MICROWAVE HEATING IN FOOD PROCESSING - Capítulo de livro

ADVANCES IN BIOPROCESSING ENGINEERING © World Scientific Publishing Co. Pte. Ltd.http://www.worldscibooks.com/lifesci/4763.html

1

CHAPTER I

MICROWAVE HEATING IN FOOD PROCESSING

Juming Tang, Feng Hao* and Ming Lau†

Department of Biological Systems EngineeringWashington State University

Pullman, WA 99164-6120, USA

INTRODUCTION

Microwave heating takes place due to the polarization effect of electromagneticradiation at frequencies between 300 MHz and 300 GHz (Decareau, 1985).Started as a by-product of the radar technology developed during World WarII, microwave heating is now used in about 92% of homes in the US (Giese,1992). Microwave heating has also found applications in the food industry,including tempering of frozen foods for further processing, pre-cooking of baconfor institutional use, and final drying of pasta products. In those applications,microwave heating demonstrates significant advantages over conventionalmethods in reducing process time and improving food quality. But in general,applications of microwave heating in industrial food processing are much lesscommon than home applications. Reasons for this difference include a lack ofbasic information on the dielectric properties of foods and their relationship tomicrowave heating characteristics and the historically high cost of equipmentand electricity. The food processing industry has been reluctant to makeexpensive investments in a technology that has not been proven thoroughlyreliable in large-scale or long-term use (Mudgett, 1989). Now, with thedevelopment of more reliable magnetrons and the invention of ferrite circulatorsto protect generating tubes, microwave equipment has a longer operating life.The cost for microwave equipment has been steadily reduced over the years

*Current address: Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, USA.† Current address: Technical Center of Kraft Foods, Glenview Il., USA.

abe1-chap01.p65 03/20/2002, 4:00 PM1


2 J. Tang, F. Hao & M. Lau

and is now comparable to that for conventional heating methods. The futureof microwave heating in food processing applications is promising, but successfulexploration of microwave heating applications relies on a thorough understandingof the interaction between microwaves and foods, and on the ability to predictand provide a desired heating pattern in foods for specific applications.Microwave heating in foods is a complicated physical process which dependsupon the propagation of microwaves governed by Maxwell’s equations forelectromagnetic waves, on the interactions between microwaves and foodsdetermined by dielectric properties, and on heat dissipation governed by basicheat and mass transfer theories. This chapter will provide a general review anddiscussion on the interactions between microwaves and food materials and givea brief introduction of the current commercial applications of microwave heatingin food processing. It will also describe some recent research results on microwavedrying, pasteurization, and sterilization at Washington State University.

MECHANISMS OF MICROWAVE HEATING

Food materials are, in general, poor electric insulators. They have the ability tostore and dissipate electric energy when subjected to an electromagnetic field.Dielectric properties play a critical role in determining the interaction betweenthe electric field and the foods (Buffler, 1993). The dielectric properties of amaterial are given by:

δεεεε jej −||=−= "' (1)

where ε = the complex relative dielectric constant'ε = the relative dielectric constant"ε = the relative dielectric loss factor

δ = dielectric loss angle (tan δ = "ε / 'ε )

1−=j

'ε is related to the material’s ability to store electric energy (for vacuum 'ε = 1),while "ε indicates dissipation of electric energy due to various mechanisms.

The magnetic permeability for most biological materials is the same as thatof free space (µo = 4π × 10−7 W/Am). Therefore, those materials do not interactwith the magnetic field component of electromagnetic waves. Magnetic materialssuch as ferrite, often used in susceptors and browning dishes, however, interactwith the magnetic field, which results in substantial heating (Buffler, 1993).

Conversion of the electric component of microwaves into thermal energy ina lossy material (Goldblith, 1967) can be calculated by:

21110565 Ef.vP "ε××= − (2)

abe1-chap01.p65 03/20/2002, 4:00 PM2


Microwave Heating in Food Processing 3

where Pv = the power conversion per unit volume (W/m3)f = frequency (Hz)

"ε = relative dielectric loss factorE = electric field (V/m)

In theory, electric conduction and various polarization mechanisms (includingdipole, electronic, atomic and Maxwell-Wagner) all contribute to the dielectricloss factor (Metaxas and Meredith, 1993; Kuang and Nelson, 1998). But in themicrowave frequency range of practical importance to food applications (e.g.2450 MHz and 915 MHz in North America), conduction and dipole rotationare the dominant loss mechanisms (Fig. 1). That is:

ωεσεεεε σo

dd +=+= """" (3)

where subscribes “d” and “σ ” stand for contribution due to dipole rotation and dueto ionic conduction, respectively; ω represents angular frequency of themicrowaves, and εo is the permittivity of free space (8.85 × 10−12 F/m). In thefrequency range between 1 kHz to 100 MHz, Maxwell-Wagner1 polarizationplays a very important role, but it is usually not considered in microwave heating.

1Maxwell-Wagner polarization arises from charge build-up in interface between components inheterogeneous systems (Metaxas and Meredith, 1993). It peaks at about 100 kHz at room temperatureof 20°C.

Fig. 1 Contributions of various mechanisms to the loss factor of moisture materials as a functionof frequency and temperature (based on Roebuck and Goldblith, 1972; Harvey and Hoekstra, 1972;Metaxas and Meredith, 1993; Kuang and Nelson, 1998).

Log ( f )

Log (ε$)

20,000 MHz0.1 100

Contribution byionic conductio

Maxwell-Wagnereffect

Free water relaxation

Bound water relaxation

Effect of increasingtemperature

Effect of increasingtemperature

Contribution byionic conduction

abe1-chap01.p65 03/20/2002, 4:00 PM3



FACTORS AFFECTING DIELECTRIC PROPERTIESOF FOODS

Dielectric properties of food materials are affected by many factors, includingfrequency of the microwaves, food temperature, moisture content, salt content,and other constituents.

Effects of Frequency and Temperature

In a food system, the change of dielectric properties with respect to temperaturedepends upon frequency, bound water to free water ratio, ionic conductivity,and composition of the material. For example, at microwave frequencies usedby the food industry, both the dielectric constants and the loss factor due topolarization of bound water in foods would increase with temperature. On theother hand, these two properties of free water would decrease when temperatureincreases (Calay et al., 1995). An important concept in understandinghow frequency and temperature affect dielectric properties due to dipolerotation, "dε in Eq. (3), is the relaxation time τ. It is defined as the time requiredfor preferentially oriented molecules, under a static external electric field, torelax back to 1/e (or 36.8%) of the original condition on sudden removal ofthe external field. In general, the larger the molecules, the longer the relaxationtime. For a pure liquid, such as water, the dielectric loss factor "dε reaches themaximum at the relaxation frequency )=( πτ2

1cf . The relaxation time τ of free

water at 20°C was measured to be between 0.0071 to 0.00148 ns, whichcorresponds to a peak in "dε at around 16 GHz (Mashimo et al., 1997). Watermolecules are polar and are the most important constituent that contributes tothe dielectric properties of moist foods. Water molecules bound to the surfaceof food solids in mono- or multi-layers have much longer relaxation timesthan free water molecules. For example, the relaxation time of bound waterin different food materials at 20°C was determined to be between 0.98 ns to2.00 ns, which corresponds to a peak in "dε at about 100 MHz. Harvey andHoekstra (1972) found that "dε of monolayer bound water in lysozyme peakedat 200 MHz (2 × 108 Hz) and "dε for the second layer bound water peaked atabout 10 GHz (1010 Hz) (Figs. 2 and 3).

Debye related the relaxation time for the spherical molecule to viscosityand temperature as a result of randomized agitation of the Brownian movement(von Hippel, 1954):

kTv

V3=τ (4)

abe1-chap01.p65 03/20/2002, 4:00 PM4



where v is viscosity, T is absolute temperature, V is volume of the sphere, andk is a constant. For non-spherical water molecules, we may have the followingrelation:

Tv∝τ (5)

Fig. 2 Dielectric constant ( 'ε ) and loss factor ( "ε ) as a function of frequency for packed lysozymesamples containing slightly more than one monolayer of bound water at 25°C (Harvey and Hoekstra,1972).

Fig. 3 Dielectric constant ( 'ε ) and loss factor ( "ε ) as a function of frequency for packed lysozymesamples containing nearly two layers of bound water at 25°C (Harvey and Hoekstra, 1972). Thetwo dispersions correspond to the first and second layers of bound water, respectively.

abe1-chap01.p65 03/20/2002, 4:00 PM5



while the viscosity of all fluid decreases with increasing temperature (Macosko,1994):

RTEa

oevv =(6)

where Ea is activation energy and R is the universal gas constant. Therefore,as temperature rises, relaxation time for water decreases. The shifting ofthe relaxation time toward a smaller value (thus the frequency at themaximum "dε shifts toward a larger value as temperature increases) reduces thevalue of "dε for water at a fixed microwave frequency (Fig. 4). For example, asthe relaxation time τ decreases with increasing temperature, the dispersion peakmoves to higher frequencies, and the loss factor of pure water at 2450 MHz(2.45 × 109 Hz) decreases with increasing temperature.

The dielectric constant 'ε of free water also decreases with increasingtemperature as the result of increased Brownian movement.

