FST4102 Literature Review on Radio Frequency Heating
34
technology in industrial applications, it requires higher costs and may give more industrial
complications. As so, it is forecasted that RF heating will not be popularized by food industries
in the near future. Therefore, RF heating may need to combine with other thermal treatment
although this may negate the original goal of non-thermal processing to eliminate the use of
elevated temperatures during processing to avoid the adverse effects of heat on the flavour,
appearance and nutritive value of foods (Barbosa-Canovas et al., 1999). RF heating like other
novel non-thermal technologies can be used solely only when the technology has reached a
certain advanced stage where it can confidently claim to produce safe and commercially sterile
food.
Given the current limited research on RF heating, it is suggested that more research
should be done to find out dielectric properties of more foods. Furthermore, there should be more
studies to investigate the effect of RF processed foods on human health. To date there are no
studies done on investigating the effects on human health after consuming food processed by RF
heating or safety of system operators during processing. Lastly, studies should also be done in
combining RF heating systems with other food processing systems to offset the disadvantages of
this new heating technology.
8 References
2005. Statutory Instrument 2005 No. 281. In The Electromagnetic Compatibility Regulations. ANONYMOUS. 1993. Radio frequency ovens increase productivity and energy efficiency. In
Prepared Foods, p. 125. APV. 1995. Cooking with Radio Frequency. Meat International, 5, 10–11. AWUAH, G. B., RAMASWAMY, H. S., ECONOMIDES, A. and MALLIKARJUNAN, K.
2005. Inactivation of Escherichia coli K-12 and Listeria innocua in milk using radio frequency (RF) heating. Innovative Food Science & Emerging Technologies, 6, 396-402.
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RF heating like most other novel non-thermal technologies are still in their early stages of
development although some of these emerging non-thermal processes have now been
implemented in industrial-scale systems for commercial and research applications (Mermelstein,
1997, Kempkes, 2001, Satin, 2002). Therefore, in order to increase the acceptability of
consumers to this new technology, it is recommended to use RF technology to complement
traditional heat processes for production of safer foods (Ohlsson, 1994), as the advantages of
several heating methods can be articulated while the drawbacks of one heating method can be
offset by the other processes. Wild-Indag Process Technology in Germany combined RF heating
with ohmic and microwave heating in a prototype machine (ElAmin, 2006). This allowed liquid
parts of food heated quickly via current, whilst the chunks in the product being heated via RF
waves. In addition, the combining of lethal heat treatments with RF processes might help
eradicate problematic microbial subpopulations that show high resistance to RF heating
(Patterson et al., 1995)
7 Conclusion
This literature review has discussed the principles of RF heating, types of processes and
equipments, compared the advantages and disadvantages of RF heating with microwave heating
and ohmic heating, reviewed successful applications of RF heating in both laboratory testing and
commercialisation, and forecasted the future developments of RF technology.
RF heating has been successfully employed in food industries for cooking of meats, post-
baking, drying, tempering, thawing, pasteurization, sterilization and pest control. RF heating is
advantageous over other heating technologies as it can heat foods volumetrically and uniformly,
with greater penetration power and lower process time. However, as RF heating is still a new
FST4102 Literature Review on Radio Frequency Heating
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However, food processing systems that combine RF with other heating methods may have a
larger possibility of being employed.
Various reasons contributed to the low popularity of RF heating. Firstly, the equipment,
processing and operational costs of RF heating systems are high, but the food that are processed
often have low retail price, like vegetables and bakeries. With low research budget on RF
heating, there are little related studies; information like the dielectric properties of foods is
scarce. Moreover, as RF heating system is relatively new in the industry, there are many
complications during industrial application. Larger floor space is required to give the same
energy output.
Given the current research, majority of the paper focuses on the efficiency of RF heating
on specific food items. There were limited studies that mentioned about the equipment design of
the RF heating system to optimise the heating process. Take for instance, different food might
require an alternative frequency to achieve uniform heating. Moreover, the formulation of the
food product will also affect the heating patterns in the RF system, which would be useful if
these topics can be looked deeper into.
At the same time, food industries and consumers are often conservative towards new food
processing technologies (Garcia et al., 2007). It is likely that novel technologies like RF heating
maybe shunned from the public due to lack of awareness and understanding. Taking from the
example of MW heating, which also employs electromagnetic wave for heating, there are many
concerns on health implications: from side-effects of consuming microwave-processed foods to
leakage of radiation from microwave ovens.
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methods are (1) fast and potentially more uniform heating that can lead to the development of
continuous treatment processes, (2) ability to treat walnuts sealed in plastic containers to avoid
post-treatment contamination, and (3) leave no residues on products and no chemicals to dispose
off (Tang et al., 2000).
Table 2: Summary of successful application of RF heating for food processing Process Frequency, MHz Food Items References
9 Boned Ham Pircon et al.,1953 60 Lean Ham Bengtsson and Green, 1970 27 Sausage emulsion Houben et al., 1991 & 1994 27.12 Milk G.B.Awuah et al. , 2005
Pasterization & sterilization
27 Macaroni and cheese Y.Wang et. al., 2003 13.56 Ham Tulip International, 1995 27.12 Beef rolls X. Tang et al., 2006
Cooking
27.12 Communited pork product N.P. Brunton et al., 2004 60 Cocoa beans Cresko and Anantheswaran, 1998 Drying 70 Decaffeinated coffee bean United States Patent 3989849 27 Cookies Tom’s Food, 1993 27.12 Cereal Weetabix, 1994 40 Biscuits and crackers Radio Frequency, Inc.
