Dielectric Properties of Food

LWT - Food Science and Technology 43 (2010) 1169e 1169 e1179

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Review

Dielectric properties of foods: Reported data in the 21st Century and their potential applications M.E. Sosa-Morales a b , L. Valerio-Junco b, A. López-Malo b, H.S. García a ,

a b

,

*

Unidad de Investigación y Desarrollo de Alimentos, Instituto Tecnológico de Veracruz, M.A. de Quevedo 2779, Col. Formando Hogar, Veracruz, Ver. 91897, Mexico Departamento de Ingeniería Química y de Alimentos, Universidad de las Américas Puebla, Ex-Hacienda Santa Catarina Mártir, Cholula, Pue. 72820, Mexico

a r t i c l e

i n f o

a b s t r a c t

 Article history: Received 14 December 2009 Received in revised form 27 March 2010 Accepted 29 March 2010

Dielectric Dielectric properties properties (DP) are the main parameters parameters that provide provide information information about about how materials materials interact interact with electroma electromagneti gneticc energy energy during dielectric dielectric heating. heating. These properties properties have gained great great importance and applications for foods that are subjected to novel microwave (MW) or radio frequency (RF) heating treatments. The knowledge of the DP of a determined foodstuff is fundamental in order to understand understand and model the response of the material to the electroma electromagneti gneticc field, at certain certain desired desired frequencies and temperatures. Through the last years, many potential applications of electromagnetic heating for foods have emerged and been published in the literature; however, new uses or research in food products to be treated with MW or RF may be limited due to lack of DP data. This review provides an overall introduction and de finition of the DP, factors that affect them, methods for their determination, as it also includes reported DP data for foods after the year 2000. DP values were grouped depending on the nature of foods, such as: 1) fruits and vegetables, 2) flour, dough and bread, 3) nuts, 4) coffee grains, 5) meats, fish and seafood, 6) dairy products, 7) eggs and egg products and 8) liquid fluids. We consider that this paper is a useful reference that contains current and valuable information on the DP of foods, which can be available and used for further developments employing MW or RF heating food technologies. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Dielectric properties of foods Dielectric constant Loss factor

1. Introduction

Maxwell s equations. equations. From an engineerin engineering g viewpoint, viewpoint, dielectric properties are the most important physical properties associated with RF and MW heating, since the dielectric behaviour of foods affects their heating characteristics. It is critical to have available data of the dielectric properties of materials in product and process develo developme pment nt and, and, especi especiall ally, y, in modern modern design design of dielect dielectric ric heatin heating g systems to meet desired process requirements. The need for such knowle knowledge dge become becomess even even more more eviden evidentt with with the advanc advance e of  computer modeling tools, used in the design of RF and MW application systems systems and heating processes (Tang, (Tang, 2005). 2005). Fortunately, many studies on the dielectric properties of agricultural and biological materials have been reported for different frequency ranges, temperatures, and moisture contents. In order to have current data available, the objective of this review article is to offer a compilation of recent information (from the year 2000 until present), related to the dielectric properties of different foods, in order to make available experimental data as a useful reference for further research and applications. ’

The dielectric properties of foods and biological products have become valuable parameters in food engineering and technology (Içıer & Baysal, 2004). 2004 ). The interest in the dielectric properties of  agricultural materials and food products has centered primarily to predict heating rates describing the behaviour of food materials when subjected subjected to high-freq high-frequency uency fields elds in dielect dielectric ric heatin heating g applications, or so called novel thermal treatments (Venkatesh (Venkatesh & Raghavan, Ragha van, 2004 2004). ). The in fluence uence of the dielec dielectri tricc proper propertie tiess on food heating by absorption of energy through radio frequency or microwave frequencies, has been known for some time, and many potential applications have been explored (Metaxas ( Metaxas & Meredith, 1993). 1993 ). For instanc instance, e, some some electr electrohe oheati ating ng process processes es have have been been recent recently ly applied applied in the industr industry, y, while while micro microwa wave ve heatin heating g is commer commercia cially lly employ employed ed and is also widely widely used used in househo households lds (Marra, Zhang, Zhang, & Lyng, 2008). 2008 ). The distribution of electromagnetic energy in radio frequency (RF) (RF) and micro microwa wave ve (MW) (MW) heatin heating g system systemss is control controlled led by Corresponding author. Departamento de Ingeniería Química y de Alimentos, Universidad de las Americas Puebla, Ex-Hacienda Santa Catarina Martir, Cholula, Pue. 72820, Mexico. Tel.: 52 222 229 2126; fax: 52 222 229 2727. E-mail address: [email protected] (M.E. Sosa-Morales). Sosa-Morales). *

þ

0023-6438/$ e see front matter doi:10.1016/j.lwt.2010.03.017

þ

Ó

2010 Elsevier Ltd. All rights reserved.

2. Definition of dielectric properties

Knowledge of the dielectric properties of foods is essential in research, modeling and development of thermal treatments based

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M.E. Sosa-Morales et al. / LWT - Food Science and Technology 43 (2010) 1169 e1179

on radio radio frequ frequenc ency y (RF) (RF) and microw microwav ave e (MW) (MW) energy energy.. These These properties provide information about the interaction between the foodstuff and electric fields (Ikediala, (Ikediala, Tang, Drake, & Neven, 2000 ).

 2.1.  2.1. Permittivity, Permittivity, dielectric constant and loss factor 

The dielectric properties of materials that are of interest in most applications can be defined in terms of their relative permittivity. Permittivity is a complex quantity generally used to describe the dielectric properties that in fluence reflection of electromagnetic waves at interfaces and the attenuation of the wave energy within materials. The relative complex permittivity, 3r , describes describes permitpermittivity related related to free space and it is represented represented as:

3r 

¼ 3r 0 À j3r 00

(1)

where 3r 0 and 3r 00 arecommonly arecommonly called called the dielec dielectri tricc constan constantt and loss factor, respectively and j 1. The real part, the dielectric constant ( 3r 0 ), describes the ability of  a material to store energy when it is subjected to an electric field and and influences uences the electr electric ic field eld distr distribu ibuti tion on and and the the phas phase e of wave wavess travelling through the material. The imaginary part, the loss factor (3r 00 ), influences uences both both energy energy absorpt absorption ion and attenu attenuati ation, on, and describes the ability to dissipate energy in response to an applied electric field or various various polarization polarization mechanisms, mechanisms, which commonly commonly result resultss in heat heat gener generati ation on (Ike Ikedial diala a et al. al.,, 2000; Mudg Mudgett ett,, 1986 986). ). The The amount of thermal energy converted in the food is proportional to the value of the loss factor (Tang, (Tang, 2005). 2005). Mechanisms that contribute to the dielectric loss factor include dipole, electronic, ionic and Maxwell-Wagner responses (Metaxas ( Metaxas & Mer Meredi edith, th, 1993 993). ). At RF and and micr microw owav ave e freq freque uenc ncie iess (RF (RF of  1e50 MHz and microwave frequencies of 915 and 2450 MHz), ionic conductivity conductivity and dipole rotation are predominant predominant loss mechanisms mechanisms (Ryynä Ryynänen, nen, 19 1995 95): ):

p  ¼ À

 ffiffi ffi ffi

300

¼ 3d00 þ 3s00 ¼ 3d00 þ 3 su

(2)

0

where subscripts d and s stand for contributions due to dipole rotation and ionic conduction, respectively; s is the ionic conductivity in S m À1 of the material, u is the angular frequency of the waves in Hz and 30 is the permittivity of free space or vacuum (8.854 10À12 F mÀ1). Besides, Besides, Maxwell-W Maxwell-Wagner agner polarization polarization arises from a charge build-up at the interface between components in heterogeneous systems. The Maxwell-Wagner polarization effect peaks at about 0.1 MHz (Metaxas (Metaxas & Meredith, 1993), 1993 ), but in general, its contribution is small compared to that of ionic conductivity. For foods with low moisture content, bound water plays a major role in dielectric heatin heating g in the frequ frequenc ency y range range from from 20 to 30,000 30,000 MHz (Wa ( Wang, ng, Wig, Tang, & Hallberg, 2003). 2003). Dielectric Dielectric materials, materials, such as food products, convert electric energy at RF and microwave frequencies into heat. The increase in temperature of a material due to dielectric heating can be calculated as:

Â

rC  p

dT  dt 

¼ 55:63 Â 10À12 fE 2300

(3)

where C  p is the specific heat of the material in J kg À1  CÀ1, r is the density of the material in kg/m 3, E  is the rms electric field intensity i n V mÀ1, f  is the the freq freque uency ncy in Hz, Hz, dT /d /dt  is the the time time rate rate of  temperatur temperature e increase increase in  C sÀ1. It is clea clearr fromEq. fromEq. (3) that that the the rise rise in temperature is proportional to the loss factor of the material, in addition to electric field intensity, frequency and treatment time (Komarov, Wang, & Tang, 2005; Nelson, 1996). 1996 ).

