency may be selected based on one or
more
Debye resonances of the medium as described above.
In step 172, the temperature at the medium is measured. In step 174,
the measured temperature and set frequency are compared to a predetermined
CA 02720494 2010-11-05
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relationship of frequency and temperature for the medium. The relationship may
be
stored in the computer 38, e.g., in the form of a look-up table.
If the comparison between the set frequency and the predetermined
frequency indicates that the set frequency must be changed (step 176; YES),
the
process advances to step 178, the set frequency is automatically changed by
control
signals sent to the signal generator 30, and step 174 is repeated. If no
change in the
set frequency is required (step 176; NO) the process advances.
As indicated by the dashed line, an automatic impedance matching
process 181 follows step 176. For an exemplary implementation, automatic
impedance matching begins with step 182. In step 182, the magnitude and phase
of
the actual load impedance are measured using the voltage and current
measurement
equipment 35, and the measured values are relayed to the computer 38. In step
184,
the phase angle difference between the measured voltage and current is
determined
to tune out the reactance component of the impedance. One element of
controlling
impedance match is therefore to tune out the capicitive reactance component of
the
actual load resulting in zero phase shift between the voltage and current.
In step 186, the impedance match between the signal generating unit
and the effective load is measured. Optionally, impedance match can be
controlled
through measuring the power waveforms supplied to and reflected from the
effective
load (the "forward and reverse powers") (optional sub-step 188), assuming the
Fig. 5
system is configured to include a measurement instrument 156 and directional
coupler 150 as shown in Fig. 9. (Measurement of the forward and reverse powers
is
described in the following section.)
Following completion of the step 186, the process advances to step
190. In step 190, the effective load impedance is compared to the
predetermined
impedance of the signal generating unit. If the impedance match is not
sufficient,
the process proceeds to step 192. If the impedance match is sufficient, the
process
proceeds to step 194.
In step 192, the effective load impedance is adjusted. In the
implementation of Fig. 5, the effective load impedance is adjusted by
automatically
CA 02720494 2010-11-05
23
tuning the tunable impedance matching network 34 based on control signals sent
from the computer 38 (step 193). Following step 192, the process returns to
step
186.
In step 194, the measured temperature is compared to a desired final
temperature. If the measured temperature equals or exceeds the desired final
temperature, the heating process in completed (step 196). Otherwise, heating
is
continued and the process returns to step 172.
Heating to pasteurization and sterilization temperatures can be rapid,
with increased rates resulting in decreased degradation of food quality (e.g.,
protease
enzyme in seafoods inactivated). The rapid heating capability is due to the
same
uniform heating advantage described above and the maximum power input to the
heated load by the matching of generator frequency or composite of frequencies
to
the Debye resonance frequency groups of the various food products and/or
packaging, and tracking those Debye resonance frequency groups with
temperature.
Power control capability of the generator/heating system allows for the
ability to set
heating rates to optimize heating processes.
In some implementations, higher overall energy efficiency is obtained
by matching the generator frequency or composite of frequencies of the RF
waveform to the Debye resonance frequency groups of the various food products
and by tracking those resonances with temperature resulting in a shorter
heating time
per unit volume for a given energy input.
Complete control of the heating process is achieved by the selective
heating of various constituents of the medium, including the food product
and/or
packaging material. Protein molecules contain peptide chains with amino-acid
side
groups that often are polar. In addition various hydrated interfaces (bound
water) of
complex tissue molecules can also be polar. For example, in implementations
where
Debye resonances are monitored, this technology can be set up to target the
Debye
resonances of those constituents of food for which heating is desired and
avoid the
Debye resonances of other constituents (e.g., packaging materials) of which
heating
is not desired by setting the generator frequency or frequency groups of the
RF
CA 02720494 2010-11-05
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waveform to target the appropriate Debye resonances and track them with
temperature and avoid other Debye resonances.
Heating rates can be increased by the matching of the generator
frequency or composite of frequencies of the RF waveform to the Debye
resonance
frequency groups of the various heated media and tracking those Debye
resonance
frequency groups with temperature.
Overall energy efficiency is improved due again to the matching of
the generator frequency or composite of frequencies to the Debye resonance
frequency groups of the various heated media and tracking those Debye
resonance
frequency groups with temperature. Efficiency is also improved by selective
heating
of the various individual constituents of a medium (e.g., glue between layers
of
plastic packaging laminates) by targeting the Debye resonance profiles of
those
constituents and setting up the generator to excite them and track them with
temperature.
The characterization of the dielectric properties of food as a function
of frequency and temperature and the search for Debye resonances of the
various
food constituents are of great interest. If sufficient information is
available, the
heating apparatus can be programmed with great precision. Such information can
be
obtained by conducting preliminary experiments on food products of the type to
be
heated.
Several of the Examples relate to testing involving aspects of the first
approach.
Second Approach
According to the second approach, enhanced feedback and automatic
control are used to match the effective adjusted load impedance with the
output
impedance of a signal generating unit that produces an amplified variable
frequency
RF waveform.
The system of FIG. 9 is similar to the system of FIG. 5, except that
the system of FIG. 9 provides for direct measurement of the power output from
the
amplifier, and this result can be used to match the load impedance to the
signal
CA 02720494 2010-11-05
generating unit output impedance, as is described in further detail below.
Specifically, the system of FIG. 9 provides for measuring the forward and
reflected
power, as well as the phase angle difference between the voltage and the
current.
Also, the temperature of the medium during the process is not used as
5 a variable upon which adjustments to the process are made, although it
may be
monitored such that the process is ended when a desired final temperature is
reached. Elements of FIG. 9 common to the elements of FIG. 5 are designed by
the
FIG. 5 reference numeral plus 100.
Similar to FIG. 5, FIG. 9 shows a variable RF frequency
10 generator 130 connected to a broadband linear power amplifier 132, with
the
amplifier 132 output being fed to a tunable impedance matching network 134. As
in
the case of the amplifier 32, the amplifier 132 is a 2 kW linear RF power
amplifier
with an operating range of 10 kHZ to 300 MHz, although a 500 W-10 kW amplifier
could be used. Positioned between the amplifier 132 and the matching network
134
15 is a tunable directional coupler 150 with a forward power measurement
portion 152
and a reverse power measurement portion 154.
The tunable directional coupler 150 is directly connected to the
amplifier 132 and to the matching network 134. The forward and reverse power
measurement portions 152, 154 are also each coupled to the connection 133
(which
20 can be on a coaxial transmission line) between the amplifier 132 and the
matching
network 134 to receive respective lower level outputs proportional to the
forward
and reverse power transmitted through the connection 133. These lower level
outputs, which are at levels suitable for measurement, can be fed to a
measurement
device 156.
25 If a 25 W sensor is used in each of the forward and reverse power
measurement portions 152, 154, the measurement capability for forward and
reverse
power will be 2.5 kW with a coupling factor of -20 dB.
The measurement device 156 allows a voltage standing wave ratio
(SWR) to be measured. The voltage SWR is a measure of the impedance match
CA 02720494 2010-11-05
26
between the signal generating circuitry output impedance and the effective
load
impedance.
As described above, the matching network 134 can be tuned to
produce an impedance adjustment such that the effective adjusted load
impedance
matches the signal generating circuitry output impedance. A voltage SWR of 1:1
indicates a perfect match between the signal generating circuitry output
impedance
and the effective load impedance, whereas a higher voltage SWR indicates a
poorer
match. As alluded to above, however, even a voltage SWR of 2:1 translates into
nearly 90% of the power reaching the load.
The measurement device 156 can also determine the effective load
reflection coefficient, which is equal to the square root of the ratio of the
reverse (or
reflected) power divided by the forward power. In specific implementations,
the
measurement device 156 can be an RF broadband dual channel power meter or a
voltage standing wave ratio meter.
Alternatively or in addition to the methods described above, it is also
possible to provide for control heating by controlling for a minimum reflected
power, e.g., a reflected power of about 10% or less of the forward power.
Similar to FIG. 5, an AC RF power waveform 136 is fed from the
network 134 to the load, which includes electrodes 120, 122 and a medium 124
to be
heated in the product treatment zone between the electrodes 120, 122. As in
FIG. 5,
the system of FIG. 9 includes voltage and current measurement equipment,
indicated
in Fig. 9 at block 135, to measure the voltage applied across the capacitive
load and
current delivered to the capacitive load, which can be used to determine load
power
and the degree of impedance match. The voltage, current and optional
temperature
measurement block 135 includes inputs from an RF current probe 137a, which is
shown as being coupled to the connection between the network 134 and the
electrode 120, and an RF voltage probe 137b, which is shown as being connected
(but could also be capacitively coupled) to the electrode 120. As indicated,
there
may be an additional sensor for measuring the temperature or other suitable
environmental parameter at the medium. Superior results are achieved with
CA 02720494 2010-11-05
27
probes 137a and 137b that are broadband units, and a voltage probe 137b that
has a
1000:1 divider. A capicitively coupled voltage probe with a divider having a
different ratio could also be used.
The voltage and current measurements are also used in determining
the effect of capacitive reactance. Capacitive reactance in a circuit results
when
capacitors or resistors are connected in parallel or series, and especially
when a
capacitor is connected in series to a resistor. The current flowing through an
ideal
capacitor is -90 out of phase with respect to an applied voltage. By
determining the
phase angle between the voltage and the current, the capacitive reactance can
be
"tuned out" by adjusting the tunable network 134. Specifically, inductive
elements
within an output portion of the tunable matching network 134 are tuned to tune
out
the capacitive component of the load.