Post-Baking
- Pasta United States Patent 6428835 Tempering 27.12 Butter Keam Holdem ,1993
14-17 Egg, vegetable and fish Cathcart et al., 1947 36-40 Fish Jason and Sanders, 1962
Thawing
36-40 Meat Sanders, 1966 27.12 Persimmon Fruit Monzon et al., 2007 27 Cherries Ikediala et al., 2002 27.12 In-shell walnuts Wang et al., 2007a,b
Pest Control
27 Apples Hansen et al., 2006 6 Future of radio frequency heating
After comparison of RF heating with other food processing methods and reviewing of the
application of RF heating in both laboratory scale and industrial scale, it is suggested that RF
heating would not be highly accepted and utilized in the food industry at the current moment.
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Furthermore, results had also shown that the most promising RF protocol to obtain
phytosanitary control of Mexican fruit fly in persimmon fruit is RF heating to 48oC for 6 mins,
whereby the fruits were able to tolerate exposure of 12 min without significant injury. Ikediala et
al. (2002) had also showed the RF treatment mentioned in the study using a 6 kW, 27 MHz pilot-
scale RF system (COMBI 6-S, Strayfield-Fastran Ltd., Wokingham, UK) was able to achieve
100% codling moth larvae mortality in cherries with little or no quality reduction. Large scale
and confirmatory tests are needed to enable the establishment of a quarantine protocol for fresh
fruits using RF technique.
In the studies by Wang et al. (2007a, 2007b), an A25 kW, 27.12 MHz industrial-scale RF
unit system (Model S025/T, Strayfield International Limited,Wokingham, UK) is used to
conduct the industrial-scale confirmatory treatments of commercial insect control technologies
for in-shell walnuts using RF energy. RF treatments provide a major advantage over hot air
heating for in-shell walnuts, because of significant thermal resistance in the porous walnut shell
and the in-shell void that hinder the transfer of thermal energy from external hot air to the walnut
kernel. In addition, heating uniformity is one of the most important considerations in scaling-up
the established treatment protocol for walnuts. However, temperature variations after RF heating
may result from variations in thermal properties and moisture contents of walnuts and a non-
uniform electromagnetic field. Nonetheless, results showed that mixing of the product between
two RF exposures and circulated hot air were required to optimize heating uniformity.�With the
treatment, the TDT curve showed that 5 min exposure to 52oC or 1 min exposure to 54oC should
result in 100% mortality of insects without adversely affecting product quality, thus
demonstrated efficacy of RF treatments as an alternative to methyl bromide fumigation (Wang et
al., 2007b). The advantages of RF heating for walnuts compared with conventional heating
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submerged in a saline solution were heated with a 27 MHz, 12 kW batch RF machine (Strayfield
International Limited, Wokingham, U.K.). Figure 7 illustrated the temperature distributions
inside an orange measured with the infrared thermal imaging technique when subjected to RF
heating for 5 and 10 min, and to hot water and hot air heating at 53 �C for 10 and 20 min (initial
fruit temperature, 20 �C). Birla and co-workers had shown that RF heating resulted in fairly
uniform temperatures over the entire orange and achieved the target temperature in a short time.
On the other hand, with the hot water and hot air treatments, a large temperature gradient was
observed from the surface to the core.
Figure 7: Illustration of temperature distributions inside an orange for various treatments.
RFHeating
HotWater
HotAir
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end-user industries for products, such as meat, fish, poultry, cheese and butter blocks, and whole
or pulped fruit. It is recommended to choose RF energy at 13 MHz, 27 MHz or 40 MHz if the
product has significant moisture content.
5.5 Pest Control
RF energy has long been used in studies to kill insect pests by heating them beyond their
thermal limits (Headlee and C., 1929, Frings, 1952, Nelson and Payne, 1982), with one of the
chief problems being lack of uniform heating (Tang et al., 2000). In recent years, interest in
using non-chemical control methods such as heat treatments for pest control in harvested fresh
and stored agricultural commodities increases in the wake of regulatory actions over the use of
pesticides, especially the limitation on the use of methyl bromide in fruits and nuts. RF heating
has been proposed as a potential alternative to chemical fumigation (Tang et al., 2000).
There are a number of laboratory and pilot scale studies focused on fresh fruits (Monzon
et al., 2007, Hansen et al., 2006, Birla et al., 2004, Wang et al., 2003b, Ikediala et al., 2002). RF
heating has the advantage of direct heating of internal pest, thus shortening the exposure of fruits
to high temperature. Fresh fruits easily suffer thermal damage at the points of contact with the
container or with other fruit when heated with RF energy in air due to overheating caused by a
concentration of electric fields around the contact areas (i.e. contact surfaces have the least
resistance to RF energy). Hence, the fruits have to be placed in a medium (e.g. saline water) that
has similar dielectric properties to fruit to overcome the markedly large temperature differential
problem associated with RF treatments in air, thus avoiding overheating of fruits and improve
heating uniformity (Wang et al., 2003a). In the study by Monzon et al. (2007), persimmon fruits
FST4102 Literature Review on Radio Frequency Heating
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more uniform and often self-limiting. Other advantages include improved quality (colour and
flavour), no temperature differential, selective heating, moisture equilibration, space saving,
higher efficiency and precise power control and quick response.