 2.2. Other properties related related to dielectric parameters: penetration penetration depth and electrical conductivity

Food materials materials are, in general, general, poor electrical electrical conductors conductors as they have have the ability ability to store store and dissipat dissipate e electr electric ic energy energy when when expose exposed d to an electromagnetic field (Buf  (Buf fler, 1993). 1993). The penetration depth (d p) is usually defined ned as the the dept depth h into into a sample sample where where the microwave and RF power has dropped to 1/e (e 2.718) 2.718) or 36.8% of  its transmitted value. The penetration depth is a function of  3r 0 and 3r 00 :

¼

d p

¼

p 

 ffiffi ffi

l0 30 2p300

(4)

where l0 is the free space microwave wavelength (for 2.45 GHz, l0 12.2 cm). Other expression to calculate the d p is

¼

d p

¼

c 

(5)

s  ffiffi ffi 0ffiffi q  ffi ffi ffi ffi ffi ffi ffi ffi ffi00ffi ffi ffi 0ffi ffi ffi ffi ffi ffi ffi ffi

2p f  23

1

2

þ ð3 =3 Þ À 1

where c  is the speed of light in free space (3 108 m/s) and f  is the freque frequency ncy (Hz). (Hz). Common Common food food produc products ts have have 3r 00 < 25, which which implies implies a d p of 0.6 0.6e1.0 cm (Ven Venkat katesh esh & Rag Raghav havan, an, 2004 2004). ). According According to Tang, Wang, and Chan (2003) and Wang, Wig, et al. (2003),, the penetration of microwaves at 915 and 2450 MHz in (2003) foods with high moisture content at room temperature is typically betw betwee een n 0.3 and and 7 cm, cm, depe depend ndin ing g on the the salt salt conte content nt and and frequency. After obtaining the dielectric properties, the penetration depths of electromagne electromagnetic tic energy in selected selected materials can be calculated. Given fixed dielectric properties, the penetration depth of a material is inversely inversely proportional proportional to frequency frequency ( f ), ) , as Eq. (5) states. Deeper Deeper penetration penetration corresponds corresponds to lower frequencies, frequencies, and that higher higher frequ frequenc encies ies result result in greate greaterr surface surface heatin heating. g. Thus, Thus, the pene penetr trat ationdept iondepth h of RF ener energy gy in foods foods canbe as larg large e as oneorder oneorder of magnitude compared with MW; for example, at 27.12 MHz a six times times greate greaterr penetr penetrati ation on depth depth in mangoe mangoess was was calcula calculated ted compar compared ed to micro microwa wave ve energy energy at 1800 MHz at 20  C (SosaMorales et al., 2009). 2009).

Â

3. Methods for determining determining dielectric properties properties

There are several techniques to measure the dielectric properties ties of the materials materials.. Içıer an and d Ba Bays ysal al (2 (2004 004)) cited different different measur measureme ements nts techni techniqu ques, es, and their their main main charac character terist istics ics are summarized summarized in Tab Table le 1. In gener general, al, the choice of measure measuremen mentt equipment depends on the material, the required frequency range and accurac accuracy, y, and both both avail availabil ability ity and costs costs of equipm equipment entss (Nelson & Kraszewski, 1990). 1990). The three three most most popular popular method methodss for measur measuring ing dielec dielectri tricc proper propertie tiess of foods foods and commodit commodities ies are: are: open-e open-ende nded d coaxia coaxiall probe, transmission line and, resonant cavity method. The probe method is based on a coaxial line ending abruptly abruptly at the tip that is in cont contac actt with with the the mate materi rial al bein being g test tested ed (Fig.1 Fig.1). ). This This method method offers offers broadband broadband measuremen measurements ts while minimizes sample disturbance. The measur measured ed reflection ection coef  coef ficient cient is rela relate ted d to the the sample sample permittivity (Sheen (Sheen & Woodhead, 1999). 1999). The probe method is the easiest to use because it does not require a particular sample shape or special containers (Feng, (Feng, Tang, Cavalieri, Cavalieri, & Plumb, 2001; Ikediala et al., 2000; Nelson, 2003; Wang, Tang, et al., 2003). 2003 ). The transmission line method involves placing a sample inside an enclose enclosed d transm transmissi ission on line (Fig. 1). 1). The cross-sec cross-sectio tion n of the transmission line must be precisely filled with the sample. This method method is usually usually more more accurat accurate e and sensit sensitive ive than than the probe probe

M.E. Sosa-Morales et al. / LWT - Food Science and Technology 43 (2010) 1169 e1179

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method, but it is dif ficult to implement and time consuming. In contra contrast, st, the resona resonant nt cavity cavity method method uses uses a single single-mo -mode de cavity cavity.. The sample of known geometry is placed in the cavity, the changes in reflected power of the cavity and the frequency of resonance are used to compute the dielectric property of the sample. The cavity method can be accurate and is especially suited for samples with a very low dielectric loss factor; however, this method provides dielec dielectri tricc proper propertie tiess at only only one fixed xed frequ frequenc ency y (Enge Engelde lderr & Buf fler er,, 199 1991 1). 4. Factors Factors in influencing dielectric properties of foods

Several important factors are involved in the dielectric properties values determined for a given material. Some of these factors are related to the nature of the material (composition, (composition, structure) structure),, while while others others are associa associated ted with with the conditi conditions ons when when electr electrooheatin heating g is applied applied (tempe (tempera ratur ture, e, frequ frequenc ency), y), and other otherss are are involved involved with the age or maturity stage of the food material. material. 4.1. 4.1. Composition Composition

The dielectric properties of materials are dependent on their chemic chemical al composi composition tion.. In foods, foods, water water is gener generally ally the predom predomina inant nt component. Moreover, the in fluence of water, or the content of salt and other minerals depends to a large extent on the manner in which which they they arebound or restri restricte cted d in their their moveme movement nt by other other food compone components. nts. This This complica complicates tes the predic prediction tion of the dielect dielectric ric properties of a mixture, based on data for each ingredient. The organic constituents of foods are dielectrically inert ( 3r 0 < 3 and 3r 00 < 0:1) and, compared to aqueous ionic fluids or water, may be considered transparent to energy ( Mudgett, 1986). 1986). Microwave heating is greatly affected by the presence of water in foods (Mudgett, (Mudgett, 1986; 1986; Nelson & Kraszewski, 1990; Von Hippel, 1954). 1954 ). Water is the major absorber of microwave energy in foods, and consequently, the higher the moisture content, the better the heating. In its pure form, water is a classic example of a polar dielectric (Venkatesh (Venkatesh & Raghavan, 2004). 2004 ). In general, higher moisture content results results in higher dielectric constant and loss factor of  the food (Komarov (Komarov et al., 2005). 2005). High temperatures can, however, increas increase e the mobilit mobility y of bound bound water water,, by reducin reducing g this this critica criticall moisture level (Tang, (Tang, 2005). 2005). Because of the reduced loss factor with decreasing moisture content, content, dehydrat dehydrated ed foods have less ability to convert convert electromagnetic energy into thermal energy. Conversely, during a microwave drying process, the wet part of the product is able to convert more microwave energy into thermal energy compared to the dry part, which tends to uniform uniform the uneven moisture distribution distribution commonly experienced in hot air drying processes, where the core has higher moisture content than the surface. This phenomenon could significantly reduce drying times (Feng ( Feng et al., 2001). 2001). Ionic Ionic compone components nts have have signi significant effects effects in the dielect dielectric ric properties. Increasing in salt content (from 0.8 to 2.8%, wet basis) resulted in an augment for loss factor of mashed potatoes, while dielect dielectric ric constant constant was was not affected affected by the salt content content (Guan, ( Guan, Cheng, Wang, & Tang, 2004). 2004). 4.2. Density Density

Physic Physical al structu structure re also affect affectss the dielec dielectri tricc proper propertie tiess of  materials materials (Ryynänen,1995 Ryynänen,1995). ). The The amou amount nt of mass mass per per unit unit of volu volume me (density) has certain effect on the interaction of the electromagnetic field and the involved mass (Nelso (Nelson, n, 19 1992 92). ). For example, bulk density density and moisture content affect the dielectric properties properties of  coffee grain, lower permittivities were observed at lower density, while higher permittivity permittivity values were achieved achieved for larger bulk

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Fig. 2. Dependence of moisture and temperature on dielectric constant and loss factor of chickpea flour at 27 MHz (from Guo et al., 2008). 2008 ).

constant is quite complex, and it may increase or decrease with temper temperatu ature re depend depending ing on the mater material. ial. The temper temperatu ature re of  a mate materi rial al has has a sign signiificant cant effect effect on the dielect dielectric ric proper propertie ties. s. Generally, the loss factor increases with increasing temperature at low frequencies due to ionic conductance (Guan ( Guan et al., 2004) 2004) and decreases with increasing temperature at high frequencies due to free water dispersion (Wang, ( Wang, Wig, et al., 2003). 2003). Fig.1. Schematic diagram of methods to measure dielectric properties: (a) open-ended coaxial probe, from Wang, Tang, et al. (2003) , (b) transmission line-waveguide and (c) free-space transmission technique, ports are connected to a network analyzer, from Venkatesh and Raghavan (2005). (2005) .