Signals from the probes 137a, 137b indicate the current delivered to
the capacitive load and voltage applied across the load, respectively, to the
computer 138. The block 135 includes a computer interface that processes the
signals into a format readable by the computer 138. The computer interface may
be
a data acquisition card, and may be a component of a conventional
oscilloscope. If
an oscilloscope is used, it can display one or both of the current and voltage
signals,
or these signals may be displayed by the computer.
The system of FIG. 9 includes feedback control as indicated by the
arrows leading to and from the computer 138. Based on input signals received
from
the measurement instrument 156 and the block 135 and algorithms processed by
the
computer 138, control signals are generated and sent from the computer 138 to
the
frequency generator 130 and the matching network 134.
The control algorithm executed by the computer may include one or
more control parameters based on properties of the specific food product being
heated, as well as the measured load impedance, current, voltage, forward and
reverse power, etc. For example, the algorithm may include impedance vs.
temperature information for the specific food product as a factor affecting
the
CA 02720494 2010-11-05
28
control signal generated to change the frequency and/or to tune the impedance
matching network.
FIG. 11 is a flowchart illustrating steps of capacitive (RF) heating
methods using impedance matching techniques.
In step 200, the signal generating unit is set to an initial frequency,
which, as in the case of step 170 in Fig. 10, may be based on a predetermined
frequency vs. temperature relationship, and the heating process is initiated.
As indicated by the dashed line, an automatic impedance matching
process 208 follows step 200. For an exemplary implementation, automatic
impedance matching begins with step 210. In step 210, the magnitude and phase
of
the actual load impedance are measured using the voltage and current
measurement
equipment 135, and the measured values are relayed to the computer 138. In
step
212, the phase angle difference between the measured voltage and current is
determined to tune out the reactance component of the impedance.
In step 213, the impedance match between the signal generating unit
and the effective load is measured. For this implementation, measuring the
impedance match includes measuring the forward and reverse powers (sub-step
214), and a voltage SWR is calculated as described above. The calculated
voltage
SWR is fed back to the computer 138. In step 220, the effective load
impedance is compared to the impedance of the signal generating unit, which is
a
constant in this example. If the match is not sufficient, e.g., as determined
by
evaluating the voltage SWR, the process proceeds to step 222. If the impedance
match is sufficient, the process proceeds to step 228.
In step 222, the effective load impedance is adjusted. As described
above, adjusting the effective load impedance, i.e., raising or lowering it,
may be
accomplished in two ways. As shown in sub-step 224, the impedance matching
network (e.g., the network 134) can be tuned to produce an impedance
adjustment
such that the effective adjusted load impedance matches the signal generating
unit
output impedance. As an alternative to, or in conjunction with sub-step 224,
the
frequency at which the RF waveform is applied can be changed (sub-step 226) to
CA 02720494 2010-11-05
29
cause a change in the effective adjusted load impedance. If the frequency is
changed, it may be necessary to tune out the capacitive reactance again by
repeating
steps 210 and 212, as indicated by the control line 225 leading from sub-step
226 to
step 210, before reaching step 213. If step 222 involves only tuning the
impedance
matching network, the process can return directly to step 213.
Step 228 is reached following a determination that an acceptable
impedance match exists. In step 228, a monitored temperature is compared to a
desired final temperature. If the measured temperature equals or exceeds the
desired
final temperature, the heating process in completed (step 230). Otherwise,
heating is
continued (step 229) and the process returns to step 210.
The feedback process of steps 210, 220 and 222 continues at a
predetermined sampling rate, or for a predetermined number of times, during
the
heating process. In specific implementations, the sampling rate is about 1-5
s.
Thus, as the food product is heated, the change in effective adjusted load
impedance
is periodically monitored and automatically adjusted to the constant signal
generating unit output impedance, thereby ensuring that maximum power is used
to
heat the food product. As a result, the food product is heated quickly and
efficiently.
The measured temperature may be used as an added check to assist in
monitoring the heating process, as well as for establishing temperature as an
additional control parameter used in controlling the process, either directly
or with
reference to temperature-dependent relationships used by the control
algorithm.
To permit operation of the system on non-ISM (Industrial, Scientific
and Medical) RF bands, shielding can be used to isolate various components of
the
system from each other and the surrounding environment. For example, as shown
schematically in FIG. 9, a resonant cavity 158 can be provided to shield the
capacitive load and associated circuitry from the surroundings. Other
components
may also require shielding.
Shielding helps prevent interference. Even though the frequency
changes during the heating process, it resides at any one frequency value long
enough to require shielding. An alternative approach is to use dithering
(varying the
CA 02720494 2010-11-05
frequency very quickly so that it does not dwell and produce sensible
radiation) or
spread the spectrum to reduce the shielding requirement.
As shown in FIG. 9, a secondary impedance matching device, e.g., a
capacitive coupling network 159, is connected in series between the network
134
5 and the electrode 120. Varying the capacitance of the capacitance
coupling network
aids in impedance matching.
A conventional servo motor (not shown) may be connected to the
capacitor coupling network to change its capacitance. The servo motor may be
connected to receive control signals for adjusting the capacitance from the
computer
10 138. Generally, the capacitance coupling network 159 is used for
relatively coarse
adjustments of load impedance.
A network analyzer (not shown) may also be used in determining
impedance levels. Usually, the network analyzer can only be used when the
system
is not operating. If so, the system can be momentarily turned off at various
stages in
15 a heating cycle to determine the impedance of the capacitive load and
the degree of
impedance matching at various temperatures.
System Components
Suitable components for the systems of FIG. 5 and FIG. 9 are
available from or are likely to be designed by:
20 = National Instruments-- GPIB (IEEE 488) data acquisition interface card
for
computer system
= Agilent Technologies-- frequency generator 8648B (9 Khz-4 Ghz) with
option
lEA (high power), oscilloscope 54615B (2 channel 500 Mhz) or 54602B (4
Channel 150 Mhz), E4419B dual channel RF power meter, 8753ES Network
25 Analyzer 30 kHz-6 Ghz with Option 006, 8482B 25 W sensors (100 khz-4.2
Ghz) for directional coupler
= Kalmus-- power amplifier
= Heatwave Drying Systems, Ltd., with consulting assistance from Flugstad
Engineering-- directional coupler, tunable matching network, resonant cavity,
30 electrodes
CA 02720494 2010-11-05
31
= Flugstad Engineering/Oregon State University--software for measurement
and
control algorithms and hardware for interfacing computer system with network,
voltage and current measurement equipment and measurement device
= Tektronix--Model A6312 current probe (DC 200 MHz, measurement capability
to 40 A peak-to-peak, used with AM3050 GPIB-based current probe amplifier)
and Model P6015A voltage probe (1000:1 20kV DC/40 kV pulse peak, DC
bandwidth 75 MHz) or Model P5100 voltage probe (100:1 2.5 kV, DC
bandwidth 250 MHz)
Electrode Construction
As shown in FIGS. 6-7, the systems of FIG. 5 or FIG. 9 can employ
gridded heating electrodes on the capacitive load for precise control of
heating of the
food medium 24 by the computer 38, especially to assist with heating
heterogeneous
media. At least one of the electrodes, for example top electrode 20 in the
embodiment of FIGS. 6-7, has a plurality of electrically-isolated electrode
elements 40. The bottom electrode 22 also has multiple electrically-isolated
electrode elements 44. Most favorably, each top electrode element 40 is
located
directly opposite a corresponding bottom electrode element 44 on the other
electrode. A plurality of switches 46, under control of the computer 38, are
provided
to selectively turn the flow of current on and off between opposing pairs of
electrode
elements 40, 44. And/or, an individual computer-controlled variable resistor
(not
shown) can be included in the circuit of each electrode pair, connected in
parallel
with the load, to separately regulate the current flowing between the elements
of
each pair. These arrangements provide the ability to heat individual areas of
a food
product at different rates than others and to protect against thermal runaway
or "hot
spots" by switching out different electrode element pairs for moments of time
or
possible providing different field strengths to different portions of the
sample.
It is also advantageous to provide one or more heat sensors on at least
one of the electrodes 20, 22. FIGS. 6-7 show a compact arrangement where
multiple
spaced heat sensors 42 are interspersed between the electrode elements 40 of
the top
electrode 20. The thermal sensors 42 acquire data about the temperatures of
the
CA 02720494 2010-11-05
32
food sample 24 at multiple locations, which data is sent as input signal to
the
computer 38. The computer uses the data from each sensor to calculate any
needed
adjustment to the frequency and power level of the current flowing between
pairs of
electrode elements located near the sensor and produces corresponding output
control signals which are applied to the RF signal generator 30, network 34,
and
switches 46.
The electrodes should be made of an electrically conductive and non-
corrosive material, such as stainless steel or gold, that is suitable for use
in a food
processing apparatus. The electrodes can take a variety of shapes depending on
the
shape and nature of the food product to be processed and its packaging. A goal
in
choosing the shape of the electrodes is to conform the shape of the electrodes
to the
shape of the item to be processed, so as to minimize air gaps. Although FIGS.