5.4 Tempering and thawing
RF heating at the 10–300 MHz range can be used to raise the temperature of product
rapidly and precisely from frozen solid to a higher temperature (i.e. 0°C) so the food matter can
then be processed. Unlike conventional heating which has the problem of overheating on the
product surface due to the poor thermal conduction of frozen foods, RF heating produces a
uniform temperature rise throughout the entire volume of the food. RF heating also suits well for
tempering frozen food in its packaging due to its large volumetric penetration depth.
Thawing of frozen eggs, fruits, vegetables, meat and fish using RF heating had long been
studied with both pilot and commercial scale RF unit operated at frequencies of 14-17 MHz
(Cathcart et al., 1947) and 36-40 MHz (Jason and Sanders, 1962), respectively. Results from
both Cathcart et al. (1947) and Jason and Sanders (1962) showed that RF thawing times were in
minutes as compared to hours in conventional thawing; and better resulting quality was reported
due to lower drip losses, minimal discoloration and loss of flavour for RF thawing.
Keam Holdem Associated Ltd (1993) has done on the tempering of both salted and
unsalted 25 kg butter blocks using a continuous radio frequency tunnel at RF frequency of 27.12
MHz. Results showed that frozen butter blocks was tempered directly in the package from -14oC
to 0oC, whereby they were frost free and ready for further processing. Currently, companies such
as Keam Holdem (KHA) has a wide range of tempering equipment suitable for a wide range of
FST4102 Literature Review on Radio Frequency Heating
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RF heating has also been proven effective in post-baking drying of biscuits such as
cookies, crackers, appetizer snacks, sponge cakes, puff pastry and breakfast cereals
(Mermelstein, 1998). The vast majority of machines installed for post-baking applications over
the years have operated in the 27.12 MHz internationally accepted (ISM) frequency band, while
a number of machines have also been installed operating at 40.68 MHz.
Tom’s Food (Cresko, J.W. et al., 1998) installed a RF oven used to remove the moisture
from post-baking crackers and cookies. The oven operates at a frequency of 27 MHz, with
crackers having 3.5-4.5% entering into the 17-foot RF oven, at a approximate temperature of
212oF, that resulted in an efficient and uniform removal of excess moisture from the crackers
with repeatability in mass production and had moisture-level accuracy of ±0.2% also without any
discolouration or flavour damage. Weetabix breakfast cereals (Nelson, 1994) also uses RF heater
at 27.12 MHz that results in a rapid heating or drying of food. The procedure of using the RF
heater, involves the passing of the cereals after moulding in rows of twelve onto the feed belt of
the initial bake oven which are 2 units of 50 kW RF ovens in sequence, which helps in the
removal of residual moisture from the centre of the central biscuit. To add on, as the heating rate
in RF heating is proportional to the amount of moisture, this thus provides control over moisture
uniformity throughout the entire thickness of a baked good and also eliminates cracking
(checking), caused by the stresses of uneven shrinkage in drying.
Macrowave™ 7000 is a low voltage operating commercial post-baking dryer operated at
40 MHz. It generates heat inside of baked goods, resulting in a completely uniform moisture
profile. This capability enables the baker to increase processing speed and improve throughput
by as much as 30 and 40% (Clark, 1997) without sacrificing quality because drying action is
FST4102 Literature Review on Radio Frequency Heating
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was found to be unfeasible and therefore it was necessary to surround products with heated water
during RF cooking. As such, a cell made from high-density polyethylene was required to hold
the product and allow water circulation to facilitate uniform heating of the product.
5.3 Drying
RF drying applications in the food industry include the drying of food ingredients (e.g.
spices, herbs, and vegetables), heat sensitive granular foods (e.g. packaged flours, coffee beans,
cocoa beans, corn, grains and nuts), potato products and a number of pasta products (Ohlsson
and Bengtsson, 2002, Punidadas et al., 1998).
Preliminary result from a vertical RF unit operated at 6.5 to 7.8 kW at 60 MHz showed
that RF heating is capable of roasting cocoa beans at 130oC, and reduced the moisture content
from 6% to 1% (Cresko and Anantheswaran, 1998). Furthermore, improved flavour of
commercially decaffeinated coffee can be accomplished by rapidly drying the wet beans from
initial moisture content of 52% to 10% in a dielectric unit with maximum output of 5 kW,
operating at 70 MHz (Debbouz and Matuszak, 2002). RF drying has been proposed as an
alternate method for drying American ginseng (Information Resources Inc. 2004). It is possible
that RF drying may result in superior retention of nutrients, flavours and medical components,
along with food safety. Pending issues of the use of RF heating to dry certain products such as
soy and coffee is that these products dry slowly as they contain a substantial amount of oil and
tars that hinder the diffusion of moisture. Also, they are less responsive to the electric field
strength.
