radio frequency treatment of foods: review of recent advances

Journal of Food Engineering 91 (2009) 497–508

Contents lists available at ScienceDirect

Journal of Food Engineering

journal homepage: www.elsevier .com/locate / j foodeng

Review

Radio frequency treatment of foods: Review of recent advances

Francesco Marra a,*, Lu Zhang b, James G. Lyng c

a Dipartimento di Ingegneria Chimica e Alimentare, Facoltà di Ingegneria, Università degli Studi di Salerno, via Ponte Don Melillo, I-84084 Fisciano, SA, Italyb Department of Chemical and Materials Engineering, School of Engineering, University of Auckland, PB 92019 Auckland, New Zealandc UCD School of Agriculture, Food Science and Veterinary Medicine, College of Life Sciences, UCD Dublin, Belfield, Dublin 4, Ireland

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 December 2007Received in revised form 13 October 2008Accepted 15 October 2008Available online 1 November 2008

Keywords:Radio frequencyFood qualityDielectric propertiesModelling

0260-8774/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.jfoodeng.2008.10.015

* Corresponding author. Tel.: +39 089 962012; fax:E-mail address: [email protected] (F. Marra).

Radio frequency (RF) heating is a technology on which quite a number of publications have emerged inrecent years. The current paper reviews the history of this form of heating and introduces the basic prin-ciple of this technology including how it is applied and how it differs from other forms of heating. Recentliterature on RF heating applications is examined, highlighting the impact of this form of heating on gen-eral quality aspects of foods. An understanding of physical characteristics which influence food heatingincluding geometry, shape, product position and dielectric properties is extremely important in thedesign of RF heating systems and recent publications in these areas are discussed in addition to the mostrecent developments on mathematical modelling. In the final section the authors give their opinion onfuture trends and prospects for this form of heating.

� 2008 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4982. History of RF heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4983. Overview of RF heating technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499

3.1. Principles of RF heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4993.2. RF heating vs. conventional and other electroheating methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500

4. The use of RF heating in food processing and the quality of RF heated products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500
4.1. Meat processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5004.2. Post-harvest treatment and disinfestation of fruits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5014.3. Liquid foods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5024.4. Other applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502
5. The role of geometry, shape and product position on RF heating and temperature distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5026. Dielectric properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503

6.1. Definition of dielectric properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503
6.1.1. Permeability and permittivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5036.1.2. Electrical conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5046.1.3. Loss tangent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5046.1.4. Penetration depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504
6.2. Factors influencing dielectric properties of foods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504
7. The mathematical modelling of RF heating in food processing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505
7.1. Recent advances in mathematical modelling of RF heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5057.2. Mathematics of RF heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505

8. Future trends and prospective. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506

ll rights reserved.

+39 089 964057.

mailto:[email protected]

http://www.sciencedirect.com/science/journal/02608774

http://www.elsevier.com/locate/jfoodeng

Nomenclature

Ap plate area (m2)C speed of propagation of waves in vacuum (m s�1)C0 capacitance (F)Cp specific heat (J kg�1 K�1)d plate distance (m)dp penetration depth (m)D electric flux density (–)E electric field (–)f frequency (Hz)H magnetic field (–)j imaginary unit (–)k thermal conductivity (W m�1 K�1)n unit vector (–)P power (W)Qv density of power (W m�3)R resistance (X)t time (s)

T temperature (K)tan d dielectric loss tangent (–)U overall convective heat transfer coefficient (W m�2 K�1)V electrical potential (V)Zc electrical impedance (X)a thermal diffusivity (m2 s�1)d dielectric loss angle (rad)e0 free space permittivity (F m�1)e permittivity (–)e0 dielectric constant (F m�1)e00 dielectric loss factor (F m�1)l magnetic permeability (H m�1)q density (kg m�3)r electrical conductivity (S m�1)r� divergence operator (m�1)r� curl operator (m�1)

498 F. Marra et al. / Journal of Food Engineering 91 (2009) 497–508

1. Introduction

Research in novel heating of foods, for applications such ascooking, pasteurisation/sterilisation, defrosting, thawing and dry-ing, often focuses on areas such as the assessment of processingtime, the evaluation of heating uniformity, the appraisal of the im-pact on quality attributes of the final product as well as the predic-tion of the energy efficiency of these heating processes. Researchon electroheating accounts for a considerable portion of both thescientific literature and commercial novel heating applications.Electroheating can be subdivided into either direct electroheatingwhere electrical current is applied directly to the food (e.g. ohmicheating (OH)) or indirect electroheating (e.g. microwave (MW) orradio frequency (RF) heating) where the electrical energy is firstlyconverted to electromagnetic radiation which subsequently gener-ates heat within a product. Some of those electroheat processes,(e.g. OH and RF heating), are used only in industrial situationswhile MW heating can be applied commercially but is also verycommonly used domestically. Of these forms of electroheating inrecent years there has been an increased interest in the area ofRF heating, as evidenced by the increasing number of publicationsin this area.

There have been a number of recent reviews in the area of RFheating which include publications by Zhao et al. (2000), who con-sidered the major technological aspects and applications of RFheating and indicated the major engineering challenges in theuse of RF technology for food processing and preservation whilealso discussing the potential use of mathematical modelling inthe design of such systems. Another review by Piyasena et al.(2003a), reported main industrial applications for RF heating infood processing and discussed dielectric properties of a range offood products suitable for RF heating.

The aim of the current review is to establish the state of art inrelation to RF electroheating applications, as evidenced by the ref-ereed publications which have appeared in this area in the pasteight years. In addition future trends for research in this field willalso be discussed. To ensure a comprehensive overview is pro-vided, this paper will include a description of the mechanism ofRF heating (which will be differentiated from conventional andMW heating); an overview of typical equipment used for RF heat-ing; examples of the wide range of RF heating applications in foodprocessing which have been proposed in the scientific literature inrecent years, with related description of the effects of RF treatmenton quality attributes of products. In addition factors which influ-

ence RF heating, such as shape and orientation of the load and itsdielectric properties will also be overviewed and fundamentals ofmathematical modelling of both electromagnetic fields and heattransfer during RF heating will also be presented and discussed.

2. History of RF heating

In 1832 Michael Faraday postulated the existence of electro-magnetic fields. Forty-one years later, James Clerk Maxwell math-ematically predicted the existence and behaviour of radio waves.Then followed the work of Heinrich Hertz who experimentally ver-ified Maxwell’s theory in 1885. Jacques Arsene dÁrsonval subse-quently used Hertz’s first high frequency oscillator to conductexperiments on the effects of high frequency (500–1500 kHz),low voltage alternating current on animals. dÁrsonval found thatthe main effect of RF on animals was the production of heat andthis discovery led to the first high frequency heat therapy unit inthe Hotel Dieu Hospital in Paris in 1895 under dÁrsonval’s direc-tion. The potential use of this technology for food processing wasrecognized after World War II. Sherman (1946) described ‘electricheat’, how it is produced, and suggested possible applications forthe processing of food. These early efforts employed RF energyfor applications such as the cooking of processed meat products,heating of bread, dehydration and blanching of vegetables. How-ever, the work did not result in any commercial installations, pre-dominately due to the high overall operating costs of RF energy atthat stage. By the 1960s, studies on the application of RF energy tofoods focused on the defrosting of frozen products, which resultedin several commercial production lines (Jason and Sanders,1962a,b). Demeczky (1974) also showed that juices (peach, quinceand orange) sealed in bottles and carried on a conveyer beltthrough an RF applicator had better bacteriological and organolep-tic qualities than the juices treated by conventional thermal meth-ods suggesting potential applications in heat processing forpreservation of foods. The next generation of commercial applica-tions for RF energy in the food industry was post-bake drying ofcookies and snack foods which began in the late 1980s (Rice,1993; Mermelstein, 1998). Later in 1990s, the area of RF pasteuri-sation was studied with attempts made to improve energy effi-ciency and solve technical problems such as run-away heating(Houben et al., 1991; Zhao et al., 2000). This in turn has led to re-cent investigations on RF applicator modifications and dielectricproperties of food at RF frequencies (Laycock et al., 2003; Zhanget al., 2004b, 2006, 2007; Birla et al., 2005).

f = 300-30000 MHz

Microwave Heating

f = 1-300 MHz

Radio Frequency heating

a b

Fig. 2. Schematic arrangement for (a) MW and (b) RF heating.

