principles of microwave combination heating

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Principles of Microwave Combination HeatingA.K. Datta and V. Rakesh

Abstract: Faster, automated cooking of foods with improved quality can be realistically achieved only by a combi-nation of heating modes such as microwave, infrared, and hot air, which, by themselves, have limitations. Combiningheating modes poses many technical challenges. To meet these challenges, comprehensive understanding of microwavecombination heating is needed. This article is the synthesis of the most fundamental-based approaches (theoretical andexperimental) in an attempt to provide the most succinctly said principles that can be useful to a product or processdesigner in a very practical sense. To obtain these principles, characteristics of various individual modes of heating arediscussed and principles of combining them are deduced based on the behavior of the individual modes.

IntroductionQuality of prepared (processed) food can take a quantum leap

through development of intelligent ovens (Datta and Others 2005).For automation in such ovens to succeed, cooking process andquality development need to be understood, predicted and con-trolled. As illustrated in Figure 1, the quality of foods, like texture(for example, sogginess) or avor (such as through browning),is a multifaceted attribute resulting from chemical and physicalchanges that depend on such parameters as rates and uniformityof heating, and also moisture transport and loss which in turn de-pend, in a complex way, on process parameters such as power level,mode, and duration of heating. To have more control over quality,the oven needs to be predictable and programmable. Using severalmodes of heating simultaneously provides for an extensive degreeof control over quality (Figure 2). Each heating mode affects qual-ity parameters in a very different way. There is great potential inexploiting this difference, but it must 1st be understood.

While in industry the main reasons for considering the use of microwave energy is to accelerate the process, improve quality,reduce costs, and increase yield (Decareau 1985), at home the rea-sons for considering microwave energy is to save time, improvequality for some situations, and obtain a higher degree of automa-tion (as is evident from the newer and more automated combi-nation ovens being introduced annually, as well as long-standingindustry interests (Anonymous 1998; Datta 1998b). Although the

literature contains much information on microwave processing,both at the applied level (Decareau 1985; Bufer 1992) and at amore fundamental level (Datta and Anantheswaran 2002; Schu-bert and Regier 2005), they are often not principle-oriented.In particular, they certainly do not provide principles of com-bination heating in a succinct manner, particularly as it relatesto domestic microwave ovens. We can divide these principles

MS 20120780 Submitted 6/4/2012, Accepted 9/21/2012. Authors Datta and Rakesh are with Dept. of Biological and Environmental Engineering, Cornell Univ.,Riley-Robb Hall, Ithaca, NY 14853, U.S.A. Direct inquiries to author Datta(E-mail: [email protected] ).

into 2 parts: principles of quality development as they relate totimetemperature and timemoisture proles and principles of obtaining a desired prole from a combination of heating modes.Such relationships between quality and processing parameters areoften complex and are the prime cause for frustrations when de-siring repeatable good quality. The 1st principles are generallyunavailable and this article will focus on the physical principles of obtaining various proles using combinations of heating modes.We have tried to distill these principles from sources that in-clude primarily our recent research but also literature sourcesthat include cookbooks (Reingold and Chaback 1990), free-lance works that integrate cooking and its science (McGee 1984;Barham 2000), food science (Moore and others 1980), and spe-cialized microwave heating aspects of food engineering (Datta andAnantheswaran 2002).

For example, a combination heating cookbook, of which thereare only a few (Bowen 1991), will primarily suggest the benetsof combination heating as combining the faster heating of themicrowave ovens with the surface browning ability with hot air or the grill. One combination-heating book suggests using stan-dalone modes for many food items. For example, when boilingor steaming vegetables, microwave only heating is preferred. For meat cooking, microwave with grill is recommended (perhaps for the browning effect). There is enormous variation between man-ufacturers of 2 similar ovens in terms of how to combine modes,

such as higher temperature with lower microwave setting in one ascompared to medium temperature with higher microwave settingin another.

This article grew out of a synthesis of the most fundamental-based approaches (theoretical and experimental) to combinationheating. However, all details of modeling and experimentationare removed in a conscious way, in an attempt to provide themost succinct principles that can be useful to a product or processdesigner in a very practical sense. Even though microwave com-bination heating is used both in home kitchens and in industry,and the same principles should be applicable in both cases, certainapplications (like drying) are emphasized in industrial processes.The scope of this article will be microwave combination ovens

24 ComprehensiveReviewsinFoodScienceandFoodSafety Vol.12,2013c 2012 Institute of Food Technologists

doi: 10.1111/j.1541-4337.2012.00211.x


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Principles of microwave combination heating . . .

Desired QualityHeating Modes Compositionhistory

Moisturehistory

Temperaturehistory

Hot airRadiationMicrowave

Steam

ColorFlavor Texture

Power levelhistory

for each mode

Power levelfor a mode

Sequenceof modes

C o n

t r o

l o f t h e

M o

d e s

Figure 1Temperature, moisture, and composition histories (and their proles) lead to what we perceive as quality. Power level history of eachheating mode in a combination heating process can be controlled to obtain the desired quality.

MicrowaveCombinationHeating

Microwave Fast Internal

Infrared Slower Surface

Hot Air Slower Surface

Steam

Factors

Power level Wavelength range Emitter con guration Food absorption,emission

Factors

Air temperature Air velocity Food thermalconductivity, density

and speci c heat

Factors

Power level/cycling Geometry & con guration Food size, shape Food dielectricproperties

Factors

Amount of steam Steam temperature

F a c t

o r s

S e q

u e n c

i n g

L e v

e l

Figure 2Schematic of possible components of a combination heatingprocessheating modes of microwaves, infrared, hot air, and steam.Factors underneath each mode denote the controls available for thatmode.

that are typically available for home and food service use. Thus,drying applications will not be discussed at length. Although dis-cussion of microbiological growth (food safety) would parallel thesame physical principles (transport with biochemical reactions) asin the development of food quality, the kinetics of microbiologicalchanges are completely different. Discussion on safety will be con-sidered outside the scope of this article. Also, packaging can bring