6-7
show a preferred embodiment of the electrodes; other arrangements of electrode
elements and sensors could be used with similar results or for special
purposes.
Packaging Considerations
To design satisfactory packaging materials for food to be heated by a
capacitive (RF) dielectric heating system according to the present invention,
it is
best to consider factors such as electric field levels, frequency schedules,
geometries, and surrounding media. In particular, it is helpful to have a full
understanding of dielectric properties of packaging materials and the food
product to
be heated, over a range of frequencies and temperatures. And, it is important
to
avoid any factors that may cause high local intensities of field strength or
high local
concentrations of lossy materials, the latter of which could be caused by
saline
moisture accumulation in the corners of vacuum packages, because salty water
is
more lossy than other components of the food.
It is possible to select packaging materials that are essentially
transparent to the RF energy over all or a portion of the 1 MHz-300 MHz normal
operating range, so that heating of the food can be accomplished without
injuring the
packaging.
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33
The capacitive (RF) dielectric heating system is particularly useful to
heat food products inside packaging that comprises multiple polymers having
different properties at different stages in the food processing flow.
Multiple design methodologies can be used to take advantage of the
sealing and preservation characteristics of certain plastic polymers used in
combination with plastic polymer components that are RF insensitive. A
multiple-
staged process could occur where the product is packaged first with RF
insensitive
materials and then run through the capacitive (RF) heating pasteurization
process
and then in turn packaged with another layer of a different polymer that has
better
sealing or preservation characteristics. A variety of sealing and bonding
methods
can be used for laminated plastic packaging materials. Packaging materials can
be
chosen or modified to make bonding agents or bonding zones of laminated
plastics
more insensitive to the effects of RF exposure.
The product to be heated can be surrounded with a non-conductive
dielectric coupling medium (e.g., de-ionized water) that itself will not be
heated
(Debye resonance at much higher frequency) but will increase the dielectric
constant
of the gaps between the electrodes and the medium to be heated thus lowering
the
gap impedance and improving energy transfer to the medium.
It may also be helpful to supply greater heat to outer edges of the
medium (e.g. by convection from pre-heated deionized water) to help compensate
for the greater heat losses that occur in those areas. The pre-heated water
may be at
a temperature of 75-80 degrees C.
Examples
Following are examples that describe how to devise particular
apparatuses, data tables, algorithms and operating procedures.
Example 1
Tests can be conducted to measure and characterize dielectric
properties, including Debye resonances, of various constituents of muscle
foods and
CA 02720494 2010-11-05
34
potential packaging materials, as functions of frequency (100 Hz - 100 MHz)
and
temperature (0 - 90 C).
The experiments are to measure the impedance (parallel capacitor and
resistor model) of muscle food samples, and potential packaging materials
sandwiched in a parallel electrode test fixture placed within a
temperature/humidity
chamber. The equipment used for these experiments is as follows:
HP 4194A: 100 Hz - 100 MHz Impedance/Gain-Phase
Analyzer
HP 41941A: 10 kHz - 100 MHz RF Current/Voltage
Impedance Probe
HP 16451B: 10 mm, 100 Hz - 15 MHz Dielectric Test
Fixture for 4-Terminal Bridge
HP 16453A: 3 mm, 100 Hz - 100 MHz RF/High
Temperature Dielectric Test Fixture
Damaskos Test, Inc. Various specially-designed fixtures
Dielectric Products Co. 9 mm, 100 Hz - 1 MHz Sealed High
Temperature Food/Semi-Solids LD3T
Liquid-Tight Capacitive Dielectric Test Fixture
HP 16085B: Adapter to mate HP16453A to HP 4194A 4-
Terminal Impedance Bridge Port (40 MHz)
HP 16099A: Adapter to mate HP16453A to HP 4194A RF
IV Port (100 MHz)
Temperature/Humidity Chamber Thermotron Computer Controlled
Temperature/Humidity Chamber -68 - +177 C
, 10% - 98% RH, with LN2 Boost for cooling
Each of the capacitive dielectric test fixtures is equipped with a
precision micrometer for measuring the thickness of the sample, critical in
calculating the dielectric properties from the measured impedance. The
different
test fixtures allow for trading off between impedance measurement range,
frequency
range, temperature range, sample thickness and compatibility with foods/semi-
solids
and liquids.
Various samples of comminuted muscle are prepared to have
moisture and salt contents representative of commercial products (e.g., 78%
and
84% moisture content and 0%, 2% and 4% salt content for surimi seafoods).
Three
different moisture and salt content values covering both ends of these ranges
and a
CA 02720494 2010-11-05
mid-range value are chosen for the samples. A minimum of four replications of
each muscle food type and preparation are tested with each dielectric probe
for a
total of 12 test cases for each muscle food type or preparation. Different
groups of 4
replicated samples are prepared in advance to be compatible with one of the
three
5 dielectric probes. In addition to the "macroscopic" samples making up
commercial
food products, properties are evaluated on such individual constituents as
starch,
water, and sugar. These find application in later stochastic food property
models.
The frequency range has been chosen to cover the typical industrial
capacitive heating range (300 kHz to 100 MHz) as well as the lower frequencies
10 (down to 100 Hz) to determine DC or low frequency electrical
conductivity. This
range also identifies Debye resonance locations of the packaging materials and
very
complex polar sidechains in the food (e.g., protein molecules-peptide chains
with
amino acid residues in the side groups). The temperature range of 0 C to 90 C
has
been chosen to overlap the likely pasteurization temperature range of 20 C to
85 C.
15 Impedance is measured on the samples (both shunt resistance and
capacitance) and then electric permittivity s', permittivity loss factor c"
and electrical
conductivity a is calculated based on the material thickness, test fixture
calibration
factors (Hewlett Packard. 1995. Measuring the Dielectric Constant of Solid
Materials-HP 4194A Impedance/Gain-Phase Analyzer. Hewlett Packard Application
20 Note 339-13.) and swept frequency data. For details on the technical
background
covering the dielectric properties of foods including Debye resonances, please
refer
to the following discussion for Example 2.
Example 2
25 A mathematical model and computer simulation program can model
and predict the capacitive heating performance of packaged comminuted muscle
foods based on the characterized dielectric properties.
There are underlying mathematical models that form the basis of the
overall simulation. The electric permittivity has been classically modeled
using
30 Debye equations (Barber, H. 1983. Electroheat. London: Granada
Publishing
CA 02720494 2010-11-05
36
Limited; Metaxas, A. C. and Meredith, R.J. 1983. In Industrial Microwave
Heating.
Peter Peregrinus Ltd.; Metaxas, A.C. and Meredith, R.J. 1983. In Industrial
Microwave Heating. Peter Peregrinus Ltd.; and Ramo, S., J.R. Whinnery, and T.
Van Duzer. 1994. Fields and Waves in Communications Electronic, 3' edition.
New
York: John Wiley & Sons, Inc.). These equations can be used to model a variety
of
relaxation processes associated with dielectric alignments or shifts in
response to
external varying electric fields. Each of these alignment processes has a
corresponding relaxation time To that is a function of several parameters of
the
atomic and molecular makeup of a medium, and therefore is a measure of the
highest frequency for which these phenomena can occur. At a frequency which
equals 1/27r To , a Debye Resonance occurs which results in a peak in the loss
factor
E". A model for the permittivity using a Debye function for a single
relaxation
process is shown in Equation (5):
Ed -
6 = Eo[E.0 1 + jC0T0
(5)
where
Low Frequency Dielectric Constant of a Medium (f
Ed
Debye Resonance).
= High Frequency Dielectric Constant of a Medium (f
>> Debye Resonance).
Permittivity of Free Space (8.854e-12 F/m).
so
Therefore, from Equation (1) it can be shown that the real and imaginary
components of the permittivity are given for a single Debye resonance as
follows:
Ed 1
= 80[Ex co 2 T 02
(6)
CA 02720494 2010-11-05
37
oToso (Ed -
E 11 = ____________________________
co2To2
(7)
Ed is typically an order of magnitude or more larger than and so
from inspection
of equations (6) and (7), it is seen that in the vicinity of a Debye
resonance, c' drops
off rapidly and there is a peak in the loss factor c". When a composite medium
containing multiple relaxation times exists, then the more general purpose
model can
be represented as a summation of Debye terms as given by Equation (8) (loss
term
only) (Metaxas and Meredith, 1983):
(
E tf = g.(0 _______
+ 2T 2 Ar
T = TO
(8)
where g(r) is the fraction of orientation polarization processes in each
interval AT .
This summation assumes a linear combination of polarizations or
Debye resonances. More complex mathematical models also exist for multiple
Debye resonances if linearity is not assumed, and for complex composite
dielectric
materials with varying geometrical arrangements of the constituents
(Neelakanta,
P.S. 1995. Handbook of Electromagnetic Materials. Monolithic and Composite
Versions and Their Applications. New York: CRC Press). In the case of
heterogeneous foods, stochastic variables need to be included to model the
relative
concentrations and spatial distributions of the various constituents, and a
Monte
Carlo analysis performed to determine the statistical composite dielectric
behavior in
each block of a 3-D finite element partitioning model of the medium.