F. Marra et al. / Journal of Food Engineering 91 (2009) 497–508 499

3. Overview of RF heating technology

3.1. Principles of RF heating

While a comprehensive explanation of physical theory of RFheating is available in Jones and Rowley (1997) this section willbriefly overview how materials are heated in RF fields while alsoproviding the basic equations which govern this heating. The RFportion of the electromagnetic spectrum occupies a region be-tween 1 and 300 MHz as shown in Fig. 1 although the main fre-quencies used for industrial heating lie in the range 10–50 MHz(J. Tang et al., 2005). Within the latter range only selected frequen-cies (namely 13.56 ± 0.00678, 27.12 ± 0.16272 and40.68 ± 0.02034 MHz) are permitted for industrial, scientific andmedical applications. In Fig. 2 a schematic arrangement for MWheating is compared with a typical RF heating parallel plate elec-trode configuration. In MW heating (Fig. 2a), special oscillatortubes known as magnetrons or klystrons emit microwaves whichare transferred by a waveguide into a metal chamber or cavitywhere the target material to be heated is placed. Resonant electro-magnetic standing wave modes are then established within thecavity (Piyasena et al., 2003a) although turntable trays and/or stir-rers can be used to improve the uniformity of the electromagneticfield within the chamber and around the target material. In con-trast RF energy is generated by a triode valve and is applied tomaterial via a pair of electrodes (Rowley, 2001). In the parallelplate RF system described in Fig. 2b, one of these electrodes isgrounded which sets up a capacitor to store electric energy. Thetarget material to be heated is placed between but not touchingthe parallel electrodes. It must be noted that while the use of par-allel plate electrodes (or ‘‘through-field” applicators) is the mostcommonly used electrode configuration for heating thicker materi-als, two other configuration types are included in Jones and Rowley(1997). These are ‘‘fringe-field” applicators (which consist of a ser-ies of bar, rod or narrow plate electrodes which are most suited forheating or drying thin layers (<10 mm)) or ‘‘staggered through-field” applicators (consisting of rod or tube shaped electrodes stag-gered on either side of a belt which are used for heating products ofintermediate thickness).

When an alternating electrical field is applied to a food, onephenomenon that occurs is the movement of positive ions in thematerial towards negative regions of the electric field and themovement of negative ions towards positive regions of the field(Buffler, 1993). This movement of ions in this fashion is often re-ferred to as ionic depolarization and is essentially resistance heat-ing as found in OH. Heating occurs because this field is not static,with polarity continually changing at high frequencies (e.g.27.12 MHz for RF or 2450 MHz for MW), which is in contrast toOH where the field polarity changes at much lower frequencies(i.e. 50 Hz in Europe or 60 Hz in USA). However, irregardless ofthe frequency, the continued reversal of polarity in the electricalfield leads to the oscillation of ions forwards and backwards inthe product with the net effect of this being the internal generationof heat within the product by friction (Buffler, 1993) (thereby

3×10-11 3× 10-9 3× 10-7 3× 10-5 3

rays

x rays

UV I

Visible

Wavel

1019 1017 1015 1013Frequency (H

Fig. 1. The electroma

avoiding the temperature lag between the surface and the centreof solid products). In addition to the movement of ions, dipolarmolecules such as water in a material will also attempt to alignthemselves appropriately with the changing polarity of an electri-cal field (a phenomenon know as dipole rotation). The movementof these dipoles can also cause friction between molecules whichcan also lead to heat generation. While MW and RF heating areboth classed as dielectric heating methods (i.e. heating due to en-ergy absorbtion by a lossy dielectric when it is placed in a high fre-quency electrical field (Rowley, 2001)) it is generally accepted thationic depolarization tends to be the dominant heating mechanismat the lower frequencies encountered in the RF range while bothionic depolarization and dipole rotation can both be dominant lossmechanisms at frequencies relevant to MW heating (i.e. 400–3000MHz) depending upon the moisture and salt content within a prod-uct (Tang, 2005). Therefore in RF range dissolved ions are moreimportant for heat generation than the water dipoles in which theyare dissolved (Ohlsson, 1983). In the case of an RF heater, whenfoods are placed between the applicator electrodes, there is a com-plex electrical impedance introduced into the RF electrical field(Jones and Rowley, 1997)

Zc ¼1

2pfC0

e00 � je0

e002 þ e02ð1Þ

where Zc is the capacitance of the material, f is the frequency of theelectric field, e0 is dielectric constant and e00 is the dielectric loss fac-tor of the material respectively. C0 is the capacitance of free spaceand j ¼

ffiffiffiffiffiffiffi�1p

. From Eq. (1), a finite resistance,

R ¼ 12pfC0e00

ð1aÞ

has appeared across the capacitor.Taking the power, P, dissipated in an electrical resistance to be

equal to V2/R, then for a capacitor containing a dielectric material,

P ¼ 2p f C0e00V2 ð2Þ

For a parallel plate capacitor, C0=e0Ap/d where Ap is the plate area, dis the plate separation and e0 is the permittivity of the free space. Asthe voltage V is equal to the electron field strength E multiplied bythe distance between the two electrodes d, Eq. (2) can be rewrittenas,

× 10-3 3× 10-1 3× 10 3×103

R

MW

RF

ength (m)

1011 109 107 105 Z)

gnetic spectrum.


P ¼ 2p f e0 e00r E2ðAp dÞ ð3Þ

Since (Apd) is the volume, the power dissipation per unit volume orpower density, Qv, is then

Qv ¼ 2p f e0e00r E2 ð4Þ

The power density is proportional to the frequency of the appliedelectric field and the dielectric loss factor, and is proportional tothe square of the local electric field, which plays key role in deter-mining how a material will absorb energy in the AC electric field.

3.2. RF heating vs. conventional and other electroheating methods

Conventional heating on an industrial-scale can be performed inbatch or continuous basis and products can be either packaged orunpackaged during heating. Leaving these factors aside, once heatreaches the outer surfaces of a foodstuff it is transferred to theproduct interior by either conduction (e.g. in solids such as meat),convection (e.g. in liquids such as milk) or in products which dis-play broken heating curves (e.g. some starch containing soups)convection and conduction can alternately dominate at differentstages during the heating process. Of these two heat transfer mech-anisms, convection is much faster than conduction. The net effectis that in solid foodstuffs, to ensure the interior is heated to appro-priate temperatures, it is necessary to heat the product for muchlonger times than liquids which in turn can lead to overheatingin the outer regions of solid products. In contrast electroheating(e.g. OH, MW and RF) differs from conventional heating in that heatis generated volumetrically within the material by the passagethrough, and its interaction with, either alternating electricalcurrent (as in OH) or electromagnetic radiation (formed by theconversion of electrical energy to electromagnetic radiation atMW (300–3000 MHz) or RF (1–300 MHz) frequencies.

Electroheating technologies differ in terms of their methods ofapplication. In MW heating, waves (generated by a magnetron)pass via a waveguide into an oven cavity in which they essentiallybounce around off the metal walls of the cavity interior impingingon the product from many directions. In ohmic heating the productis placed in direct contact with a pair of electrodes through whichgenerally a low frequency (traditionally 50 or 60 Hz) alternatingcurrent is passed into the food product. Low frequency alternatingcurrent is used in OH because the cyclic change in current directionhelps to prevent electrolysis although higher frequencies (e.g. 10 or4 kHz) have been shown by Samaranayake et al. (2005) to furtherreduce these electrochemical reactions. RF heating also involvesthe use of electrodes (with the product being placed either midwaybetween or on top of one of a pair of electrodes) between which ahigh frequency directional electrical field is generated by highpower electrical valves which transfer energy to the electrodesby a transmission lines. However, RF heating does not have anyrequirement for direct contact between the product and electrodesas RF waves will penetrate through conventional cardboard orplastic packaging. In OH, the product needs to be either unpack-aged and in direct contact with the electrodes and subsequentlypackaged, or alternatively be in a sealed pack which has conductiveregions which allow electrical current into the product.

Rowley (2001) reviews the methods for producing and trans-mitting RF power into materials and categorises systems as ‘con-ventional’ or the more recently introduced ‘50 ohm’ (or ’50 X’)systems which are becoming increasingly popular due to theirmany advantages.

MW and RF heating also differ in a number of other respects. Asfrequency and wavelength are inversely proportional, RF (lowerfrequency) wavelengths (i.e. 11 m at 27.12 MHz in free space) aremuch longer than MW (higher frequency) wavelengths (i.e.0.12 m at 2450 MHz in free space). As electrical waves penetrate

into materials attenuation occurs, with the result that the energyof the propagating wave decreases exponentially. Penetrationdepth (dp) is defined as the depth into the material to which the en-ergy is reduced to 1/e (1/2.72) of the surface energy value. This dp isproportional to wavelength. During RF heating, electromagneticpower can penetrate much deeper into samples without surfaceover heating or hot/cold spots developing which are more likelyto occur with MW heating. Generally, RF heating offers advantagesof more uniform heating over the sample geometry due to bothdeeper level of power penetration and also simpler more uniformfield patterns compared to MW heating.

4. The use of RF heating in food processing and the quality of RFheated products

Food processing applications for RF heating are less commonthan for MW heating, though a number can be found in the litera-ture. The two reviews mentioned in Section 1 (Zhao et al., 2000;Piyasena et al., 2003a) extensively discussed RF heating applica-tions published up to 1999. However, since their publication, morework has appeared in the literature showing several additionalapplications for RF heating in food processing. In this section theseapplications will be reviewed together with their reported impacton general quality aspects of foods.

4.1. Meat processing

Meat and meat products is the area on which most informationhas been published to date, both in terms of processing and in rela-tion to the effects of RF treatment on the quality of foods. Laycocket al. (2003) compared the heating rate, time temperature profilesand quality of three meat products (ground, comminuted and non-comminuted muscle) cooked in a water bath or by a 1.5 kW RFheater (a cylindrical chamber set between two electrodes wrappedaround the RF applicator). This system operated at 27.12 MHz andRF cooking was found to reduce cooking times to up to 1/25 of con-ventional cooking times in a water bath. The results of these work-ers also indicated that the surface of the RF cooked products heatedat a faster rate than the centre, with differences in temperatures of10–20 �C at the end of the process, which the authors attributed toan uneven salt distribution. Instrumental quality measurementsindicated that RF cooked samples had lower juice losses and alsoacceptable in terms of colour and water holding capacity. Instru-mentally measured textural attributes of RF cooked whole musclewere not significantly different to water bath cooked samples.However, some textural differences were noted between RF andwater bath cooked ground and comminuted meats with RF cookedground beef having higher springiness and chewiness and RFcooked comminuted meat having lower hardness and higherspringiness than water bath cooked samples. Power efficiencywas highest for comminuted meat with muscle having the lowestpower efficiency. Overall the authors postulated that well mixedcomminuted and ground meat products appeared to be the mostpromising for RF cooking.