densities (Berbe (Berbert rt et al., 2001). 2001). A similar conclusion was reached reached by Guo, Tiwari, Tiwari, Tang ang,, and Wa Wang ng (200 (2008) 8) who found that both the dielectric dielectric constant and loss factor of chickpea flour increased with increases in both density and moisture content from 1.265 g/cm 3 for flour with 1.9% of moisture, to 1.321 g/cm3 for chickpea chickpea flour with a moisture content of 20.9% ( Fig. 2). 2). The authors also suggested gested simple simple relati relations ons to estima estimate te dielect dielectric ric proper propertie tiess of chickpe chickpea a flour from its density and estimation of density from dielectric properties of the flour. 4.3. Temperature Temperature

The influence of temperatu temperature re on the dielectric properties properties of  foods foods depend dependss on many many factor factors, s, includi including ng food composi compositio tion, n, especially moisture and salt contents, and the involved frequencies (Tang, 2005). 2005). Thus, the temperature dependence of the dielectric

4.4. Frequency Frequency

With the exception of some materials with extremely low loss (mate (materia rials ls that that absorb absorb essenti essentiall ally y no energy energy from RF and MW elds), the dielect dielectric ric proper propertie tiess of most materi materials als vary vary conside considerab rably ly fields), with the frequency frequency of the applied electric electric fields. elds. Thus, Thus, an import important ant phenomenon phenomenon contributing contributing to the frequency frequency dependence of the dielectric properties is the polarization of molecules arising from the orient orientati ation on with with the impose imposed d electr electric ic field, eld, whic which h have have permanent dipole moments (Venkatesh ( Venkatesh & Raghavan, 2004). 2004). At low frequencie frequenciess ( <200 MHz) ionic conductivity plays a major role, role, wherea whereass both both ionic ionic conduct conductivi ivity ty and the dipole dipole rotat rotation ion of free free water are important at microwave frequencies. For example, ionic conduction was the dominant mechanism for dielectric dispersion in eggs eggs at freq freque uenc ncie iess lower lower than than 200 MHz MHz ( Ragni, Al-Sha Al-Shami, mi, Mikhaylenko, & Tang, 2007), 2007 ), while ionic conduction dominated until 300 MHz in mangoes (Sosa-Morales ( Sosa-Morales et al., 2009). 2009). For pure liquids with polar molecules, like alcohols or water, polar dispersion dominates dominates the frequency frequency characteristics characteristics of dielectric dielectric properproperties and the Debye model can be used to describe the general frequency-dependent behaviour of pure liquids (Decareau, ( Decareau, 1985). 1985). For example, Liu, Tang, and Mao (2009) used a modified Debye

M.E. Sosa-Morales et al. / LWT - Food Science and Technology 43 (2010) 1169 e1179

equation in order to know the frequency-dependent behaviour of  the loss factor in breads. From 1 to 1800 MHz, ionic conduction exhibited exhibited the major major contribution; contribution; the dipole relaxation relaxation of free water was was moderate at high-frequency values. The combined effect of temperature and frequency can be observed in Fig. 3. 3. 4.5. Storage Storage time time

The storag storage e time, time, when when ripeni ripening ng proces processes ses take take place, place, may affect affect the DP of fruits fruits.. Guo Guo,, Nel Nelson, son, Tr Trabe abelsi lsi,, and Kay Kayss (200 (2007) 7) measur measured ed the dielec dielectri tricc proper propertie tiess of fresh fresh apples apples (Fuji, (Fuji, Pink Pink Lady and Red Rome) at 24  C, from 10 to 1800 MHz and over 10 weeks of storage at 4  C in order to determine if these properties could be used as quality factors. The dielectric constant and loss factor remained essentially constant during the proposed refrigerated erated storag storage e period, period, and furthe furtherr resear research ch employ employing ing wider wider frequency ranges is necessary to assess the potential for sensing quality factors in apples through radio frequency electromagnetic elds. Furt Furthe herm rmor ore, e, diele dielect ctri ricc prope propert rtie iess of mang mangoe oess were were fields. measured by Sosa-Morales et al. (2009) at 0, 4, 8, 16 days days of storage at 21  C. Both 3r 0 and 3r 00 values decreased with storage time, caused mainly by the reduced moisture content and the increased pH observed during that period; while the electrical conductivity of  mangoe mangoess increas increased ed with with increa increasin sing g tempe temperat rature ure during during the storage time. Shell eggs undergo significant changes during storage, most of  them them relate related d to their their freshn freshness. ess. Rag Ragni, ni, AlAl-Sha Shami, mi, Mikh Mikhayl aylenk enko, o, et al. (2007) investigat investigated ed the DP of hen shell eggs using an open-ended open-ended coaxial probe technique. Measurements were carried out on intact eggs in the 10 e1800 MHz frequency band, after storage at 22  C for 1, 2, 4, 8, and 15 days and at three selected points on the shell eggs. Both dielectric properties ( 3r 0 and 3r 00 ) increased increased with storage storage time; for for exam exampl ple, e, the the loss loss fact factor or incr increa ease sed d by 22 22% % from from day day 1 to day day 15 at 20 MHz. DP of eggs could be used for predicting basic quality parame parameter terss (aircell height height,, thick thick albume albumen n height height,, yolk yolk index index among among other others) s) and, and, in genera general, l, to know know produc productt freshn freshness ess (Ragni, AlShami, Berardinelli, et al., 2007). 2007). 5. Reporte Reported d values values for dielectric dielectric properties properties of foods in the 21st century 

Characterization of dielectric properties, as mentioned above, is key for understanding the response of a material when subjected to RF or MW fields for the purposes of heating, drying or processing. Recent data are presented and classi fied depending on food nature.

Fig. 3. Mechanisms involved in the loss factor of materials with high moisture content as functions of frequency and temperature (from Wang, Wig, et al., 2003). 2003 ).

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5.1. 5.1. Fruits and vegetables vegetables

Recent data for dielectric constants and loss factors of fresh fruits and vegetables at common frequencies are shown in Table 2. 2. Dielectric Dielectric heating heating using RF and MW energy energy has been studied studied as a possible disinfestation disinfestation treatment treatment for several several commodities. RF energy energy has been been studied studied in pilot pilot scale scale system systemss against against codling codling moth moth in cherri cherries es (Ike Ikedia diala, la, Han Hansen sen,, Tang ang,, Dra Drake, ke, & Wa Wang, ng, 2002 2002)) and apples (Wang, (Wang, Birla, Tang, & Hansen, 2006). 2006 ). To develop a treatment protoc protocol ol based based on RF or MW heatin heating, g, the first step is to gain knowledge of the dielectric properties of the fruit. Wang,, Tang, Wang Tang, et al. (2003 (2003)) and Wan Wang, g, Wig, Wig, et al., (2003) published an extensiv extensive e analysis analysis of dielectri dielectricc propertie propertiess of fruits fruits and insect insect pests in the context of RF and MW treatments. When a selection of fruits and insect larvae were examined the authors found that the loss factor factorss at RF freq frequen uencie ciess of commonpest commonpest insect insectss wereclear wereclearly ly great greater er thanthat of nuts,suggesting nuts,suggesting possible possible different differential ial and fasterheatingof  fasterheatingof  insect insectss versus versus nuts nuts when when treat treated ed simult simultane aneous ously ly in an RF applica applicator tor.. An interesting application for vegetable products was proposed by Zhong, Sandeep, and Swartzel (2004) who considered RF heating as a potential alternative to conventional heating for liquids contain containing ing particu particulat lates. es. Using Using a 30 kW, kW, 40.68 40.68 MHz, MHz, continuo continuous us flow RF unit, the authors processed carrot and potato cubes using a 1% CMC (carbo (carboxym xymeth ethyl yl cellulo cellulose) se) solution solution as carrie carrierr. Based Based on thermal images captured by an infrared camera, small temperature gradients were observed inside the carrots and potato cubes that were heated in a short residence time. Likewise, a 600 W, 27.12 MHz, RF applicator was used by Orsat, Gariépy, Raghavan, and Lyew (2001) to determine the potential for RF to improv improve e and exten extend d the storab storabili ility ty of vacuum vacuum-pa -packa ckage ged d carrot sticks. Despite the fact that the quality of RF-treated samples was higher than that of both control (chlorinated water) and hotwater-treated carrot samples, and that the RF treatments maintained tained colour colour,, taste taste and the vacuum pressure pressure of the packages, packages, which was not the case for the the control control or hot-wate hot-waterr-treat treated ed carrots, carrots, authors concluded that RF heating should not be recommended as a sole treatment to improve safety and storability of minimally processed processed ready-to-eat ready-to-eat carrot sticks. Instead RF should be considered as a part of an integrated approach, including proper packaging and adequate refrigeration. Recently, microwaves treatments have been used in extraction processes. Solid-liquid extraction of oils and bioactive compounds from plants is being studied with microwave assisted extraction (MAE) or solvent free microwave extraction (SFME). Opposite to conventional solvent extraction, presence of water improves the extraction when MAE or SMFE are employed. In fact, water absorbs the electromagnetic energy and the generated heat becomes easy the extraction of chemical constituents from the plant tissue. When MAE is applied, solvents with high dielectric constant should should be chosen. chosen. Polar Polar molecu molecules les and ionic ionic solutio solutions ns absorb absorb microwave energy since they have permanent dipole moment. The dielectric constant of ethanol, methanol and water (24.3, 32.6 and 78.3 at 20  C in microwaves region) is enough to consider them adequate solvents for MAE processes (Takeuchi ( Takeuchi et al., 2009). 2009). SMFE is considered a green technology, which reduces the time extraction without affecting the quality of the extracted oil ( Wang, Ding, et al., 2006). 2006). When SMFE is used, the dielectric properties of the material are very important, because of a lack of solvent. Lucchesi, Smadja, Bradshaw, Louw, and Chemat (2007) reported the dielectric properties properties of cardamom cardamom seeds. 3r 0 and 3r 00 for cardamom seeds with 5% of moisture content were 2.203 and 0.060, respectively; while that when seeds had 60% of moisture, moisture, 3r 0 was 3.3 and 3r 00 had a value of 2.2. Loss factor increased when the moisture content was was higher, which is desirable for SMFE as the plant is more absorber of  the MW energy.