Conduction Convection from ow Absorption from microwave Evaporation

Pressure driven ow Diffusion in air Evaporation as gain

Pressure driven ow Diffusion in vapor

Pressure driven ow Capillarity driven Evaporation as a loss

Water vaportransport inpores

Air transportin pore

Energy transport

Liquid water transport

Figure 3Modes of transport of energy, water, and water vapor in food asa porous medium, illustrated using a scanning electron microscope (SEM)image of bread. Most foods can be modeled as a porous medium withmulticomponent (and multiphase) transport in it during a heatingprocess. This study shows computations from several such models. Aircontributes as an inert phase in the process. Modes of transport forvarious components of liquid water, water vapor, and air are shown.

another dimension to heating, moisture transfer, and their control,as discussed by Bohrer and Brown (2001), but such effects will beexcluded from this report due to the lack of scientic studies andalso to focus on the basic heating process.

c 2012 Institute of Food Technologists Vol. 12, 2013 Comprehensive Reviews in Food Science and Food Safety 25


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Hot air

Infrared

Microwave(no heatingconcentration)

Microwave(heatingconcentration)

T e m p e r a t u r e

M o

i s t u r e

Center Surface Center Surface

A

D

C

B

Figure 4Simplied qualitative representations of temperature andmoisture proles in various individual modes of heating. These proleshave been compiled based on insight from results obtained in detailedcomputation or experimentationsome of which are presentedelsewhere in this article. For example, Figure 10 shows how highertemperatures due to heating concentration in microwave ovens(corresponding to gure d above) are combined with more surfaceheating (corresponding to gure a above).

Primer on the Physics of Food Heating

The most comprehensive understanding of a food heating pro-cess can be obtained by imagining the food as a porous medium(Ni and Others 1999; Datta 2007), consisting of a solid skeletonmade up of solids such as carbohydrates and proteins and withwater absorbed in the matrix. This is illustrated in Figure 3. In thepore space, there can be any combination of liquid water, water vapor, and air. Transport of energy, liquid water, and water vapor affects the chemical and physical changes during a heating processthat we perceive as quality changes. Although there can be sev-eral complex modes of transport, they can be divided into a few.Energy is conducted into the matrix from the surface (as in hotair heating) or absorbed in the volume (as in microwave heating).As the temperature is raised, vapor pressure of water increases andthus the rate of evaporation increases taking away some of theheat. Heat is also convected due to the (pressure-driven) ow of water and water vapor, although contribution of water vapor toenergy transport is generally small. Liquid water moves by capillarypressure differences between locations due to amount of moisturepresent, temperature, or simply due to differences in the material.As evaporation increases, the increasing amount of vapor generatespressure inside the material. The magnitude of pressure dependson the balance between rate of evaporation and the ability of thefood to release this pressure, namely, its permeability. In a lesspermeable material, higher pressures develop. This pressure thencauses liquid water to ow. Water vapor diffuses through the gas(vapor plus air) lled pores. Water vapor can also ow due to thepressure developed.

Time (s)

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Figure 5Example of experimentally measured heat uxes in (A) acombination jet-impingement oven (Geedipalli and Others 2008) and (B)a combination halogen (infrared) oven (Dhall and Others 2009). Theuxes are without any addition of microwave energy.

Temperature and moisture behavior are quite different in vol-umetric heating such as by microwaves or halogen, as comparedto conventional hot air heating. In microwave heating, internalevaporation causes more signicant pressure-driven ow of mois-ture that can push a signicant amount of moisture to the surface.Depending on the porous structure of the food and airow condi-tions at the surface, this increased moisture ow toward the surfacecan make the surface soggy or it can cause the product to lose toomuch moisture, making it tough and causing it to shrink. It is pos-sible to judiciously combine conventional hot air or other modesof heating with microwave heating to obtain custom temperatureand moisture proles, but this needs careful study of the food andoven parameters and also their interactions.

Desired Food Quality as It Relates to HeatingThe aim of different cooking processes is to achieve different

temperature and moisture distributions inside the food. For exam-ple, baking leads to high surface temperatures that are needed for

26 Comprehensive Reviews in Food Science and Food Safety Vol. 12, 2013 c 2012 Institute of Food Technologists


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Wavelength,nm

N o r m a

l i z e

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1Measuredspectrum

Blackbodyspectrum,4198 K

Figure 6Radiation spectrum of the halogen source in amicrowave-halogen combination oven with blackbody (theoretical idealemitter) spectrum at 4198 K superimposed on it for contrast (Dhall andOthers 2009). The signicant part of the radiation in case of halogen is inthe near-infrared range.

surface browning. Quality is multifaceted and it is not easily de-ned. For example, 5 factors are identied in one report (Samueland Lovingood 1986) to describe the quality of the nal product:brownness, density, moisture content, drip loss, and volume index.It is certainly difcult to nd an engineering denition of desiredquality since quality involves not only the timetemperature his-tory but also the kinetics of complex reactions involving color and avor changes. Since timetemperature and timemoisturehistories vary throughout a food material (surface is always hotter than the inside in hot air heating), quality as we perceive it isa composite of local qualities that vary throughout the material(Thussu and Datta 2011). This section attempts to elaborate someof the physical and chemical changes that make up desired foodqualitychemical reactions that lead to color and avor and phys-ical changes. Good examples about quality and its relationship tophysical principles of cooking can be found in at least 2 generalinterest books on the science of cooking (McGee 1984; Barham2000).

Important chemical reactions during cookingChemical reactions such as enzymatic reactions, caramelization,

and specically the Maillard reactions help develop avors dur-ing cooking (Yaylayan and Roberts 2001). These reactions causebrowning in the food and may be desirable or undesirable. Foodscontain enzymes and the natural chemical reactions in the fooddue to the presence of these enzymes are called enzymatic reac-tions. The more relevant reactions in the context of this study thatlead to browning due to heating are caramelization and Maillardreactions.

Caramelization. Caramelization is one of the most importanttypes of browning processes in foods. It is needed to obtain de-sirable color and avor in various food processes. The process of caramelization may also lead to undesirable effects such as burnedsugar odor andblackening. Caramelization occurs during dry heat-ing and roasting of foods with a high concentration of carbohy-drates and is due to oxidation of sugars in the food. Caramelization

is temperature-dependent and starts at relatively high temperaturesas compared to the other browning reactions. The temperatureneededfor caramelization depends on the types of sugars present inthe food ( http://www.food-info.net/uk/colour/maillard.htm ).