Orsat et al. (2004) pasteurised vacuum-packaged ham slices in anRF applicator (600 W at 27.12 MHz) and subsequently examinedsamples for moisture loss, colour changes, total bacterial surfacecounts and sensory quality attributes, such as off odours, and slimi-ness. The ham samples were packed in three different plastic films(composed of nylon–polyethylene, polypropylene and high densitypolyethylene), and were brought to internal temperatures of 75 �Cand 85 �C in 5 min by RF heating and maintained at those tempera-tures for an additional 5 min by adjusting the RF power. The authorsconcluded that RF heating, coupled with appropriate packaging,could improve the storability of repacked hams by decreasing the


bacterial load, reducing the moisture loss and maintaining an overallgreater product sensory and quality acceptance. Also in the area ofmicrobial quality, Guo et al. (2006) compared the effectiveness ofRF vs. hot water bath cooking on the inactivation of Escherichia coliK12 in ground beef. These workers, who used an RF apparatus sim-ilar to the system used by Laycock et al. (2003), reduced cookingtime to 1/30 of immersion cooking time and found lower tempera-ture variations. They noted that both methods significantly reducedE. coli and extended the product shelf life and suggested it had greatpotential as an alternative method to immersion cooking.

The most extensive investigations on the processing and thequality of RF cooked meats were published between 2004 and2007 (Brunton et al., 2005; Lyng et al., 2007; McKenna et al.,2006; X. Tang et al., 2005, 2006; Zhang et al., 2004a, 2006) by ateam from UCD Dublin, Ireland, who heated products using a27.12 MHz RF applicator with a maximum power output of600 W. The remainder of this subsection attempts to summarisethis information with no aspect described in great detail as a moreextensive overview of this work has been given by Lyng (2007).These workers developed systems for RF cooking of cased meatsamples which avoided arcing of the casings which occurred whenRF cooking in air. These systems involved submerging the casedproducts in circulating water at 80 �C during cooking in custombuilt polyethylene cells. The examined quality attributes of theRF cooked products included yield, texture, colour and flavour. Incomminuted products, no yield differences were noted betweenconventional and RF cooked samples. However, for non-commi-nuted products (i.e. ham and beef), in all cases RF cooked productshad higher yields than steam cooked products, with the differencebeing about 1–1.5% in the case of ham and 4–6% in beef. A range ofinstrumental texture measurement techniques (including WarnerBratzler, Kramer Shear and Penetrometer) were also used by theseworkers. When these methods were used to compare convention-ally or RF cooked products no definite trends emerged, and wheredifferences occurred, generally they were small (Lyng, 2007). Sim-ilarly when RF and steam cooking protocols were compared interms of instrumental colour, overall, samples were reasonablysimilar with some significant differences but no definite trendbeing evident in any of the colour attributes assessed. Variousinstrumental flavour analyses were also performed on a numberof products. Some work was conducted using gas chromatographywhile other work involved the Tbars test. Differences were notedthough they were very slight and unlikely to have an impact onsensory analysis. Rates of oxidation in beef and turkey wereslightly less in RF compared to steam cooked samples but the mag-nitude of this difference was so low that it was unlikely to be de-tected by sensory analysis. In addition, no major differences werenoted in sulphur volatiles in RF vs. steam cooked samples. Zhanget al. (2004a) developed an optimised cooking protocol for pasteu-rising meat emulsion samples. Authors reported reductions of upto 79% in pasteurisation times for meat products compared toequivalent steam cooked samples though claimed the use of ahigher powered RF source could further reduce cooking times.

4.2. Post-harvest treatment and disinfestation of fruits

Nowadays RF radiation is often considered as an alternativemethod for the disinfestation of fruit and nuts. This is largely dueto the fact that processors are seeking alternatives to some of themore traditional fumigation agents such as methyl bromide, whichmay be discontinued because of environmental concerns (as it isbelieved to have a role in ozone depletion) and also issues regardinghealth and safety. Researchers from Washington State University,Pullman, WA, USA, have performed the most extensive investiga-tions on this topic (Birla et al., 2004, 2005; Hansen et al., 2005,2006a,b; Ikediala et al., 2002; Mitcham et al., 2004; Monzon et al.,

2006; Tang et al., 2000; Wang et al., 2002, 2003, 2006a), includingdiscussions on the scaling-up of RF plants and the design of com-mercial RF processes (Wang et al., 2006b, 2007a,b). A kinetic modelstudy describing the intrinsic thermal mortality of insect pests(Tang et al., 2000) demonstrated the potential to develop high-tem-perature-short-time thermal treatments for controlling codlingmoth in fruits. Wang et al. (2003a,b) published an extensive analy-sis of dielectric properties of fruits and insect pests in the context ofRF and MW treatments. When a selection of fruits and insect larvaewere examined, they found that the loss factors at RF frequencies ofcommon pest insects was clearly larger than that of nuts, suggest-ing possible differential and faster heating of insects vs. nuts whentreated simultaneously in an RF applicator. Wang et al. (2002) pro-cessed walnut loads by means of an RF applicator (27 MHz–12 kW),heating up the load to temperatures which were lethal to importantinsect pests. Heating walnuts to 55 �C or higher resulted in 100%mortality of fifth instar navel orangeworm (Amyelois transitella),which was the most resistant of the three insect pests (Cydia pomo-nella, Plodia interpunctella, Juglans regia) considered by the authors.In 2004, Mitcham et al. conducted a follow up study to their previ-ous work in which they analysed the heating of a heavier walnutload in term of heating uniformity. As moisture content is an impor-tant factor affecting heating rates during RF treatments, theseauthors measured the variability in moisture content of walnutsduring the process and concluded that the control of moisture con-tent in the samples was a key factor for ensuring uniform heating,since ions dissociate and better distribute in moisturized sampleand it was in this form that they underwent uniform ionicdepolarization.

An interesting improvement of RF based treatments was pro-posed by Birla et al. (2004) who developed a laboratory scalefruit-mover which was capable of continuously rotating a freshfruit (an orange and an apple were the test samples used) via a ser-ies of water jets, with the purpose of achieving a uniform RF heat-ing. Published results showed that rotation and movement of fruitminimized the adverse effect of non-uniformity within the RF fieldand the irregular geometry of the fruit and lead to an overall im-proved heating uniformity. It is important to emphasise that theseauthors found the fruit geometry influenced the heating pattern.Apples with typical geometry (oval and dimples on ends) showedmore variation in temperature within a fruit than oranges, whichwere almost spherical in shape. The role of shape and geometryof loads was often not considered in the past but, as discussed inSection 5, they are often key parameters for the heating patterns.Some of previous authors, with co-workers, applied RF heating tofresh apples ‘‘Red delicious” (Wang et al., 2006a), ‘‘Gala”, ‘‘Fuji”and ‘‘Delicious” (Hansen et al., 2006a,b) and to ‘Bing’ sweet cherry(Hansen et al., 2005; Monzon et al., 2006) always claiming theeffectiveness of RF treatment for pest control, with no majorchanges in the quality attributes of processed fruits. Birla et al.(2005) assessed also the impact on orange quality (weight loss, lossin firmness, colour change, total soluble solids, acidity and changein volatiles) of RF treatments designed to control Mediterraneanfruit flies. Aside from greater retention of volatile compounds inthe RF treated compared to conventional hot water processedproducts no other quality parameters were significantly affectedafter 10 days of 4 �C storage. While RF was found to successfullyinactivate the larvae, some changes in fruit and stem colour wereobserved in addition to some pitting and bruising of products.Monzon et al. (2007) evaluated the impact of RF heating protocolsused to control Mexican fruit fly larvae on Fuyu persimmons fruitand found the treatments used (48–52 �C followed by holdingtimes of 0.5–18 min) had no significant effect on firmness, solublesolids content, titratable acidity or product weight loss with RFtreated products attaining a deeper orange red peel than controlfruits. Due to the apparently mixed impact of RF treatments on


product quality as suggested by the studies above it would appearthat the use of RF for disinfestation will need to be considered on aproduct by product basis.

Wang et al. (2006b, 2007a,b) also proposed considerationsabout designing and scaling-up plants and processes for indus-trial-scale post-harvest RF treatment of walnuts, working withcontinuous apparatus characterized by high power (up to 25 kW)and high product load (up to 187 kg of product moving on a belt).Design and running of industrial-scale RF processes is one of themain challenges in this field since Wang et al. (2007b) claimed thatmany factors (e.g. walnut orientation, differential heating betweenopen and closed shell walnuts, actual post-storage, and energycosts) influencing the commercial RF processes must be taken intoaccount. Energy costs may be comparable to traditional fumigationones, but at present the use of RF installations will incur consider-ably higher capital costs.