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 Table 2

Dielectric properties of fruits and vegetables. Tem Tempera peratu ture re (  C)

Frui ruit or vegetabl table e

Apple Apple (Gold (Golden en Delici Delicious ous)) Appl Apple e (Red (Red Deli Delici ciou ous) s) Avocado Banana Cantaloupe Carrot Cherimoya Cherry Cucumber Grape Grapefruit Kiwifruit Lemon Lime Longan Mango Mango Tommy Atkins ‘

Onion Orange Papaya Passion fruit Peach Pear Persimmon Potato Radish Squash Strawberry Sweet potato Turnip White sapote

’

20 50 20 50 20 50 23 23 23 20 50 20 50 23 23 20 50 e e e 20 50 e 20 50 e 20 50 e 20 50 e e 20 50 e e e e e e 20 50

Moist isture ure content (% w.b.)

e e e e e e 78 92 87 e e e e 97 82 e e 87 91 90 e e 86 86

92 e e 88 e e 90 84 e e 79 96 95 92 80 92 e e

Dielectric constant

Loss factor

Frequency

Frequency

Reference

27.12 MHz

915 MHz

27.12 MHz

915 MHz

72.5 68.1 74.6 68.7 115.7 137.9 e e e 71.5 72.0 91.2 89.6 e e 89 93.8 e e e 75.2 69.7 e 83.1 79.8 e 84 78 e 82.7 91.6 e e 79.8 76.6 e e e e e e 76 74.5

74.3 67.8 77.0 68.9 59.9 57.9 64.0 68.0 59.0 59.0 57.5 73.7 66.7 71 69 72.7 66.1 70 73 72 68.2 60.8 64 74 67.3 61 72.9 66.1 69 59.7 55.5 70 67 68.4 68.5 62 68 63 73 55 63 62.6 60.3

120.4 202.2 92.0 153.8 699.6 1136.2 e e e 238.5 406.4 293.0 501.0 e e 202.4 345.3 e e e 230.1 377.7 e 250.1 404.6 e 223.3 367.7 e 264.1 441.2 e e 207.5 346.4 e e e e e e 258.6 433.1

8.5 8.3 10.0 9.8 27.4 39.8 19.0 14.0 18.0 25.4 29.1 16.4 19.3 11 15 12.1 14.2 18 15 18 13.3 15 13 13.8 16.0 12 16.5 17.5 10 15 17.6 12 11 21.1 16.1 22 20 15 14 16 13 24 24.9

5.2. Bread Bread

Shel Shelff life life of fres fresh h brea breads ds is limi limite ted d due due to mouldgro mouldgrowt wth. h. In orde orderr to design ef ficient MW or RF treatments that assure mould control and retent retention ion of produc productt qualit quality, y, it is desira desirable ble to underst understand and dielect dielectric ric proper propertie tiess of bread bread product products. s. The dielec dielectri tricc constan constants ts and loss factors of white breads at four different moisture contents between 34.0 and 38.6% and five temperatures from 25 to 85  C were reported by Liu, Tang, and Zhihuai (2009) and are depicted in Table 3. 3. In their work, they also found some valuable equations equations to calculate both the dielectric constant and dielectric loss factor. In other report, Liu, Tang, and Mao (2009) focused on the analysis of  the influence of frequency, temperature and moisture content of  white bread on the dielectric loss factor, which is more directly related to conversion of electromagnetic energy to thermal energy during dielectric heating.

Wang, Tang, Johnson, et al. (2003) Wang et al. (2005) Venkatesh and Raghavan (2004) Venkatesh and Raghavan (2004) Venkatesh and Raghavan (2004) Wang et al. (2005) Wang, Tang, et al. (2003) Venkatesh and Raghavan (2004) Venkatesh and Raghavan (2004) Wang, Tang, et al. (2003) Venkatesh and Raghavan (2004) Venkatesh and Raghavan (2004) Venkatesh and Raghavan (2004) Wang et al. (2005) Venkatesh and Raghavan (2004) Sosa-Morales et al. (2009) Venkatesh and Raghavan (2004) Wang, Tang, et al. (2003) Venkatesh and Raghavan (2004) Wang et al. (2005) Venkatesh and Raghavan (2004) Venkatesh and Raghavan (2004) Wang et al. (2005) Venkatesh and Raghavan (2004) Venkatesh and Raghavan (2004) Venkatesh and Raghavan (2004) Venkatesh and Raghavan (2004) Venkatesh and Raghavan (2004) Venkatesh and Raghavan (2004) Wang et al. (2005)

and Simunovic (2004) for several several densities, densities, temperatu temperatures, res, and moistur moisture e conten contents ts in the microw microwav ave e region region (300e30 3000 00 MHz)of MHz)of the the electromagnetic spectrum. Dielectric mixture equations were used to correlate the dielectric properties with density and the coef ficients of quadratic and linear dielectric mixture equations were tabulat tabulated ed for 91 915 5 and 2450 2450 MHz, MHz, differe different nt temper temperatu atures res and

 Table 3

White bread dielectric properties (adapted from Liu, Tang, Zhihuai, 2009). 2009 ). Moisture Temperature Temperature ( C) Dielectr Dielectric ic constant constant content (% wb) Frequency

Dielectr Dielectric ic loss factor factor Frequency

27.12 27.12 MHz 915 MHz MHz 27.12 27.12 MHz 915 MHz MHz 38.6

37.1

5.3. 5.3. Nuts Nuts 34.6

Dielectric Dielectric properties properties of ground ground samples samples of in-shell and shelled shelled peanuts ( Arachis hypogaea L.) were measured by Boldor, Sanders,

Wang, Tang, Johnson, et al. (2003)

25 55 85 25 55 85 25 55 85

2.83 3.15 3.55 2.68 3.02 3.50 2.35 2.80 3.45

2.08 2.17 2.26 2.03 2.11 2.23 1.81 1.94 2.13

4.95 8.00 13.26 3.90 6.74 12.55 2.32 5.09 11.98

0.69 0.83 1.15 0.59 0.78 1.13 0.47 0.67 1.07

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moisture contents. The values of the dielectric constant and loss factor of bulk in-shell and shelled peanuts were determined by extrapolat extrapolation ion of the first and secondsecond-ord order er polynom polynomial ialss that that relate related d 3r 0 and 3r 00 with density. An equation that determines the dielectric properties of  nominal peanut pods (in-shell peanuts) and kernels (shelled (shelled peanuts) as a function function of their temperature temperature and moisture content was also determined by using multiple linear regression. For in-shell peanuts, the dielectric loss factor ranged from 0.005 to 0.05 0.05 and the dielec dielectri tricc constan constantt from from 0.01 0.01 to 0.2. 0.2. For shelled shelled peanuts, DPs ranged from 0.002 to 0.1 (dielectric loss factor) and from 0.05 to 0.5 (dielectric constant). As the density increased, the dielectric properties increase for both ground in-shell and shelled peanuts. At higher moisture contents, the signi ficance of temperature effects on 3r 0 and 3r 00 was reduced by the high dependence of  the dielectric properties on moisture content. Mean values of the dielectric constant and the loss factor for almond and walnut walnut at 20, 30, 40, 50 and 60  C are showed in Table 4. Dielectric properties of almonds and walnuts were below 7 and this was because these nut kernels had a low moisture content (3% wet wet basis basis)) and and high high oil cont conten entt (ca. (ca. 50 50%) %);; resu result ltss that that were were comparable with the values of the dielectric constant (2.7) and loss factor factor (0.3) for walnut walnutss at 2000e3000 MHz obtained obtained by other other authors. A similar trend was observed for the dielectric constant and loss factor factor for the two nuts. nuts. Ther There e was also a peak peak value value for for loss  factors factors at ca. ca. 590 MHz MHz at 20 C, which might have been the result result of  the presence of bound water in the nuts ( Wang, Tang, et al., 2003). 2003). Protocols Protocols against worms in walnuts have been proposed using RF energy (Wang (Wang,, Tang, et al., 2006 2006). ). “