Maillard reactions. Maillard reactions are a set of reactions thatoccur, more strongly on heating, between amino acids and reduc-ing sugars. Different types of food have a distinctive set of avor compounds that are formed during the Maillard reaction. Maillardreactions are important in baking, frying, or other food processesthat involve heating. Maillard reactions are partly responsible for the avor in most of such food products as bread, cookies, cakes,meat, beer, chocolate, popcorn, and cooked rice. In many cases,such as in roasted coffee beans, the avor is a combination of Maillard reactions and caramelization. Although caramelizationonly takes place above 120150 C, Maillard reactions can alsooccur at room temperature. Some of the Maillard end productsmay be toxic or carcinogenic. One example of such a product isacrylamide, a potentially toxic compound that is formed at temper-atures above 180 C, especially in baked or fried products (Frenchfries). However, studies have also shown that acrylamide may beformed at 100 C and very high temperatures are not necessary(Mottram and others 2002).

Important physical changes due to heatingImportant physical changes include changes in a commonly

known parameter called texture. Texture itself is a difcult, mul-tifaceted concept that is within the realm of psychorheologyand texture should therefore be assessed by taste panels. Tex-tural attributes such as hardness and softness can be related tothe engineering quantity known as Youngs modulus, but in nostraightforward manner. The structural elements in the food thatcontribute to texture, such as cell walls, colloidal particles, andbiopolymer networks, change due to temperature, moisture, andchemical composition, thus changing texture. Some of the mostobvious effects of heating on texture are thermal softening due toheating and the increase in cr ispiness due to moisture loss.

Few Common Food Processes Relevant to MicrowaveCombination Heating

Of the large number of food processes that are possible, a fewcommon operations are amenable to automation, the ultimate goalof combination heating. Following is a brief overview of some of these: broiling, baking, steaming, and simple reheating. Each typeof heating is divided into dry heat and wet heat (Moore and Others1980).

Broiling and roastingBroiling is the process of cooking food with high heat applied

directly to the food, most commonly from above. Heat transfer tothe food is primarily through radiation. The temperature of theoven walls is typically around 500 F (260 C) during broiling,but the direct heat ux is perhaps one of the highest possible. Theproblem associated with broiling is that the surface of the food isheated at a much faster rate (due to radiation) compared to the in-terior of the food, which is heated by conduction. Greater heatingat the food surface leads to the desirable browning. However, itneeds to be ensured that the food interior is also heated adequatelywhile broiling.

Roasting is cooking food using dry heat from an open ame,oven, or by using another heat source. Broiling, described above,has been referred to as the modern, more controlled version of roasting (McGee 1984). Like in broiling, the food needs to be



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Figure 7Example of electric elds inside a microwave oven cavity across different sections. The color red represents high electric elds and bluerepresents lower values. Plots are for the same oven as in Figure 10, details are provided by Rakesh and Others (2012).

turned frequently during the process. The heat during roastingis transferred to the food mostly by conduction. Roasting causescaramelization on the food surface.

BakingBaking is the process of prolonged cooking of food by dry

heat inside an oven. The food during baking is heated due to acombination of radiation from surrounding heating surfaces andconvection from the surrounding air. Among other factors suchas ratio of ingredients and sequence in which the ingredientsshould be mixed, the optimal time and temperature for bakinghave been designated as most critical in the 4 cardinal rules of baking (Masi and Carlos 2007). The typical baking temperaturesare above the boiling point of water and in the range of 150to 260 C (McGee 1984). Temperature during baking is impor-tant as it plays a critical role in caramelization and nal moisturecontent in the product during the process. The high tempera-ture used in baking dehydrates the surface of the food causingbrowning.

The 5 factors that affect the changes during baking are heat,moisture content, enzyme activity, and changes in starch and pro-tein contents and structure (Yin and Walker 1995). The mostimportant physical and biochemical changes that occur duringbaking are volume increase, structural uidity, moisture removal,crumb resilience, and crust formation (Yin and Walker 1995).

SteamingSteaming is a method of cooking involving application of moist

heat using steam. Variants of steaming are boiling and stewing.Here the temperature is likely to stay around 100 C (if not pres-surized), substantially lower than in broiling or baking. As a result,the caramelization and Maillard browning reactions mentionedabove cannot happen in a steaming process. Condensing steam

provides much higher heat uxes than when using hot water (fur-ther discussed later). During steaming, thermal softening occursand the moisture loss should be low.

ReheatingReheating refers to heating of previously cooked food. The

primary aim of reheating is to heat the food uniformly.

Various Modes of Heating and the Parameters In-volved

Various modes of heating are summarized here to help focusattention to similarities and differences between them.

Conventional hot air jet impingement heatingHot air is perhaps the most common type of heating. Here

heating is from the surface of the food, which stays the hottest atany time, compared to any interior location. Figure 4A illustratesthe temperature prole in the food during hot air heating, withthe highest temperatures at the surface and progressively lower temperatures inside. Rate of heating is decided by the ambient uidtemperature, transfer coefcient at the surface, and the thermalproperties of the food. Thermal properties of food include thermalconductivity, density, and specic heat. Thermal conductivity of the food is the lowest for a dry, porous food and highest for a frozenfood having large amounts of water. Jet impingement heating, aswell described by Wahlby and others (2000), is a special form of hot air heating in which jets of hot air at high velocities impingeon the product for faster heat transfer. An example of heat ux ina jet-impingement oven is shown in Figure 5A.

The surrounding medium (typically air) is at a high enoughtemperature near the surface causing rapid evaporation. Pressure-driven ow is small initially from lower internal temperatures dueto lack of heat penetration. Thus evaporation at the surface can



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Steamoutlet

Waterreservoir

Hot air jetot air jet

Hot airot airGrillrill

Halogenalogen

Inductionheating

A

E

C D

B

Figure 8Examples of combination ovens: (A) microwave plus infrared (halogen), GE Advantium oven Model SCA2000BBB 03; (B) microwave plus hotair jet, showing the openings at the bottom for air (Model CJ302UB, technology licensed from Enersyst Development Center, Dallas, TX, USA); (C)microwave plus infrared plus hot air fan (Prole Trivection, Model no. JT930BHBB, General Electric Co., Louisville, KY, USA.); (D) microwave oven withsteam heating (Sharp 2011); (E) schematic from patent literature of microwave plus induction heating (LG Electronics 1997)

more than match the water transport due to capillarity and smallamounts of pressure-driven ow, and there is no accumulation of moisture at the surface initially. This remains true even at later times when the internal temperatures (and thus pressures) have in-creased since pressure-driven ow is still small. Thus conventionalheating can nicely contribute to surface crispness, as desired inmany baked and fried foods.