4.3. Liquid foods

Investigations into RF heating of liquid foods have also beenperformed because the rapid heating and high penetration of RFenergy make it an attractive alternative for processing liquid foodsunder continuous flow conditions. Heating of starch solutions andof guar solutions in continuous flow was investigated respectivelyby Awuah et al. (2002) and Piyasena and Dussault (2003), in orderto evaluate the effect of system parameters (flow rate and RFpower) and product parameters (starch or guar concentration,pH, added salt and sugar) on temperature change across the RFapplicator (1.5 kW, 27.12 MHz) tube and to develop correlationsfor estimating exit temperature for the considered solutions.Awuah et al. (2005) used a 2 kW, 27.12 MHz, RF applicator in orderto find best conditions to inactivate surrogates of both Listeria andE. coli cells in milk under continuous laminar flow conditions. Theimpact of RF treatment on microbial inactivation in orange andapple juices and in apple cider was assessed by Geveke and Brunk-horst (2004, 2008) and by Geveke et al. (2007). The used RF treat-ments were applied across frequency a range (which varied from aminimum of 15 kHz to a maximum of 41 kHz which varied withthe product type) and were relatively mild, increasing producttemperatures to a maximum of 65 �C at the outlet. The main focusof their work was on microbial inactivation and microbial qualityof beverages processed using this system. Under their experimen-tal conditions they demonstrated the potential of RF towards theinactivation of E. coli K12 by up to 3, 3.3 and 4.8 log cycles in applejuice, orange juice and apple cider respectively. In addition toassessing microbial inactivation these workers also noted no lossin ascorbic acid or enzyme browning in RF treated orange juice.Zhong et al. (2003) reported uniform heating pattern during con-tinuous flow of tap water in a 30 kW, 40.68 MHz RF applicator,whereas 1% carboxymethylcellulose (CMC) solution, in sameequipment, exhibited higher temperature values at the tube innerwalls, given the low penetration depth characterizing the CMCsolution at the specific frequency used.

Notwithstanding the above quoted works, further studies arenecessary to justify the industrial application of RF for heating liq-uids under continuous flow conditions, including an in-depth anal-ysis of temperature distribution and penetration depth for largediameter pipes under more realistic flow conditions.

4.4. Other applications

The number of other possible applications for RF heating offoodstuffs has grown in the last seven years. After some trials to as-sess heating uniformity using a model food (whey protein gelcharged with a chemical marker, M-1), Wang et al. (2003) com-pared RF heating (6 kW, at 27.12 MHz) with a conventional retort

process for sterilizing trays of macaroni and cheese. They foundthe RF treated samples to be much closer in colour and flavour tothe control, while the retorted samples were darker in appearanceand had a more burnt flavour. The use of chemical marker M-1 asmethod to map heating patterns was discussed by Wang et al.(2004). Previously a number of authors (Luechapattanporn et al.,2004, 2005) successfully validated the use of RF equipment forthe sterilisation of samples (mashed potatoes and scrambled eggs)inoculated with Clostridium sporogenes (PA 3679), and achievedsufficient microbial inactivation while producing products whichhad a higher quality than conventionally retorted products.

Another interesting application was proposed by Zhong et al.(2004) who considered RF heating as a potential alternative to con-ventional heating for liquids containing particulates. Using a30 kW, 40.68 MHz, continuous flow RF unit, authors processed car-rot and potato cubes using a 1% CMC solution as carrier. Based onthermal images captured by an infrared camera, small temperaturegradients were observed inside the carrots and potato cubes thatheated in a short residence time. A 600 W, 27.12 MHz, RF applica-tor was used by Orsat et al. (2001) to determine the potential for RFto improve and extend the storability of vacuum-packaged carrotsticks. Despite the fact that the quality of the RF treated sampleswas higher than for either the control (chlorinated water) or hot-water-treated carrot samples and that the RF treatments main-tained colour, taste and the vacuum of the packages, which wasnot the case for the control or hot-water-treated carrots, authorsconcluded that RF heating should not be recommended as a soletreatment to improve the storability and food safety of minimallyprocessed ready-to-eat carrot sticks. Instead RF should be consid-ered as a part of an integrated approach, including proper packag-ing and adequate refrigeration.

Ahmed et al. (2007) studied rheological and gelation character-istics of RF heated egg white dispersions and found RF heated pro-tein dispersions produced stronger gels in particular whenacidified with no gel formation when in an alkaline condition.

In addition to publications which explored the use of RF for dis-infestation, a paper by Nelson et al. (2003) evaluated RF as amethod for reducing Salmonella, E. coli O157:H7 and Listeria mono-cytogenes contamination in alfalfa seeds. Short RF exposures(several seconds) produced reductions in the target organism with-out adverse effect on seed germination. However, extending RFexposure to produce the desired level of microbial reduction hadan adverse effects on germination.

In contrast to the aforementioned studies, Schuster-Gajzágóet al. (2006) exposed white mustard seed to RF with the intentionof inactivating the endogenous enzyme myrosinase which wasresponsible for the development of pungent sharp flavour. In addi-tion, these workers also assessed the impact of RF on compoundswith health beneficial effects found in these seeds. They found RFcould effectively inactivate myrosinase to a sufficient level to inhi-bit pungent flavour development while causing no damage to com-pounds of nutritional significance. Previously Cserhalmi et al.(2001) had also reported successful reduction of pungency of yel-low mustard seed after treatment in a 10 kW, 13.5 MHz, RF system.The RF treated yellow mustard seed (Sinapsis alba L. cv Tilney)exhibited a small reduction of fat binding ability and nitrogen sol-ubility index but the water binding ability and emulsifying activitypractically did not change as a result of treatment and the emul-sion stability increased.

5. The role of geometry, shape and product position on RFheating and temperature distribution

Several papers published in recent years show a growing inter-est in understanding the role of geometry and shape (both sample


and applicator) and also of position and orientation of product withrespect to electrodes, in RF heating. In a capacitor, the air gap be-tween the electrodes and load and the air space between lateralsurfaces of the load and electrode edges work as preferential path-ways for the electric field vectors to reach from one electrode tothe other due to the low permittivity of air. Furthermore, the inci-dence of the electric field on the exposed surfaces of the load canbe responsible for a differential heating rate according to the shapeof the load itself.

Orsat et al. (2001) processed layers of carrot sticks between par-allel plate electrodes (8 cm from each other) in an RF system. Thefirst experiment was performed on a 1–2 cm layer of carrot sticksand produced poor RF coupling and a very slow temperature in-crease within the product (5 min to reach 40 �C from a startingtemperature of 5–6 �C). In a second experiment the layer thicknesswas increased from 1–2 cm to 4–5 cm, without moving the elec-trodes: authors reported that internal temperature increased from6 to 60 �C in a time ranging between 2 and 7 min, depending on po-sition (i.e. some regions of the sample heated at different heatingrates). Finally, a third experiment was performed with a 6.5 cmlayer of carrots: in this case, reaching an internal temperature of60 �C took between 80 and 140 s. Overall it would appear that anincrease in the mass of the load brought about better RF coupling.However, this increase in mass corresponded to an increase in vol-ume, a change in the shape of the load (from a thin layer to a thick-er box shape) and to a reduction in the gap between the top surfaceof the sample and the upper electrode.

Wang et al. (2005) observed that for in-shell walnuts to success-fully undergo RF heating intermittent stirring was required to avoidnon-uniform heating (due to differences in orientation and loca-tion) encountered when walnuts were heated in a static state. Insubsequent research by these workers (Wang et al., 2006b,2007a,b), considerations in the design of commercial RF treatmentsfor post-harvest pest control in in-shell walnuts were discussed.These workers, proposed that in-shell walnuts were non-isotropicmaterials due to the irregular shape of the shell and kernel and insuch products, variations in walnut temperature after RF heatingwere caused not only by differences in both thermo-physical anddielectric properties but also by the shape of individual walnutsand different orientations of walnuts in the applicator. They alsoconcluded that heating uniformity in RF treated walnuts was a ma-jor concern in developing a commercial scale process.

Birla et al. (2004) observed non-uniform heating in oranges(a typical spherically-shaped sample) and apples, when water as-sisted RF treatment was used. As described in Section 4, a waterassisted RF system was used in order to prevent overheating andto promote a more uniform temperature distribution within the or-anges. However, in this case, authors found consistent hot spots atthe naval end of the oranges and at the stem and bottom sides of ap-ples, when the fruits were placed with the axis in vertical position(i.e. stem end on top). Therefore in this case the axis was the short-est route for RF energy to pass through. The failure of a water as-sisted system in promoting a more even temperature distributionis due to the shape of the sample: in fact, authors stated that theconsistent hot spots on those two ends suggests that the shape ofa fruit highly influences the temperature uniformity within it, withor without water recirculating about the sample. In order to obtaina more uniform heating within the sample, the same authors suc-cessfully proposed a fruit-mover, to continuously rotate and movethe fruit in a water bath placed between two electrodes.