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Tang et al. (2006), has done further investigation on baking of meat, where it uses RF at
500 W, 27.12 MHz to heat 1kg of encased beef rolls under the recirculation of H2O at 80oC.
Results shows significant reduction of cooking time to 23% and 31% of steam and water cooking
times respectively in non-injected meat and in rolls prepared with curing brines having similar
dielectric properties. Higher cooking yield was obtained and toughness of meat was lower than
other method without any sensory difference from that of steam and water cooking.
Studies have been done on the effect of radio frequency heating on the sensory aspects of
different types of food, to evaluate the acceptability of the food quality as compared to other
cooking method. For instance, Tang et al. (2005) did a study on the effect of RF heating on
chemical, physical and sensory aspects of quality in turkey breast rolls compared against
conventional steam oven cooking. From the study, the texture profile analysis (TPA) revealed
that there is no significant difference (P � 0.05) between the two cooking methods. However,
from the sensory test, the sensory panel could distinguish between RF-cooked and steam-cooked
rolls, but panellists did not express a preference for rolls cooked by either method. Therefore this
study showed that RF heating is capable of producing food products of similar quality to other
cooking methods and with short cooking duration.
Beside ham and beef, RF heating has also been used in the study of effects on
comminuted pork meat product (Brunton et al., 2005). It uses radio frequency at 450 W, 27.12
MHz to heat pork based white pudding. It resulted in mean end-point temperature of 73oC after 7
min 40 s which is similar to that in water bath and steam oven heated products which were
achieved only after 29 and 33 min, respectively. There was also no significant difference in
texture and colour between different cooking methods. However, RF cooking of products in air
FST4102 Literature Review on Radio Frequency Heating
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Figure 6: 2 kW with 27.12 MHz of RF heater (50� Radio Frequency System)
Wang et al. (2003c) developed high temperature short time sterilization protocols for
foodstuffs using RF dielectric heating at 27 MHz. Results showed that a lethality (F0 = 10 min)
was achieved in both food models (macaroni and cheese) and of better quality within 30 min
with relative uniform heating, compared to a 90 min conventional retort process that delivered a
similar lethality.
5.2 Cooking
Cooking is another processing method that RF heating has been explored into. In 1995,
Tulip International, a division of the Danish Company, APV Pasilac (1995) installed two RF
cooking lines for ham production in Denmark. A 50 kW unit was used for test production while a
12 kW capacity unit was used for commercial production. Both units operated at 13.56 MHz.
The meat product was heated up in a cooking system to the target temperature of 70oC. The
entire process lasted only 3 min and produces reduced juice loses and better ham texture.
FST4102 Literature Review on Radio Frequency Heating
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min. However, the process has not been used commercially, probably because of high cost and
problems of designing processing cells or cans for large-scale operation.
Following then, Bengtsson and Green (1970) developed continuous RF pasteurization of
cured hams packaged in Cryovac casings at 60 MHz. A conveyor fed the material between the
electrodes of the load condenser with 1 kW output generator. The energy efficiency obtained was
about 25% at 60 MHz. For 0.91 kg lean ham, treatment time reduced to 1/3 its initial value by
heating in a condenser tunnel at 60 MHz. Lower drip losses and better quality were obtained as
compared to traditional processing in hot water.
Houben et al. (1991, 1993) described continuous RF pasteurization of sausage emulsion.
Heating experiments were performed at 27 MHz using 2 power generators at 25 and 10 kW with
coagulated type sausage emulsion of various compositions. It resulted in treatment time of
sausage emulsion from 15 to 80oC at a mass flow rate of 120 kg/h for 2 minutes and heating rate
up to 40oC/min was achieved, compared to about 1oC/min at the center of a 50 mm diameter
sausage during a conventional heating process indicating a faster cooking period and better
quality products are achieved.
Besides meat, RF heating has also been used for the pasteurization for other foods such as
milk (Awuah et al, 2005). In the research, a 2 kW with 27.12 MHz of RF heater as shown in
Figure 6 was used to evaluate the effectiveness in inactivating surrogates of both E. coli and
Listeria innocua in milk under continuous flow. RF heating has shown its capability in
inactivation of the both microbes, giving a total residence time of 55.5 s and up to 5 and 7 log
reduction of Listeria innocua & E. coli respectively at 1200W with 65oC.
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5 Application of radio frequency heating in food processing
Application of RF heating has been in the food industry for more than 60 years since
1940s (Anonymous, 1993, McCormick, 1988). The first attempts for RF heating application
were done on processed meat, then to bread and vegetables (Moyer and Stotz, 1947, Kinn, 1947)
and followed by thawing of frozen products during 1960s (Anonymous, 1993, Jason and
Sanders, 1962). With further investigation and development, juices (peach, quince and orange) in
bottles moving on a conveyer belt through an RF applicator had shown better bacteriological and
organoleptic qualities than juices treated by conventional thermal methods (Demeczky, 1974).
From the various applications as shown above, it is evident that RF heating has the potential to
improve the quality of food products and is more superior than other conventional methods.
However, there are still no full commercialization RF heating in food processing due to the high
operational cost of using RF and technical problems such as dielectric breakdown and thermal
runaway heating. Following is a collection of information on RF applications in food categories
based on the different processing methods.