The dielectric loss factor "σε due to ionic conduction decreases withincreasing frequency as shown in Eq. (3). The contribution of "σε to the over-all loss factors is smaller at 2450 MHz (2.45 × 109 Hz) than at 915 MHz(0.915 × 109 Hz) (Fig. 5).

Fig. 4 Effect of temperature on dielectric behavior of free water (ω = 2π f, f is frequency in Hz)(from Mudgett, 1985).

(a)

(b)

abe1-chap01.p65 03/28/2002, 10:22 AM6



Fig. 6 Effect of temperature and frequency on dielectric properties of cottage cheese (11% protein,4% lactose, 2% fat and 0.5% NaCl) (Herve et al., 1998).

Fig. 5 Effect of temperature on dielectric properties of 0.5 N aqueous sodium chloride at threetemperatures (from Roebuck and Goldblith, 1972).

εεεεε d

εεεεε σσσσσ

0

20

40

60

80

0 20 40 60

Temperature, C

Die

lect

ric c

onst

ant

915 MHz

2450 MHz

0

20

40

60

80

0 20 40 60

Temperature, C

Loss

fact

or

915 MHz

2450 Mhz

abe1-chap01.p65 03/28/2002, 10:32 AM7



The electric conductivity σ in ionic solutions increases with temperature dueto decreased viscosity and hence increased mobility of the ions (Trump, 1954).Therefore, based on Eq. (3), "σε also increases with temperature (Fig. 5). Forexample, at 915 MHz the dielectric constant of ionic solutions generally increaseswith temperature. Figure 6 shows the effect of frequency and temperature onthe dielectric constant, 'ε , and loss factor, "ε , of cottage cheese with about0.5% NaCl.

Ice is almost transparent to microwaves (Table 1). When a food is frozen,both dielectric constant and loss factor are significantly reduced, the degree ofreduction depends, to a large extent, upon the amount of water in the unfrozenstate and the ionic conductivity of the free water.

Figures 7 and 8 show the effect of temperature on the dielectric propertiesof different foods at 2450 MHz (Bengtsson and Risman, 1971). The high saltcontent in cooked ham makes the dielectric properties of this product quitedifferent from those of the rest of the materials in the graphs. Due to ionicpolarization, both dielectric constant and loss factors of cooked ham increasewith temperature above the freezing point, which is contrary to the trend ofdielectric properties of other foods in which loss mechanisms are mostlydetermined by the dipole polarization of free water. One advantage of thedecreased loss factor with increasing temperature in low salt foods at microwavefrequencies is the so-called temperature leveling effect. That is, when a certainportion of a food is overheated, the loss factor of that part is reduced, whichresults in less conversion of microwave energy to heat at that part of the foodand helps to reduce non-uniform spatial temperature distribution.

On the other hand, if the dielectric loss factor increases with increasingtemperature, the foods would experience a phenomenon called thermal runaway.For example, when thawing frozen foods at relatively high microwave powerlevels, certain areas of the food are overheated while the other areas are stillfrozen. This is because faster thawing of a portion of food due to uneven heatingdramatically increases the loss factor of that part of the food due to the highloss factor of free water (Figs. 7 and 8), which in turn increases microwaveabsorption, causing more uneven heating. In practice, a low microwave powerlevel is often used in micro-wave thawing so that heat conduction can reduce

Table 1 Dielectric properties of water and ice at 2450 MHz (Schiffmann, 1986)

State of water Relative dielectricconstant ( 'ε )

Loss factor ( "ε ) Loss tangent(tan δ)

Water (25°C)

Ice

78

3.2

12.5

0.0029

0.16

0.0009

abe1-chap01.p65 03/20/2002, 4:01 PM8



non-uniform temperature distribution. In industrial tempering of large blocksof meat or fish (a process that brings deep frozen products from −30°C to a fewdegrees below freezing point for further processing), convective surface coolingat below freezing temperature is often used to prevent possible thermal runaway.

Fig. 7 Dielectric constant of selected foods as affected by temperature (from Bengtsson and Risman,1971).

Fig. 8 Dielectric loss factor of selected foods as affected by temperature (from Bengtsson and Risman,1971).

abe1-chap01.p65 03/20/2002, 4:01 PM9



Effect of Moisture Content

Due to the dipole nature of water molecules, food moisture content is animportant determinant of the dielectric properties. In general, the higher themoisture content, the larger the dielectric constant and loss factor of thematerial. At temperatures above freezing, moisture exists in foods in one of thetwo forms — free water and bound water. The free water components havedielectric properties similar to those of liquid water, while the bound waterexhibits ice-like dielectric properties. Dielectric properties of foods, in general,decrease rapidly with decreasing moisture content to a critical moisture level.Below this moisture level, the reduction in loss factor is less significant due tothe bound water (Fig. 9). During microwave drying, the wetter parts of foodsabsorb more microwave energy and tend to level off the uneven moisturedistribution. The moisture leveling effect is less pronounced when the moisturecontent is below the critical moisture, as the reduction of loss factor withreducing moisture content is not as significant.

Fig. 9 Rate of evaporation and dielectric loss factor as affected by food moisture content (Metaxasand Meredith, 1983).

abe1-chap01.p65 03/20/2002, 4:01 PM10



Other Factors

Salting reduces free water and depresses the dielectric constant and the dipolarloss, while increasing the conductive loss (Calay et al., 1995). Sugar moleculesare relatively large and non-polar. An increase in sugar content reduces thedielectric constant. Hydration of sugar in water solutions shifts relaxation timeto lower frequencies, thus increasing the dielectric loss factor at microwavefrequencies (Roebuck and Goldblith, 1972). Similarly, hydration of protein andstarch in solutions made up of 50% solids reduces the dielectric constant andincreases the dielectric loss factor (Roebuck and Goldblith, 1972).

Predictive Models

Values of dielectric constant and loss factor of dry food solids, fats andoils are small and are relatively independent of frequency and temperature.

Table 2 Dielectric properties of oil and solids (Kent, 1986)

F(Hz)

Products T(°C) 106 107 108 109

Soybean salad oil

25

49

82

'ε"ε'ε"ε'ε"ε

2.850.1592.88

0.1382.86

0.092

2.620.1682.71

0.1742.72

0.140

Flour(mc = 3.2%)

0

40

70

'ε"ε'ε"ε'ε"ε

2.80.184

3.50.196

4.00.160

2.80.184

2.70.235

3.20.275

Cotton oil25

49

'ε"ε'ε"ε

2.830.1742.87

0.134

2.640.1752.70

0.174

Skimmed milkpowder

0

40

70

'ε"ε'ε"ε'ε"ε

2.10.0382.1

0.0442.4

0.067

1.90.0401.8

0.0540.2

0.072

abe1-chap01.p65 03/20/2002, 4:01 PM11



The dielectric properties of selected oils and low moisture solids are listed inTable 2.

A food material of high moisture content can be considered a mixture ofdielectrically active ionic solutions and dielectrically inert solids (Mudgett, 1985).The dielectric properties of two-phase mixtures of aqueous ions and colloidsolids are related to the dielectric properties of each component and their volumefractions, shown in the following distributive model (Mudgett, 1985):

)1( scssm VV −+= εεε (7)

where εm = relative permittivity of the mixture,εs = relative permittivity of the suspended solids,εc = relative permittivity of the continuous aqueous phase,Vs = volume fraction of the solids.

According to Mudgett et al. (1977), reasonable estimation of dielectricproperties for various liquid and solid foods can be obtained from the aboverelation. However, Eq. (7) requires an estimation of the volume fraction of thesolid or liquid phases, which is often difficult to obtain. Newer empirical modelshave been developed in which dielectric properties are related to the massfraction of various components. For example, based on selected groups of datain the literature, Sun et al. (1995) developed the following empirical relationshipsto correlate the dielectric properties of a meat product to temperature, moisturecontent and ash content:

97.05452.8)7947.4()0018485.00707.1( ashwater =++×−= RmTm'ε (8)

99.05985.3)23109.0093.57(

)000025.001868.04472.3(2

ash

2water

=−×+−+×+×−=

RTm

TTm"ε(9)

Calay et al. (1995) also developed empirical polynomial correlation to relatedielectric properties to moisture content and temperature for selected foods.Due to the influences of two different loss mechanisms, and the effect oftemperature and food composition, it is difficult to develop a general predictiveequation that can accurately take into account the influence of various factorsmentioned in the previous sections.

PENETRATION DEPTH OF MICROWAVES

When microwaves propagate through a lossy material, a fraction of microwaveenergy is converted into heat and the remaining power decreases with the

abe1-chap01.p65 03/20/2002, 4:01 PM12



distance from the surface (Fig. 10). Lambert’s law describes microwavepower reduction as a function of the distance that microwaves travel into asemi-infinite lossy body:

zePzP α2o)( −= (10)

where Po is incident microwave power at the surface, P(z) is microwave powerat distance z in the direction of microwave propagation within the lossy material,and α is the attenuation constant.

Attenuation factor can be calculated according to von Hippel (1954):

21

2

o11

212

−

+=

'

"'

εεε

λπα (11)

where λo is the wavelength of microwaves in free space. For 2450 MHzmicrowaves, λo = 12.24 cm, and for 915 MHz microwaves, λo = 32.77 cm.