Effects of load shape, orientation and position on the heatingpatterns during RF treatments were also investigated by com-puter simulation (modelling details are discussed in Section 7).Romano and Marra (2008), on the basis of previous work devel-oped by Marra et al. (2007), proposed a theoretical analysis ofmulti-physics phenomena during RF heating of a foodstuff,

shaped as cube, cylinder or sphere. The objective of their paper(2008) was to analyze the effects of sample shape and orientationon heating rate and temperature distribution, once some param-eters (such as distance between electrodes, electrode to productair gap, volume occupied by the sample with respect to cavityvolume) were defined. For this purpose, they built and solved a3D multi-physic mathematical model, based on the heat-conduc-tion equation plus a power generation term and on the Gausslaw. They considered and inhomogeneous material with non-uni-form permittivity and built and solved a 3D multi-physics math-ematical model that was based on the heat-conduction equationin addition to a power generation term: Gauss Law stated foran electro-quasi-static conduction was used in order to obtainthe electric field distribution. Considered loads were meat battersas sample food regularly shaped as cube, cylinder and sphere,with dielectric and physical properties as functions of tempera-ture. The reported results showed a great influence of the sampleshape on heating rate and on temperature distribution within thesample. Among the shapes investigated by the authors, regularcubes were found to be more suitable for RF treatment since cu-bic shaped products exhibited a fast and more uniform heating,with a good absorption of power. In case of cylindrically shapedproducts, authors recommended a vertical orientation duringtreatment, since horizontally oriented cylinders showed a slowerheating, characterized by uneven temperature distribution. Spher-ical shapes were found to be the less favoured to RF heating.

Birla et al. (2008) focused on role of sample shape and position(with respect to electrodes) during the RF heating of sphericalshaped object. As Marra et al. (2007) and Romano and Marra(2008), authors followed the electro-quasi-static approach, butthey enriched the computer simulation of RF heating by taking intoaccount the fluid dynamics of the medium surrounding the foodsample to be heated using Navier–Stoke equations. The modelsample was prepared from 1% gellan gel for experimental valida-tion of the simulation results. Authors showed that spherically-shaped samples surrounded with air between RF electrodes andplaced in the proximity of electrodes would not heat uniformly.Immersing the model fruit in water helped to reduce uneven heat-ing within the model fruit, but created a new problem because themodel fruits were found to heat unevenly at different horizontalpositions. Horizontal and vertical model fruit positions with re-spect to electrodes significantly influenced the heating patterns in-side the model fruit. Non-uniform heating was attributed to shape,dielectric properties, and relative fruit position in the container.The study suggested that movement and rotation of the sphericalobject was the only plausible solution for improving heating uni-formity. The authors proposed the prediction of temperature pro-file by means of computer simulation as a useful tool fordesigning an RF heating process for disinfestation of fruits.

6. Dielectric properties

In this section definition of dielectric properties and factorsinfluencing dielectric properties of foods are discussed. A reviewof dielectric property measurement techniques was given by Re-gier and Schubert (2005).

6.1. Definition of dielectric properties

6.1.1. Permeability and permittivityThe dielectric properties of food materials can be divided into

two parts known as the permeability and permittivity (e). Perme-ability values for foodstuffs are generally similar to that of freespace and as a result are not believed to contribute to heating(Zhang and Datta, 2001). However, the permittivity – which deter-mines the dielectric constant (e0) and the loss factor (e00) – influ-


ences RF heating. The e0 and e00, which are the real and imaginaryparts, respectively of e, are given by

e ¼ e0 � je00 ð5Þ

e0 is a characteristic of any material and is a measure of the capacityof a material to absorb, transmit and reflect energy from the electricportion of the electrical field (Engelder and Buffler, 1991) and is aconstant for a material at a given frequency under constant condi-tions. The e0 is a measure of the polarizing effect from applied elec-tric field (i.e. how easily the medium is polarized). e00 measures theamount of energy that is lost from the electrical field, which is re-lated to how the energy from a field is absorbed and converted toheat by a material passing through it (Engelder and Buffler, 1991).A material with a low e00 will absorb less energy and could be ex-pected to heat poorly in an electrical field due to its greater trans-parency to electromagnetic energy (Decareau, 1985). However, itis important to emphasise that dielectric properties are but one ofa range of properties influencing heating and other properties(e.g. specific heat capacity) will also have an influence on the mag-nitude of the temperature rise obtained.

6.1.2. Electrical conductivityElectrical conductivity (r) indicates the ability of a material to

conduct an electric current. In a dielectric food system, r is relatedto ionic depolarization. It contributes to e00 and in RF ranges can becalculated from the following equation (Piyasena et al., 2003):

r ¼ 2pf e00 ð6Þ

It is also worth noting that at RF frequencies, e00 in liquid or semi-li-quid foods can be estimated with reasonable accuracy by measuringr value of the material using an electric conductivity meter (Guanet al., 2004).

6.1.3. Loss tangentThe tangent of dielectric loss angle (tand) is often called the loss

tangent or the dissipation (power) factor of the material. For a gi-ven material this is equivalent to the ratio of the e00 over e0:

tan d ¼ e00

e0ð7Þ

6.1.4. Penetration depthBengtsson and Risman (1971) defined the penetration depth

(dp) as the depth in a material where the energy of a plane wavepropagating perpendicular to the surface has decreased to 1/e(1/2.72) of the surface value

dp ¼C

2pfffiffiffiffiffiffiffi2e0p

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ ðtan dÞ2

q� 1

r ð8Þ

where C is the speed of propagation of waves in a vacuum(3 � 108 m s�1) and dp is expressed in meters. When tand is low

Table 1Publications after 2000 which present dielectric property data at radio frequencies.

Authors and year Frequency (MHz) Products a

Wang et al. (2003b) 27 and 40 Whey proWang et al. (2003a) 1�1800 FruitsPiyasena et al. (2003b) 10, 20 and 30 Starch solZhang et al. (2004b) 27.12 LuncheonGuan et al. (2004) 1�1800 Mashed pAl-Holy et al. (2005) 27 Salmon (0Lyng et al. (2005) 27.12 Ranges ofBrunton et al. (2006) 27.12 Beef muscSacilik et al. (2006) 0.05�10 FlaxseedsRagni et al. (2007) 20�1800 Egg durinSacilik et al. (2007) 0.05–10 SafflowerFarag et al. (2008) 27.12 Frozen me

(i.e. far less than 1) penetration depth is well described by the fol-lowing simpler form:

dp ¼C

2pfffiffiffiffie0p

tan d¼ 4:47� 107

fffiffiffiffie0p

tan dð9Þ

Eqs. (8) and (9) illustrate the effect of f, e0 and e00 on dp. Bengtssonand Risman (1971) found that the greatest dp was experiencedwhen both e0 and e00 were low.

6.2. Factors influencing dielectric properties of foods

The dielectric properties of food play an important role in bothRF and MW heating (Piyasena et al., 2003) but these properties areinfluenced by a variety of factors. The content of moisture is gener-ally a critical factor (Tang, 2005), but the frequency of the appliedalternating field, the temperature of the material and also the den-sity, chemical composition (i.e. fat, protein, carbohydrate and salt)and structure of the material all have an influence (Piyasena et al.,2003). In terms of bulk density samples of an air-particle mixturewith higher density generally have higher e0 and e00 values becauseof less air incorporation within the samples (Nelson and Datta,2001). In relation to composition, Nelson and Datta (2001) statedthat the dielectric properties of materials are dependent on chem-ical composition and especially on the presence of mobile ions andthe permanent dipole moments associated with water. While aconsiderable amount of work on dielectric properties has beenpublished at MW frequencies a more limited number of studieshave examined the dielectric properties of food and agriculturalproducts at RF frequencies. Work completed up to the year 2000was summarised by Piyasena et al. (2003). Table 1 lists post2000 publications which present dielectric property data for food-stuffs at RF frequencies.

Research conducted at Washington State University (USA) pub-lished dielectric data on fruits, mashed potatoes, whey protein gel,macaroni, cheese, egg and salmon. Results show that e00 of wheyprotein products, and macaroni and cheese increased sharply withincrease temperature at 27 and 40 MHz. The e0 of mashed potatoincreased with temperature at 27 MHz but stayed stable at40 MHz while the e00 increased with temperature at both 27 MHzand 40 MHz. Moisture content did not affect the dielectric proper-ties but added salt had a significant influence on both dielectricproperties which in turn influenced dp.

Another group at UCD Dublin (Ireland) conducted investiga-tions on the dielectric properties of meat, meat products and ingre-dients used in meat product manufacture at 27.12 MHz. Overalltheir results showed that most of additives which have free ionscan change the dielectric properties of final product and in turninfluence the RF heating process. Within the meat species studied,turkey and chicken breast had the higher values of e00 than pork,lamb and beef muscle which was attributed to compositional dif-ferences between the products. Farag et al. (2008) evaluated the

ssessed Temperature range (�C)

tein gel, macaroni noodles, cheese sauce 15�12020�52

ution (1–4%, w/w) 20�80roll and white pudding meat batter 5�85

otato 20�120.8% and 2.3% total salt) 20�80meats and ingredients Room temperaturele 5�90

22 ± 3g storage Room temperatureseed Room temperatureat blends �18 to 10


dielectric properties of three different beef meat blends (lean, fatand 50:50 mixture) over a temperature range �18 to +10 �C. Inthe region of thawing (�3 to �1 �C), e0 and e00 values at27.12 MHz were significantly higher (P < 0.05) than at other mea-sured temperatures for the three blends. Composition also signifi-cantly influenced (P < 0.05) the measured dielectric properties atall temperatures used and, thus, the heating patters during thaw-ing (Farag et al., in press).

7. The mathematical modelling of RF heating in food processing

As in many other technological applications, computer simula-tion is a powerful tool which can be used to facilitate the design ofRF heating systems for food processing. Although modelling resultscannot replace experimental work and on-site data measurements,computer simulation can be used as a virtual laboratory wherenumerical experiments can be run in order to get useful informa-tion for designing RF heaters.