Radiative heatingRadiative heating uses electromagnetic waves. In the context

of food processing, the entire range of infrared electromagneticwaves is typically further divided into near-infrared (0.753 m),mid-infrared (325 m), and far-infrared (251000 m) regions.

Infrared heatingof foods is used in processes such as drying, baking,roasting, grilling, and reheating. When electromagnetic radiation,such as infrared, strikes a surface, part of it is reected, part of it isabsorbed, and the remaining, if any, is transmitted. The wavelengthof radiation incident on the food depends on the emission char-acteristics of the source of the radiation. An example of spectraldistribution of infrared radiation in a microwave-halogen combi-nation oven is seen in Figure 6, showing much of the radiationin the near-infrared range. As the infrared radiation penetrates, itsenergy level drops exponentially. Penetration depth, dened as thedistance over which the ux has dropped to 37% of its value at thesurface, is one of the critical heating parameters as it determinesthe spatial variation in the rate of infrared heating. Penetration



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Heating from variousmodes, leading totemperature changes

Evaporation andmoisture changes

Dimension changes(Shrinkage/swelling)

Dielectric propertychanges

Biochemicalchanges

Modifies microwaveenergy absorption

Modifies latent heatand sensible heatcontributions

Modifies distancefor heat or masstransport

Thermophysicalpropertychanges

Modifies heat andmass transport

Figure 9Schematic showing coupling of different types of physics withheat transfer. The connecting solid lines stand for coupling due totemperature itself, whereas the dashed lines stand for additional couplingthat can arise in a heating process suchas moisture loss.

depth depends on the food composition (and its changes duringprocessing) and the source of infrared radiation (see Datta andAlmeida 2005 for more information). Penetration depth in foodsis typically of the order of 110 mm.

When radiant heat does not penetrate sufciently, its effect issomewhat like hot air heating (Figure 4b), except the heat uxesare higher. In this case, radiative heating can cause the surface to

lose moisture and eventually increase the temperature to the levelswhere color and avor developments take place. An example of heat ux from a halogen lamp in a combination oven is shownin Figure 5B. However, when radiative heating has signicantpenetration, as with halogen, it can mimic microwave heating andthus the rules for microwave heating (without any focusing effect)will start to apply in that moisture accumulation at the food surfacemight actually increase (Datta and Ni 2000).

Microwave heatingFundamentally, microwave heating is volumetric and nonuni-

form. Inside the metallic walls of a microwave oven, microwavesform resonant patterns that are regions of high-and-low intensityelectric elds, as illustrated in Figure 7 for a particular oven. This isone factor leading to hot and cold spots in the food being heated.As the waves penetrate a material that absorbs microwave energy,less energy remains to penetrate further. Thus, energy absorptionis nonuniform. The shape, size, and properties of the load, as wellas the design of the microwave oven, complicate this scenario for energy absorption, but nonuniformity remains the rule. Its char-acteristics compare to those of conventional heating as follows:

(1) Its quick, the rates of heating are much higher than in con-

ventional heating.(2) Its generally more nonuniform than conventional heating.(3) Its selective, moist areas heat more than the dry areas. Such

selectivity is absent in conventional heating.(4) Unlike conventional heating, signicant internal evapora-

tion inside the microwave-heated material leads to additionalmechanisms of moisture transport that enhance moisture lossduring heating.

(5) It can be turned on or off instantly, unlike conventionalheating.

Oven factors that lead to heating rate and nonuniformity of heating include placement inside the oven (Rakesh and oth-ers 2010), effect of oven size and geometry, use of turntables

0 10 20 30 40 50 60 70 80

0 10 20 30 40 50 60 70 Temp (C)

Convection and radiant heating

Microwave, convection and radiant heating

Measured

Computed

Measured

Computed

Temp (C)

Figure 10Temperature maps comparing the measured (using MRI) and computed values at different slices of a cylindrical food analog after 10 min of heating in convection and radiant heating without and with microwaves. The temperature maps demonstrate 2 distinct features of heating: surfaceheating for combination hot air and infrared heating (top gure) and heating at the interior locations due to volumetric heating and focusing effect of the microwaves in addition to surface heating for combination microwave (cycled), hot air, and infrared heating (bottom gure). The oven is fromProle Trivection, Model no. JT930BHBB, General Electric Co., Louisville, KY, USA. Further details of heating are provided by Rakesh and Others(2012). The gure is reproduced with permission, also from the work of Rakesh and Others (2012).



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Time (seconds)0 20 40 60 80 100 120

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jet impingement heating

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Figure 11Numerically calculated and experimentally measured temperatures in microwave, jet impingement, and combination heating. The locationfor microwave-only and combination heating are hot spots whereas the location for jet impingement is just below the surface of the food. The ovenmodel is CJ302UB, technology licensed from Enersyst Development Center, Dallas, TX, USA. Brick-shaped food (potato) dimensions are 0.047 m 0.022 m 0.036 m. Further details of heating are provided by Geedipalli and Others (2008).

(Geedipalli and others 2007), oven power, use of power cycling,use of mode stirrers, and effect of feed location (Zhang and Datta2003). Food factors that affect the energy absorption are food di-electric properties, food volume, food shape, and food aspect ratio(Zhang and Datta 2001). Nonuniformity of energy absorption ismanifested in terms of corner and edge over-heating, focusingeffects (enhanced internal heating) due to curved surfaces of thefood, and resonance inside the food (where the food itself be-haves like a cavity, with regions of high and low electric elds).These factors have led to microwaves often being used only for reheating of food rather than actual cooking. Heating uniformitywithin a microwave oven chamber can be improved by using aturntable, power cycling, having a mode stirrer in the oven, andactive microwave packaging. Note that power onoff cycling isapproximately equivalent to using continuous power at an averagepower level that is given by the fraction of the on-time multipliedby the maximum power.