5.1 Pasterization and sterilization
According to Sun (2006), no commercial RF heating systems for the purpose of food
pasturization or sterilisation systems are known to be in use. RF pasteurization was first done on
meat since 1950 with further researches and improvement to be done so as to allow the
commercialization of the technology. Firstly, Pircon et al. (1953) described a process for
sterilizing boned ham at 9 MHz using an industrial model of a 15 kW oscillator, 56.6% of energy
conversion efficiency was achieved, and that 2.7 kg of meat could be heated to 80oC in about 10
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microbiology and economics) and government was formed in 1992 to develop products and
evaluate the capabilities of the OH system (Parrott, 1992). A wide variety of shelf-stable low and
high-acid products, as well as refrigerated extended-shelf-life products were found to have
texture, colour, flavour, and nutrient retention that matched or exceeded those of traditional
processing methods such as freezing, retorting, and aseptic processing (Parrott, 1992; Zoltai and
Swearingen, 1996). A later study by Caristo et al. (2004) shows that the vitamin level of the food
was not compromised when using OH in strawberry products. The presence of low intensity
electric fields (< 20 V.cm-1) does not affect the ascorbic acid degradation. There were no
literature making direct comparison on the effect of food quality undergoing OH and RF heating.
Skudder (1993) suggests that OH is able to better retain flavour and particulate integrity than
conventional processes.
Overall, RF heating presents similar advantages to ohmic and microwave heating which
are essentially due to the generation of heat throughout the volume of the material to be
processed. However, it possesses additional strengths that other electroheating technologies do
not have as mentioned above. The utilization of electroheating technologies in the food industry
are still at its early stage of development. Additional researches are needed to fully understand
dielectric properties of different food products and improve the sensory quality of food processed
by RF heating.
FST4102 Literature Review on Radio Frequency Heating
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evidence that OH may be useful for shortening the time during yoghurt processing and cheese
production.
Although, the OH technology are used to mainly process liquids, the application to solid
meat products has not yet found industrial application (Piette et al., 2004). Brunton et al. (2006)
studied OH in meat products and revealed the difficulties to apply OH due to the uneven
distribution of fats and lean meat which caused the complexity of the electrical conductivity of
the product. Though meat products are more efficiently heated using RF heating, increased
hardness of the meat product and poor appearance (less well done colouration) are associated
with RF heated meat product. Therefore, OH of meat product will be more favourable than RF
heating but less favourable than conventional heating due to the unpredictability of the electrical
conductivity.
The major drawback of OH is that food product needs to be in contact with electrodes,
which will pose safety problem because of the shortage of inert electrodes. On the contrary, RF
heating can be easily applied to both solid and liquid foods. In addition, as mentioned earlier the
heat generation rate is influenced by the electrical heterogenetity of the particle, heat channeling,
complex coupling between temperature and electrical field distributions, and particle shape and
orientation. All these make the process complex and contribute to non-uniformity in temperature,
which may be difficult to monitor and control (Ruan et al., 2001). Electrode degradation and
uneven heating of the product are also associated to the early commercialization attempts.
4.3.2 Impact on the food quality
In the United States, a consortium of 25 partners from industry (food processors,
equipment manufacturers, and ingredient suppliers), academia (food science, engineering,
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RF heating technology has its own strength and weaknesses. Laycock et al. (2003)
revealed RF heating at 27.12 MHz could reduce cooking time by up to 90% in whole, minced
and comminuted beef, however the eating quality and texture is adversely affected. On the other
hand, RF heating was found to be capable of inactivating both Listeria innocua and E. coli in
milk, with E. coli being the most heat sensitive of the two (Awuah et al., 2005), however more
study is needed. Therefore, more comprehensive studies are needed so as to find out the effects
of RF heating on the quality of foodstuffs.
4.3 Ohmic heating
Ohmic heating (OH) uses the resistance of liquid or solid products to convert the electric
energy into heat (Fellows, 2000). The rate of heat is directly proportional to the intensity of the
electric field and to the electric conductivity of the sample. The efficiency of OH is dependent on
the conductive nature of the food to be processed (Zoltai & Swearingen, 1996) and hence
knowledge of the conductivity of the food as a whole and its components is essential in
designing a successful heating process. Possible applications include most of the heat treatments
such as blanching, evaporation, dehydration, fermentation as well as pasteurization and
sterilization. The OH is widely applied to liquid-particulate food system (e.g. low acid ready-to-
eat meal) as well as fresh produces such as fruits, vegetables, meat products and surimi.
4.3.1 Advantages and disadvantages of the ohmic heating and radio frequency heating
In terms of cost, an OH system is equivalent to RF; whilst MW heating system has a
lower investment cost (Vicente and Castro, 2007). The even and rapid heat distribution
throughout the product reduces processing time. A study done by Cho et al. (1996) provided
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In general, faster heating rates can be achieved in MW than in RF heating. However, RF
heating has much greater penetration ability, hence making this technology ideal for large, thick
foods compared to only small, regular-shaped for MW heated foods. For normal wet foods, the
penetration depth from one side in MW heating is approximately 1-2 cm at 2450 MHz. In
unfrozen meat, the penetration depth in MW frequencies is only a few millimeters whilst ten
centimeters in radio frequencies.