It can be shown (Roussy, G., J.A. Pearce. 1995. Foundations and
Industrial Applications of Microwaves and Radio Frequency Fields. Physical and
Chemical Processes. New York: John Wiley & Sons; Barber, 1983; Metaxus and
Meredith, 1983) that the power per unit volume (P) delivered to a medium for a
given electric field intensity is represented by the following:
CA 02720494 2010-11-05
38
Pv = Qgen = 6 " 0E12
(9)
This reduces to the following when (DE" >> a :
Qgen(x,y,z,t) = Pv = E2wE"
(10)
where E is again the RMS value of the electric field intensity. So for a given
electric
field intensity, peaks in the permittivity loss factor E" results in peaks in
the energy
imparted to a medium, resulting in more efficient and rapid heating. Assuming
for
the moment that there is no heat transfer into or out of a medium due to
convection
or conduction, the heating time t h for a given temperature rise (AT) due to
dielectric
heating is then given by Equation (11) (Orfeuil, 1987):
Cp p AT Cp p AT
th =
E2wE" Pv
(11)
where
C. = Specific Heat of the Medium (J/Kg C)
p = Density of Medium (Kg/m3)
and all the other parameters are as previously defined.
The more general purpose conservation of energy equation that
accounts for heat transfer (convection or conduction from adjacent areas) and
heat
generation (dielectric heating source term) is given as follows (Roussy and
Pearce,
1995):
T
p Cp
t - V = ( kT VT) = gen (X,Y, t)
(12)
where KT = thermal conductivity of the medium and t = time; all other
parameters
are as previously defined.
CA 02720494 2010-11-05
39
In a similar fashion, the general purpose governing equation solving
for the electric field (from Maxwell's equations in differential form) is as
follows
(Roussy and Pearce, 1995):
2v
Pv
V2V - t2 ¨ -
(13)
where pv = Charge Density; V = Electric Potential or Voltage
Equation (13) is also referred to as the Helmholtz equation, and in
cases where the time derivative is zero, it reduces to Poisson's Equation.
When the medium is a passive source-less medium such as food and
when the frequency of operation is low enough where the wavelength is long
compared to sample dimensions such as in the case of capacitive heating (i.e.,
quasi-
static model), Equation (13) reduces to the following:
V2V = 0
(14)
The electric field is related to the voltage by the following equation:
E = -VV
(15)
Or simply stated, the electric field is the negative gradient of voltage in
three
dimensions.
Equations (8), (9), (12), (14) and (15) form the basis for an
electromagnetic dielectric heating model which can be applied to a composite
dielectric model, to model a food substance having several subconstituents.
In addition, it is possible to make a composite series model for a food
sample sandwiched top-and-bottom by a packaging layer, an air or water layer,
and
electrodes. From earlier discussion it is apparent that the dielectric
parameters are
all functions of temperature and frequency. It is also true from Equations (9)
and
(10) that the power generated for heating is a function of the dielectric loss
factor
and electric field intensity. Finally it can be deduced from Equations (13)-
(15) that
the electric field intensity is a function of the dielectric parameters which
in turn are
CA 02720494 2010-11-05
functions of temperature and frequency. Therefore an iterative solving
algorithm
can be developed to solve for all the desired parameters in this model, one
that also
sequences in time, cycling back and forth between the electromagnetic and
thermal
solutions and solves them as a function of frequency.
5 There are several options for developing a simulation model. One
is
to adapt existing electromagnetic models developed in MathCAD and MATLAB.
Another is to employ various examples of electromagnetic field FEM programs
for
complex composite geometrical structures, such as the High Frequency Structure
Simulator (HFSS) developed by Agilent Technologies and the Maxwell Extractor
10 electromagnetic field solver programs developed by Ansoft, Inc. of
Pittsburgh, PA.
HeatWave Drying Systems, Ltd., working with Dr. W. Hoefer at the University of
Victoria, has developed a third approach to solve for both the electromagnetic
and
thermal processes (Herring, J.L., W.J. Hoefer, and R.L. Zwick. 1995. Time
Domain
Transmission Line Matrix (TLM) Modeling of a RFNacuum Wood Drying Kiln.
15 Progress in Electromagnetic Research Symposium. Seattle, WA). One can
start
with this 3-D TLM electromagnetic field solver which has already been combined
with heat transfer simulation models to address the RF wood drying process.
Findings on Debye resonances both with the food samples of interest, and on
stochastic models representing the heterogeneous spatial distribution of
constituents
20 within a food volume, can then be incorporated. Also, apart from Debye
resonances,
other properties, including both ohmic and dielectric properties other than
Debye
resonances, can be modelled for study. Thus, the initial model work leverages
off of
models already developed by HeatWave and their collaborators.
25 Example 3
Thermal and non-thermal effects of radio-frequency pasteurization on
microbiological lethality, color, and texture in representative comminuted
muscle
foods are examined.
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41
Sample preparation
Two types of comminuted muscle samples are investigated: beef
frankfurter and surimi seafood. Since all food ingredients mixed into samples
contribute to every aspect of quality, to include microbiology, sensory, and
physical
properties, a commercial formulation for both samples is developed. For surimi
seafood, the formulation is adjusted to maintain 75% moisture and 1.65% salt.
Overall chopping procedures are based on the teachings in Yongsawatdigul, J.,
Park,
J.W., Kolbe, E., AbuDagga, Y. and Morrissey, M.T. 1995, Ohmic heating
maximizes gel functionality of Pacific whiting surimi. J. Food Sci. 60:10-14.
Surimi paste is stuffed into stainless steel tubes (1.9 cm I.D. x 17.5
cm long). Initial heating is conducted in a 90 C water bath until internal
temperature reaches 70 C. At this temperature, fish myosin and actin complete
denaturation and gelation (Oakenfull, D.G. 1996. Gelation mechanisms. Foods
and
Food Ingredients J. Japan. 167:48-68). Initial cooking for beef franks is
suggested
at155 F (68.2 C) for 30 min (Hanson, R. 1995. Design and function of batch
meat
processing ovens. A textbook of Viskase and AMSA Meat Science School. Aug.
16-18. Chicago, IL). Therefore, it is assumed that this process mimics initial
cooking in the commercial processing of surimi seafood and beef franks.
Cooked gels, without chilling, are removed from the tubes and
vacuum-packaged in plastic bags for various thermal treatments for
pasteurization.
Pasteurization/Heating method
Pasteurization is conducted in a radio frequency (RF) heating device.
Heating at the same rates under a range of frequencies enables
measurement of the nonthermal effects of RF on aerobic plate counts (APC). As
a
control, a sample is heated in a water bath (90 C) for 60 min. Each heat
treatment is
repeated three times. Changes of internal temperatures as a function of time
are
monitored using a 21X data logger (Campbell Scientific, Logan, UT) in the
water
bath heater, and by fiber-optic sensors in the RF heating device.
CA 02720494 2010-11-05
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Microbiological assay
Raw paste, initial cooked gels, and pasteurized gels are aseptically
collected for aerobic plate count (APC). The microbial assay are conducted by
spread-plating on tryptone-peptone-yeast extract (TPE) agar and incubated at
30 C
for 48 hr (Lee, J.S. and Howard, L.A. 1970. Comparison of two procedures for
enumeration of microorganisms from frozen foods. J. Milk and Food Technol.
33:237-239).
Textural properties
Longer time and higher temperature cooking generally causes
textural destruction especially in surimi seafood products made with reduced
surimi
content and higher starch content. Changes of textural properties are
monitored as
shear stress and shear strain using a torsion test (NFI. 1991. A Manual of
Standard
Methods for Measuring and Specib;ing the Properties of Surimi, T.C. Lanier, K.
Hart, and R.E. Martin (Ed.), National Fisheries Institute, Washington, DC).
Shear
stress denotes gel strength, while shear strain indicates the cohesive nature
of gels.
Color properties
Longer time and higher temperature cooking causes discoloration in
surimi seafood (development of yellow hue), especially when some protein
additives
and sugar are present. Color properties (L*, a*, b*) of gels are measured
using a
Minolta chroma meter (Minolta USA, Ramsey, NJ).
Non-thermal killing effects by radio frequency
Microbial (APC) destruction is measured in samples heated in a
water bath, and in a radio frequency heater for which heating rates are
adjusted
similar to those measured in the water bath. Various frequencies are used in
the
radio frequency heater. The difference between the two microbial destruction
measurements is used as non-thermal kills.
CA 02720494 2010-11-05
43
Model study
A pasteurization method for the maximum thermal treatment is
selected for the model study. Enterococcus faecium is selected as the target
organism because of its high thermal resistance in sous vide products
(Ghazala, S.
Coxworthy, D., and Alkanani, T. 1995. Thermal kinetics of Streptococcus
faecium
in nutrient broth sous vide products under pasteurization conditions. J. Food
Processing and Preservation 19:243-257; Magnus, C.A., McCurdy, A. R., and
Ingledew, W.M. 1988. Further studies on the thermal resistance of
Streptococcus
faecium and Streptococcus faecalis in pasteurized ham. Can. Inst. Food Sci.