”

5.4. Coffee Coffee

The dielectric properties of several coffee varieties were determined and analyzed by Berbert et al. (2001), (2001) , for frequencies from  75 kHz to 5 MHz at 21 C. Permit Permittiv tivity ity of parchm parchmen entt coffee coffee increased with moisture and bulk density (Fig. ( Fig. 4), 4), but decreased with with frequ frequenc ency. y. Loss factor factor also decreas decreased ed when when frequ frequenc ency y increa increased sed,, but the behavi behaviour our was was less less regula regularr than than that that observ observed ed for permittivity, ranging from 0.75 to 0.08, which was dependent on the frequency and moisture content. According to the authors, the potential application of the knowledge of the relative permittivity of coffee coffee grains grains is the indire indirect ct moistu moisture re contentdeter contentdetermin minati ation on or an on-line moisture meters in automatic control of coffee dryers, as a non-destructive technique for this valuable agricultural product. 5.5. Meats Meats and seafood seafood

Cooking and heating of meat and meat products is an area where MW and RF radiation has found applications at household  Table 4

Dielectric properties of almonds and walnuts ( Wang, Tang, et al., 2003). 2003 ). Type of nut nut Temperat Temperature ure ( C) Dielec Dielectri tricc const constant ant Frequency

Loss Loss factor factor

Fig. 4. Effect of bulk density and moisture content on the permittivity of parchment coffee variety Catuai Vermelho: 415 kg/m 3 and 11.2% (lower), 414 kg/m 3 and 12.3%; 423 kg/m3 and 13.9%, 426 kg/m 3 and 15.1%; 426 kg/m 3 and 15.9%; 427 kg/m 3 and 17.9%; 425 kg/m 3 and 19.3%; 424 kg/m 3 and 20.5%; 429 kg/m3 and 21.3% and 429 kg/m 3 and 22.5% (upper). From Berbert et al. (2001). (2001) .

Frequency

27.12 27.12 MHz MHz 915 MHz 27.12 27.12 MHz MHz 915 MHz Almond

20 30 40 50 60

5.9 5.7 5.8 5.8 6.0

1.7 3.2 3.3 3.4 3.1

1.2 0.6 0.6 0.6 0.7

5.7 6.4 6.0 5.7 6.4

Walnut

20 30 40 50 60

4.9 5.0 5.1 5.2 5.3

2.2 2.1 3.0 3.4 3.8

0.6 0.5 0.4 0.3 0.4

2.9 2.6 2.3 2.0 1.8

level, level, but that that also has industr industrial ial potent potential ial.. A vast vast amount amount of  inform informati ation on has been been publishe published d on the dielect dielectric ric proper propertie tiess of meat meat and meat meat produc products ts using using differe different nt methods methods,, frequ frequenc encies ies and temperatures, which made it somewhat dif ficult to cross compare results. With the aim of gathering information related to microwave (MW) and radio frequency (RF) radiation of meat products, Lyng, Ly ng, Zha Zhang, ng, and Bru Brunto nton n (200 (2005) 5) publi publish shed ed a surve survey y on the the dielec dielectri tricc proper propertie tiess of meats meats (chick (chicken, en, lamb, lamb, beef, beef, pork pork and turke turkey) y) and typical ingredients used in meat products manufacture (salt, nitrite, soy protein isolate, deionised water, potato starch). In the

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study, dielectric properties at 27.12, 915 and 2450 MHz of lean, fat, aqueous solutions and meat blends of ingredients were measured. In addition, temperature rises of ingredient and meat blends were compared following RF or MW heating. They found that frequency and composition composition affected dielectric properties, properties, with fat having having lower dielectric activity than lean tissue. Also, dielectric properties at MW frequencies were more sensitive to changes in composition. What is more, when low and high dielectric activity ingredients were incorporated into lean meat cuts, dielectric properties did not correlate well with temperature rises, which indicated the importance tance of other other factor factorss in additio addition n to dielect dielectric ric proper propertie tiess that that 0 determine determine temperatur temperature e rise. The 3r  values obtained for all lean meats were in the same order of magnitude, with values ranging from 70.5 to to 77.8; 77.8; while while 3r 00 values values for lean meats meats were were ranked ranked in the following order: lamb < pork < beef  < turkey < chicken; though pork fat was dramatically lower than all lean meats. Table 5 shows the dielectric properties of different species and types (lean or fat) of meat at 27.12 and 2450 MHz. Regarding fish products, Wang, Tang, Rasco, Kong, and Wang (2008) measured measured the dielectric dielectric properties properties for anterior anterior,, middle, middle, tail, and belly portions of Alaska pink salmon (Oncornynchus gorbuscha) fillets at frequencies between 27 and 1800 MHz from 20 to 120  C to provide insights for improvement of the modeling of  microwav microwave e (MW) and radio frequency frequency (RF) commercial commercial sterilizatio sterilization n processes processes of salmon products. Compositional differences differences contribcontributed to the observed slight differences differences in the dielectric properties properties for different parts of salmon fillet. For all portions of the fillet, similar trends in dielectric constant and loss factor measurements were observed as a function of temperature (20 e120  C). At RF frequencies of 27 and 40 MHz, the dielectric constant decreased with increasing temperature. But at microwave frequencies (915 and 1800 MHz), the opposite trend was observed. The dielectric loss factor increased increased with increasing increasing temperatu temperature re over the tested frequ frequenc ency y range range.. Calcula Calculatio tions ns from from electr electrical ical conducti conductivit vity y of  minced salmon fillets measured at different temperatures suggest

that ionic conductivity conductivity was the major contributor to temperat temperature ure dependent behaviours of dielectric properties at RF frequencies. Table 5 reviews the findings for salmon fillets at two frequencies and three temperatures. Caviars are ready-to-eat aquatic food products made by brining and curing fish roe. Caviar is heat labile and dif ficult to pasteurize, so radio radio frequ frequenc ency y (RF) (RF) and micro microwa wave ve (MW) (MW) heatin heating g provide provide a possible alternative to the conventional thermal processing for caviar products. The objectives of a study made by Al-Holy et al. (2005) were to determine the dielectric properties of untreated stur sturge geon on and and salm salmon on cavia caviar; r; to study study the the effe effect ct of produ product ct  temperat temperature ure (20e80 C) on dielec dielectri tricc proper propertie tiess in connect connection ion with with a potential use in the development development of pasteurization pasteurization protocols; protocols; and to investigate the impact of commercially used salt concentrations on the dielectric properties. properties. The dielectric dielectric constant constant and dielectric dielectric loss factor for salmon and sturgeon caviar increased markedly with increasing temperature at 27 MHz but not at 915 MHz ( Table 5). 5). Microwave processing is considered to be a promising technology nology for shucked shucked oyster oysters. s. Since Since its applicat application ion is restr restricte icted d by rare rare information on the thermal and dielectric properties of oysters, the later were determined by Hu and Mallikarjunan (2005) between 300 MHz and 3 GHz, as models were developed to describe the temperature effects on thermal and dielectric properties of oysters. At microw microwav ave e frequ frequenc encies ies of 91 915 5 and 2450 2450 MHz, MHz, they they observ observed ed that that the dielectric constant decreased (64.02 e50.89 and 59.10e47.67, respec respectiv tively ely), ), while while the loss factor factor increa increased sed (13.8 (13.84 4e20.14) 20.14) at 915 MHz as temperature increased from 1 to 55  C. 5.6. Dairy Dairy products products

Recently, Nu Nune nes, s, Boh Bohig igas, as, an and d Tej ejad ada a (2 (2006 006)) studied studied the dielectric properties of UHT skim, low fat and homogenized whole milk at room temperature (17 (17 e20  C) and over the frequency range of 1 e20 GHz 3r 0 for skim and low fat milk was similar at 1 GHz and 10 GHz, 75 to 53 and 73 to 50, respectively; while 3r 0 of whole milk

 Table 5

Dielectric properties of meats, salmon and caviars. Spec Specie iess (ana (anato tomi mica call loca locati tion on))

Type Type

Beef (forequarter trimmings) Lamb (leg) Pork (shoulder) Pork (back) Chicken (breast) Turkey (breast)