7.1. Recent advances in mathematical modelling of RF heating

Starting from the mid 1990s, a number of papers have appearedin the literature and have discussed mathematical modelling andthe computer simulation of RF systems (Neophytou and Metaxas,1998; Yang et al., 2003; Chan et al., 2004; Marra et al., 2007; Birlaet al., 2008; Romano and Marra, 2008). Activities of computer sim-ulation fall into two general categories, the first being the simula-tion of heat transfer within the product (load) between theelectrodes and has focused mainly on the description of transportphenomena inside the food. The second area has been modellingRF heating in terms its electric and magnetic fields.

Neophytou and Metaxas (1998) presented a 3D FEM model forthe characterization of electric fields within RF applicators. In orderto establish the validity of electrostatic conditions, they solvedboth Laplace (for electrostatic conditions) and wave equationsand compared both in 2D and 3D. Comparison of these approacheswas done in term of a ratio between magnetic and electric energy.According to the authors, if electrostatic conditions hold, thenmagnetic energy should be much less than the electric energy,while at resonance they are equal. Furthermore, they proposed an-other comparison in terms of power loss inside the processedmaterial predicted by the Laplace and wave equations. The abovecriteria were analysed for different type of applicators loaded withpaper blocks at different frequencies. The authors concluded thatelectrostatic conditions (and then the Laplace equation) could beassumed only in very small experimental size applicators. Further-more, they recommended the use of a 3D model with wave equa-tions, since the 2D model was found to be inappropriate.

Yang et al. (2003) proposed the assessment of a program simu-lating the heating performance of radish and alfalfa sprout seedspacked inside rectangular seed boxes during RF heating, evaluatingthe distribution of electric field in a time stepping procedure bymeans of the transmission line method (TLM) method. Heat diffu-sion in their food samples was solved by standard explicit finitedifference time domain method FDTD. Simulated temperaturedistributions were compared against experimental data, andtime–temperature profiles of seeds were then validated usingexperiments conducted with seeds in an RF heating system. Theyreported discrepancies between simulated and experimentalresults, especially at the edges of the box.

A further step towards improving the understanding of RFapplicators by means of computer simulation was proposed byChan et al. (2004) who followed the method proposed by Neophy-tou and Metaxas (1999) with the addition of a means of excitationto the tank oscillatory circuit with an external, properly positioned,coaxial source, with the purpose of presenting a way to visualize

the heating pattern inside the load. These authors did not incorpo-rate heat transfer in their simulated model and evaluated theirmathematical model by comparing a simulated electric field mapfor a 1% Carboxy-Methyl-Cellulose (CMC) solution with an actualtemperature map taken with an infra red camera.

Applying the electro-quasi-static hypothesis for a small cavity,Marra et al. (2007) solved coupled EM and heat transfer equations,using the commercial FEM based software FEMLAB, for a cylindri-cal shaped meat batters placed between the electrodes of a 600 WRF system. For the meat batters chosen as sample foods, dielectricand physical property data were available as a function of temper-ature. Temperature profiles were experimentally measured alongthe sample axes, for RF output powers of 100, 200, 300 and400 W. The authors evaluated the goodness of fit of their modelby comparing numerical results with measured temperature pro-files. Results showed different heating rates within the samplesand, therefore, an uneven temperature distribution. The closerthe temperature measurement point was to the bottom of the sam-ple, the higher was the heating velocity. Unevenness of tempera-ture distribution was emphasised by the applied RF outputpower: the higher the applied power the more uneven was thetemperature distribution. This work demonstrated that the elec-tro-quasi-static hypothesis applied successfully to lab-scale RFapparatus and it can be used for further investigation on RF heatingby means of computer simulation (Birla et al., 2008; Romano andMarra, 2008; also discussed in Section 5).

7.2. Mathematics of RF heating

As reported in Section 3, the power density absorbed by theload is a function of e00 factor and electric field strength. The predic-tion of electric field distribution in a RF heating system is thenessential in order to determine the power density and hence thetemperature of the material. Electric field strength is part of a morecomplex electromagnetic field that can be described, in terms ofelectric field intensity E, magnetic field intensity H and electric fluxdensity D, by the following Maxwell equations in differential form,when the involved media are isotropic, linear and homogenous(Chan et al., 2004):

r� E ¼ �l @

@tðHÞ ð10Þ

r � H ¼ ðe0 er@

@tþ rcÞE ð11Þ

r � D ¼ qc ð12Þr � H ¼ 0 ð13Þ

where rc and qc are the effective electrical conductivity and thecharge density, respectively. Please note that Eq. (10) is known asFaraday’s Law, Eq. (11) is known as Ampere Law, Eq. (12) is knownas the Gauss’ Law for electric fields and Eq. (13) is know as theGauss’ Law for magnetic fields.

Maxwell’s equations need some assumptions on the depen-dence of fields with respect to time. A usual assumption is to con-sider the fields to be time-harmonic (Neophytou and Metaxas,1998):

E ¼ E0 ej x t ð14ÞH ¼ H0 ej x t ð15Þ

When the time-harmonic assumption is made, Eqs. (10) and (11)become

r� E ¼ �jlxH ð16Þr � H ¼ je0 er xE ð17Þ

Combining Eqs. (16) and (17), one obtains the following wave equa-tion that is used for the calculation of the frequency domain


r� 1lrr� E� x2 l0 e0 erE ¼ 0 ð18Þ

Boundary conditions are needed in order to get a unique and propersolution. External boundaries of the cavity are considered as perfectconductors and the following boundary condition is then used(Neophytou and Metaxas, 1998):

n� E ¼ 0 ð19Þ

with n being the unit vector normal to the boundary surfaces.When a quasi-static approach is considered, Maxwell’s equa-

tions collapse to the following:

r � ½ðrþ jxeÞrV � ¼ 0 ð20Þ

where V is the electrical potential, related to the electric field by

E ¼ �rV ð21Þ

In this case, as boundary conditions, a source electric potential V0 isapplied to the upper electrode of the capacitor while at the bottomelectrical ground conditions (V = 0) is considered. RF applicatorwalls are electrically insulated, so last boundary conditions are

r � E ¼ 0 ð22Þ

The mathematical description of heat transfer within the food prod-uct placed between the electrodes, is given by unsteady heat-con-duction equation (assuming that a solid-like foodstuff is processedin the RF applicator) with a generation term (Qv) represented bythe power density already seen in Eq. (4):

@T@ t¼ rarT þ Qv

qCpð23Þ

where T is the temperature within the sample, t is the process time,a is the thermal diffusivity, q is the density, Cp is the heat capacity.

The heat transport equation, to be solved, needs initial andboundary conditions: as initial condition a uniform temperatureT0 can be assumed within the food sample; on boundaries, as gen-eral conditions, convective heat transfer from the external surfaces,in accordance with the Newton law, can be set:

�krT ¼ U ðT � T1Þ ð24Þ

where U is a overall convective heat transfer coefficient and T1, isthe temperature of the medium surrounding the sample.

8. Future trends and prospective

The past 7 years have produced a substantial number of publi-cations in the area of RF heating and it is likely that this trend willcontinue for the foreseeable future. As a rapid heating method, RFheating offers a considerable speed advantage over conventionalheating methods, particularly in solid foods in which heat transferis predominantly governed by heat conduction. However, evenwith this major advantage and the fact that this technology hasbeen available for many years, its uptake by industry has been rel-atively slow. This lack of uptake to date is largely due to

(a) A lack of available in-depth information on areas such as itsimpact on all aspects of product quality. Recent years haveshown a substantial number of publications in the area ofthe quality of RF processed meats but this needs to be repli-cated across a wider range of food commodities. The avail-ability of such information will help to convince processorsof the benefits of this technology.

(b) As a potential pasteurisation/sterilisation technique, morework needs to be published on the effectiveness of RF forinactivating microorganisms and its impact on product qual-ity and shelf life. However, in addition to this a greater

understanding of temperature distribution within productsneeds to be developed. Conventional heating methods aregenerally well understood in this context and until similarknowledge is available for RF heating, food processorscharged with consumer safety are going to be cautious tochange from the methods they are most familiar with.

(c) Needs in designing and scaling-up larger systems for indus-trial application. This step is essential in order to evaluatethe actual capital costs for initial installations and to esti-mate the energy costs involved in particular treatments, inorder to further investigate the efficiency and economics ofRF processing.

(d) A continuing need for the production of more dielectricproperty data on foodstuffs and potential packaging. Thisinformation is the key to improving understanding of tem-perature distribution but also is important in the design ofRF heating systems.

(e) Mathematical modelling improves the understanding of RFheating of food and it is essential to the continued develop-ment of this technology. More efforts are needed in order todevelop computer aided engineering of processes and plantson an industrial-scale.

References

Ahmed, J., Ramaswamy, H.S., Alli, I., Raghavan, V.G.S., 2007. Protein denaturation,rheology and gelation characteristics of radiofrequency heating egg whitedispersions. International Journal of Food Properties 10, 145–161.

Al-Holy, M., Wang, Y., Tang, J., Rasco, B., 2005. Dielectric properties of salmon(Oncorhybchus keta) and sturgeon (Acipenser transmontanus) caviar at radiofrequency (RF) and microwave (MW) pasteurization frequencies. Journal ofFood Engineering 70 (4), 564–570.