Technol. J. 21:209-212). The stock culture for Enteroccocus faecium is revived
in
cooked meat broth for 24 hr at 37 C, plated on Difco trypticase soy agar (TSA)
and
incubated for 24 hr at 37 C. These plates are maintained at 1 C. An overnight
culture is prepared in a cooked meat broth (pH 7.23, 0.5% NaC1) and incubated
at
37 C for 20 hr so that the cell concentration is 107-108 CFU/ml. Surimi and
frankfurter paste are inoculated with the culture and homogenized before the
heat
treatment. Enumeration follows dilution and plating as described by (Ghazala
et al.,
1995).
Thermal inactivation trials are performed at each temperature (30, 45,
60, 75, 90 C) in triplicate. The D-value of E. faecium at each trial
temperature is
determined from a plot of the logarithm of microbial survivors versus heating
time.
The z-value is obtained by plotting the logarithm of D-values versus heating
temperatures (thermal death time, TDT plot). Quattro Pro software is used to
determine slope, intercept, and r2 values using least square linear regression
analysis.
Pasteurization values represent the minimum number of minutes, at a
specific temperature, which are required for the product's coldest point to
receive
about 13-14 times the decimal reduction of a target organism (Ghazala, S. and
Aucoin, E.J. 1996, Optimization of pasteurization processes for a sous vide
product
in rectangular thin profile forms. J. Food Quality 19:203-215). Pasteurization
values are determined by Ball's equation (Ball, C. 0. and Olson, F.C.W. 1957,
In
CA 02720494 2010-11-05
44
Sterilization in Food Technology, pp. 291, 353, 356, McGraw-Hill Book Co., New
York, NY), while cooking values are determined by Mansfield equation
(Mansfield,
T. 1962, High-temperature short-time sterilization, Proc. 1st Int. Cong. Food
Sci.
& Tech. Vol 4, Gordon and Breach, London, UK) as follows:
(T- Tõf
Pasteurization value = Pv = j10 dt (16)
0
(T - Tr,f
Z,
Cook Value = C, = j10 dt (17)
0
where Pv = Integrated pasteurization value at the point of slowest
heating
Cv= Integrated cooking value at the point of slowest heating
t = Processing time, min
T = Temperature at time t, C
Tref = Reference temperature (85 C)
z = Slope of the logarithm of the decimal reduction time versus
temperature for a specified organism, C
zc = z-value for degradation of quality of a specified quality factor,
e.g., texture and color.
The pasteurization value and cook value are determined using Eq.
(16) and (17) employing z-value for Enterococcus faecium and a reference
pasteurization temperature. Thermal kinetics of Enterococcus faecium are
calculated.
Example 4
High energy density capacitive heating experiments can be conducted
on various packaged comminuted muscle foods, validating the computer
simulation
model, testing pasteurization efficacy, testing the effects of electromagnetic
energy
on food quality, and verifying suitable packaging materials. For example, such
testing can be performed in a capacitive heating test facility.
CA 02720494 2010-11-05
The processed food samples can be analyzed for textural changes and
pasteurization efficacy. The packaging materials are analyzed for sensitivity
to RF
energy. The heating vs. time results are analyzed to validate the computer
simulation model and to make any necessary adjustments to the model, based on
5 those results. Exact quantities of samples and replication numbers, as
well as exact
RF power levels and other Example 4 experimental details are determined based
on
the results of Examples 1-3.
The various media under test are heated at frequencies in the range 1
MHz ¨ 100 MHz based on the information gained in Example 1. There are two test
10 scenarios as shown below. The first is a general test over a semi-
logarithmic
distribution of frequencies. The second concentrates on Debye resonances that
may
have been identified from Example 1. Other tests experiment, for example, with
results of controlled/constant field strength; voltage gradient can vary with
tested
sample thickness. In all cases, the food package dimensions can be varied --
from
15 single, relatively thin package to a stack representing a load up to 24
cm in
thickness. Electrode dimensions and shape can be modified in the heating
system.
For all cases, thermal sensors are placed in the media under test to determine
temperature rise and distribution.
20 Test 1: General Purpose Frequency Sweep:
Frequencies* Test Power Levels Inoculated Number of Food
Type (Max. Output) (YIN) Replications*
1 ¨ 100 MHz 200W, 2KW Y 4 Surimi
Seafood
25 1 ¨ 100 MHz 200W, 2KW Y 4
Frankfurters
1 - 100 MHz 200W, 2KW N 4 Surimi
Seafood
1 - 100 MHz 200W, 2KW N 4
30 Frankfurters
1 - 100 MHz 200W, 2KW N 4
Packaging only
CA 02720494 2010-11-05
46
* Samples are prepared and tested for each of the following frequencies in the
general sweep test: 1,2,3,5,7,10,20,30,50,70,100 MHz
Test 2: Debye Resonance Search
In this test, the frequency of the capacitive heater can be set at the
location of any Debye resonances that were identified by the results in
Example 1.
The power level is set first to a low level (100W) for the first set of
samples and the
frequency swept gradually to both search for the resonance and to test for its
dependence on temperature. Once Debye resonances are located at 100W, the
system is adjusted to 2kW and the Debye resonance experiment repeated on a
different set of replicated samples with the temperature rise measured.
Example 5
In Example 5, testing was conducted to determine the heating
efficiency of capacitive dielectric (RF) methods and systems. The testing
conducted
in Example 5 was derived from the planned testing of Example 4. Rather than
considering Debye resonance frequencies alone, the testing of Example 5 was
focused on controlling the heating process based on impedance matching.
First Tests
In the first series of tests, a capacitive dielectric (RF) heating system
with a variable frequency range of 12-132 MHz and up to 1 kW output power was
used to heat and pasteurize surimi, frankfurters, alfalfa seeds and packaging.
Samples were tested to evaluate the effects of RF frequency and electric field
intensity on heating rates.
Following adjustments in the system, including repositioning
electrodes to reduce the air gap and rounding sharp edges of surimi gels,
results
showed that relatively uniform and fast heating were achieved with both seeds
and
surimi. In addition, increased heating rates at higher frequencies and higher
electric
field intensities were observed for seeds. FIG. 16 shows much faster heating
rates at
60 MHz than at 15 MHz for a given field intensity for seeds.
CA 02720494 2010-11-05
47
Second Tests
The purpose of the second tests was to examine if faster heating rates
could be achieved with possible adjustments of air gap, sizes and shapes of
electrodes and configurations of seeds and packaging. A 1 kW capacitive
dielectric
heating system with a variable frequency of up to 50 MHz was used in the
second
tests. The operating frequency used was between 33 to 39 MHz.
The impedance match between the load impedance and the signal
generating circuitry was manually controlled by manually setting the frequency
(i.e.,
tuning four knobs on a frequency generator). The forward and reverse power
levels
were measured manually, and the voltage through the medium was also measured.
FIG. 20 shows temperature vs. time profiles when a disk-shaped
surimi sample was heated from about 26 to 77 C. Three temperature probes
measured variation across the thickness of the surimi sample. The largest
measured
temperature difference was less than 5.6 C (between the center and top
positions).
This result indicates that better temperature uniformity could be achieved
using
capacitive dielectric heating, than by using conventional hot water bath
heating.
AbuDagga and Kolbe (2000) show that hot water heating produced an initial
temperature difference of 20-30 C between the surface and the center of a
surimi
sample. Analysis of Heat Transfer in Surimi Paste Heated by Conventional and
Atomic Means, Journal of Aquatic Food Product Technology, 9 (2):43-54
FIG. 20 also shows that the heating rate appeared to slow down and
almost stopped twice during the heating process. This break in the heating
rate was
caused by impedance mismatch between the power amplifier and the load. Manual
adjustment of frequency could not be preformed quickly and accurately enough
to
keep pace with the changing dielectric or ohmic properties of this surimi as
it was
heated. This led to the loss of impedance match between the power amplifier
and
the heated surimi as its dielectric and ohmic properties or electrical
impedance
changed with temperature.
FIG. 21 is a plot of the first derivative of the temperature-time
profiles of FIG. 20. As shown, the heating rate first increases to 25 C/min at
point
CA 02720494 2010-11-05
48
A then falls to almost zero at points B and D due to loss of impedance match.
The
heating regained its maximum rates at points C and E.
FIG. 22 shows heating rate vs. time for the surimi sample when a
relatively good impedance match was maintained by manual adjustment. The
resulting heating rates in FIG. 22 were much more uniform than those shown in
FIG. 21.
As a result, automatically controlling the frequency of the frequency
generator as well as the elements in an impedance matching network, to
maintain
impedance match will produce better results. Such an automated system helps
achieve maximum heating rates for the power available and the through put of
the
food being tested.
Additional results specific to the seed testing are discussed in
Example 6.
Example 6
Example 6 concerns the use of capacitive (RF) dielectric heating for
the sterilization and/or pasteurization of seeds packaged in edible films or
alternatively standard polymer films.
Considerations in Capacitive (RF) Dielectric Heating of Seeds
Capacitive (RF) dielectric heating may also be useful to improve or
enhance the germination rates of seeds through the pretreatment of seeds prior
to
planting and sprouting with capacitive (RF) dielectric heating. The warming of
seed
shells as well as the control or reduction/elimination of germination-
inhibiting
microorganisms are the two primary methods of improving germination rates when
using capacitive (RF) heating technology as a pre-germination seed treatment
process.
Edible film packaging materials will be chosen so as to allow for the
complete isolation of seeds from external contaminating agents after being
sterilized
in the capacitive (RF) dielectric heating system. The films will be
biodegradable
and water soluble so as to naturally dissolve when planted to produce sprouts
in
either a sterile bed or hydroponic environment.
