Faster, automated cooking of foods with improved quality can be realistically achieved only by a combination of heating modes such as microwave, infrared, and hot air, which, by themselves, have limitations. Combining heating modes poses many technical challenges. To meet these challenges, comprehensive understanding of microwave combination heating is needed. This article is the synthesis of the most fundamental-based approaches (theoretical and experimental) in an attempt to provide the most succinctly said principles that can be useful to a product or process designer in a very practical sense. To obtain these principles, characteristics of various individual modes of heating are discussed and principles of combining them are deduced based on the behavior of the individual modes.
Quality 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 and quality development need to be understood, predicted and controlled. As illustrated in Figure 1, the quality of foods, like texture (for example, sogginess) or flavor (such as through browning), is a multifaceted attribute resulting from chemical and physical changes that depend on such parameters as rates and uniformity of heating, and also moisture transport and loss which in turn depend, 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 several modes of heating simultaneously provides for an extensive degree of control over quality (Figure 2). Each heating mode affects quality parameters in a very different way. There is great potential in exploiting 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 reasons for considering microwave energy is to save time, improve quality for some situations, and obtain a higher degree of automation (as is evident from the newer and more automated combination ovens being introduced annually, as well as long-standing industry interests (Anonymous 1998; Datta 1998b). Although the literature contains much information on microwave processing, both at the applied level (Decareau 1985; Buffler 1992) and at a more fundamental level (Datta and Anantheswaran 2002; Schubert and Regier 2005), they are often not principle-oriented. In particular, they certainly do not provide principles of combination heating in a succinct manner, particularly as it relates to domestic microwave ovens. We can divide these principles into 2 parts: principles of quality development as they relate to time–temperature and time–moisture profiles and principles of obtaining a desired profile from a combination of heating modes. Such relationships between quality and processing parameters are often complex and are the prime cause for frustrations when desiring repeatable good quality. The 1st principles are generally unavailable and this article will focus on the physical principles of obtaining various profiles using combinations of heating modes. We have tried to distill these principles from sources that include primarily our recent research but also literature sources that include cookbooks (Reingold and Chaback 1990), freelance works that integrate cooking and its science (McGee 1984; Barham 2000), food science (Moore and others 1980), and specialized microwave heating aspects of food engineering (Datta and Anantheswaran 2002).
For example, a combination heating cookbook, of which there are only a few (Bowen 1991), will primarily suggest the benefits of combination heating as combining the faster heating of the microwave ovens with the surface browning ability with hot air or the grill. One combination-heating book suggests using standalone modes for many food items. For example, when boiling or 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 manufacturers of 2 similar ovens in terms of how to combine modes, such as higher temperature with lower microwave setting in one as compared to medium temperature with higher microwave setting in another.
This article grew out of a synthesis of the most fundamental-based approaches (theoretical and experimental) to combination heating. However, all details of modeling and experimentation are removed in a conscious way, in an attempt to provide the most succinct principles that can be useful to a product or process designer in a very practical sense. Even though microwave combination heating is used both in home kitchens and in industry, and the same principles should be applicable in both cases, certain applications (like drying) are emphasized in industrial processes. The scope of this article will be microwave combination ovens that are typically available for home and food service use. Thus, drying applications will not be discussed at length. Although discussion of microbiological growth (food safety) would parallel the same physical principles (transport with biochemical reactions) as in the development of food quality, the kinetics of microbiological changes are completely different. Discussion on safety will be considered outside the scope of this article. Also, packaging can bring another dimension to heating, moisture transfer, and their control, as discussed by Bohrer and Brown (2001), but such effects will be excluded from this report due to the lack of scientific studies and also to focus on the basic heating process.