4.2.2 Impact on food qualities
In microwave heating, the advantage of fast heating throughout the food gives reduced
cooking time; hence lowering the temperature burden to the food. As a result, the loss in heat-
sensitive vitamins in MW heated foods comparing to conventional heating is minimized
(Ehlermann, 2002). Also, Maillard reaction may be reduced in the extent of chemical reaction
giving both positive and negative result, depending on the food product it is applied on
(Ehlermann, 2002).
RF gives uniform heating for its volumetric heating effect and selective nature with
energy being dissipated accordingly to the loss factor. Thus, the RF heating prevents the
problems with surface over-heating or hot or cold spots as encountered in MW heating. The
improved control of RF heating in responding instantaneously to the electric field makes the
controlling of food quality during processing feasible. The variation in dielectric loss factor with
moisture in foodstuffs is generally larger at radio as compared to microwave frequencies. This
allows more efficient water removal at the final baking and drying processing steps.
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4.2 Microwave heating
RF heating is commonly applied on thawing of frozen foods, tempering, post-baking
drying, pasteurization, cooking and roasting. On the other hand, MW heating is usually applied
in baking and cooking, tempering, drying, pasteurization and sterilization. The frequencies
selected for domestic, industrial, scientific and medical applications are 13.56, 27.12 and 40.68
MHz in RF heating and 915 MHz, 2450 MHz, 5.8 GHz and 24.124 GHz in MW heating (Awuah
et al., 2005).
4.2.1 Advantages and disadvantages of microwave heating and radio frequency heating
Both RF and MW heating utilize energy efficiently, hence increase throughput. They are
non-ionizing. MW heating has the advantages of high heating rate, design-freedom, less
sensitivity to food heterogeneity, much research and development (R&D) available and well
documented information on dielectric properties. Both RF and MW heating require high
investment cost. MW heating needs more engineering whilst simpler construction in RF heating.
RF heating needs greater floor space as compared to MW heating. Besides, the narrow frequency
bands in RF heating due to its waves lying in the radar range might interfere with the
communication system, limiting its frequency selection. Also, there is a lack of R&D support
and the data on dielectric properties in RF heating. There is also a risk of arching in RF.
However, the advantage of RF heating (simple, uniform field patterns) over MW heating
(complex, non-uniform standing wave patterns) is that it is more easily understood and
controlled.
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Table 1: The properties of each novel thermal technology
Types Advantage (s) Disadvantage (s) Specific use Impact on food quality
Using steam or water
� Cheap � Simple Setup
� Surface overheating on solid or viscous food
� Undesired quality change
� Reheating � Drying � Cooking � Blanching � Baking � Sterilization and
pasteurization
� Flavor loss � Quality loss
Microwave heating (MW)
� Non-ionizing � Efficient energy utilization � Rapid heating � Extensive availability of data on dielectric properties � Not quite sensitive to food heterogeneity
� Limited penetration (unsuitable for large, thick foods)
� High investment cost � More engineering
adjustments are needed
� Thawing and tempering
� Reheating � Drying � Cooking � Blanching � Baking � Sterilization and
pasteurization � Pre-cooking
� Loss in heat sensitive vitamin is minimized due to reduced heating time
� Maillard reaction may be reduced (lack browning in baked foods)
� Surface over-heating or hot or cold spots
Ohmic heating (OH)
� Uniform and rapid heating in the absence of temperature gradients (if the resistance of solids and liquids are the same) � No localised over-heating � Suitable for viscous liquids (heating is uniform and does not have the problems associated with poor convection in these materials)
� Safety (lack of suitable inert electrode materials and controls)
� Non-uniformity of the heat generation rate may be easily affected by the electrical heterogeneity of the particle, heat channeling, complex coupling between temperature and electrical field distributions and particle shape and orientation
� Calculation of heat transfer is very complex
� Drying � Cooking � Blanching � Baking � Sterilization and
pasteurization
� Superior food appearance as compared to conventional thermal heating
Radio frequency heating (RF)
� Specific advantages of RF over those alternative volumetric technologies, namely (Vicente and Castro, 2007) � No need for electrodes � Greater penetration power (suited for thick and large food) � Simpler construction of large industrial scale application as compared to MW
� Higher operational and processing costs
� Need large floor space � Risk of arching in RF � Narrow frequency bands due
to likelihood of interference with the communication system
� Limited R&D support � Lack of data on RF dielectric
properties
� Tempering � Post-baking � Drying � Pasteurization � Cooking � Roasting
� Uniform heating, hence better control on food quality (texture, colour, taste formation etc)
� Improved moisture levelling to yield better quality product at the final stage of baking and drying
FST4102 Literature Review on Radio Frequency Heating
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4 Comparison of radio frequency heating with other heating methods
The earliest means to heat food relies on the slow conduction of heat through its surface.