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The capacitive (RF) dielectric heating system will be capable of use
in heating applications to enhance seed germination effects. The system will
heat
for the purpose of killing germination-inhibiting organisms as well as
softening the
hard encasing material of seeds to prepare for germination. The seeds will be
either
packaged in edible or non-edible packaging films (individually or
collectively), or
sealed containers, or alternatively heated in non-sealed containers.
The capacitive (RF) dielectric heating system will be capable of use
in heating applications to enhance growth and flowering performance and yield
performance and disease and pest resistance along with other delayed
enhancement
effects of plants grown from seeds treated by the capacitive heating. The
system
will be capable of again targeting optimum frequencies (in this case not
necessarily
limited to Debye resonances), that have been proven to be successful in seed
treatment processes to achieve the desired delayed effects that occur during
the
plant's growth stages.
The capacitive (RF) dielectric heating system will be designed such
that there are no germination-inhibiting effects from the sterilization or
pasteurization processes for seeds. This will result from understanding the
dielectric
properties of seeds and heating at the optimum frequencies for enhancing
microbial
kill and germination and staying away from frequencies or exposure times that
may
inhibit seed germination.
The capacitive (RF) dielectric heating system will allow for various
product geometries to handle the wide variation of packaged or unpackaged seed
geometry variations.
The various seed sprout products may have optimum "Debye
resonances" or frequencies where capacitive (RF) dielectric heating will be
the most
efficient. As described in the First Approach section above, the capacitive
(RF)
dielectric heating system can be set to target those optimum frequencies.
These
possible "Debye resonances" in seed sprout products will have particular
temperature dependencies. The capacitive (RF) dielectric heating system will
be
designed to track those temperature dependencies during heating as the
temperature
CA 02720494 2010-11-05
rises. The various seed sprout products may have other "optimum" frequencies
that
are not necessarily "Debye" resonances but are still proven to be important
frequencies for achieving various desired benefits in either the seeds or
plants
growing from the seeds. The capacitive (RF) dielectric heating system will be
5 capable of targeting those frequencies and tracking any of their
temperature
dependencies.
Target micro-organisms or agents also may have "Debye" resonances
or other non Debye optimum frequencies that are proven to be especially
effective in
achieving selective killing performance of the organisms without adversely
affecting
10 the seeds that the organisms reside on any packaging materials that may
be used.
The capacitive (RF) dielectric heating system will be capable of targeting
those
optimum frequencies and tracking them with temperature to achieve selective
control of those organisms. In general various microscopic pathogens
(microbial
organisms, fungal spores, etc.) as well as macroscopic pests (e.g., insects,
insect
15 larvae, etc.) may have "Debye" resonances or other non Debye optimum
frequencies
that are proven to be especially effective in achieving selective killing
performance
of the organisms to allow for the broad use or application of the above-
described
capacitive (RF) dielectric heating technologies. The capacitive (RF)
dielectric
heating system will be designed to accommodate these optimum frequencies and
20 track them with temperature in a broad arrangement of commercial,
industrial,
laboratory and field implementations of the technology for use in the food,
agriculture and medical industries.
Under normal circumstances the seeds will be packaged in edible
and/or non-edible standard films which are invisible to the applied RF
electric fields
25 to insure that the packaging materials will not heat or burn or change
any of their
packaging performance qualities after exposure to the RF field. The packaging
materials and corresponding capacitive (RF) dielectric heating system will be
designed (including frequency selection) for such performance and
compatibility.
Under special circumstances the seeds may be packaged in certain
30 types of edible films that exhibit Debye resonances allowing for the
heating of the
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packaging film and seeds simultaneously with a complex multi-frequency profile
from the capacitive (RF) dielectric heating system. This would allow for
supplementary heating of the seeds from the packaging material over and above
the
direct dielectric heating of the seeds themselves. The location of the Debye
resonances in the films may be at much lower frequencies than those found in
the
seeds. The capacitive (RF) dielectric heating system will be designed to
target the
Debye resonances of both the edible film packaging materials and seed products
either simultaneously or in a time-multiplexed manner that approximates
simultaneous heating behavior. The frequency and heating profile would be
designed to allow for the heating of the packaging materials and supplementary
transfer of heat to the seeds without the destruction of the packaging
materials.
Alternatively, the edible films and seeds may have similar dielectric
properties, such as similar Debye resonances and/or dielectric loss factors,
allowing
for more uniform heating.
Testing and Results
In Example 6, capacitive (RF) dielectric heating apparatus was used
to pasteurize packaged alfalfa and radish seeds that are used to produce
vegetable
sprouts. Seed samples were subjected to RF heating at controlled RF frequency
and
electric field intensity. The time and temperature profiles of the seeds were
monitored during each test to determine the product heating rates. Seeds
germination rate, total aerobic plate counts, coliforms, and E. coli was also
determined.
It was found that the rapid and uniform heating in vegetable sprout
seeds could be achieved by using RF energy. The heating rate and uniformity
strongly related to RF frequency, electric field intensity, and RF system
(i.e.,
electrode design and frequency control) and sample configurations. Controlling
final seed temperature as related to seed moisture content and rapidly cooling
seeds
after RF heating will help avoid moisture loss and improve results.
Vegetable sprout seed treatments using RF energy were conducted in
two rounds of tests.
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The first tests were to study the effects of RF frequency and electric
field intensity on the heating rate of seeds, as well as their impacts on seed
germination and occurrence of microorganisms. A RF system with a variable
frequency range of 12 to 132 MHz and 1 kW output power was used for heating
alfalfa and radish seeds.
A 2 x 2 Randomly Block Design (RBD) experiment using frequency
as a block with three replications was used. Frequencies (f) of approximately
15 and
60 MHz (+10% variation) and electric field intensities (E) of approximately
0.55 and
0.85 kV/cm ( 20% variation) were applied for heating alfalfa and radish seeds
with
targeted final seeds temperature of about 75 C.
The purpose of the second tests was to examine if faster heating rates
could be achieved with possible adjustments of air gap, sizes and shapes of
electrodes and configurations of seeds and packaging.
A 1 kW capacitive dielectric heating system with a variable
frequency of up to 50 MHz was used in the second tests. The operating
frequency
used was between 31.2 to 3 MHz.
During RF heating in both tests, the system frequency was adjusted
manually on a frequency generator to achieve optimal impedance match between
the
heated seeds and signal generating circuitry. Impedance match was controlled
by
changing frequency, as well as adjusting an impedance matching network
connected
to the load, i.e., to the cavity/capacitive electrode system. The capacitive
electrodes
were adjusted and optimized to reduce the air gaps and therefore reduce the
maximum required overall electric field intensity across the electrodes so as
to
reduce the risk of arcing as well as the effects of fringing fields. The
electrodes
were also adjusted and optimized to conform to the smaller sample geometry to
increase the electric field intensity actually across the sample, which
allowed for the
higher power densities and faster heating rates. A minimum of four
replications for
each combination was tested.
Comparing Examples 5 and 6 shows how quickly samples may be
heated if impedance match is maintained. Seeds could be heated in 25 s when
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impedance match was maintained, whereas surimi required a 3 min heating time
when impedance match was not maintained as closely.
Sample Preparation
Radish seeds for the testing were provided by Dorsing Seeds (Nyssa,
OR). The radish seeds were produced in 1999 and had a moisture content of
about
6.5% and an estimated germination rate of ¨ 97%. Alfalfa seeds for the testing
were purchased from Andrews Seeds (Ontario, OR). The alfalfa seeds were
produced in 1998 and had a moisture content of about 7.3% and a germination
rate
of ¨ 92%.
In the first tests, alfalfa and radish seeds were packaged in 9" x 6" x
1.5" polystyrene seeds trays. Approximately 1600 g alfalfa seeds or 1400 g
radish
seeds were enclosed in the seed tray. The same amount of each type of seeds
was
also packaged inside water-soluble edible film bags (Polymer Inc., West
Heaven,
CT). For the second tests, one half of a polystyrene box (4 1/2" x 4 1/2"x 1")
with
polycarbonate spacers filled in the other half was used for holding the seeds
FIGS.
15A and 15B. The sample sizes used in this experiment were determined based on
the knowledge learned from the previous tests with a goal for achieving rapid
and
uniform heating.
In the first tests, seeds were kept inside the RF unit for a few minutes
and then poured into a large plastic container and stirred for rapid cooling
after
being subjected to RF heating. A 250 g sample of seeds from each treatment
were
randomly selected and packed in aseptic bags, and then send back to the
laboratory
for moisture content, seeds germination, and microbial tests. Sanitation and
personal hygiene procedures were followed to avoid further contamination of
seeds.
Untreated seeds were used as controls.
In the second tests, seed samples were kept in the cavity for one to
several minutes before they were poured into a plastic zipper bag. No
consideration
was given to cool them quickly after heating. Moisture content and germination
rate
were measured on RF heated and unheated seed samples.
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Time-Temperature Profile Monitoring
During the first tests, Luxtron probes were set up to display the
temperature of seeds every 5 s, and a laptop based datalogging system was used
to
acquire each time-temperature reading. In this case, the 5 s sample interval
rate was
sufficient due to the relatively slow heating rates.