Awuah, G.B., Ramaswamy, H.S., Piyasena, P., 2002. Radio frequency (RF) heating ofstarch solutions under continuous flow conditions: effect of system and productparameters on temperature change across the applicator tube. Journal of FoodProcess Engineering 25 (3), 201–223.

Awuah, G.B., Ramaswamy, H.S., Economides, A., Mallikarjuanan, K., 2005.Inactivation of Escherichia coli K-12 and Listeria innocua in milk using radiofrequency (RF) heating. Innovative Food Science and Emerging Technologies 6(4), 396–402.

Bengtsson, N.E., Risman, P.O., 1971. Dielectric properties of foods at 3 GHz asdetermined by cavity perturbation technique. Journal of Microwave Power 6(2), 107–123.

Birla, S.L., Wang, S., Tang, J., Hallman, G., 2004. Improving heating uniformity offresh fruit in radio frequency treatments for pest control. Postharvest Biologyand Technology 33 (2), 205–217.

Birla, S.L., Wang, S., Tang, J., Fellman, J.K., Mattinson, D.S., Lurie, S., 2005. Quality oforanges as influenced by potential radio frequency heat treatments againstMediterranean fruit flies. Postharvest Biology and Technology 38 (1), 66–79.

Birla, S.L., Wang, S., Tang, J., 2008. Computer simulation of radio frequency heatingof model fruit immersed in water. Journal of Food Engineering 84, 270–280.

Brunton, N.P., Lyng, J.G., Li, W., Cronin, D.A., Morgan, D., McKenna, B., 2005. Effect ofradio frequency (RF) heating on the texture, colour and sensory properties of acomminuted pork meat product. Food Research International 38 (3), 337–344.

Brunton, N.P., Lyng, J.G., Zhang, L., Jacquier, J.C., 2006. The use of dielectricproperties and other physical analyses for assessing protein denaturation inbeef Biceps femoris muscle during cooking from 5 to 85 �C.. Meat Science 72 (2),236–244.

Buffler, C.R., 1993. Dielectric properties of foods and microwave materials. In:Microwave Cooking and Processing. Van Nostrand Reinhold, New York, pp. 46–69.

Chan, T.V.C.T., Tang, J., Younce, F., 2004. 3-Dimensional numerical modeling of anindustrial radio frequency heating system using finite elements. Journal ofMicrowave Power and Electromagnetic Energy 39 (2), 87–106.

Cserhalmi, Zs., Márkus, Zs., Czukor, B., Baráth, Á., Tóth, M., 2001. Physico-chemicalproperties and food utilization possibilities of RF-treated mustard seed.Innovative Food Science and Emerging Technologies 1 (4), 251–254.

Decareau, R.V., 1985. Food Industry and Trade – Microwave Heating – IndustrialApplications. Academic press, INC. Frorida, US, Orlando. pp. 1–10.

Demeczky, M., 1974. Continuous pasteurisation of bottled fruit juices by highfrequency energy. Proceedings of IV International Congress on Food Science andTechnology IV, 11–20.

Engelder, D.S., Buffler, C.R., 1991. Measuring dielectric properties of food products atmicrowave frequencies. Microwave World 12 (2), 2–11.

Farag, K., Lyng, J.G., Morgan, D.J., Cronin, D.A., 2008. Dielectric and thermophysicalproperties of different beef meat blends over a temperature range of �18 to+10 �C. Meat Science 79 (4), 740–747.


Farag, K., Marra, F., Lyng, J.G., Morgan, D.J., Cronin, D.A., in press. Temperaturechanges and power consumption during radio frequency tempering of beeflean/fat formulations. Food and Bioprocess Technology. doi:10.1007/s11947-008-0131-5.

Geveke, D.J., Brunkhorst, C., 2004. Inactivation of Escherichia coli in apple juice byradio frequency electric fields. Journal of Food Science 69 (3), FEP 134–FEP 138.

Geveke, D.J., Brunkhorst, C., 2008. Radio frequency electric fields inactivation ofEscherichia coli in apple cider. Journal of Food Engineering 85 (2), 215–221.

Geveke, D.J., Brunkhorst, C., Fan, X., 2007. Radio frequency electric fields processingof orange juice. Innovative Food Science and Emerging Technologies 8 (4), 549–554.

Guan, D., Cheng, M., Wang, Y., Tang, J., 2004. Dielectric properties of mashedpotatoes relevant to microwave and radio-frequency pasteurization andsterilization processes. Journal of Food Science 69 (1), 30–37.

Guo, Q., Piyasena, P., Mittal, G.S., Si, W., Gong, J., 2006. Efficacy of radio frequencycooling in the reduction of Escherichia coli and shelf stability of ground beef.Food Microbiology 23 (2), 112–118.

Hansen, J.D., Drake, S.R., Heidt, M.L., Watkins, M.A., Tang, J., Wang, S., 2005.Evaluation of radio frequency-hot water treatments for postharvest control ofcodling moth in ‘Bing’ sweet cherries. Hort Technology 15 (3), 613–616.

Hansen, J.D., Drake, S.R., Heidit, M.L., Watkins, M.A., Tang, J., Wang, S., 2006a. Radiofrequency-hot water dips for postharvest codling moth control in apples.Journal of Food Processing and Preservation 30, 631–642.

Hansen, J.D., Drake, S.R., Watkins, M.A., Heidit, M.L., Anderson, P.A., Tang, J., 2006b.Radio frequency pulse application for heating uniformity in postharvest codlingmoth (Lepidoptera: tortricidae) control of fresh apples (Malus domestica borkh.).Journal of Food Quality 29, 492–504.

Houben, J., Schoenmakers, L., van Putten, E., van Roon, P., Krol, B., 1991. Radiofrequency pasteurisation of sausage emulsions as a continuous process. Journalof Microwave Power and Electromagnetic Energy 26 (4), 202–205.

Ikediala, J.N., Hansen, J.D., Tang, J., Drake, S.R., Wang, S., 2002. Development of asaline water immersion technique with RF energy as a postharvest treatmentagainst codling moth in cherries. Postharvest Biology and Technology 24 (1),25–37.

Jason, A.C., Sanders, H.R., 1962a. Dielectric thawing of fish. I. Experiments withfrozen herrings. Food Technology 16 (6), 101–106.

Jason, A.C., Sanders, H.R., 1962b. Dielectric thawing of fish. II. Experiments withfrozen white fish. Food Technology 16 (6), 107–112.

Jones, P.L., Rowley, A.T., 1997. In: Baker, C.G.J. (Ed.), Industrial Drying of Foods.Blackie Academic and Professional, London (Chapter 8).

Laycock, L., Piyasena, P., Mittal, G.S., 2003. Radio frequency cooking of ground,comminuted and muscle meat products. Meat Science 65 (3), 959–965.

Luechapattanporn, K., Wang, Y., Al-Holy, M., Kang, D.H., Tang, J., Hallberg, L.M.,2004. Microbial safety in radio-frequency processing of packaged foods. Journalof Food Science 69 (7), M201–M206.

Luechapattanporn, K., Wang, Y., Wang, J., Tang, J., Hallberg, L.M., Dunne, C.P., 2005.Sterilization of scrambled eggs in military polymeric trays by radio frequencyenergy. Journal of Food Science 70 (4), E288–E294.

Lyng, J.G., 2007. Rapid pasteurisation of meats using radio frequency or ohmicheating. New Food 3, 58–62.

Lyng, J.G., Zhang, L., Brunton, N.P., 2005. A survey of dielectric properties of meatsand ingredients used in meat product manufacture. Meat Science 69 (4), 589–602.

Lyng, J.G., Cronin, D.A., Brunton, N.P., Li, W., Gu, X., 2007. An examination of factorsaffecting radio frequency heating of an encased meat emulsion. Meat Science 75(3), 470–479.

Marra, F., Lyng, J., Romano, V., McKenna, B., 2007. Radio-frequency heating offoodstuff: solution and validation of a mathematical model. Journal of FoodEngineering 79 (3), 998–1006.

McKenna, B.M., Lyng, J., Brunton, N., Shirsat, N., 2006. Advances in radio frequencyand ohmic heating of meats. Journal of Food Engineering 77 (2), 215–229.

Mermelstein, N.H., 1998. Microwave and radio frequency drying. Food Technology52 (11), 84–86.

Mitcham, E.J., Veltman, R.H., Feng, X., de Castro, E., Johnson, J.A., Simpson, T.L., Biasi,W.V., Wang, S., Tang, J., 2004. Application of radio frequency treatments tocontrol insects in in-shell walnuts. Postharvest Biology and Technology 33 (1),93–100.

Monzon, M.E., Biasi, B., Simpson, T.L., Jhonson, J., Feng, X., Slaughter, D.C., Mitcham,E.J., 2006. Effect of radio frequency heating as a potential quarantine treatmenton the quality of ‘Bing’ sweet cherry fruit and mortality of codling moth larvae.Postharvest Biology and Technology 40 (2), 197–203.

Monzon, M.E., Biasi, B., Mitcham, E.J., Wang, Shaojin, Tang, Juming, Hallman, G.J.,2007. Effect of radiofrequency heating on the quality of Fuyu persimmon fruitas a treatment for control of the Mexican fruit fly. Hort Science 42 (1), 125–129.

Nelson, S.O., Datta, A.K., 2001. Dielectric properties of food materials and electricfield interactions. In: Datta, A.K., Anantheswaran, R.C. (Eds.), Handbook ofMicrowave Technology for Food Applications. Marcel Dekker, New York.