Lambert’s law is applicable to a large body of lossy material where microwavesare largely attenuated with little reflection within the material at the oppositeinterface with the air. Ayappa et al. (1991) proved that for a sufficiently thickslab, Lambert’s law applies as long as the thickness of the slab satisfies thefollowing condition:

cm08.0dp4.5crit −=≥ LL (12)

where, dp is penetration depth of microwaves in food.When Eq. (12) is satisfied, the Lambert’s law results in less than 1% error as

compared to more rigorous analysis with the Maxwell equations for plane waves.When the thickness of a slab is less than Lcrit, the interference betweentransmitted and reflected waves between the surfaces may create standing waves,causing internal hot and cold spots.

As described by the Lambert’s law, microwave intensity reduces exponentiallywith the depth into a lossy material (Fig. 10). Penetration depth of microwavepower is defined as the depth where the power is reduced to 1/e (e = 2.718) ofthe power entering the surface. That is:

.dp o

eP

)P( =

abe1-chap01.p65 03/20/2002, 4:01 PM13



From Eq. (10), a relation can be derived for dp:

.21

dpα

= (13)

In general, 915 MHz microwaves have deeper penetration depth in foods than2450 MHz, and the penetration depth of microwaves also varies with temperature(Fig. 11 and Table 3). The limited penetration depth of microwaves in foodsoften causes non-uniform heating.

0.0

1.0

2.0

3.0

4.0

0 20 40 60

Temperature, C

Pen

etra

tion

dept

h, c

m

915 MHz

2450 MHz

Fig. 11 Effect of temperature on the penetration depth of 914 and 2450 MHz microwaves in cottagecheese (11% protein, 4% lactose, 2% fat and 0.5% NaCl) (Herve et al., 1998).

Fig. 10 Definition of penetration depth of microwave in a lossy material.

Depth into the material z, m

Po

Po*1/e

dp

Decay of microwave poweraccording to Lambert’s Law

Food material

Microwaveradiation

abe1-chap01.p65 03/20/2002, 4:01 PM14



ENERGY COUPLING

The intrinsic impedance in a material is defined as (Sadiku, 1995):

εη

εεµ

η o

o== (14)

where η is a complex quality, )/( ooo εµη = is the intrinsic impedance offree space, µo(= 4π × 10−7 H/m) is the permeability of free space, andεo(= 8.854 × 10−12 F/m) is the permittivity of free space); ηo is about 377 Ω.

For example, water at 25°C has an intrinsic impedance of about 43 Ω, andice has a value of about 210 Ω at 2450 MHz. The difference between theintrinsic impedance of two media causes mismatch. This would lead to twoconsequences: 1) the microwaves will change their direction of propagationonce entering a new material, and 2) a portion of microwave power will bereflected at the interface.

Snell’s Law of Refraction describes the refraction for transmitted waves(Mudgett, 1985):

Table 3 Penetration depth (dp) of microwaves in selected foods (all data were measured in ourlaboratory, except for ham)

*From Mudgett (1986).

915 MHz 2450 MHz

Material Temp(°C)

'ε 'ε"ε "εdp(mm)

dp(mm)

Water

DeionizedIce+0.5% Salt

Ham*

Yogurt (pre-mixed)Apple (Red Delicious)Potato (raw)AsparagusWhey protein gel(20% solid)Corn oil

20−1223

2550

2222252122

25

79.5–

77.2

6150

7160

65.173.650.9

2.6

3.8–

20.8

96140

219.5

19.620.617.0

0.18

122.5–

21.5

5.13.7

21.242.721.722.222.4

481.1

78.23.2

75.8

6053

6857

53.771.3440.1

2.5

10.30.00315.6

4255

17.512

15.716

12.9

0.14

16.81162010.9

3.82.9

9.312.39.2

10.410.6

216.7

abe1-chap01.p65 03/20/2002, 4:01 PM15



o

sinsin

ηφηψ = (15)

where ψ is the angle of refraction and φ is the angle of incidence (Fig. 12).The reflected power ratio at a food surface can be calculated as (Mudgett,

1985):

.coscoscoscos

o

o

φηψηφηψη

+−=Γ (16)

Thus, if the differences between the intrinsic impedance of the foods and freespace in the microwave cavity are very large, then standing waves arepredominant in the unoccupied space. For small impedance differences, theunoccupied space is mostly traveling waves (Mudgett, 1986).

The results in Table 4 were obtained from Eqs. (15) and (16) as an exampleto show the percent of microwave power reflected over the surface of ice orwater at 2450 MHz. The intrinsic impedance for ice and water were calculatedfrom dielectric properties in Table 3 and Eq. (14).

Fig. 12 Reflection and refraction of microwaves at the interface between air and a food material.

Incidentwaves, Pi Reflected

waves, Pr (= Γ ∗3L/ ZKHUH 3? Γ< 1,

Refractedwaves

φ φ

ψ

$LU + ηο,

)RRG PDWHULDO +η)

Pi

waves, Pr (= Γ *Pi, where 0< Γ≤1)

abe1-chap01.p65 03/20/2002, 4:01 PM16



Because of the refraction, microwaves entering spherical and cylindrical foodsare directed towards the center. Buffler (1993) estimated that when 'ε > 40,which is the case for most moist foods, all incident microwaves are refracted towithin an angle of 9o of the internal normal and travel towards the center ofthe spheres (Fig. 13). For relatively small-sized foods, e.g. when the spherediameter is less than 2.5 to three times the microwave penetration depth (dp),this would cause central heating (Lu et al., 1998).

Table 4 Refraction angle and percent power reflection at a flat interface between air andice or water as function of incident angle

Material Intrinsicimpedance

Incidentangle

Refracted angle[by Eq. (15)]

Reflected power in %[by Eq. (16)]

Ice 210Ω 030°90°

016.17°33.85°

8%11%

100%

Water(at 25°C)

43Ω 030°90°

03.27°6.55°

63%67%

100%

Fig. 13 Fraction of microwaves at a convex food surface (adapted from Buffler and Stanford, 1991).

Inc identw aves, Pi

R eflectedw aves, Pr (= Γ ∗3L/ ZKHUH 3?Γ <1,

φ φ

$ LU + η ο,

)RRG PDWHULDO +ε·! 73,

ψ <<

5HIUDFWHG Z DYHV

waves, Pi

waves, Pr (= Γ *Pi, where 0< Γ<1)

°

abe1-chap01.p65 03/20/2002, 4:01 PM17



Coupling of microwaves and foods in a cavity depends upon the dielectricproperties and total volume of the food in the cavity. The actual microwavepower absorbed by the food Po is related to matched power Pm and the volumeof the food V by the following empirical relation (Mudgett, 1985):

)1(obV

m ePP −−= (17)

where b depends upon the geometry and dielectric properties of foods, andoperating characteristics of the microwave heating equipment.

As discussed above, the interactions between foods and microwave energyare complicated. Microwave heating in foods also depends upon microwavefield distribution and heat and mass transfer. Computer simulation, based onvarious numerical schemes (i.e. finite element, finite difference and time-domain,and method of moments), has become a powerful tool to study heating patternin foods when subjected to microwave radiation (Sundberg et al., 1998; Sorianoet al., 1998; Van Remmen et al., 1996; Lin et al., 1995). Commercial packagesthat run on inexpensive PCs to study microwave fields in loaded microwaveapplicators are on the market at affordable prices (e.g. Quick Wave-3D, QWEDlnc., Warsaw, Poland). Those packages will greatly facilitate microwave research.

MEASUREMENT OF DIELECTRIC PROPERTIES

As discussed in the previous sections, dielectric properties of food materialsinfluence the penetration depth of microwaves and energy coupling of foodmaterials in microwaves. Therefore, studying the microwave heatingcharacteristics requires accurate measurement of the dielectric properties forfood materials. Several methods have been developed to measure dielectricproperties in the microwave frequency range. The following three methods arecommonly used for food materials (Engelder and Buffler, 1991). Each hasadvantages and disadvantages.

Open-End Coaxial Probe System

This system is commercially available from Agilent Technologies(Englandwood, CO). During the measurement, a coaxial probe with an openend (HP 85070B) is pressed against the sample material (Fig. 14). Themicrowave signal launched by a vector network analyzer (HP 8752C, 8753,8720 or 8510 network analyzer) is reflected by the sample. The magnitude andphase shift of the reflected waves depend upon the dielectric properties of the

abe1-chap01.p65 03/20/2002, 4:01 PM18



tested material. The analyzer receives the reflected waves, and the dielectricconstant and loss factor are then calculated. A major advantage of this methodis that it is easy to use and well suited for liquids or soft solid food materials.The dielectric properties of the material can be measured over a large range offrequencies (from 0.2 to up to 20 GHz). The measurement requires little samplepreparation; the system is interfaced with a personal computer. A samplemeasurement can be completed within a few minutes. This method is mostcommonly used by the food research community (Seaman and Seals, 1991;Nelson et al., 1993; Herve et al., 1998). The accuracy (± 5% in best cases) ofthe measurement by open-end coaxial probe system is adequate for mostmicrowave heating research. The sample thickness needs, however, to be greaterthan 1 cm for typical foods and the solids must have a flat surface to allow goodcontact between the solid and the probe surface. This method is also not suitablefor measuring materials such as plastics and oil with low dielectric property.