During the second tests, four fiber-optic temperature probes (FOT-C
low temperature sensor, produced by FISO Technology Inc., of Quebec, Canada)
were used for recording the time-temperature profiles in the seed containers
every
1.2 s. The temperature probes have a tip of about 1.0 in (25.4 mm) long and
0.059
in (1.5 mm) diameter. The accuracy is 0.3 C in the temperature range of -50 to
250 C. They were precisely located in the center, upper layer, lower layer,
and
corner of the box by the spacers and the mounting holes in the spacers. All
probes
were inserted 1 inch into the container (FIG. 15B).
Seeds Germination Tests
Seeds germination rates were measured at the Oregon State
University Seeds Laboratory. The method used to germinate the alfalfa and
radish
seeds followed standards described in the Association of Official Seed Analyst
Rules For Testing Seed. Four hundred seeds were planted for each sample in 100
seed replications. The seeds were planted on brown towels moistened with
water.
The samples were placed in a 20 C germinator, and then evaluated after 7 days
germination using the Association of Official Seed Analysts Seedling
Evaluation
Handbook.
Microbial Tests
Total aerobic plate account (APC), coliforms, and E. coli of
controlled (unheated) or RF heated seeds were analyzed at the Laboratory
Services
Division, Oregon Department of Agriculture. Twenty-five grams of dry seeds
were
placed in a sterile blender jar, and 225 ml of Lactose Broth (diluent) was
added to
achieve a 1:10 dilution. Seeds and diluent was blended on high speed for 2
min, and
further decimal dilutions were prepared.
CA 02720494 2010-11-05
Seeds were plated in the tryptic soy agar (TSA) for total aerobic plate
count (APC) measurement. The plates were incubated at 32 C for 24 h. A spiral
plate method was used to determine APC in seeds as described by Maturin and
Peeler, Bacteriological Analytical Manual, 1998 (8th Ed., Rev. A), FDA
(published
5 by AOAC International).
The ColiComplete disc method (Biocontrol System, Inc., Bellevue,
WA) was used for the analysis of coliforms and E. co/i. Lauryl sulfate
tryptose
broth (LST) tubes were inoculated with appropriate sample dilution series
selected
to determine MPN levels or presence/absence of total coliforms and E. coli in
seeds.
10 One ColiComplete disc was aseptically added to each tube. Tubes were
then
incubated at 35 C (AOAC Official method 992.30, 1992). For total coliforms
reading, each tube was examined for visually detectable blue color on disc or
in
surrounding medium after 24 h incubation. Presence of blue color indicated
confirmed positive result for total coliforms. Tubes were reincubated at 35 C
for
15 additional 24 2h and reexamined. Continued absence of blue indicated
negative
result; presence of blue indicated confirmed positive result for total
coliforms. The
MPN code or presence/absence of total coliforms in the sample were read and
recorded. For E. coli reading, tubes were examined under longwave UV light
(366
nm) after 30 2h initial incubation. Fluorescence indicated confirmed positive
result
20 for E. co/i. The MPN code or presence/absence of E. coli in the seeds
were read and
recorded.
The results from the microbial tests are reported in FIG. 14.
Time-Temperature Profiles of Seeds Subjected to RF Heating
Regarding the first tests, relative fast and uniform heating was
25 achieved for both types of seeds (FIGS. 12 and 17). Heating rates
increased with
increased frequencies and electric field intensities as indicated by the
greater slopes
of temperature-time profiles obtained at 60 MHz and 0.85 kV/cm at a given
field
intensity and frequency, respectively. However, the heating rates achieved
from this
experiment were still lower than those suggested for optimal germination of
seeds
30 because of lower electric field intensity caused by limited RF power
supply, larger
CA 02720494 2010-11-05
56
physical sample size, as well as the electrode geometries and larger air gap
sizes.
Previous studies recommended to heat seeds from ambient temperature to ¨75 C
in
to 20 second in order not to kill seeds for germination. (Nelson, S.O.,
Stetson,
L.E. and Works, D.W., Hard-seed reduction in alfalfa by infrared and radio-
5 frequency treatment, Transactions of ASAE, 11(5):728-730, 1968). Thus,
the
second tests were targeted for faster heating.
By adjusting sample sizes, air gap, and configurations of seeds and
packaging, much faster heating was achieved during the second tests (compare
FIGS. 12 and 13). As shown in FIG. 13, heating rates ranged from 56 to 109
C/min
10 in the second tests, as compared to 4.66 to 20 C/min for the second
tests.
FIGS. 18 and 19 show temperature¨time responses for alfalfa seeds
exposed to RF field at the frequency range of 31.3 to 34.4 MHz. As shown in
FIG.
18, the maximum heating rate for the seeds was estimated to be 110 C/min
between
points A (108 sec, 26 C) and B (132 sec, 70 C), which compares favorably with
the
109 C/min rate shown in FIG. 13, considering measurement accuracy.
In addition to fast heating rates, high heating uniformity was also
achieved. FIG. 19 shows profiles of temperature vs. time using data from a
typical
test with alfalfa seeds. All probes were inserted 1 inch into the container.
During
the heating period (between the two dashed vertical lines), the measured
temperatures at the top, center, and bottom positions showed differences of
less than
2.5 C between the center and top position, and less than 1 C between the
center and
bottom position. The largest temperature difference was less than 6.5 C at
the end
of heating between the edge and top positions.
Seed Germination
The duration, final temperature and holding time at the final
temperature (i.e., how fast the seeds were cooled) influenced how much
moisture the
seeds lost during the heating process. A lower final moisture content in the
seeds is
directly related to a lower germination rate for the seeds.
The seed germination rates as related to RF frequency, electric field
intensity and final product temperature are shown for the first tests in FIG.
12. In
CA 02720494 2010-11-05
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general, without the ability to control impedance match, the seeds could not
be
heated as rapidly as possible, so too much moisture was lost, which decreased
the
seed germination rate. Also, without the ability to measure the seeds' final
temperature accurately, the final temperature may have risen too high, which
undesirably increased the moisture loss. In some cases, lack of temperature
uniformity throughout all of the sample(s) being heated could lead to some
samples
being heated above a desired final temperature and/or beyond an optimum
duration,
which would also undesirably increase the moisture loss.
The interactions between frequency and electric field intensity on
seed germination were found. At the lower field intensity of 0.55 kV/cm, seed
germination rate was similar at both frequencies, however, the higher
frequency of
62 MHz produced seeds having a higher germination rate than those treated at
the
lower frequency of 15 MHz. This may be caused by the rapid heating rate at
high
frequency.
The influence of final seed temperature on seed germination was not
consistent from this experiment although previous study indicated that the
final seed
temperature is critical to the seed germination rate (Nelson, 1968, 1976).
Again,
many other factors such as frequency and electric field intensity were
involved.
Thus, in the second tests, frequency was controlled to help maintain
impedance match. Specifically, the frequency was controlled within the range
of
31.2 to 31.6 MHz. Similarly, electric field intensity was also controlled to
be the
same for all runs.
The results from the second tests are reported in FIG. 13. Heating
rates were significantly improved in the second tests due to electrode
optimizations,
reduction in air gaps (to maximize the electric field intensity across the
actual
sample), reduction in sample size (and therefore thermal mass), and finally,
better
control of impedance match. As stated, frequency and electric field intensity
were
controlled for all the tests.
It was confirmed that the final temperature of seeds is certainly a
factor affecting germination rates. However, the longer holding and cooling
times
CA 02720494 2010-11-05
58
after heating finished were also a contributing factor to poor germination
rates.
Higher final temperatures plus long temperature ramp-down times all
contributed to
drying or moisture loss. Thus, cooling the seeds quickly after heating is
advantageous.
When final temperature exceeded 75 C, it caused a reduction in seed
germination rate as shown in FIG. 13 for the samples coded as RD-G4-01
(germination rate 32%), AF-G4-03 (germination rate 27%), and AF-G4-04
(germination rate 13%). This is consistent with previous findings that "the
most
important temperature is the final seed temperature raised by RF energy. For
seeds
in the normal moisture content of 6 to 7%, 75 C is close to the optimum for
increasing seeds germination" (Nelson, 1968, 1976). When final temperature is
too
high, seeds were actually killed.
Nelson also reported that the final temperature produced by optimum
RF treatment levels influenced final moisture content of seeds. Final moisture
content decreased as final temperature increased. In Nelson's findings, final
moisture content ranged from 9.8% at a final temperature of 49 C to a final
moisture
content of 2.8% at 100 C. Thus, it is important to control final seed
temperature
according to the moisture content of seeds. This shows the critical nature of
having
accurate temperature measurement capability, including accurate temperature
sensors.
Although relatively fast heating rates were achieved in the second
round of tests, germination rates on the seed samples are still much lower
than
expected. One of the possible reasons could be the seeds were held too long in
the
cavity after heating, i.e., they were not removed immediately for a quick
cooling
down.