Lean Lean Lean Fat Lean Lean

Pink salmon

Anterior

Middle

Sturgeon caviar

Salted

Unsalted

Sturgeon caviar

Salted

Unsalted

Temp Temper erat atur ure e (  C)

Diel Dielec ectr tric ic cons consta tant nt Frequency

Dielectric loss factor Frequency

27.12 MHz

27.12 MHz

70.5 77.9 69.6 12.5 75.0 73.5 20 60 120 20 60 120 20 50 80 20 20 50 80 20 50 80 20 50 80

2450 MHz 43.7 49.4 51.3 7.9 49.0 56.3

418.7 387.2 392.0 13.1 480.8 458.4

Reference

2450 MHz 13.7 15.0 15.1 0.76 16.1 18.0

Lyng et al. (2005) Lyng et al. (2005) Lyng et al. (2005) Lyng et al. (2005) Lyng et al. (2005) Lyng et al. (2005)

40 MHz 87.6 100.8 116.8 85.3 99.1 119.7

915 MHz 55.1 51.4 47.1 57.0 53.7 50.7

40 MHz 296.3 525.5 890.8 313.9 581.4 1085.2

915 MHz 22.6 33.0 47.1 22.8 34.8 60.4

Wang et al. (2008) Wang et al. (2008) Wang et al. (2008) Wang et al. (2008) Wang et al. (2008) Wang et al. (2008)

27.12 MHz 129.8 121.5 182.0 70.7 46.4 59.6 81.5 111.5 202.8 61.0 77.4 92.5

915 MHz 29.8 22.7 25.0 30.7 18.3 18.9 25.0 26.4 31.9 32.6 33.7 35.3

27.12 MHz 1349.4 1501.1 2614.5 470.8 375.9 642.7 1004.0 1769.5 2873.3 105.5 210.8 352.2

915 MHz 40.5 43.3 73.6 18.7 14.1 22.2 35.8 59.5 99.9 8.9 11.3 17.0

Al-Holy et al. (2005) Al-Holy et al. (2005) Al-Holy et al. (2005) Al-Holy et al. (2005) Al-Holy et al. (2005) Al-Holy et al. (2005) Al-Holy et al. (2005) Al-Holy et al. (2005) Al-Holy et al. (2005) Al-Holy et al. (2005) Al-Holy et al. (2005) Al-Holy et al. (2005)

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range ranged d from from 70 to 48 in the same frequ frequenc ency y region. region. Likewise, Likewise, 00 3r values were very similar among products: 15 for both skim and low low fat fat milkand milkand 14 for for whol whole e milkat milkat 1 GHz, GHz, and and arou around30 nd30 for for skim skim and low fat milk and 27 for whole milk at 10 GHz. Evera Eve rard rd,, Fa Faga gan, n, O Don Donnel nell, l, O Ca Calla llagh ghan an,, an and d Ly Lyng ng (2 (2006 006)) measured the dielectric properties of 16 processed cheeses over the frequency range 0.3 e3 GHz. The effect of temperature on the dielectric properties of those cheeses was investigated at temperature intervals of 10  C between 5 and 85  C. Results showed that the dielectric constant was highest at 5  C and generally decreased up to a temperature between 55 and 75  C. On the other hand, the dielectric loss factor generally increased with increasing temperature ture for chees cheeses es with with high high and and mediu medium m moist moistur ure/ e/fa fatt rati ratio, o, decre decreased ased with with tempe temperat rature ure betwee between n 5 and 55  C and and then then increa increased sed,, for cheese cheesess with with low moistur moisture/f e/fat at ratio. ratio. Moreov Moreover er,, partia partiall least square regression models indicated that 3r 0 and 3r 00 could be used used as a qualit quality y control control screeni screening ng applica application tion to measur measure e moistu moisture re content and salt content of processed cheeses. Amhed, Ramaswamy, Ramaswamy, and Ragha Raghavan van (2007) determine determined d the dielectric properties of salted and unsalted butter over the MW frequency range of 500 e3000 MHz. Their findings indicated that dielectric dielectric spectra of butter butter without salt differed differed significantly from the salted salted one as functio function n of tempe temperat rature ure.. The dielect dielectric ric consta constant nt of  unsalted butter was observed independent of frequency, whereas the loss factor decreased with an increase in frequency. Both 3r 0 and 3r 00 of unsalted unsalted butter butter decreased decreased as the temperature temperature increased and dielectric dielectric parameters parameters of salted butter gradually decreased decreased with increasing frequency but increased with temperature. Finally, the dielec dielectri tricc proper propertie tiess of both both salted salted and unsalt unsalted ed butter butter were were adequately described by a second-order polynomial equation and the increase increase in ionic ionic conduct conductance ance,, as a result result of salt salt presen presence, ce, appeared to be the principal reason for the differences in electrical properties of salted butter. ’

’

5.7. Eggs and egg egg products products

Eggs Eggs repr repres esen entt an inte intere rest stin ing, g, and and so far litt little le expl explor ored ed,, biomaterial biomaterial from the standpoint of their dielectric dielectric characterizat characterization. ion. For example, example, Rag Ragni, ni, AlAl-Sha Shami, mi, Mik Mikhay haylen lenko, ko, et al. (200 (2007) 7) and Ragni, Al-Sha AlShami, mi, Ber Berar ardine dinelli, lli, et al., 2007 investigat investigated ed the electrical electrical change changess of egg constit constituen uents ts (album (albumen en and yolk), yolk), throug throughout hout dielectric properties and conductivity, during storage. Moreover, data of dielectric dielectric properties properties is important important to the research research of RF and MW heating applications for egg products potential pasteurization and steriliz sterilizati ation. on. Since Since there there was a lack of informa information tion on the dielectric properties of whole eggs and egg whites, Wang, Tang, Wang, Wa ng, and Sw Swanso anson n (200 (2009) 9) studied studied the effect effectss of cooking cooking on dielectric dielectric properties properties of liquid liquid whole eggs and liquid egg whites in relation with radio frequency and microwave heating processes to prepare shelf-stable products. products. Dielectric measurements were made using an open-ended coaxial probe method over a temperature range of 20 and 120  C at radio frequencies 27 and 40 MHz, and microwave frequencies 915 and 1800 MHz. Thermal denaturation of liquid liquid egg whit whites es and and whole whole eggs in fluenced the dielectric dielectric constan constants ts and dipole loss compone component nt of eggs, eggs, as reflected ected by changes in loss factors above 60  C. In addition, loss factor of liquid wholeeggswas wholeeggswas foundgen foundgener eral ally ly smal smalle lerr than than that that of egg egg whit whites es and and larger larger than the loss factor factor of egg yolk. Ionic conducti conductivit vity y was was conside considere red d a dominan dominantt factor factor determ determini ining ng the dielec dielectri tricc loss behaviour behaviour of egg products at radio frequencies, frequencies, whereas whereas dipole water water molecules molecules playe played d an increa increasing sing role role with with an increa increase se in microwav microwave e frequencie frequenciess (Wang et al., 2009). 2009 ). Tab Table le 6 shows the dielec dielectri tricc proper propertie tiess of liquid liquid and pre-coo pre-cooked ked egg whites whites and whole whole eggs. ’

1177

 Table 6

Dielectric properties of liquid and pre-cooked egg whites and whole eggs (adapted from Wang et al., 2009). 2009). Egg State ate product

Tem Temper peratur ature e Dielectr Dielectric ic const constant ant ( C) Frequency

Dielectr Dielectric ic loss loss facto factorr Frequency

27.12 MHz 915 MHz 27.12 MHz 915 MHz Egg Liquid white

20 80 120 Pre-cooked 20 80 120

84.6 98.3 135.1 89.3 99.5 124.4

64.0 50.5 53.2 64.5 53.0 50.1

427.0 866.5 1665.8 411.8 937.1 1480.5

18.7 33.3 56.9 18.9 34.6 52.2

Whole egg

Liquid

76.3 87.5 106.1 79.6 89.0 104.8

55.5 48.9 44.7 56.5 48.5 44.3

335.9 801.8 1132.7 336.8 745.8 1020.0

15.8 30.5 42.3 16.3 29.0 39.5

20 80 120 Pre-cooked 20 80 120

Previously, Luechapattanaporn et al. (2004, 2005) successfully validated the use of RF equipment for the sterilization of samples (mashed potatoes and scrambled eggs) inoculated with Clostridium sporogenes (PA 3679), and achieved suf ficient microbial inactivation, while producing products which had a higher quality than conventionally retorted products. 5.8. Liquid Liquid foods foods