Transmission Line Method

With the transmission line method, a sample is precisely shaped to completelyfill the cross-section of a transmission line (a coaxial or a rectangular wave guide)(Goedeken et al., 1997). The change of the impedance and propagationcharacteristics as a result of the sample in the loaded transmission line ismeasured by a vector network analyzer. A personal computer interfaced withthe network analyzer calculates and displays the measured value of dielectricconstant and loss factor. The accuracy of this method is generally between thatof the open-end coaxial probe method and that of the cavity resonance method.The transmission line method requires careful sample preparation (Fig. 15). In

Fig. 14 Hewlett Packard (HP) open-ended coaxial probe dielectric property measurement system.

abe1-chap01.p65 03/20/2002, 4:01 PM19



particular, the sample shape needs to precisely fit the cross-section of thetransmission line for accurate measurement. Liquid foods are more difficult tomeasure with this method. Commercial systems are available from AgilentTechnologies. These systems, however, need precision transmission line fixturesfor narrow bands of frequencies (e.g. S-Band: 2.60–3.95 GHz; G-band: 43.95–5.85 GHz, etc.). In general, this system is more expensive for the same range offrequency than the open-end coaxial probe system, and the measurements aremore difficult and time-consuming. This method also gives limited resolutionwhen measuring low lossy foods such as oil.

Resonance Cavity Method

The resonance cavity method is based on the fact that cavities are high quality(high Q) resonance structures. A small sample material introduced inside acavity shifts the center resonance frequency fc and alters the quality factor (Q)of the cavity (ASTM, 1971). These two parameters are measured by a networkanalyzer (Fig. 16). A special software calculates the dielectric properties of thematerial. Since the change of the fc is small, it is essential that the networkanalyzer has a resolution of 1 Hz. This method gives an overall accuracy of± 2–3% (Ohlsson, 1989). The resonance cavity method is suited to measuringthe dielectric properties of low lossy materials such as oil, paper, plastics, glassor wood. This method, however, requires precise sample shape. In addition, aresonance cavity provides dielectric properties only at a fixed frequency. Theanalysis may also be complex. Commercial systems from Hewlett-Packard aremuch more expensive than the open-end coaxial probe system.

Fig. 15 Schematic diagram for HP transmission line method: a measurement system consists of anetwork analyzer, a coaxial or waveguide, a computer and software for data acquisition and conversion.

abe1-chap01.p65 03/20/2002, 4:01 PM20



Dielectric properties of meats, dairy products, cereal, fresh fruits andvegetables have been compiled by several researchers (Bengtsson and Risman,1971; Nelson, 1973; Kent, 1987; and Nelson and Kuang, 1997).

TEMPERATURE AND PRESSURE MEASUREMENTS

Temperature and pressure measurements are often necessary in process controland validation in microwave heating research. Conventional temperature sensorsbased on thermoelectric effects (e.g. thermocouples, RTD and thermistors) andpressure sensors consisting of metal parts may distort the electromagnetic fieldin the vicinity of the probes and give erroneous readings. Fiber optic sensors,on the other hand, do not interact with electromagnetic energy, and are nowcommonly used in microwave research. The principles for the design of thesensors vary among manufacturers. The probe sizes of fiber optical sensors aregenerally small (e.g. as small as 1.0 mm in diameter, FISO Technologies Inc.,Sainte-Foy, Quebec, Canada). Temperature fiber optic sensors providecomparable accuracy as thermocouples in a normal heating medium (Fig. 17).They generally have short response times (~1.5 s in water) and are particularlysuited for relatively fast microwave heating. The new FISO pressure probes canprovide 0.005 psi resolution and can be useful in studying internal vapor pressuregeneration during microwave drying. Fiber optic sensors were used very effectivelyin our laboratory to monitor temperature and pressure changes during microwavehigh temperature-short-time sterilization pilot-scale tests (Lau et al., 1998; Fenget al., 2001). Four companies in North America produced fiber optic sensors,but only two companies ceased operation over the past 3 years (Table 5), and

Fig. 16 Schematic diagram for HP resonant cavity method: a measurement system consists of anetwork analyzer, a cavity, a computer and software.

abe1-chap01.p65 03/20/2002, 4:01 PM21



the prices range from US$7000 to US$25 000, depending upon number ofchannels and sophistication of data acquisition.

COMMERCIAL APPLICATION OF MICROWAVE HEATINGIN FOOD PROCESSING

Due to congested bands of microwave and radio frequencies (RF) already beingused for communication purposes, only a limited number of bands are allocated(e.g. in the US by the Federal Communications Commissions or FCC) forindustrial, scientific, and medical (ISM) applications. Table 6 lists mostimportant bands allocated for those purposes.

Table 5 Manufacturers of fiber optic sensors

Company name Company address Type of sensors

Photonetics, Inc.

Luxtron, Co.

FISO Technologies

Nortech Fibronic, Inc.

200 Corporate Place-Suite 1A,Peabody, MA 01960-3840(cease operation)

2775 Northwestern Parkway,Santa Clara, CA 95051-0941

2014 Jean-Talon N. Suite 125Saint-Foy (Quebec) QC, CanadaG1N 4N6

240-500 St-Jean-Baptiste, Quebec,QC, Canada G2E 5R(ceased operation)

Temperature and pressure

Temperature

Temperature and pressure

Temperature

Fig. 17 Photonetics fiber optic temperature and pressure measurement system and probes (courtesyof FISO Technologies Inc.).

abe1-chap01.p65 03/20/2002, 4:01 PM22



Table 6 Important RF and microwave frequency allocations for industrial, scientific and medical(ISM) use (Decareau, 1985; Metaxas and Meredith, 1993; Buffler, 1993)

The frequency bands centered at 13.56 MHz, 27.12 MHz, 40.68 MHz, 896MHz, 915 MHz and 2450 MHz are most commonly used in ISM applications,and industrial equipment for those frequency bands is readily available fromcommercial suppliers. Other frequencies are also allocated for ISM use in variouscountries. For example, 42 MHz, 49 MHz, 56 MHz, 84 MHz and 168 MHz arepermitted in Great Britain, and 433.92 MHz is allocated in Austria, Netherlands,Portugal, Germany and Switzerland (Metaxas and Meredith, 1993). Thefollowing table shows the frequencies, the power levels, and the estimatednumber of commercial units used in the US food industry (Table 7).

*A number of US manufacturers have had 915 MHz equipment accepted for use in Europe by keeping theinterference emission below an acceptable level for the country of installation (Buffler, 1993; and, Gene Eves,Ferrite Components, Inc., Hudson, NH, personal communication, 1998).

Frequency(MHz)

Frequencytolerance(MHz)

Typicalapplications

Countries

RFFrequencies

13.56

27.12

40.68

MicrowaveFrequencies

896

915*

2375

2450

± 0.067

± 0.160

± 0.020

± 10

± 13

± 50

± 50

Wood drying, curing of ceramic,final drying of bakery products,textile drying and curing,and bonding

Same as above

Same as above

Tempering of frozen products

Pre-cooking of bacon, temperingof frozen products

Domestic microwave ovens

Domestic microwave ovens,pre-cooking of bacon,pasteurization and sterilizationof packaged foods

Worldwide

Worldwide

Worldwide

Great Britain

North and South America

Albania, Bulgaria,Hungary, Romania,Czechoslovakia,and former USSR

Worldwide, except where2375 MHz is used.

abe1-chap01.p65 03/20/2002, 4:01 PM23



Microwave Drying

It is generally not economical to use microwaves to replace conventional dryingmethods for complete drying processes. Microwave drying, however, has a majoradvantage in speeding up the final drying in the falling rate region in whichconventional drying becomes less effective and takes a long time due to reducedheat and moisture transfer. The most successful drying application is the finishingdrying of pasta products. Pasta must be dried to a moisture content of about 13%.When hot air is used, a hard case forms after the pasta loses surface moisture.This not only dramatically reduces the moisture diffusion rate and slows thedrying process, but also causes unacceptable cracking when dried with hightemperatures and dry air. This problem can be alleviated by using a slow dryingprocess at a low temperature and relatively high humidity. This process, however,may take over eight hours. Since microwaves interact directly with the moisturein the core of the pasta, microwave heating provides a positive gradient formoisture to migrate towards the surface, thus significantly reducing drying time.When microwaves were used as the finishing drying method (from 18–13.5%),the process time was reduced to 1.5 hour, and the length of the process line wasreduced from 8.3 m to 3.6 m (Microdry, 1998). This process also resulted in a20 to 25% savings in energy, and a product with better color and texture, aswell as 15 times less bacteria than products from conventional methods(Decareau, 1995).

Microwave drying under vacuum conditions has been demonstrated toproduce high quality dehydrated fruits and vegetables. Pilot-scale testing facilitiesare available for process demonstration and development at California State

Table 7 Microwave food processing systems operating in the US (Schiffmann, 1992 and 1995;Mudgett, 1989)

*Estimated by Schiffmann (1955).