During the 20th century, new technologies that no longer rely on slow conduction heat transfer
have been developed as summarized by Jamieson and Williamson (1999); Fellows et al. (2002);
Vicente and Castro (2007). These novel thermal processing technologies generate heat
volumetrically throughout a product using electromagnetic waves. Heating extends within the
entire food material independent of heat diffusivity and thermal conductivity. RF heating,
microwave heating and ohmic heating are examples of these novel technologies in food
processing. The strengths and weakness of each method are summarized in Table 1. The
following paragraphs compare the other thermal processing technologies with RF heating.
4.1 Conventional Thermal Processing
Thermal processing affects the quality of food significantly (loss of flavour, fresh
appearance, vitamins and minerals). The ability to process foods under the high temperature-
short time (HTST) concept will optimize the quality of the food. Rapid heating using RF
technology can often reduce to less than 1% of that required using conventional techniques
(Meredith, 1998). Besides that, RF heating offers several other advantages over conventional
heating methods in food application (Sun, 2006). RF heating does not generate by-products of
combustion and thus is environmental-friendly. Its efficient heat transfer increases heat
production without an increase in overall plant length, as efficient heat transfer results in faster
product transfer and reduced oven length as compared to the conventional thermal processing.
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RF food processing systems are often combined with hot air convection heating to
increase performance (Jones and Rowley, 1996). In the example of an industrial-scale RF
heating unit for heating walnuts to remove pests (Figure 5), ambient air was pumped through a 9
kW heater and was eventually introduced to conveyor belt through triode tubes. With the
increased temperature of surrounding air, less RF energy is needed to achieve the same heating
process and the size of the heating system can be reduced.
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3.3.2 Fringefield
In fringefield configuration, a series of electrodes alternatively connected to either side of
the RF voltage supply are placed on only one side of the electrodes where the food is passed
through, as shown in Figure 4(b). As the RF waves are only applied from the bottom, there may
be electric field variations within the volume of a thick food, so fringefield is more suitable for
food products that are in thin slices, like in pasta drying and cereal baking.
3.3.3 Staggered throughfield
The staggered throughfield configuration is somewhat similar to fringefield
configuration, except that the two series of electrodes that are connected to either side of the RF
voltage supply are now placed at opposite ends of the food conveyor line, as shown in Figure 4.
This configuration provides an electric field between that of throughfield and fringefield, so
staggered throughfield is often used in food products with intermediate thickness and also in
post-baking applications.
3.4 Combination with hot air convection
Figure 5: Schematic view of an industrial-scale 25 kW, 27.12 MHz RF heating unit (Wang et al., 2007a).
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3.3 Various configurations of applicator
There are three types of configurations of applicator electrodes, designed to provide
different applications on food products. The three types are throughfield, fringefield and
staggered throughfield.
(a) Throughfield (b) Fringefield
(c) Staggered throughfield
Figure 4: Three different configurations of the applicator (Richardson, 2001)
3.3.1 Throughfield
Throughfield configuration is the simplest design, where the food is passed through two
parallel electrode plates where high-frequency voltage is applied, as shown in Figure 4 (a). As
RF waves are transferred from two directions, total penetration of depth and surface area of
heating are larger, so this is particularly suitable for processing thick food products, for example
meat (Vicente and Castro, 2007).
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shown in Figure 3 (Marchand, 1989). Furthermore, the generator can be controlled on-line with a
crystal oscillator and an impedance matching network is included in the system just before the
applicator, to transform the impedance of the applicator to 50 �.
The advantages of the 50 � RF systems are many. Firstly, frequency of the generator is
fixed, at either 13.56 MHz or 27.12 MHz, making it easier to meet the regulations of
electromagnetic compatibility (EMC) (2005). Also, as the generator and applicator are separated,
the applicator circuit can be designed with more flexibility to adapt to food applications and the
cleaning process of applicator is simplified. Thirdly, advanced process control can be done to
modify RF power, conveyor speed and air temperature in the applicator, using on-line
monitoring information from impedance matching network on dielectric load.
Figure 3: 50� RF heating system (Richardson, 2001)
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3 Types of radio frequency heating process
A RF heating system consists of two major components: the generator, that generates RF
waves, and the applicator, that applies the RF waves to food. Depending on the positioning of the
generator and applicator, there are two types of RF heating systems used in the food industry: the
conventional RF heating equipment, which is widely used for many years, and the more recently
developed 50 � RF heating system.
3.1 Conventional radio frequency heating system
In conventional RF heating systems, the applicator is a component of the generator
circuit, as shown in Figure 2. More precisely, the primary circuit is the output circuit of the
generator, while its secondary circuit contains the applicator (Hulls, 1992). The amount of RF
power supplied to the food product is controlled by electrodes in the applicator circuit and is
demonstrated by the DC current flowing through the high power valve within the generator.
Figure 2: Conventional RF heating system (Richardson, 2001)
3.2 50 � radio frequency heating system
Compared to conventional RF systems, the generator and applicator in 50 � RF heating
systems are physically separated, but are connected by a 50 � high-power coaxial cable, as
FST4102 Literature Review on Radio Frequency Heating
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The penetration depths of almonds and walnuts have been calculated based on measured
dielectric properties at 538 and 654 cm at 27 MHz versus only 2 – 3 cm at 915 and 2450 MHz
(Wang et al., 2003b). This limited penetration depth in nuts would suggest that large scale
pasteurization at the microwave frequencies of 915 and 2450 MHz is an impractical solution.