Dielectric Dielectric properties properties of pumpable pumpable food materials have been measured at 915 MHz in the temperature range of 10 e90  C for continuous flow microwav microwave e heating heating applications. applications. The products products teste tested d by Cor Corone onel, l, Sim Simuno unovik vik,, San Sandee deep, p, and Ku Kumar mar (200 (2008) 8) included milk, ready-to-eat ready-to-eat puddings, soy beverage beveragess and avocado avocado paste product products. s. The result resultss these these author authorss obtain obtained ed showe showed d that that the dielec dielectri tricc proper propertie tiess of skim skim milk milk and 3.2% fat milk were were very very similar within the studied range of temperatures, demonstrating that the fat content has a negligible effect on these properties. The value of the dielectric constant ranged from 70 to 57.7, while the dielectric dielectric loss factor varied from 14 to 28 in these products. products. In the case of chocolate flavoured milk (1.5% fat), a different trend was observed at 30  C, with changing values of  3r 0 from 65 to 23 and 3r 00 from 16 to 5, as the temperature raised from 20 to 30  C (Coronel et al., 2008). 2008). The dielectric properties of soy beverages followed the same genera generall trend: trend: a decrea decrease se in the value value of 3r 0 and and incr increa ease se in the the valu value e 00 of  3r  as temperatu temperature re raised. raised. Dielectric Dielectric constant constant values values (ranging (ranging from 73 to 61) were very similar between products (1% fat, fat-free and lactose-free soy beverages), but 3r 00 values, ranging from 9 to 14, were were smaller smaller than than those those obtain obtained ed for skim milk milk as a result result of  different composition and nature of solutes found in both liquid foods (Coronel (Coronel et al., 2008). 2008). For puddings (tapioca and cornstarch), they both had similar diel dielec ectr tric ic prop proper erti ties, es, 64 and and 52 at 10 and and 90  C for for 3r 0 and and for 3r 00 at 10 and 90  C were 17.2 and 22.7, respectively. Values of  3r 0 for freshfresh00 made avocado avocado paste paste ranged ranged from 51 to to 39 and the values values of 3r  were calcu calcula late ted d betw betwee een n 16 and and 26 in a temp temper erat atur ure e inter interva vall of  15e85  C. Fresh-made avocado paste dielectric constant was lower than those tested for commercial avocado paste at 70 and 80  C, as commercial cial paste paste was much much higher and increased increased quickly quickly 3r 00 for the commer with temperature than that of the fresh-made product ( Coronel et al., 2008). 2008). The typical characterization of musts and wines is based on chemical chemical composition composition and sensory sensory analysis. analysis. García, Torres, Prieto, and De Blas (2001) aimed to describe grape juice by dielectric

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parameters and to examine the obtained data by means of statistical methods in order to find out a possible connection among dielectric values and grape varieties. Red wine grape juice samples (from Merlot, Tempranillo Tempranillo and Cabernet Sauvignon grape varieties) were used for measuring dielectric constant and loss factor at 0.2 and 3 GHz. Cabernet Cabernet grapes presented presented the highest mean values of  the the diele dielectr ctric ic const constan antt at 0.2 0.2 GHz, GHz, 3 GHz GHz and and tota totall losse lossess at 0.2 0.2 GHz; GHz; whereas, Merlot grapes had the highest mean values of total losses at 3 GHz and dipolar losses at both 0.2 and 3 GHz. Tempranillo grapes grapes had the lowest lowest mean mean values, values, excep exceptt for dielect dielectric ric constan constantt at 3 GHz (Merlot (Merlot)) and dipolar dipolar losses losses at 0.2 GHz (Caberne (Cabernet). t). The invest investiga igator torss observe observed d the exist existenc ence e of differe difference ncess among among the three three varieties in relation to the values of total and dipolar losses at 3 GHz; GHz; howeve howeverr, those those prelim prelimina inary ry result resultss were were not decisiv decisive e enough and more measurements were considered as necessary. Nevertheless, the use of this electromagnetic radiation would have some advantages advantages such as a clean and fast operation, operation, in addition to the non-destructive character, as it also offers the possibility of  making continuous measurements (García ( García et al., 2001 2001). ). 6. Final remarks remarks

Dielect Dielectric ric proper propertie tiess of a wide wide divers diversity ity of foods foods are are being being needed needed to underst understand and the behaviou behaviourr of the material material when when is submitted to electromagnetic heating. Their importance as basic concepts must not escape to the current engineering background. Valuabl Valuable e data data have have been been publish published ed in this this Centur Century, y, and more more studies will be conducted in order to explore other food materials. Depending on the values of both dielectric constant and loss factor, new food processes or measurements procedures based on RF or MW could be developed to improve the food quality (disinfestations tions prot protoc ocols ols)) and/ and/or or becom become e fast faster er the the food food prod product uction ion (paste (pasteuri urizat zation, ion, steril steriliza ization tion)) and quality quality assuran assurance ce based based on determ determina ination tionss of moistur moisture e content content,, densit density, y, egg age, age, among among others. others. Also, dielectric properties properties have an important important role when extraction extraction process of phytochem phytochemical ical compounds from plants is developed involving microwaves.  Acknowledgement

Author M. E. Sosa-Morales thanks the financial support from CONACyT (Consejo Nacional de Ciencia y Tecnología, Mexico) for a scholarship to support her PhD studies. References Al-Holy, M., Wang, Y., Tang, J., & Rasco, B. (2005). Dielectric properties of salmon (Oncorhynchus keta) and sturgeon (  Acipenser transmontanus) caviar at radio frequency (RF) and microwave (MW) pasteurization frequencies. Journal of Food Engineering, 70, 564e570. Amhed, J., Ramaswamy, S., & Raghavan, V. (2007). Dielectric properties of butter in Journal of Food the MW frequency range as affected by salt and temperature. Journal Engineering, 82, 351e358. Berbert, P. A., Queiroz, D. M., Sousa, E. F., Molina, M. B., Melo, E. C., & Faroni, L. R. D. (2001). Dielectric properties of parchment coffee. Journal of Agricultural Engineering Research, 80(1), 65e80. Boldor, D., Sanders, T., & Simunovic, J. (2004). Dielectric properties of in-shell and Transact action ionss of the ASAE, ASAE, 47 , shelled shelled peanuts peanuts at microw microwave ave frequenc frequencies. ies. Trans 1159e1169. Microwavee cooking cooking and processin processing. g. Microwav Microwavee cooking cooking and Buf fler, er, C. R. (1993) (1993).. Microwav  processing . New York: Van Nostrand Reinhold. pp. 47 e68. Coronel, P., P., Simunovik, J., Sandeep, K. P., & Kumar, Kumar, P. (2008). Dielectric properties properties of  pumpable food materials at 915 MHz. International Journal of Food Properties, 11, 508e518. Microwaves es in the food processing processing industry industry. Orlando: Decareau Decareau,, R. V. (1985). (1985). Microwav Orlando: Academic Press. pp. 2 e37 37.. Engelder Engelder,, D. S., & Buf fler, er, C. R. (1991) (1991).. Measurin Measuring g dielectr dielectric ic properti properties es of food products at microwave microwave frequencies. Microwave World, 12 , 6e15.