Process Frequency (MHz) Power Range(kW)

No. of systems*

Tempering of meat, fish and poultry

Bacon pre-cooking

Pasts drying

Meat, sausage and chicken cooking

Pharmaceutical drying

Drying (snacks and vegetables)

Pasteurization/sterilization

915

915

915

2450

2450

2450

2450

30–80

50–300

30–50

30–80

10

40

10–30

>400

25

20

5

25

5

>10 (Europe)

abe1-chap01.p65 03/20/2002, 4:01 PM24



University Fresno, California and at the University of British Columbia,Vancouver, Canada. The food industry is, however, slow to adopt this technology,possibly due to a lack of understanding of the microwave and vacuum drying,inexperience in the process control, and relatively high capital investment.

Tempering

In many food production processes, incoming meat or fish is frozen in thickblocks and stored at −30° to −10°C until ready to use. For further processing,such as cooking or drying, this material needs to be diced, sliced or separatedinto smaller pieces. This mechanical operation requires that the blocks be“tempered” from their solid frozen state to a point just below freezing (−4°Cto −2°C) at which point cutting or separation can be done without damage tothe product and the machinery. In conventional tempering operations, thetemperature of frozen food in solid blocks of up to 100 pounds is raised to justbelow freezing with the application of water or air. In most plants, the frozenblocks are simply thawed in warm water, which takes several hours. Others usehot air. Many use floor tempering alone, without any heat aid, which may takeup to three to four days. The drip loss with conventional methods is oftenaround 5%. Long tempering time and drip loss may result in contamination bypsychrotrophic bacteria. Microwaves have a large penetration depth in frozenproducts, and thus provide relatively uniform heating throughout frozen blocks.Microwave tempering is generally accomplished in continuous units in less thanfive minutes. It is critical that the temperature of the products remain belowthe freezing point to avoid thermal runaway. The advantages of microwavetempering include: higher product quality at lower overall cost (0.33 to 0.20cents per pound), space requirements are reduced up to 90%; and time is reducedup to 98% (Microdry, 1998). The capital investment is about $150 000 forproduction rates of up to 7500 lb/hr.

Pre-Cooking of Bacon

Microwave pre-cooking of bacon for institutional use and for use in the foodservice industry is yet another successful industrial application of microwaveheating. Pre-cooked bacon is rid of most fat and requires only heating beforeserving. This product is easy to use and there is little need for disposal of therendered fat. Pre-cooking also reduces the flight costs and the warehouse spacefor bacon by up to 50%. Microwave pre-cooking for bacon takes about 1.5 to 2minutes (Decareau, 1985). A continuous commercial microwave heating unitincorporates hot air to remove moisture and can process up to 50 000 strips per

abe1-chap01.p65 03/20/2002, 4:01 PM25



hour. Microwave pre-cooked products retain about 75% of their original length,as compared to a 40 to 50% size retention when cooked with conventionalmethods. The yield is increased by 30 to 40%, because no product is lost throughovercooking (Mudgett, 1989). The fat rendered from the bacon during themicrowave pre-cooking process is of high quality and can be sold as a highvalue by-product. Additional advantages include energy savings of about 75%and a reduction of processing floor space by about 50%, compared with theconventional pre-cooking method.

Microwave Pasteurization

Plastic packaging materials are transparent to microwaves. Microwave can,therefore, be used to heat a food product in a sealed package. The majoradvantage of microwave pasteurization of packaged foods is the elimination ofpossible post-treatment contamination. Another advantage of microwavepasteurization is the shorter heat-up time and is thus more energy efficientthan conventional heating methods. Microwave pasteurization of packaged foodsis commercialized in Europe, Japan and the US (Ohlsson, 1989; Schiffmann,1992). Those units operate at 2450 MHz, normally with 10–40 kW of installedmicrowave power. Packaged cakes, breads, fresh pasta and refrigerated ready-to-serve meals are pasteurized in a process during which the temperature of theproduct is raised to 80°C in 3 to 5 minutes and then held for several minutes.This process provides protection against molds, yeast, and thermolabile bacteria(Giese, 1992).

RECENT RESEARCH ACTIVITIES AT WASHINGTONSTATE UNIVERSITY

Renewed interest on microwave processing in food applications is evident fromincreasing reports in recent literature. Those activities are, in part, driven byconsumers’ desire for high quality foods and by a general need for more energyefficient and environmentally friendly processes. The following subsectionsdescribes some of the recent results on microwave drying and sterilization fromour laboratory.

Microwave Drying in a Spouted-Bed

An inherent problem associated with microwave drying is non-uniform heatingcaused by an uneven spatial distribution of the electromagnetic field inside the

abe1-chap01.p65 03/20/2002, 4:01 PM26



drying cavity. Non-uniform heating may cause partial scorching in high sugarproducts. Fluidization provides pneumatic agitation for particles in the dryingbed. It also facilitates heat and mass transfer due to a constantly renewedboundary layer at the particle surface. Coarse food particles such as diced applesare, however, difficult to fluidize, especially when their moisture content isrelatively high and surface is sticky due to high sugar content. We developed alaboratory system to study the feasibility of combining microwave heating witha spouted-bed to finish-dry high sugar fruits such as blueberries and diced apples.A spouted-bed is a special fluidized technique suitable for handling Group-Dparticles in the Geldart classification of particles (Geldart, 1973). Group-Dparticles are coarse particles that cannot be fluidized well in ordinary fluidizedbeds. Many coarse dices of agricultural products, including diced apples, fallinto the Group-D particle category (Feng et al., 1999). A major distinctionbetween a spouted-bed and an ordinary fluidized-bed lies in the particle flowpattern (Fig. 18). In an ordinary fluidized-bed, particles experience a localizedoscillatory and somewhat random movement. In a spouted-bed, on the otherhand, the particles are moved through a macro-scale circulation featured byupward “spouts” and downward annulus. The trajectory of an individual particleforms a three-dimensional pattern in the spouted bed over a certain period, butthe position of the particle at any moment is random. This particle circulationpattern provides a uniform heating in the microwave field. A schematic diagramof the laboratory microwave-spouted-bed drying system is shown in Fig. 19. Itconsists of: 1) a variable microwave power source (from 0 to 1.4 kW) operatingat 2450 MHz; 2) an air supply system with an electric heater and a temperaturecontroller; and 3) a spouted-bed. A circulating water load was used to protectthe magnetron from overheating when the product moisture was low. The

Ordinary fluidized bed Spouted bed

Air Air

Fig. 18 Comparison between an ordinary fluidized-bed and a spouted-bed.

abe1-chap01.p65 03/20/2002, 4:01 PM27



Fig. 19 Schematic diagram of microwave and spouted-bed drying system (Feng and Tang, 1998).

V

T

T

F

D

Heater

Computer

Microwave power controller

Temperaturecontroller

Blower

Electric balance

Magnetron

Spouted bed

Sample

Microwave cavity

Sink

Water pump

D -- Dew point temperature; F -- Flowrate; T -- Temperature; V -- Velocity;

bypass

microwave power absorbed by the water load was calculated by temperatureincrease of the circulating water. Microwave input into the drying was estimatedby the difference between the microwave power from the generator and theabsorbed power in the circulating water. A more detailed description of thesystem is provided in Feng and Tang (1998).

Temperature distribution among sample particles in microwave-spouted-beddrying process was very uniform, as indicated by the error bars in Fig. 20.Figure 21 shows a comparison of center temperature variation in ten appledices after 2.5 minutes of drying with the microwave-spouted-bed drying systemand in a stationary bed during microwave heating. In the microwave-spouteddrying bed, the variation in the product core temperature was between ± 4°Cabout the average. This variation was reduced to about ± 1.4°C toward the endof a 25-minute drying (Fig. 20). With a stationary bed and a horizontal flow ofhot air at 70°C, however, microwave heating caused severe localized heating.For example, the center temperature of one dice was recorded as 193°C, whileanother was at 65.5°C. Some apple dices were charred, while others were stillvery moist. Thus, the spouted-bed helped to overcome the drawback ofnon-uniform heating in a microwave cavity.

abe1-chap01.p65 03/20/2002, 4:01 PM28



Fig. 21 Variation of the core temperature among ten randomly sampled diced apples 2.5 minutesafter the start of microwave-spouted-bed drying using 70°C hot air and 4.9 W/g (raw material)microwave power (Feng and Tang, 1998).

Fig. 20 Temperature and moisture content changes of diced apples during microwave andspouted-bed drying using 70°C hot air and 4.9 W/g (raw material) microwave power (Feng andTang, 1998).

-10

-5

0

5

10

Sampling sequence

0 1 2 3 4 5 6 7 8 9 10 11

Tem

pera

ture

dev

iatio

n o C

-50

-25

0

25

50

75

100

MW&SB drying

MW & parallel flow hot-air druying

T(average) = 101.32 oC

T(average) = 74.29 oC

MW & parallel flow hot-air drying

Tem

per

atur

e d

evi

atio

n (°

C)

Drying time (min)

0 5 10 15 20 25

Te

mp

era

ture

(o C)

0

20

40

60

80

100

Mo

istu

re c

on

ten

t (w

b)

%

0

5

10

15

20

25

I

II III

temperature

moisture

Tem

per

atu

re (

°C)

abe1-chap01.p65 03/28/2002, 10:54 AM29



In large commercial drying operations, diced apples are usually dried in twosteps: 1) from fresh to 24% moisture content to produce so-called evaporatedapples that can be used in pie filling or in other bakery products; and 2) from24% to about 5% to produce low moisture diced apples used in breakfast cereals.Hot-air drying in the first stage is very effective. In the second stage, hot airdrying is, however, much less efficient as the drying enters into the falling rateperiod. Very high air temperatures (sometimes over 100°C) are used to reducethe drying time, and the drying is still lengthy (over one hour). Microwave-spouted-bed drying may be particularly useful in this stage to reduce dryingtime. Figure 22 shows that the time needed to dry diced apples from an initialmoisture content of 24% to 5% was reduced from over 2.5 hour in a spouted-bed at an air temperature of 70°C to only 10 minutes when assisted by microwaveheating at 6.1 w/g (raw material). Generation of internal vapor pressure isbelieved to contribute to significantly increased drying rate in microwave-spoutedbed drying system (Feng et al., 2001). The product produced in the microwave-spouted-bed drying system had brighter color, lower bulk density and a higherrehydration rate than the hot air-dried products (Feng and Tang, 1998).