FST4102 Literature Review on Radio Frequency Heating
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p
r
CtfVT
����� tan'2 0
2
� Equation 2
where �T is temperature increase (°C), t is temperature rise time (s), �0 is dielectric constant of a
vacuum (considered equal to 8.85419 10-12- F/m), f is frequency, �r’ is relative dielectric
constant or permittivity of the material to be heated, V is the electric strength (equal to
voltage/distance between plates, V/cm), Cp is specific heat of the material to be heated (J/kg°C),
and � is the density of the material to be heated (kg/m)
As equation 2 shows, �T can be increased by increasing the loss factor. However, if the
loss factor is too high, current leakage takes place through the material. On the other hand, if the
loss factor is too low, heating takes place slowly and it becomes difficult to reach the desired
temperature due to heat losses. Therefore, for dielectric heating to be successful, the loss factor
should lie between 0.01 < �” < 1.
RF heating is also influenced by means of the penetration depth (d), which is defined
(Bengtsson and Risman, 1971) as depth in a material where the energy of a plane wave
propagating perpendicular to the surface has decreased to 1/e (1/2.72) of the surface value and is
represented by
Equation 3 Where c is the speed of propagation of waves in a vacuum (3×10-8m/s) and d is in meter.
FST4102 Literature Review on Radio Frequency Heating
6
placed in a high frequency electric field. In foods, at radio frequencies, this loss principally arises
from the electrical conductivity of the food, and the heating mechanism is simply resistance
heating (i.e. similar to ohmic heating). Although microwave heating also relies on a dielectric
loss to provide the heat, the principal loss mechanism in food products at microwave frequencies
is different (resonant dipolar rotation) (Metaxas and Meredith, 1983).
The RF band of electromagnetic spectrum covers a broad range of high frequencies,
typically either in kHz range (3 kHz < f � 1 MHz) or MHz range (1 MHz < f � 300 MHz). The
microwaves which are similar to RF waves in heating behaviour are of further higher frequency
range, between 300 MHz and 300 GHz. Both RF and MW are considered to be part of non-
ionizing radiation because they have insufficient energy (10 eV) to ionize biologically important
atoms. Since these waves are within the radar range where it is mostly used for communications,
the frequencies that can be used for heating applications are strongly limited. The allowed
frequencies for RF heating application are 13.56, 27.12 and 40.68 MHz (Piyasena et al., 2003).
2.1 Factors influencing radio frequency heating
The relevant properties in RF heating are the relative dielectric constant (�r’), the relative
dielectric loss factor (�r”), and the electrical conductivity (�), which are the so-called dielectric
properties. The first two can be combined to yield the loss tangent (tan)
'"tan
r
r
��� � Equation 1
These properties affect RF heating, for example, in terms of their influence on temperature
increase
FST4102 Literature Review on Radio Frequency Heating
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2 Principles of radio frequency heating
Unlike conventional systems where heat energy is transferred from a hot medium to a
cooler product resulting in large temperature gradients, RF heating involves the transfer of
electromagnetic energy directly into the product, initiating volumetric heating due to frictional
interaction between molecules (i.e. heat is generated within the product) (Piyasena et al., 2003).
In RF heating, the food is placed between two metal capacitor plates, where it plays the role of a
dielectric to which a high frequency alternating electric field is applied (Figure 1). Polar
molecules, such as water, try to align themselves with the polarity of the electric field. Since the
polarity changes rapidly (due to high frequency of the alternating electric field), the molecules
try to continuously realign themselves with the electric field in a flip-flop motion. The resulting
kinetic energy and friction caused by colliding neighbouring molecules generate heat within the
product.
Figure 1: A radio frequency heating system with a product between the electrodes. Polar molecules within the product are represented by the spheres with + and - signs connected by bars
The term dielectric heating can be equally applied to RF and microwave (MW) systems –
in both cases the heating is due to the fact that energy is absorbed by a lossy dielectric when it is
FST4102 Literature Review on Radio Frequency Heating
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1 Introduction
Radio frequency (RF) heating is a food processing method that involves electromagnetic
waves to generate heat in the food item. During RF heating, heat is generated within the product
due to molecular friction that is caused by the oscillation of the molecules and ions resulting
from the applied alternating electric field. It directly increases the temperature of the entire
product, without heating up heat transfer surfaces and requiring less time to come up to the
desired temperature as compared to the conventional method. Due to its rapid and uniform heat
distribution, large penetration depth and lower energy consumption (Zhao et al., 2000), RF
heating emerges as a promising technology for food application.
RF heating applications in the food industry have been recognized since the 1940s. Early
efforts attempted to apply RF energy to cook various processed meat products, heat bread and
dehydrated vegetables. Thawing of frozen products was the next step on the application of RF
energy in 1960s. The primary application in the late 1980s was the post-baking of cookies and
crackers. Compared to conventional ovens, such RF systems have been recognized to be more
efficient in removing moisture. By the 1990s, great attention has been given to the use of RF
energy for pasteurization and sterilization of particulate products due to its several advantages as
mentioned above.
This review will focus on the principles of RF heating, types of processes and
equipments, comparison between different types of heating technologies as well as its
application in food industry.