Distribution of heat inside a food is highly dependent on the sizeand dielectric properties of the food as well as characteristics of theoven. Two extremes of temperature proles in microwave heat-ing are mostly surface heating and highly focused internal heating(Figure 4C and D, respectively). Compositional and temperaturevariation in the food, present initially or developed during theheating, can contribute to spatially nonuniform energy absorption.For example, in microwave thawing, the outside layer will thaw1st since the outside typically absorbs more energy. Once thematerial thaws, its energy absorption (dielectric loss) increasestremendously. This thawed outer layer essentially shields muchof the microwave energy, and heating rates inside drop signi-cantly. Nonuniformity of heating due to temperature variationduring heating is also evident in salty foods where the microwaveabsorption (a dielectric property) increases with temperature. Ina drying process, typically more moisture is lost from the outer regions. The microwave absorption in the drier outer regions will



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Microwave Jet Impingement Combination

Figure 12Average temperature rise (patterns) and nonuniformity (solidregions) in heating after 2 min of heating under different heating modesin the microwave-hot air jet combination oven shown in Figure 8B, detailsare in Datta and Others (2005). Difference-over-rise is dened as theratio of the difference between the 10th and 90th percentile values of temperature to the mean rise in temperature. This is similar to coefcient

of variance and is considered an improved representation of nonuniformity.

reduce subsequently, and the microwaves will be preferentially ab-sorbed in the wetter regions, leading to more efcient evaporationof the moisture (faster drying)a very desirable situation.

In microwave heating, the surrounding medium (air) is at roomtemperature as it is not directly heated by microwave energy. Thus,evaporation from the surface is small. Early in the heating process,pressure pockets have not developed and thus the pressure-drivenow is small. Therefore, during the early heating process, thesurface can lose some water and there is insignicant moistureaccumulation on the surface. However, not long into the heating

process, due to the high rate of internal heating by the microwaves,internal evaporation occurs and pressure develops that cause in-creased pressure-driven ow. Since the moisture removal capacityof the surrounding unheated air is small, it is eventually overpow-ered by the moisture transport to the surface from inside due toincreased pressure-driven ow, and moisture accumulates at thefood surface.

Induction heatingInduction heating is part of electromagnetic heating. Unlike

microwave heating in induction heating, it is the cookware that isheated 1st. The cookware needs to be made of magnetic material(steel, iron, nickel, or various alloys). In one oven model whereinduction is combined with microwave heating (discussed later),below the smooth glass, coils produce a high-frequency alternatingmagnetic eld, which heats the cookware placed on the glass.Molecules in the cookware are excited by the alternating magneticeld, causing the cookware to become hot and cook the food.The cooktops glassceramic surface is unaffected by the magneticeld since it contains no magnetic material, only the heat of thecookware warms the glass heating surface.

Steam heatingAddition of steam inside a microwave oven has become available

(Sharp 2011). No research article could be located on microwave-steam combination heating. In steam heating, the (surface) tem-peratures involved are likely to be lower than in dry heating (with

Time (s)

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40/60

30/60

50/60

60/60

M e a s u r e o

f N o n - u

n i f o r m

i t y

Time (s)

A

B

Figure 13(A) Average temperature rise and (B) nonuniformity forvarious cycling of microwave power in a microwave-hot air combinationheating with the goal of reaching a temperature of 40 C. The oven airtemperature is 80 C and the heat transfer coefcients are 22 W / m 2 K atthe top and 20 W / m 2 K at the side. The measure of nonuniformity isdened in terms of deviation from 40 C, details are in Rakesh and Others(2010). The notation 10/60 denotes microwaves being on for 10 s andoff for 60 s, which can be interpreted as an average power level of 1/6the full power.

hot air) or the other cooking techniques mentioned above. Heat

ux due to steam heating is not readily available (Braud andOthers 2001). A range of heat transfer coefcients due to lmcondensation of water vapor is provided (Michailidis and oth-ers 2009): 50006700 W / m2 K. Heat ux, obtained from q =h c(T sat T s) (see, for example, Sa-adchom and others 2011) whereq is ux, h c is the heat transfer coefcient due to condensation,T sat is the saturation temperature of water, and T s is the surface tem-perature; and that would put ux values perhaps in the vicinity of 100000 W / m2, a value signicantly higher than infrared or hot air.

Combination Heating AppliancesMicrowave combination heating appliances currently in use can

be grouped into:



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Time (s)

A v e r a g e

t e m p e r a

t u r e

( o C )

0 100 200 300

25

30

35

40

45

50

55

60

65

70

75

80

B- Center, 10/60,h = 40, T = 100

D- Back, 10/60,h = 40, T = 100

A- Center, 10/60,h = 20, T = 100

C- Back, 10/60,h = 20, T = 100

F- Top, 10/60,h = 40, T = 100

E- Top, 10/60,h = 20, T = 100

Time (s)

M e a s u r e o

f N o n - u n

i f o r m

i t y

0 100 200 300

2.0E-04

3.0E-04

4.0E-04

5.0E-04

6.0E-04

7.0E-04

8.0E-04

9.0E-04

1.0E-03

A

B

C

D

E

F

A

B

Figure 14(A) Average temperature rise and (B) heating nonuniformityfor different combinations of heating as shown in the gure. In the gure,units of heat transfer coefcient and temperature are W/m 2 K and C,respectively. The oven air temperature is 100 C for all cases. The heattransfer coefcients at all surfaces of the food are 20 W / m 2 K forCombinations A, C, and E, and 40 W / m 2 K for B, D, and F. ForCombinations A and B, the food is placed at the center; for C and D, thefoodis placed10 cmtoward the backof the oven; and for E and F,thefoodis placed 10 cm towards the top of the oven. The notation 10/60 denotesmicrowaves being on for 10 s and off for 60 s. The goal is to obtain a naltemperature between 45 and 55 C for a heating time of 5 min. Themeasure of nonuniformity is dened in terms of deviation from thistemperature range. Combination F results in the fastest heating, whereasCombination C leads to the least nonuniformity in heating. Combination Eis the optimum combination for the process providing a fast heating rateand small nonuniformity in heating. The computations were performedusing the computational setup presented by Rakesh and Others (2010).

(1) microwave only,(2) microwave with infrared (halogen lamp or grill),(3) microwave with hot air (jet or otherwise),(4) microwave with steam,(5) microwave with induction heating,(6) various combinations of the above.