Everard, C., Fagan, C., O Donnell, C., O Callaghan, D. J., & Lyng, J. G. (2006). Dielectric properties of process cheese from 0.3 to 3 GHz. Journal of Food Engineering, 75 , 41 415 5e422. Feng, H., Tang, J., Cavalieri, R., & Plumb, O. (2001). Heat and mass transport in microwave microwave drying of hygroscopic porous materials in a spouted bed. AIChE   Journal, 47 , 1499e1512. García, A., Torres, J., Prieto, E., & De Blas, M. (2001). Dielectric properties of grape  juice at 0.2 and 3 GHz. Journal of Food Engineering, 48 , 203e211. Guan, D., Cheng, M., Wang, Y., & Tang, J. (2004). Dielectric properties of mashed potatoes relevant to microwave and radio-frequency pasteurization and sterilization processes. Journal of Food Science, 69 (1), FEP30eFEP37. Guo, W.-C., Nelson, S. O., Trabelsi, S., & Kays, S. J. (2007). 10 e1800 MHz dielectric Journal of Food Engineering, Engineering, 83 , properti properties es of fresh fresh apples apples during during storage. storage. Journal 562e569. Guo, W., Tiwari, G., Tang, J., & Wang, S. (2008). Frequency, moisture and temperature-dependent dielectric properties of chickpea flour. Biosystems Engineering, 101, 217e224. Hu, X., & Mallikarjunan, P. (2005). Thermal and dielectric properties of shucked LWT  e Food Science and Technology, 38 , 489e494. oysters. LWT e Ikediala, J. N., Hansen, J. D., Tang, J., Drake, S. R., & Wang, S. (2002). Development of  a saline water immersion technique with RF energy as a postharvest treatment Postharvest est Biology Biology and Technolo Technology, gy, 24, against against codling codling moth in cherries cherries.. Postharv 209e221. Ikediala, J. N., Tang, J., Drake, S. R., & Neven, L. G. (2000). Dielectric properties of  Transact action ionss of the ASAE, ASAE, 43, apple apple culti cultiva vars rs and codlin codling g moth moth larva larvae. e. Trans 1175e1184. Içıer, F., & Baysal, T. (2004). Dielectric properties of food materials-2: measurement techniques. Critical Reviews in Food Science and Nutrition, 44 , 473e478. Komaro Komarov, v, V., Wang, Wang, S., & Tang, Tang, J. (2005). (2005). Permitt Permittivit ivity y and measurem measurement ents. s. In (3693e 3711). New K. Chang (Ed.), Encyclopedia of RF and microwave engineering (3693e York: John Wiley and Sons, Inc. Liu, Y., Tang, J., & Mao, Z. (2009). Analysis of bread loss factor using modi fied Debye equations. Journal of Food Engineering, 93 , 453e459. Liu, Y., Tang, J., & Zhihuai, M. (2009). Analysis of bread dielectric properties using mixture equations. equations. Journal of Food Engineering, 93 , 72e79. Lucchesi, M. E., Smadja, J., Bradshaw, S., Louw, W., & Chemat, F. (2007). Solvent free microwave extraction of  Elletaria cardamomum L.: a multivariate study of a new technique for the extraction of essential oil. Journal of Food Engineering, 79 , 1079e1086. Luechapattanaporn, K., Wang, Y., Wang, J., Al-Holy, M., Kang, D. H., Tang, J., et al. (2004). (2004). Microbia Microbiall safety safety in radio radio frequenc frequency y processi processing ng of packaged packaged foods.  Journal of Food Science, 69(7), 201e206. Luechapa Luechapattan ttanaporn aporn,, K., Wang, Y., Wang, J., Tang, Tang, J., Hallberg, Hallberg, L. M., & Dunne, Dunne, P. (2005). (2005). Steriliz Sterilizatio ation n of scrambled scrambled eggs in military military polymeric polymeric trays by radio frequency energy. Journal of Food Science, 70 (4), E288eE294. Lyng, J., Zhang, L., & Brunton, N. (2005). A survey of the dielectric properties of  meats and ingredients used in meat product manufacture. Meat Science, 69, 589e602. Marra, F., Zhang, L., & Lyng, J. (2008). Radio frequency treatment of foods: review of  recent advances. Journal of Food Engineering, 91, 497e508. Metaxas, A. C., & Meredith, R. J. (1993). Industrial microwave heating . London: Peter Peregrinus Peregrinus Ltd. Mudgett, Mudgett, R. E. (1986). Electrical properties of foods. In M. A. Rao, & S. S. H. Rizvi (Eds.), Engineering properties of foods (pp. 329e390). New York: Marcel Dekker, Inc. Nelson, Nelson, S. O. (1992). (1992). Correlating Correlating dielectric dielectric properti properties es of solids solids and particul particulate ate Transact action ionss of the ASAE, ASAE, 35(2), samples samples through through mixture mixture relation relationships ships.. Trans 625e629. Nelson, S. O. (1996). Review and assessment of radio-frequency and microwave Transactions tions of the ASAE, ASAE, 39(4), energy for stored-grain and insect control. Transac 1475e1484. Nelson, S. O. (2003). FrequencyFrequency- and temperature-depen temperature-dependent dent permittivities of fresh Transactions ions of the ASAE, ASAE, 39(1), fruits and vegetables from 0.01 to 1.8 GHz. Transact 281e289. Nelson, S. O., & Kraszewski, A. W. (1990). Dielectric properties of materials and measurement techniques. Drying Technology, 8(5), 1123e1142. Nunes, A., Bohigas, X., & Tejada, J. (2006). Dielectric study of milk for frequencies between 1 and 20 GHz. Journal of Food Engineering, 76 , 250e255. Orsat, Orsat, V., Gariépy Gariépy,, Y., Y., Raghav Raghavan, an, G. S. V., & Lyew Lyew,, D. (2001). (2001). Radio-fre Radio-freque quency ncy Research Internatio International, nal, 34(6), treatment treatment for ready-to-eat fresh carrots. Food Research 527e536. Ragni, L., Al-Shami, A., Berardinelli, A., Mikhaylenko, G., & Tang, J. (2007). Quality evaluation evaluation of shell eggs during storage using a dielectric technique. Transactions of the ASABE, 50, 1331e1340. Ragni, L., Al-Shami, A., Mikhaylenko, Mikhaylenko, G., & Tang, J. (2007). Dielectric characterization characterization of hen eggs during storage. Journal of Food Engineering, 82 , 450e459. Ryynänen, S. (1995). The electromagnetic properties of food materials: a review of  basic principles. Journal of Food Engineering, 26 , 409e429. Sheen, N. L., & Woodhead, I. M. (1999). An open-ended coaxial probe for broadband permittivity measurement of agricultural agricultural products. Journal of Agricultural Engineering Research, 74, 193e202. Sosa-Morales, M. E., Tiwari, G., Wang, S., Tang, J., López-Malo, A., & García, H. S. (2009). Dielectric heating as a potential post-harvest treatment of disinfesting mangoes I: relation between dielectric properties and ripening. Biosystems Engineering, 103, 297e303. ’

’

M.E. Sosa-Morales et al. / LWT - Food Science and Technology 43 (2010) 1169 e1179

Takeuchi, T. M., Pereira, C. C., Braga, M. E. M., Maróstica, M. R., Jr., Leal, P. F., & Meireles, A. A. (2009). Low-pressure solvent extraction (solid-liquid extraction, extraction, microwave microwave assisted, and ultrasound assisted) from condimentary plants. In A. A. Meireles (Ed.), Extracting bioactive Componds for food products. Theory and applications (pp. 137e218). Boca Raton: CRC Press/Taylor and Francis Group. Tang, J. (2005). Dielectric properties of foods. In H. Schubert, & M. Regier (Eds.), The microwave processing of foods (pp. 22e38). Cambridge: Woodhead Publishing Limited. Tang, J., Wang, Y., & Chan, T. V. (2003). Radio frequency heating in food processing. In G. Barbosa-Cánovas (Ed.), Novel food processing technologies (pp. 501e524). New York: Marcel Dekker, Inc. Venkate Venkatesh,M. sh,M. S.,& Raghav Raghavan,G. an,G. S.V. (2004).An (2004).An overvi overviewof ewof microw microwaveproc aveprocessi essingand ngand dielectric properties of agri-food materials. Biosystems Engineering, 88(1), (1), 1e18. Venkatesh, M. S., & Raghavan, G. S. V. (2005). An overview of dielectric properties measuring techniques. techniques. Canadian Biosystems Engineering, 47 , 7.15e7.30. Von Hippel, A. R. (1954). Dielectric properties and waves . New York: John Wiley. Wang, S., Birla, S. L., Tang, J., & Hansen, J. D. (2006). Postharvest Postharvest treatment to control codling moth in fresh apples using assisted radio frequency heating. Postharvest  Biology and Technology, 40 (1), 89e96. Wang, Z., Ding, Ding, L., Li, T., T., Zhou, X., Wang, Wang, L.,Zhang, H., et al. (2006). Improved Improved solventsolventfree microwave microwave extraction of essential oil from dried Cuminum cyminum L., and  Zanthoxylum bungeanum Maxim. Journal of Chromatography A, 1102, 11e17 17..

1179

Wang, S., Monzon, M., Gazit, Y., Tang, J., Mitcham, E. J., & Armstrong, J. W. (2005). Temper Temperatur ature-de e-depend pendent ent dielectr dielectric ic properti properties es of selected selected subtropic subtropical al and Transactions tions of the ASAE, ASAE, 48(5), tropica tropicall fruit and associate associated d insect insect pests. pests. Transac 1873e1881. Wang, S., Tang, J., Johnson, J. A., Mitcham, E., Hansen, J. D., Hallman, G., et al. (2003). Dielectric properties of fruits and insect pests as related to radio frequency and microwave treatments. Biosystems Engineering, 85(2), 201e212. Wang, Y., Tang, J., Rasco, B., Kong, F., & Wang, S. (2008). Dielectric properties of  salmon fillets as a function of temperature and composition. Journal of Food Engineering, 87 (2), (2), 236e246. Wang, S., Tang, J., Sun, T., Mitcham, E. J., Koral, T., & Birla, S. L. (2006). Considerations in design design of commerc commercial ial radio radio frequenc frequency y treatme treatments nts for post-har post-harvest vest pest control in in-shell walnuts. Journal of Food Engineering, 77 (2), (2), 304e312. Wang, J., Tang, J., Wang, Y., & Swanson, B. (2009). Dielectric properties properties of egg whites and whole eggs as in fluenced by thermal treatments. LWT  e Food Science and Technology, Technology, 42, 1204e1212. Wang, Y., Wig, T., Tang, J., & Hallberg, L. (2003). Dielectric properties of food relevant to RF and microwave pasteurization pasteurization and sterilization. sterilization. Journal of Food Engineering, 57 , 257e268. Zhong, Q., Sandeep, K. P., & Swartzel, K. R. (2004). Continuous flow radio frequency heating of particulate foods. Innovative Food Science & Emerging Technologies, 5 (4), 475e483.