Group 2 Radio Frequency Heating Report Print

FST4102 Literature Review on Radio Frequency Heating

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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


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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


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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


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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


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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


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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.


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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.


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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


17

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


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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.


13

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.


12

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).


11

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).


10

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)


9

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


8

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.


7

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.


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


5

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


4

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.

Group 2 Radio Frequency Heating Report Print

Documents

fst4102 literature review

impedance matching network

lower drip losses

greater penetration power

radio frequency heating3

rf voltage supply

temperature distributions inside

conventional thermal processing