In microwaves with infrared heating, a source of infrared heatis provided inside the oven, using halogen lamps or heated rods,as shown in Figure 8A and C, respectively. In microwaves with

3 min

2 min1 min

0 min

Sample

LeftDistance

Figure 15Heating prole at different times along a horizontal line at thecenter of the sample (as shown in the gure) for combination hot air andinfrared heating. The oven air temperature and heat transfer coefcientare 110 C and 20 W / m 2 K , respectively. At a particular time,temperatures increase toward the surface, as would be characteristic of such heating. Additional computational details can be found in Rakeshand Others (2010).

hot air, typically forced hot air is provided that emulates simulta-neous hot air heating. Note that airow in a typical microwaveoven without forced air is low, leading to very low rates of heatand moisture transfer (Verboven and others 2003). In jet impinge-ment technology, air jets of much higher velocities impinge ona food surface, increasing the rates of heat and moisture trans-port (see Figure 8B). Microwaves with grill or a source of in-frared are sometimes combined with hot air as well, an exampleof which is shown in Figure 8C. Microwaves with steam is a veryrecent featurehere steam is generated and fed to the oven cavity(Figure 8D). In one of the few models available with microwavesand steam, as of this writing, steam appears to be used not simulta-neously with microwaves, but as a mode available in the same ap-pliance. It should be possible to design combinations that sequencemicrowave and steam in such an appliance. As no known scien-tic study exists, combination of microwaves with steam will notbe discussed further. However, producing secondary steam withheat from microwaves within a package or a food container placedinside a microwave oven is well known and several patents existin this area (Matsuba 2011; Unwin 2011). This also has not beenreported in the research literature. Finally, a microwave oven incombination with an induction-heating cooker has been reportedin the patent literature (Figure 8e). A shielding plate is mountedon the bottom surface and an induction coil is provided below the

shielding plate in order to selectively choose between microwaveand/or induction cooking (transmit a high-frequency magneticeld). As no known commercially available unit or its scienticstudy exists, this combination will also not be discussed further.

Coupling of Physics in Combination HeatingWhether intended or not, temperature increases due to heating

lead to other physical changes in the food material, such as mois-ture loss, and together they result in changed dielectric, thermal,or moisture transport properties (see Figure 9). These changes, inturn, inuence heat and moisture transport due to any of the indi-vidual heating modes. During combination heating, not only arethe heat transfer, moisture transfer, and electromagnetics coupled



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20

25

30

35

40

45

50

55

60

0 200 400 600 800 1000 1200 1400

T e m p e r a

t u r e

( o C )

Time (s)

High MCLow MC

Convection and radiant

Cycled microwave,convection and radiant

Full microwave,convection and radiant

74

76

78

80

82

84

0 200 400 600 800 1000 1200 1400

M o i s

t u r e c o n

t e n

t ( %

w b )

Time (s)


Cycled microwave,convection an d radiant


32

34

36

38

40

42

0 200 400 600 800 1000 1200 1400

M o i s

t u r e c o n

t e n

t ( %

w b )

Time (s)


Cycled microwave,convection an d radiant


High moisture Low moisture

A

B C

Figure 16Computed (A) average temperature history; (B) average moisture histories for high-moisture food (4.6 dry basis); and (C) average moisturehistories for low-moisture food (2.3 dry basis) heated using 3 different combinations of heating modes. There is a marked increase (more than 2-fold)in moisture loss in the low-initial-moisture food compared to the high-initial-moisture food (15.4%, 17.5%, and 22.1% lost for low moisture and 7%,7.6%, and 9.4% lost for high moisture, respectively, for the 3 combinations). This is due to higher average temperatures reached in lower moisturematerial for all combinations of heating modes and for the same oven and food as in Figure 10 (Rakesh and others 2012).

together, contributions from individual modes of heating arealso coupled. For example, a microwave oven provides volume-

heating, whereas a hot air oven heats the food surface. One impor-tant aspect of the coupling of physics is whether the coupling existsas one-way or both ways. An example of one-way coupling can bemicrowave heating where the dielectric properties do not changewith temperature, for example, in heating of raw potatoes over smaller changes in temperature; whereas electromagnetic heat-ing leads to temperature changes and, therefore, is coupled withheat transfer; if the dielectric properties are unchanged, temper-ature changes do not affect the electromagnetics. Another exam-ple of one-way coupling can be biochemical and microbiologicalchanges (quality aspects) that are affected by temperature but thesechanges do not inuence the heat transfer process. On the other hand, if the dielectric properties change signicantly with temper-ature or moisture, the electromagnetic aspect is affected by heatand mass transfer and the coupling is two-way, as during thawing.

Studying Combination HeatingAlthough a number of experimental studies of combination

heating exist (Riva and others 1991; Li and Walker 1996; Renand Chen 2000; McMinn and Others 2005; Sumnu and Others2005, 2007), they primarily focus on point measurements for tem-perature and overall moisture loss measurements. Many studies alsoexist that focus on the nal food quality in microwave combinationheating, without the details of temperature and moisture informa-tion (Yin and Walker 1995; Sakiyan and Others 2011). Obtaining

proles (spatial variations) for temperature is difcult but possiblewith an infrared camera, while for moisture, obtaining proles

has been nearly impossible, except using the complex and veryinvolved process using magnetic resonance imaging (MRI). Anexample of temperature proles using MRI is shown in Figure 10.