A relatively large air velocity (>1.9 m/s) in the microwave-spouted-bed dryeralso helps to maintain a constant product temperature (10–14°C above the airtemperature), and to eliminate the possibility of the product being overheatedwhen product moisture content is low near the end of drying. We measured

Fig. 22 Drying curves of diced “Red Delicious” apples dried in a microwave and spouted-bed(MW & SB) or spouted-bed only using 70°C hot air (Feng and Tang, 1998).

abe1-chap01.p65 03/20/2002, 4:01 PM30



the dielectric properties of diced apples at various moisture content levels usingan open-end coaxial probe system; and observed a significant drop of the die-lectric loss factor as the moisture was reduced. The product, therefore, absorbedmuch less microwave energy at a lower moisture content than at a highermoisture content. This compensates for lower energy loss from the product dueto reduced moisture evaporation when at a lower moisture content, and thuscontributes, to some degree, the more or less constant temperature near the endof drying.

We are studying the energy efficiency of the microwave-spouted-bed dryingprocess and the feasibility of scaling-up to commercial applications.

Microwave Sterilization

Microwave sterilization has a major potential advantage over retorting becausethe heat-up time of microwave processes can be very short (Buffler, 1993).Ohlsson (1987) demonstrated that a high-temperature-short-time microwaveprocess (128°C and 3 minutes cooking time) produced products superior to thosefrom canning (120°C retort temperature and 45 minutes processing time) andretorting foil pouches (125°C and 13 minutes cooking time). Earlier studies byStenstrom (1974) and O’Meara et al. (1977) also showed that the microwaveprocess produced better products than conventional sterilization processes. Severalcommercial microwave sterilization systems have been reported in the literature,including that of OMAC (Harlfinger, 1992) and Berstorff (Schlegel, 1992). Butcommercial applications of microwave sterilization processes can only be found inBelgium (TOPS Foods, Belgium) and Japan (Otsuka Chemical Co., Osaka, Japan).Reasons for the slow adaptation of microwave sterilization processes include non-unifor heating and a lack of reliable methods to validate commercial thermalprocesses for food safety. Large temperature variations in microwave heating area result of excessive heating at the corner or edge of food stuff, due to localizedconcentrations of the microwave field. The means of providing a more uniformheating need to be investigated. All the work reported to date on microwavesterilization has been at the microwave frequency of 2450 MHz (Decareau,1995). Potential advantages of using 915 MHz microwaves frequency, comparedto 2450 MHz, are the deeper penetration depth in foodstuffs and, possibly,more uniform field distribution over a confined surface area of packaged foods.

A pilot-scale (5 kW) pressurized 915 MHz microwave heating system hasbeen developed in our laboratory (Fig. 23) to study high-temperature-short-temperature sterilization of packaged foods. During the sterilization process,vacuum-packaged food is immersed in a water solution with a selected saltconcentration (based on dielectric property measurement) in a pressurized vessel

abe1-chap01.p65 03/20/2002, 4:01 PM31



placed in the 915 MHz microwave cavity. The vessel wall is made of aluminumand the top and bottom plates are transparent to microwaves. The solutionreduces refraction of microwaves at the interface between air and the foodwhich helps to enhance microwave heating uniformity. Four fittings are installedin the top plate to allow insertion of fiber optic temperature and pressure sensors(1.5 mm dip diameter and 0.2 to 1 second response time) (Photonetics, Inc.,

Fig. 24 The 5 kW pilot-scale 915 MHz unit at Washington State University.

Fig. 23 Schematic diagram of pressurized vessel in 915 MHz microwave system.

Circulator Waveguide

Stirrer

Pressurized vessel

Product

TurntableCold water inlet

Water outlet

Pressure gauge Pressurerelease-valve

Pre-pressureregulator

Air supply

915 MHzmicrowavegenerator

Water-holdingtank

Hot watersupply

Steam kettle

Pump

Photoneticsystem

Micro-processor

Fiber optical temperature &pressure sensors

Water-levelindicator

abe1-chap01.p65 03/20/2002, 4:02 PM32



Peabody, MA). One fiber optic sensor (inside a thermal well inserted in thepackaged food) is used to measure the temperature in the vacuum-sealed food.A pressure regulating system outside of the microwave cavity controls the over-pressure in the vessel during microwave heating. This over-pressure preventsfood packages from bursting.

Before sterilization tests, warm water at 70°C was introduced to the vesselso that the temperature of the water solution and the product reached thesame temperature of about 121°C (the dielectric loss factor of water decreaseswith increasing temperature at 915 MHz, and thus hot water absorbs less energythan foods at high temperatures). Tap water was introduced into the vessel tocool the food after sterilization. Typical temperature and pressure profiles duringa microwave sterilization is shown in Fig. 25. The product temperature wasmeasured at the center of a model food gel slab (12 cm × 6.5 cm × 2 cm) madeof 20% whey proteins. Figure 25 shows a short come-up time (< 4.5 minutes),a steady temperature (~ 121°C) during the holding period, and rapid coolingafter the 915 MHz microwave sterilization process. The temperature differencebetween any two locations in the salt solution within the pressurized vessel wasless than 1°C. The gel center temperature was also very close to that of the saltsolution temperature. The pressure inside the vessel was fairly stable over time.Figure 26 compares temperature-time profiles in an 8 oz packaged food during

Fig. 25 Time-temperature profile during the sterilization process of vacuum-packaged rectangularwhey protein gel containing 1% ribose.

Time (min)

0 5 10 15 20 25 30

Tem

pera

ture

(°C

)

20

40

60

80

100

120

140P

ress

ure

(PS

I)

-5

0

5

10

15

20

25

30

Ch1 - Temp. inside food

Ch3 - Temp. inside pressurized vesselCh 4 -Temp. inside pressurized vessel

Ch2 - Pressure inside pressurized vessel

Ch 4 Ch 1 Ch 2

Ch 3

Pressurized-vessel

Sample

Stand

abe1-chap01.p65 03/20/2002, 4:02 PM33



microwave sterilization with those during conventional thermal processes toachieve an Fo = 6.

Microwave heating uniformity within whey protein gels was studied usingthe intrinsic chemical marker method developed by researchers at the US ArmyNatick Laboratories (Kim and Taub, 1993; Kim et al., 1996a and b). Threemarkers have been identified in various food systems, M1 [2,3-dihydro-3,5-dihydroxy-6-methyl-4(H)-pyran-4-one) and M2 [4-hydroxy-5-methyl-3 (2H)-furanone] are particularly useful. M1 is formed by Maillard reaction betweenglucose and protein, and M2 is formed through the reaction between riboseand protein (Fig. 26) (Prakash et al., 1997). Both marker formations follow firstorder reactions, and the formation M1 is slower than that of M2. M1 and M2yields generated within whey protein gels at sterilizing temperatures weredetermined by an HPLC system (equipped with an anionic exclusionchromatographic column and a photodiode array detector) after the thermalprocessing to show the integrated time-temperature effect at various locations.M1 and M2 yield ratios in a whey protein gel after 3 minutes heating at about121°C in the microwave sterilization are expressed yield ratios and shown inFig. 27. The maximum marker yields were determined in an oil bath at 121°Cin a separate test. Given the accuracy for M1 and M2 yield determination ofabout 5%, the marker yields were relatively uniform (Fig. 28). The resultsindicate that when heated in a suitable submerging solution using 915 MHzmicrowaves, the gel went through a very uniform heating.

The relative uniform heating in our 915 MHz microwave sterilization systemmade it possible to sterilize food in a much shorter time than with conventional

Fig. 26 Comparison of microwave and conventional sterilization history to achieve Fo = 6 minutes.

Te

mp

era

ture

(°C

)

2 0

40

60

80

100

120

140

M icrow ave ab le P ouchR eto rt P ouchC an 211 x 400

abe1-chap01.p65 03/20/2002, 4:02 PM34



retorting. A fast-cooking macaroni cheese product developed by Kraft Foods,Inc. (Glenview, Illinois) was used to assess the effect of high-temperature-short-time sterilization on food quality. Kraft Macaroni and Cheese Dinner(Family Size) was cooked as instructed on the package, and sterilized in pouches(16.2 cm × 13.6 cm × 2 cm) in the 915 MHz microwave sterilizing system or

Fig. 27 Reaction pathways leading to the formation of chemical markers (Kim et al., 1996b).