In both experiments, it was found that seeds lost some of their
moisture as evidenced by decreased moisture contents after RF heating. As
shown
in FIG. 13, the moisture content of radish seeds reduced from its original of
6.27%
to about 4.5-5.4% and alfalfa seeds from 7.12% to 5.5-6.7%, depending on the
actual heating rate and final seed temperature. This was confirmed by observed
CA 02720494 2010-11-05
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moisture condensation on the inside of the top lid of the seed tray as a
result of
moisture accumulation. Previous studies indicated that moisture content is
important for seed tolerance to final heating temperature, in turn affecting
germination rate (Nelson, 1968). Generally, seeds of lower moisture content
responded more favorably to treatment than high moisture content seeds. This
finding indicated the need for rapid cooling of seeds to maintain their
original
moisture contents in order to achieve desired seeds germination rates. Also
this
finding shows the need to accurately determine the final temperature and allow
the
system to reach this temperature with precision.
Microbial Results
Total aerobic plate count (APC), coliforms and E. coli were measured
in the first round of experiment, and the results are summarized in FIG. 14.
No
significant differences were observed on APC numbers on radish seeds between
RF
treated and control samples. However, RF heating resulted in one log reduction
of
APC on alfalfa seeds, i.e., reduced from 105 to 104. Frequency did not affect,
but
higher electric field intensity of 0.85 kv/cm further reduced APC number from
4 x
104 to 1.3 x 104 at 15 MHz, and from 5.2 x 104 to 2.4 x 104 at 60 MHz,
respectively.
RF treatment has no significant effects in coliforms on both types of seeds.
The
number of E. coli was less than 3.6 no matter what the type of seeds,
frequency or
electric field intensity applied.
The microbial tests conducted in this study were not the challenge
tests. That is, seeds were not intensively inoculated with any bacteria. As
seeds used
for this study were relatively clean, it was difficult to see the exact
effects of RF
heating on microorganisms. According to seed sanitation standards from
International Sprout Grower Association, the number of E. coli on vegetable
sprout
must be less than 10/g. Seeds used for the tests did meet this standard.
Previous
research indicated that the major impacts of RF energy on microorganisms is
simply
based on heating, i.e., raised temperature kills bacteria. Kozempel, Michael
F.;
Annous, Bassam A; Whiting, Richard C., Inactivation of Microorganisms with
CA 02720494 2010-11-05
Microwaves at Reduced Temperatures, J. Food Prot., 1998, vol. 61(5), 582.
Thus,
the challenge tests did not apply in the second round of tests.
As stated above, there is strong potential for using capacitive
dielectric heating in pasteurizing packaged seeds. HPMC- or CC-based edible
films
5 could be a good choice of packaging films because of their reduced
sensitivity to a
radio frequency field. The inverse square root relationship of the loss
tangent with
frequency (at higher temperatures) would point towards the use of high
frequencies
(>30 MHz) for seeds. This is due to the positive linear dependence of the
power
density on both the frequency and the loss component of permittivity (loss
factor)
10 when dielectric loss mechanisms dominate.
Potential Manufacturing Process Flow Applications
There are several potential manufacturing process flow applications
of this technology in the food industry. These are shown in FIG. 8 in
schematic
15 form. The four manufacturing process flows shown in FIG. 8 could
represent the
following types of process applications:
A. Capacitive (RF) dielectric heating of a mixed particulate slurry (e.g.,
diced
vegetables in soup) where the device can be tuned to preferentially or
selectively heat the particulate material by targeting its Debye resonances.
20 Conversely, the device may be tuned to preferentially or selectively
heat the
formulated carrier medium by targeting its Debye resonances instead.
B. Capacitive (RF) dielectric heating of foods within a package (e.g.,
pasteurization of packaged surimi seafoods) where a medium can be heated
even though it has already been packaged within an electrical and thermal
25 insulator.
C. Capacitive (RF) dielectric heating action targeted toward in-situ
surrounding
micro-environment of packaged products. In such cases, secondary
influences motivated by and resulting from RF interaction with in-package
atmosphere (e.g., elemental gases, gaseous molecular compounds, aerosols,
CA 02720494 2010-11-05
61
liquids and/or fluids) result in the generation of quality enhancing,
preserving
and/or pasteurization effects.
D. Elicitation of live culture with RF energy to stimulate production
of cellular
biomass, intra- or extra-cellular metabolites, and/or fermentation products.
General Aspects
The capacitive (RF) dielectric heating system will have power control
and voltage/electric field level control capabilities as well as potentially
contain a
gridded electrode arrangement (see FIGS. 6-7) to allow for precise control of
the
field strength vs. time and position in the heated sample. This will allow for
the
heating of small and large product geometries as well as composite and non-
uniform
product configurations to prevent arcing, burning and thermal runaway
problems.
This may be especially important for thawing applications, and for surimi and
packaged seed products that may come in non-regular product geometries.
The capacitive (RF) dielectric heating system will be compatible with
all of the existing production flow schemes described for meat and fish
products and
extend those industrial and production process capabilities where applicable
to allow
for the treatment of seeds.
The capacitive (RF) dielectric heating system will be designed in
such a way as to be compatible with export sterilization and pasteurization
processes
that insure that quality foods are shipped abroad.
The capacitive (RF) dielectric heating technology will be developed
so as to allow for household "counter top" solutions to allow for the
home/kitchen
broad based sterilization and pasteurization of all types of foods that
consumers use.
This solution will be a competitive product to microwave ovens but will
operate at
lower frequencies where electric field penetration is deeper and more uniform.
In addition to the above examples of various manufacturing process
flows, there also exists the potential of using this technology in combination
with
other heating technologies to improve product quality, process productivity,
and/or
energy efficiency. This technology will allow production floor space to be
reduced
and throughput to be increased compared to conventional hot water heating
systems.
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62
Processes that might benefit from the superposition of RF exposure
include liquid-solid and liquid-liquid extraction, miscella and phase
separation,
pressure and vacuum treatments, aeration, and the like.
By certain processes, it is possible to obtain uniform heating over the
entire volume of a food product. This technology heats the polar dielectric
molecules evenly and directly over the entire volume of the sample as opposed
to
hot water and steam heating or cold water thawing technologies that rely on
thermal
conduction from the surface of the medium. Capacitive (RF) dielectric heating
offers more uniformity than higher frequency microwave ovens due to the lower
RF
frequency range which, for the purpose of this disclosure, is defined as being
about
300 KHz to 300 MHz, with best results at about 1 MHz-100 MHz, allowing for
deeper penetration into saline media as well as the fact that the media is
small
compared with a wavelength and so capacitive heating does not rely on the
complex
wave propagation and reflection modes required in a microwave oven. The field
patterns are generally simple 2-D patterns between parallel plates again
resulting in
more uniformity in heating.
As one example, it is difficult to thaw large packages of foods
quickly and uniformly with minimal degradation of the quality of the food,
such as
in the case of thawing large pieces of frozen meat. Conventional hot water
bath
thawing processes not only take a long time, but also could alter the quality
of the
meat. Microwave heating could not be used for large packages because it does
not
penetrate deeply enough. With capacitive (RF) dielectric heating, however,
sufficient penetration is possible, and the automatic impedance matching with
a
gridded electrode system ensures that heating is fast and uniform. These
characteristics of RF heating may be particularly advantageous in the real-
time
preparation of food, such as by restaurants and others in the food service
industry,
since RF heating outperforms other known technologies.
Capacitive (RF) dielectric heating of food is a clean process with no
generation of wastewater. Capacitive (RF) dielectric heating systems according
to
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63
the present invention offer numerous advantages over hot water and steam
heating
(for pasteurization or sterilization) and cold water thawing technologies and
other
heating technologies.
Capacitive (RF) dielectric heating is less expensive (on a cost per
pound of treated food basis) than irradiation or high pressure technologies
for heat
pasteurization.
Capacitive (RF) dielectric heating will heat a food sample even when
packaged in a thermal and electrical insulator as well as if the sample is
thick and
unusually shaped due to the fact that the internal polar molecules are being
heated
and again therefore no reliance on thermal conduction from the surface. Hot
water
and steam technologies require flat geometries with very thin packaging or
insulating layers.
Capacitive (RF) dielectric heating as described above allow for
heating of food within packaging without developing high local power
densities,
such as the concentration of a small amount of food/water within the seam of a
packaging material, which might result in a small thermal mass combined with a
large dielectric loss factor being exposed to a potentially large localized
voltage
gradient.
With the methods and apparatuses described herein, it is possible to
avoid the potential disadvantages of capacitive (RF) dielectric heating
methods
mentioned above. According to the first approach, the potential limitations
are
addressed by providing frequency control to match Debye resonances or other
parameters of the dominant constituents of the medium, track them with
temperature, control field strengths and optimize product geometries to
prevent
arcing. According to the second approach, automatic impedance matching ensures
that the effective adjusted load impedance is matched to the output impedance
of the
signal generating unit, thereby ensuring that the load is heated with maximum
energy (thus yielding a shorter heating time).
To prevent or reduce the risk of thermal runaway, a gridded electrode
system can be used with an infrared scanner to monitor the entire body of a
food
CA 02720494 2013-03-28
. .
64
product being heated. And, the electrode system may be constructed to lower
the
field strengths on the packaging seams (and packaging methodologies can be
used to
keep food out of seams to avoid heating the seams). In response to signals
from the
scanner, individual components of the food product can be independently heated
by
adjusting local field strengths or by switching some portions of the grid off
in
different duty cycles to prevent "hot spots".
The scope of the claims should not be limited by the preferred
embodiments set forth in the examples, but should be given the broadest
interpretation consistent with the description as a whole.