Complexities of mathematical models to study combinationheating range from simple transport models that assume simpledistribution of microwave energy (Khraisheh and Others 1997; Jumah and Raghavan 2001) to multiphase transport models of water and energy in porous media, with simple solution usingMaxwells equations (Marra and others 2010) to combined solu-tionusing Maxwells equations for electromagnetics and heat trans-fer (Wappling-Raaholt and Others 2002) to perhaps the most com-plex model to date of multiphase transport in a porous mediumwith complete solution using the Maxwells equations in a mi-crowave cavity with two-way coupling between electromagneticsand heat and moisture transfer (Rakesh and Others 2012). Infraredheating inside an oven has also been modeled comprehensively,providing infrared heat ux on a food surface (Dhall and Others2009), but such a model has not been coupled with microwaveheating, with the exception of perhaps the model by Haala andWisebeck (2000). Some models exist for heating with superheatedsteam alone (Sa-adchom and others 2011) but not coupled withmicrowave heating. Detailed models are the primary means bywhich one can obtain distributions of temperature and moistureand how change occurs with time (see the Principles of Com-bining Modes of Heating section): transient measurements of inside temperature and moisture are extremely difcult. Also, the



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Temperature (C)

Convection and radiantCycled microwave,

convection and radiantFull microwave,

convection and radiant

Moisturecontent (% wb)




A

B

Figure 17Computed (A) spatial temperature and (B) moisture maps at various sections of a cylindrical high-moisture food analog (4.6 dry basis) after20 min of heating. Plots are for the same oven and food as in Figure 10 (Rakesh and others 2012).

detailed models that can separate the contributions from variousmodes of heating are ideal for relating the precise effects of variousmodes of heatingsomething not possible with the simpler mod-els that lump various transport processes. The focus of this articleis the synthesis of what has been learned from detailed modelsand experiments. Due to the review nature of this article, theexperiments or the models will not be elaborated on further, butthe reader will be referred to the detailed studies at appropriateplaces.

Principles of Combining Modes of HeatingAs combination heating is affected by all the food-related and

equipment related factors (Figure 2) that affect individual modesof heating, illumination of all the principles will not come easy.Even the principles of heating in a microwave oven only are hardto come by (Dodson 2001). This article is an attempt, using thedominant characteristics of each type of heating, to develop someguidelines for combining various heating modes.

Goals (when we say that we have reached the right combina-tion) are not dened in engineering terms but whatever leads tothe right quality, as has been mentioned earlier. In more com-plex processes, such as microwave pufng, goals are even harder todescribe in terms of temperature and moisture proles. In general,coupling, discussed in relation to Figure 9, increases complexi-ties in microwave combination heating. In heating multicompo-

nent foods, such as a multicompartment dinner (Zhang and Datta2003), for example, relative contributions of microwave and other modes of heating on one component in comparison to another can change during heating.

Thus, for engineering purposes, perhaps we can have 3 goals incombining modes of heating that cover a large number of uses for combination heating:

(1) Goal A: achieve rapid heating.(2) Goal B: achieve desired temperature prole.(3) Goal C: achieve desired moisture prole.

Combining modes to achieve rapid heating (Goal A)At any instant during heating, microwave heating is additive

with other modes of heating, meaning that the contribution of microwave heat and another mode of heating are independent of each other and can be added. This can be seen in Figure 11 wherethe transient temperature increase in Figure 11C is close to be-ing the summation of increases in Figures 11A and B. The stair stepping of the temperature prole in microwave only heating andin combination heating occurs due to cycling of the microwaves.Figure 12 shows how the speed of heating, dened as the rate of increase of average temperature of the food with time, increaseswhen modes are combined. Over the duration of heating, how-ever, the contribution from individual modes can change (theyare still additive) due to the coupling effects of heat and moisturetransfer with properties.



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Temperature (C)




Moisturecontent (% wb)




A

B

Figure 18(A) Spatial temperature and (B) moisture maps at various sections of a cylindrical low moisture food analog (2.3 dry basis) after 20 min of heating. Plots are for the same oven and food as in Figure 10 (Rakesh and others 2012).

Thus, with the microwave mode being typically additive, timeto reach the desired temperature proles will depend on the con-tribution from each of the modespower level (or equivalentpower level in the case of cycling of the microwaves), hot air tem-perature (in the case of convection heating), and heat ux (in thecase of radiant heating).

Combining modes to achieve desired temperature proles(Goal B)

Desired temperature proles in food can be broadly divided into2 categories:

(1) Uniform temperature, as in most cooking operations,(2) temperature that increases near the surface, often to obtainreduced surface moisture.

Combination of heating modes can potentially provide de-sired temperature proles for specic food processes. This canbe seen using conceptual plots of temperature proles for in-dividual heating modes, as in Figure 4. Assuming contributionsdue to 2 heating modes are additive, when we combine 2 some-what complementary temperature proles, we can predict ap-proximately what the combined temperature prole will looklike.

Obtaining uniform temperature. In microwave heating situa-tions where heating concentrations occur inside the food (see

Desired Food Quality as It Relates to Heating section), hotair or infrared addition can make the resulting temperature pro-les more uniform. Figure 10 illustrates this; here the signicantlyhigher internal temperatures near the center when microwavesare present would make the heating much more nonuniform if the other modes of heating (convection plus radiant) were absent.Another typical nonuniformity in microwave only heating is thatit leads to a colder surface due to unheated surrounding air. Thisalso can be made more uniform by having a small extent of con-vective heating (air temperature does not have to be too warm) atthe surface so the surface is not cold. Usage of a specialized food

container (package) to enable steam produced from water heatedby the microwaves to surround the food should also reduce thenonuniformity of heating (Matsuba 2011).

Figure 12 illustrates how combining microwaves with hot air can make heating more uniform while signicantly increasing thespeed of heating. Temperature nonuniformity is quantied usingthe coefcient of variation (COV) that is the standard deviationtemperature divided by the mean temperature. Higher COV valueswould signify more nonuniformity of heating. Similar trends of faster heating and increased uniformity can be seen with other combinations of heating modes such as microwaves with infrared(Datta and others 2005), with actual changes depending on theheating parameters involved, as noted in Figure 2.



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A more detailed example of how power cycling improves theuniformity of heating in a microwavehot air combination heatingprocess is shown in Figure 13. The microwave power level tobe used is a compromise between faster heating and increaseduniformity. When the power level is high, heating is rapid butmore nonuniform, as shown by the 60/60 line. When the power level is low, as in the 10/60 line, heating is slow but more uniform.

In addition to microwave power cycling, other processing con-ditions, such as rate of hot air heating and positioning of food inthe oven, can be changed to obtain uniform temperatures duringcombination heating. For example, in Figure 14, 3 different posi-tions of the sample inside the oven along with 2 different surfaceheating rates and with a microwave onoff cycle of 10/60 weresimulated. The example demonstrates that a judicious combina-tion heating mode should be used to aim for uniform temperaturesduring combination heating as certain combinations may help infaster heating but may also lead to more nonuniformity in heating.