Weak acid2,3-enolization

D-glucose + amine

Amadori Compound

Strong acid1,2 - enolization

0 CHOH0H2C

5-hydroxymethylfurfural(M-3)

0

OHHO

H3C

O

2,3-dihydro-3,5-dihydroxy-6-methyl-4(h)-pyran-4-one

(M-1)

D-ribose + amine

Amadori Compound

Strong acid1-2-enoliza tion

2-furaldehyde

0

OHO

H3C0 C HO

4-hydroxy-5-methy l-3(2H)-furanone (M -2)

w eak ac id 2 ,3-enolizat ion

abe1-chap01.p65 03/20/2002, 4:02 PM35



retorted in cans (size: 303 × 406) to Fo∼5.2 No significant product texture orflavor was lost during the microwave sterilization process. Drained weightmeasurement was also performed for these samples following the instructionsdeveloped by the US Army Natick RD&E Center. The macaroni and cheesesterilized with microwaves retain the original structure while the canned became“lumpy and gooey” (Fig. 29). Similar research on peas also indicated a bettercolor and texture retention when sterilized with microwaves compared toconventionally canned products. We are researching on the effects of storageon the quality of microwave sterilized products.

Microwave Pasteurization

FDA regulations (FDA, 21 CFR parts 102, 110, 113 and 114, 1998) require thatpickled products be pasteurized to inactivate pathogenic and spoilage micro-organisms. Many vegetables such as asparagus are, however, heat-sensitive andthe texture usually severely degrade during thermal treatments (McGlynn et al.,1993). During pasteurization of pickled products, it is important to minimizequality losses. A study was carried out to investigate the effect of 915 MHz

2Fo value was calculated using the general method (Ball and Olson, 1959) based on the temperatureof the coldest spot Tc in the package:

∫ −=t zTT dtF0

/)(o refc10

where Tref = 121.12oC, Z = 10oC (Z value for Clostridium botulinum), and t is process time.

Fig. 28 M1 (from 5% glucose) and M2 (from 1% ribose) yields in the top and bottom half-layersof rectangular whey protein gels (6.5 cm × 12 cm × 2 cm) after 915 MHz microwave heating for3 minutes at 121°C.

0

0.2

0.4

0.6

0.8

1

M-2

yie

ld

Top

0

0.2

0.4

0.6

0.8

1

M-2

yie

ld

Bottom

abe1-chap01.p65 03/20/2002, 4:02 PM36



microwave pasteurization on the textural quality of pickled asparagus in 64 ozglass bottles.

The pickled asparagus in 64 oz glass bottles was treated with three differentpasteurization processes: 1) Treatment I — bottled asparagus was filled with 80°Cbrine to reach 50°C, the bottle was then heated to 70°C in a water bath (80°C)

Fig. 29 Comparison of microwave and retort sterilized macaroni and cheese. The product was notspecially formulated to withstand severe thermal processing. (a) Products after sterilization and (b)after the drained weight tests.

(a)

(b)

abe1-chap01.p65 03/20/2002, 4:03 PM37



and then further heated to 88°C (determined at the coldest spot) using a pilotscale 915 MHz microwave oven at 2 kW power level; 2) Treatment II —similar to Treatment I, except that 1 kW was used to reach the pasteurizationtemperature, instead of 2 kW; 3) Treatment III (conventional heating) — thepickled asparagus in 64 oz bottles was filled with brine at 80°C to reach anequilibrium temperature of 50°C. The bottles were heated in water bath set at98°C until the temperature at the coldest spot reached 88°C. After each heattreatment, the bottle was first tempered with lukewarm water (40°C) for 30seconds, to reduce the checking of the glass bottles, and then further cooled withrunning tap water (10°C). For microwave treatment, the top one-third of thebottle was covered with aluminum foil to shield microwave radiation to enhanceheating uniformity, and the coldest location in the bottle was pre-determinedby measuring temperature at eight different locations using fiber optical sensors.

The reason for pre-heating to 70°C in water bath prior to microwavetreatments was to reduce the use of microwave energy or increase the capacityof microwave equipment. Our kinetics study (Lau et al., 2000) indicates thatthe texture loss in asparagus at or below 70°C is negligible compared to thetexture loss at 80~90°C (Fig. 30, only shown for the butt section, similar resultswere obtained for the middle and bud sections).

Cook value (C-value) was used to evaluate the thermal effect of differentprocesses on the quality of pickled asparagus (Table 8). The conventional heating

Time (min)

0 20 40 60 80 100 120 140

Ln

(S

he

ar

Str

ess

)

10

11

12

13

14

15

70°C, r2 = 0.576

80°C, r2 = 0.819

90°C, r2 = 0.889

98°C, r2 = 0.986

Fig. 30 Effect of heating temperature and time on texture of asparagus as indicated by the maximumshear stress to cut through asparagus spears (Lau et al., 2000).

abe1-chap01.p65 03/20/2002, 4:03 PM38



method resulted in a much higher C-value (more severe cooking) than the twomicrowave heating treatments which significantly reduced thermal degradation.This was further confirmed by our textural study.

The maximum shear stresses for pickled asparagus after conventional andmicrowave heating treatments are shown in Fig. 31 (only shown for the bud

Table 8 C-value for pickled asparagus using different pasteurization processes(Lau and Tang, 1998)

*The most severely heated locations in each treatment detected by temperaturemeasurement.† The C-value was evaluated from:

∫ −=t ZT dt0

/)( ,10 refTC where Tref = 100°C, Z = 33°C

Pasteurization process Location in bottle* C† -value (min)

Hot-filled, boiling water bath andrapid-cooled

Hote-filled, water bath, 1 kWmicrowave and rapid-cooled

Hot-filled, water bath, 2 kWmicrowave and rapid-cooled

Surface

Top surface

Bottom surface

8.66

3.16

2.64

T re a tm e n t

T 1 T 2 T 3

Sh

ea

r s

tre

ss

(k

Pa

)

0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

4 0 0

4 5 0

C = F re s h a sp a ra g u s

T 1 = C o m b in a tio n o f h o t fi lle d , w a te r b a th h e a te d to 7 0 °C ,9 1 5 M H z m ic ro w a ve h e a te d (2 kW ), a n d h yd ro c o o le d .

T 2 = C o m b in a tio n o f h o t fi lle d , w a te r b a th h e a te d to 7 0 °C ,9 1 5 M H z m ic ro w a ve h e a te d (1 kW ), a n d h yd ro c o o le d .

T 3 = H o t fille d , h e a te d in b o ilin g w a te r b a th , a n d h y d ro c o o le d .

M e a s u re d s h e a r s tre s s

P re d ic te d s h e a r s tre s s

C

Fig. 31 Effect of different pasteurization on texture of pickled asparagus (Lau and Tang, 1998).

abe1-chap01.p65 03/20/2002, 4:03 PM39



section, similar results were obtained for the middle and butt sections). Ingeneral, the shear stress values for pickled asparagus in the bud, middle andbutt segments heated with microwave treatments were higher than those heatedby the conventional method. For microwave treatments, the shear stress valuesfor the pickled asparagus treated with 2 kW microwave heating were higherthan those treated with 1 kW microwave heating. The predicted maximumshear stresses (based on kinetic model using the parameters shown in Table 9)were within the range of the experimental values.

ACKNOWLEDGEMENTS

Our research projects on microwave drying and sterilization were funded byWashington State University IMPACT Center, Northwest Small Fruit ResearchCenter, and the US Army Natick RD&E Center. The reported experimentresults at Washington State University were gathered by Ph.D. students, FengHao, Ming Lau and Julian N. Ikediala.

REFERENCES

American Society for Testing and Materials (ASTM) (1971). Standard test methodsfor complex permittivity (dielectric constant). Designation D 2520-86. Method B.

Ayappa, K.G., H.T. Davis, G. Crapiste, E.A. Davis and J. Gordon (1991). Microwaveheating, an evaluation of power formulations. Chem. Eng. Sci. 46(4):1005–1016.

Ball, C.O. and F.C.W. Olson (1959). Sterilization in Food Technology — Theory, Practiceand Calculations. McGraw-Hill, New York.

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

Buffler, C. and M. Stanford (1991). The effects of dielectric and thermal properties onthe microwave heating of foods. Microwave World 9(4):15–24.

Table 9 Kinetic data for texture degradationof green asparagus during pasteurization(Lau et al., 2000)

Lacation Ea (kcal/mol) ka (min−1)

ButtMiddleBud

24.924.823.7

0.01530.02240.0278

abe1-chap01.p65 03/20/2002, 4:03 PM40



Buffler, C.R. (1993). Microwave Cooking and Processing. Van Nostrand Reinhold, NewYork.

Calay, R.K., M. Newborough, D. Probert and P.S. Calay (1995). Predictive equationsfor the dielectric properties of foods. Int. J. Food Sci. Tech. 29:699–715.

Decareau, R.V. (1985). Microwaves in the Food Processing Industry. Academic Press, Inc.,New York.

DeCareau, R.V. (1995). The microwave sterilization process. Microwave World 16(2):12–15.

Engelder, D.S. and C.R. Buffler (1991). Measurement of dielectric properties of foodproducts at microwave frequencies. Microwave World 12(2):6–15.

Feng, H. and J. Tang (1998). Microwave finish drying of diced apples in a spouted bed.J. Food Sci. 63(4):679–683.