Nelson, S.O., Lu, C.Y., Beuchat, L.R., Harrison, M.A., 2003. Radio-frequency heating ofalfalfa seed for reducing human pathogens. Transactions of the ASAE 45 (6),1937–1942.

Neophytou, R.I., Metaxas, A.C., 1998. Combined 3D FE and circuit modeling of radiofrequency heating systems. Journal of Microwave Power and ElectromagneticEnergy 33 (4), 243–262.

Neophytou, R.I., Metaxas, A.C., 1999. Combined tank and applicator design of radiofrequency heating systems. Microwaves, Antennas and Propagation, IEEEProceedings 146 (5), 311–318.

Ohlsson, T., 1983. Fundamentals of microwave cooking. Microwave World 4, 4–9.Orsat, V., Gariépy, Y., Raghavan, G.S.V., Lyew, D., 2001. Radio-frequency treatment

for ready-to-eat fresh carrots. Food Research International 34, 527–536.Orsat, V., Bai, L., Raghavan, G.S.V., Smith, J.P., 2004. Radio-frequency heating of ham

to enhance shelf-life in vacuum packaging. Journal of Food Process Engineering27, 267–283.

Piyasena, P., Dussault, C., 2003. Continuous radio-frequency heating of a modelviscous solution: influence of active current, flow rate, and salt content ontemperature rise. Canadian Biosystem Engineering 45, 327–334.

Piyasena, P., Dussault, C., Koutchma, T., Ramaswamy, H.S., Awuah, G.B., 2003a. Radiofrequency heating of foods: principles, applications and related properties. Areview. Critical Reviews in Food Science and Nutrition 43 (6), 587–606.

Piyasena, P., Ramaswamy, H.S., Awuah, G.B., Defelice, C., 2003b. Dielectricproperties of starch solutions as influenced by temperature, concentration,frequency and salt. Journal of Food Process Engineering 26, 93–119.

Ragni, L., Al-Shami, A., Mikhaylenko, G., Tang, J., 2007. Dielectric characterization ofhen eggs during storage. Journal of Food Engineering 82 (4), 450–459.

Regier, M., Schubert, H., 2005. Measuring dielectric properties of foods. In: Schubert,H., Regier, M. (Eds.), The Microwave Processing of Foods. Woodhead PublishingCambridge, UK, pp. 41–60.

Rice, J., 1993. RF technology sharpens bakery’s competitive edge. Food Processing 6,18–24.

Romano, V., Marra, F., 2008. A numerical analysis of radio frequency heating ofregular shaped foodstuff. Journal of Food Engineering 84, 449–457.

Rowley, A.T., 2001. Radio frequency heating. In: Richardson, P. (Ed.), ThermalTechnologies in Food Processing. Woodhead Publishing Cambridge, UK, pp.162–177.

Sacilik, K., Tarimci, C., Colak, A., 2006. Dielectric properties of flaxseeds as affectedby moisture content and bulk density in the radio frequency range. BiosystemsEngineering 93 (2), 153–160.

Sacilik, K., Tarimci, C., Colak, A., 2007. Moisture content and bulk densitydependence of dielectric properties of safflower seed in the radio frequencyrange. Journal of Food Engineering 78 (4), 1111–1116.

Samaranayake, C.P., Sastry, S.K., Zhang, H., 2005. Pulsed ohmic heating – a noveltechnique for minimization of electrochemical reactions during processing.Journal of Food Science 70 (8), E460–E465.

Schuster-Gajzágó, I., Kiszter, A.K., Tóth-Márkus, M., Baráth, A., Márkus-Bednarik, Z.,Czukor, B., 2006. The effect of radio frequency heat treatment on nutritional andcolloid-chemical properties of different white mustard varieties. InnovativeFood Science and Emerging Technologies 7, 74–79.

Sherman, V.W., 1946. Food Industry 18, 506–509. 628.Tang, J., 2005. Dielectric properties of foods. In: Schubert, H., Regier, M. (Eds.), The

Microwave Processing of Foods. Woodhead Publishing Cambridge, UK, pp. 22–40.

Tang, J., Ikediala, J.N., Wang, S., Hansen, J.D., Cavalieri, R.P., 2000. High-temperature-short-time thermal quarantine methods. Postharvest Biology and Technology21, 129–145.

Tang, J., Wang, Y., Chan, T.V.C.T., 2005. Radio frequency heating in food processing.In: Gustavo Barbosa-Cánovas, G.V., Tapia, M.S., Cano, M.P. (Eds.), Novel FoodProcessing Technologies. Marcel Dekker, New York, NY, USA, pp. 501–524.

Tang, X., Cronin, D.A., Brunton, N.P., 2005. The effect of radio frequency heating onchemical, physical and sensory aspects of quality in turkey breast rolls. FoodChemistry 93 (1), 1–7.

Tang, X., Lyng, J.G., Cronin, D.A., Durand, C., 2006. Radio frequency heating of beefrolls from Biceps femoris muscle. Meat Science 72 (3), 467–474.

Wang, S., Tang, J., Johnson, J.A., Mitcham, E., Hansen, J.D., Cavalieri, R.P., Bower, J.,Biasi, B., 2002. Process protocols based on radio frequency energy to controlfield and storage pests in in-shell walnuts. Postharvest Biology and Technology26 (3), 265–273.

Wang, S., Tang, J., Johnson, J.A., Mitcham, E., Hansen, J.D., Hallman, G., Drake, S.R.,Wang, Y., 2003a. Dielectric properties of fruits and insect pests as related toradio frequency and microwave treatments. Biosystems Engineering 85 (2),201–212.

Wang, Y., Wig, T.D., Tang, J., Hallberg, L.M., 2003b. Sterilization of foodstuffs usingradio frequency heating. Journal of Food Science 68 (2), 539–544.

Wang, Y., Lau, M.H., Tang, J., Mao, R., 2004. Kinetics of chemical marker M-1formation in whey protein gels for developing sterilization processes based ondielectric heating. Journal of Food Engineering 64, 111–118.

Wang, S., Yue, J., Tang, J., Chen, B., 2005. Mathematical modelling of heatinguniformity for in-shell walnuts subjected to radio frequency treatments withintermittent stirrings. Postharvest Biology and Technology 35, 97–107.

Wang, S., Birla, S.L., Tang, J., Hansen, J.D., 2006a. Postharvest treatment to controlcodling moth in fresh apples using water assisted radio frequency heating.Postharvest Biology and Technology 40 (1), 89–96.

Wang, S., Tang, J., Sun, T., Mitcham, E.J., Koral, T., Birla, S.L., 2006b. Consideration indesign of commercial radio frequency treatments for postharvest pest control inin-shell walnuts. Journal of Food Engineering 77 (2), 304–312.

Wang, S., Monzon, M., Johnson, J.A., Mitcham, E.J., Tang, J., 2007a. Industrial-scaleradio frequency treatments for insect control in walnuts I: heating uniformityand energy efficiency. Postharvest Biology and Technology 45, 240–246.

Wang, S., Monzon, M., Johnson, J.A., Mitcham, E.J., Tang, J., 2007b. Industrial-scaleradio frequency treatments for insect control in walnuts II: insect mortality andproduct quality. Postharvest Biology and Technology 45, 247–253.

Yang, J., Zhao, Y., Wells, J.H., 2003. Computer simulation of capacitive radiofrequency (RF) dielectric heating on vegetable sprout seeds. Journal of FoodProcess Engineering 26, 239–263.


Zhang, H., Datta, A.K., 2001. Electromagnetics of microwave heating: magnitude anduniformity of energy absorption in an oven. In: Datta, A.K., Anantheswaran, R.C.(Eds.), Handbook of Microwave Technology for Food Applications. MarcelDekker, New York, pp. 1–28.

Zhang, L., Lyng, J.G., Brunton, N.P., 2004a. Effect of radio frequency cooking ontexture, colour and sensory properties of a large diameter comminuted meatproduct. Meat Science 68 (2), 257–268.

Zhang, L., Lyng, J.G., Brunton, N., Morgan, D., McKenna, B., 2004b. Dielectric andthermophysical properties of meat batters over a temperature range of 5–85 �C.Meat Science 68, 173–184.

Zhang, L., Lyng, J.G., Brunton, N.P., 2006. Quality of radio frequency heated pork legand shoulder ham. Journal of Food Engineering 75 (2), 275–287.

Zhang, L., Lyng, J.G., Brunton, N.P., 2007. The effect of fat, water and salt on the thermaland dielectric properties of meat batter and its temperature following microwaveor radio frequency heating. Journal of Food Engineering 80 (1), 142–151.

Zhao, Y., Flugstad, B., Kolbe, E., Park, J.E., Wells, J.H., 2000. Using capacitive (radiofrequency) dielectric heating in food processing and preservation – a review.Journal of Food Process Engineering 23, 25–55.

Zhong, Q., Sandeep, K.P., Swartzel, K.R., 2003. Continuous flow radio frequencyheating of water and carboxymethylcellulose solutions. Journal of Food Science68 (1), 217–223.

Zhong, Q., Sandeep, K.P., Swartzel, K.R., 2004. Continuous flow radio frequencyheating of particulate foods. Innovative Food Science and EmergingTechnologies 5 (4), 475–483.

radio frequency treatment of foods: review of recent advances

Documents