Obtaining higher surface temperature. To obtain a tempera-ture prole that increases toward the surface, modes that provideincreased surface heating need to be present. As illustrated inFigures 4 and 15, these would include infrared and hot air heat-ing. When the goal is to make the surface dry, steam heating,

although providing surface heating, is not a choice as it wouldmake the surface moist.

Combining modes to achieve desired moisture proles(Goal C)

Desired moisture proles in food can be divided into 3 broadcategories:

(1) keep surface crispy, that is, moisture level drops (perhapssharply) near the surface,

(2) keep moisture level uniform (typically with minimal overallloss),

(3) lose enough moisture overall, as in a drying process or per-haps as a preprocess to applications such as pufng.

Again, the conceptual plots of moisture proles in Figure 4can help us navigate in reaching a desired moisture prole bycombining modes. It must also be noted that the rate of moistureloss and the moisture proles obtained after heating depends onthe initial moisture content of the material. For example, Figure 16shows that average temperatures are higher for a drier food heatedusing the same combinations resulting in greater moisture loss bythe drier food for the same heating time.

Obtainingacrispysurface. When the goal is to make the surfacecrispy (that is, keep the moisture low or bring it down), microwavepower would need to be low and combined with a high rate of re-moval from the surface (such as by use of high surface temperatureand/or air ow rate, or the use of infrared, Datta and Ni 2000).This can be seen in Figure 17B, between cycled microwave andfull power microwave heating of a high-moisture material, wherethe full power microwave causes more accumulation of water to-ward the surface. For a low-moisture material, as shown in Figure18B, even full-power microwave heating does not lead to mois-ture accumulation at the surface, which is desirable for obtaininga crispy surface.

Keeping uniform moisture level. When the goal is minimal lossof moisture, namely to keep initial moisture levels, low-power microwaves can be used that would reduce the pressure-drivenow of moisture. In long-term heating, such as cooking, wherethe goal is not a crispy surface, it seems the goal would be tomaintain somewhat uniform temperature over time and, near theend, use radiant or hot air heat to dry and eventually brown the

surface. Use of steam, by having the food heat in a bag, can alsokeep the moisture level uniform, as has been suggested in patents(Matsuba 2011).

Lose signicant moisture. When the goal is to lose water quickly, as in a drying process, high-power microwave heatingcan produce signicant pressure-driven ow and thus loss of water out of the surface (compare moisture loss in cycled and full-power microwave heating in Figure 17B). Thus, one can go to as high of a rate of heating as feasible (depending on the materials moisturecontent, permeability, and so on) without causing explosion or any undesired pufng.

Controls available in a combination heating ovenControls available in combination heating (also shown in

Figure 2) are:(1) Power levels used for each individual mode in a combination.

For hot air heating, for example, this would mean controlof air temperature and perhaps air ow (which controls thesurface heat transfer coefcient).

(2) Sequence of the combination, such as microwaves followedby convection or vice versa.

These two controls can be alternatively viewed as having a control

over combining the power level history for each mode, as illus-trated in the bottom portion of Figure 1. Based on the discussionso far, some guidelines on power level history to achieve the goalsmentioned earlier can be provided.

Goal A. To achieve Goal A (rapid heating alone), all power levels can be at their maximum levels and simultaneous, lim-ited by available power at the wall, especially where the supplylines are 110 V. Since microwaves heat the fastest, more mi-crowave power rather than hot air or infrared would be the mostappropriate.

Goal B. For Goal B, when trying to achieve a uniform temper-ature prole while increasing the rate of heating, a low microwavepower setting from the start that does not make the heating too

long would be appropriate to add to any surface heating mode.When trying to achieve temperatures that are higher near the sur-face, primarily surface heating modes (hot air and low penetrationinfrared) can be added from the start. Provided there is no focusingeffect of microwaves for the particular geometry and properties,microwaves can also be added.

Goal C. For Goal C, to achieve a crispy surface, higher mi-crowave power can be more useful initially to raise the tempera-ture quickly (something not possible with conventional heating)with minimal effect on moisture transport since pressure-drivenow due to microwave heating increases at later times. Microwavepower can be decreased with increasing infrared/hot air to increasethe moisture removal capacity as an increasing amount of moisturereaches the surface. The same power level history would be pur-sued when the goal is surface-browning by increasing temperatureand reducing moisture. In some drying applications, microwavescan selectively heat areas of higher moisture near the core of afood materialmoisture in these areas are normally more difcultto remove in surface-heating than in hot air or infrared heating.In such situations, microwaves are added in the latter part of thedrying process, as has been known in the literature for some time(Decareau 1985).

Sensors for humidity and temperature, as are provided in manynewer ovens, can provide feedback control capabilities and makeit signicantly easier to reach the goals. Also, packaging, espe-cially active packaging, can also be used to reach the goals moreeffectively.



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It is clear from these discussions on controls that many factorswill affect the decision on power level history needed for a par-ticular product for using combination heating. In addition to thefactors just mentioned, for example, power level history wouldalso depend on the product dielectric and thermophysical proper-ties. Although the desired power level history can be theoreticallyapproached as an optimization problem (Balsa-Canto and others2005), realistically, the exact combination power level history thatworks for a particular product is obtained through experimen-tation. The intention in this study was to provide rational andfundamentals-based guidelines to reduce the amount of experi-mentation, knowing there are a myriad of factors that affect theprocess. Although preprogrammed in many of todays combina-tion heating ovens (where we only need to press one button thatselects a recipe), these histories are generally arrived at throughtrial and error by the appliance manufacturers and are proprietaryinformation to them (which made it impossible to include anexample of a power level history in use).

Summary and the FutureCombination of heating modes is the holy grail of customized

quality that can be automated. This article is an attempt to pro-

vide succinctly the principles by which they can be combined.Every food composition, shape, size, and every mode of heating,equipment design, and extent and nature of combinations willproduce different results that are impossible to summarize. On theother hand, this leads to nearly innite possibilities and thus novelcombinations will keep coming out forever. This article is simplyan attempt to make this search process a little more methodical.Other aspects not included in this study include product formu-lation (Shukla and Anantheswaran 2001) and packaging (Bohrer and Brown 2001) that will continue to play a signicant role bymodifying the food dielectric and thermophysical properties.

AcknowledgmentsThis project was supported by National Research Initiative

Grant no. 20033550313737 from the USDA Cooperative StateResearch, Education, and Extension Service Competitive Grantsprogram.

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