Feng, H., J. Tang and R.P. Cavalieri (1999). Combined microwave and spouted beddrying of diced apples: effect of drying conditions on drying kinetics and producttemperature. Drying Tech. 17(10):1981–1998.

Feng, H., J. Tang, R.P. Cavalieri and O.A. Plumb (2001). Heat and mass transportin microwave drying of porous materials in a spouted bed. AICHE J. 47(7):1499–1511.

Food and Drug Administration (FDA) (1998). 21 CFR parts 102, 110, 113 and 114.1998. National Records Administration. http://www.access.gpo.gov/nara/about.cfr.html.

Geldart, D. (1973). Types of fluidization. Powder Tech. 7:285–290.

Giese, J. (1992). Advances in microwave food processing. Food Tech. 46(9):118–123.

Goedeken, D.L., C.H. Tong and A.J. Virtanen (1997). Dielectric properties of apregelatinized bread system at 2450 MHz as a function of temperature, moisture,salt and specific volume. J. Food Sci. 62(1):145–149.

Goldblith, S.A. (1967). Basic principles of microwaves and recent developments. Adv.Food Res. 15:277–301.

Harlfinger, L. (1992). Microwave sterilization. Food Tech. 46(12):57–61.

Harvey, S.C. and P. Hoekstra (1972). Dielectric relaxation spectra of water adsorbedon lysozyme. J. Phys. Chem. 76(21):2987–2994.

Herve, A.-G., J. Tang, L. Luedecke and H. Feng (1998). Dielectric properties of cottagecheese and surface treatment using microwaves. J. Food Eng. 37(4):389–410.

Kent, M. (1987). Electrical and Dielectric Properties of Food Materials. COST 90bisProduction, Science and Technology Publishers, Hornchurch, Essex, UK.

Kim, H-J. and I.A. Taub (1993). Intrinsic chemical markers for aseptic processing ofparticulate foods. Food Tech. 47(1):91–99.

abe1-chap01.p65 03/20/2002, 4:03 PM41



Kim, H-J., Y.M. Choi, T.C.S. Yang, I.A. Taub, P. Tempest, P. Skudder, G. Tucker andD.L. Parroott (1996a). Validation of ohmic heating for quality enhancement offood products. Food Tech. 50(3):253–261.

Kim, H-J., I.A. Taub, Y-M. Choi and A. Prakash (1996b). Principles and applicationsof chemical markers of sterility in high temperature-short-time processing ofparticulate foods. In Chemical Markers for Processed and Stored Foods (ed.)T.-C. Lee and H.-J. Kim, American Chemical Society, Washington.

Kuang, W. and S.O. Nelson (1998). Low-frequency dielectric properties of biologicaltissues: a review with some new insights. Trans. ASAE 41(1):173–184.

Lau, M.H., J. Tang and B.G. Swanson (2000). Kinetics of textural and color changes ingreen asparagus during thermal treatments. J. Food Eng. 45:231–236.

Lau, M.H. and J. Tang (1998). Pasteurisation of pickled asparagus using 915 MHzmicrowaves. IFT Ann. Meeting, Atlanta, Georgia, paper 34B-6.

Lau, M.H., J. Tang, I.A. Taub, T.C.S. Yang, C.G. Edwards and F.L. Younce (1998).Microwave heating uniformity of foods during 915 MHz microwave sterilizationprocess. Procs. 33rd Microwave Power Symp., 78–81.

Lin, Y.E., R.C. Anantheswaran and V.M. Puri (1995). Finite element analysis ofmicrowave heating of solid foods. J. Food Eng. 25:85–112.

Lu, L., J. Tang and L. Liang (1998). Moisture distribution in spherical foods in microwavedrying. Drying Tech. 16(3–5):503–524.

Macosko, C.W. (1994). Rheology, Principles, Measurements, and Applications. VCHPublishers, Inc., Cambridge, UK.

Mashimo, S., S. Kuwabara, S. Yagihara and K. Higasi (1997). Dielectric relaxationtime and structure of bound water in biological materials. J. Phys. Chem. 91:6337–6338.

McGlynn, W.G., D.R. Davis and F. Honarmand (1993). Gluconic acid influences textureand color of canned asparagus. J. Food Sci. 58:614–615.

Metaxas, A.C. and R.J. Meredith (1993). Industrial Microwave Heating. Peter PeregrinusLtd., London.

Microdry (1998). Website: http://www.microdry.com/micro5.htm.

Mudgett, R.E., S.A. Goldblith, D.I.C. Wang and W.B. Westphal (1977). Prediction ofdielectric properties in solid foods of high moisture content at ultrahigh andmicrowave frequencies. J. Food Proc. Preserv. 1:119–151.

Mudgett, R.E. (1985). Dielectric properties of foods, In Microwaves in the Food ProcessingIndustry (ed.) R.V. Decareau. Academic Press, Inc., New York.

Mudgett, R.E. (1986). Electric Properties of Foods. In Engineering Properties of Foods(eds.) M.A. Rao and S.S.H. Rizvi. Marcel Dekker, Inc., New York.

Mudgett, R.E. (1989). Microwave food processing — A scientific status summary bythe IFT expert panel on the food safety and nutrition. Food Tech. 43(1):117–126.

abe1-chap01.p65 03/20/2002, 4:03 PM42



Nelson, S.O. (1973). Electrical properties of agricultural products (a critical review).A presentation at the 1993 Annual Meeting of the ASAE. ASAE, St. Joseph,Michigan.

Nelson, S.O., W.R. Forbus Jr. and K.C. Lawrence (1993). Microwave permitivities offresh fruits and vegetables. Paper No. 933079. ASAE 1993 Ann. Meeting, ASAE,St. Joseph, Michigan.

Nelson, S.O. and W. Kuang (1997). Dielectric relaxation of fresh fruits and vegetablesat microwave frequencies. Paper No. 976060, ASAE 1997 Ann. Meeting, ASAE,St. Joseph, Michigan.

Ohlsson, T. (1987). Sterilization of foods by microwaves. Presented at Int. Sem. NewTrends in Aseptic Processing and Packaging of Foodstuffs. Munich, October 22–23.

Ohlsson, T. (1989). Dielectric properties and microwave processing. In Food Propertiesand Computer-Aided Engineering of Food Processing Systems (eds.) R.P. Single andA.G. Medina. Kluwer Academic Publishers, Dordrecht, The Netherlands.

O’Meara, J.P., D.F. Farkas and C.K. Wadsworth (1977). Flexible pouch sterilizationusing combined microwave-hot water hold simulator. Contact No. (PN)DRXNM77-120, US Army Natick Research and Development Laboratories, Natick, MA01760 (unpublished report).

Prakash, A., H.-J. Kim and I.A. Taub (1997). Assessment of microwave sterilization offoods using intrinsic chemical markers. J. Microwave Power Electromagnetic Energy32(1):50–57.

Roebuck, B.D. and S.A. Goldblith (1972). Dielectric properties of carbohydrate-watermixtures at microwave frequencies. J. Food Sci. 37:199–204.

Sadiku, M.N.O. (1995). Electromagnetics. Oxford University Press, Oxford, UK.

Seaman, R. and J. Seals (1991). Fruit pulp and skin dielectric properties for 150 MHzto 6400 MHz. J. Microwave Power Electromagnetic Energy 26(2):72–81.

Schiffmann, R.F. (1986). Food product development for microwave processing. FoodTech. 40(6):94 –98.

Schiffmann, R.F. (1992). Microwave processing in the U.S. food industry. Food Tech.46(12):50–56.

Schiffmann, R. (1995). Guidelines for the adoption of industrial microwave processes.30th Microwave Power Symp. Procs., International Microwave Power Institute.

Schlegel, W. (1992). Commercial pasteurization and sterilization of food products usingmicrowave technology. Food Tech. 46(12):62–63.

Soriano, V., C. Devece and E. de los Reyes (1998). A finite element and finite differenceformulation for microwave heating laminar material. J. Microwave PowerElectromagnetic Energy 33(2):67–76.

Stenstrom, L.A. (1974). Heating of products in electromagnetic field. US patent3.809.845.

abe1-chap01.p65 03/20/2002, 4:03 PM43



Sun, E., A. Datta and S. Lobo (1995). Composition-based prediction of dielectricproperties of foods. J. Microwave Power Electromagnetic Energy 30(4):205–212.

Sundberg, M., P. Kildal and T. Ohlsson (1998). Moment method analysis of a microwavetunnel oven. J. Microwave Power Electromagnetic Energy 33(1):36–48.

Trump, J.G. (1954). Dielectric materials and their applications, In Dielectric Propertiesand Waves (e.d.) A.R. Von Hippel. John Wiley, New York.

van Remmen, H.H.J., C.T. Ponne, H.H. Nijhuis, P.V. Bartels, P.V. Bartel and P.J.A.M.Kerkhof (1996). Microwave heating distribution in slabs, spheres, and cylinderswith relation to food processing. J. Food Sci. 61(6):1105–1117.

von Hippel, A.R. (1954). Dielectric Properties and Waves. John Wiley, New York.

abe1-chap01.p65 03/20/2002, 4:03 PM44

Documents

MICROWAVE HEATING IN FOOD PROCESSING - Capítulo de livro