Primer on the Physics of Food Heating
The most comprehensive understanding of a food heating process can be obtained by imagining the food as a porous medium (Ni and Others 1999; Datta 2007), consisting of a solid skeleton made up of solids such as carbohydrates and proteins and with water absorbed in the matrix. This is illustrated in Figure 3. In the pore 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 process that we perceive as quality changes. Although there can be several complex modes of transport, they can be divided into a few. Energy is conducted into the matrix from the surface (as in hot air heating) or absorbed in the volume (as in microwave heating). As the temperature is raised, vapor pressure of water increases and thus the rate of evaporation increases taking away some of the heat. Heat is also convected due to the (pressure-driven) flow of water and water vapor, although contribution of water vapor to energy transport is generally small. Liquid water moves by capillary pressure differences between locations due to amount of moisture present, temperature, or simply due to differences in the material. As evaporation increases, the increasing amount of vapor generates pressure inside the material. The magnitude of pressure depends on the balance between rate of evaporation and the ability of the food to release this pressure, namely, its permeability. In a less permeable material, higher pressures develop. This pressure then causes liquid water to flow. Water vapor diffuses through the gas (vapor plus air) filled pores. Water vapor can also flow due to the pressure developed.
Temperature and moisture behavior are quite different in volumetric heating such as by microwaves or halogen, as compared to conventional hot air heating. In microwave heating, internal evaporation causes more significant pressure-driven flow of moisture that can push a significant amount of moisture to the surface. Depending on the porous structure of the food and airflow conditions at the surface, this increased moisture flow toward the surface can make the surface soggy or it can cause the product to lose too much moisture, making it tough and causing it to shrink. It is possible to judiciously combine conventional hot air or other modes of heating with microwave heating to obtain custom temperature and moisture profiles, but this needs careful study of the food and oven parameters and also their interactions.
Desired Food Quality as It Relates to Heating
The aim of different cooking processes is to achieve different temperature and moisture distributions inside the food. For example, baking leads to high surface temperatures that are needed for surface browning. Quality is multifaceted and it is not easily defined. For example, 5 factors are identified in one report (Samuel and Lovingood 1986) to describe the quality of the final product: brownness, density, moisture content, drip loss, and volume index. It is certainly difficult to find an engineering definition of desired quality since quality involves not only the time–temperature history but also the kinetics of complex reactions involving color and flavor changes. Since time–temperature and time–moisture histories vary throughout a food material (surface is always hotter than the inside in hot air heating), quality as we perceive it is a composite of local qualities that vary throughout the material (Thussu and Datta 2011). This section attempts to elaborate some of the physical and chemical changes that make up desired food quality—chemical reactions that lead to color and flavor and physical changes. Good examples about quality and its relationship to physical principles of cooking can be found in at least 2 general interest books on the science of cooking (McGee 1984; Barham 2000).
Important chemical reactions during cooking
Chemical reactions such as enzymatic reactions, caramelization, and specifically the Maillard reactions help develop flavors during cooking (Yaylayan and Roberts 2001). These reactions cause browning in the food and may be desirable or undesirable. Foods contain enzymes and the natural chemical reactions in the food due to the presence of these enzymes are called enzymatic reactions. The more relevant reactions in the context of this study that lead to browning due to heating are caramelization and Maillard reactions.
Caramelization is one of the most important types of browning processes in foods. It is needed to obtain desirable color and flavor in various food processes. The process of caramelization may also lead to undesirable effects such as burned sugar odor and blackening. Caramelization occurs during dry heating and roasting of foods with a high concentration of carbohydrates and is due to oxidation of sugars in the food. Caramelization is temperature-dependent and starts at relatively high temperatures as compared to the other browning reactions. The temperature needed for caramelization depends on the types of sugars present in the food (http://www.food-info.net/uk/colour/maillard.htm).
Maillard reactions are a set of reactions that occur, more strongly on heating, between amino acids and reducing sugars. Different types of food have a distinctive set of flavor compounds that are formed during the Maillard reaction. Maillard reactions are important in baking, frying, or other food processes that involve heating. Maillard reactions are partly responsible for the flavor 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 flavor is a combination of Maillard reactions and caramelization. Although caramelization only takes place above 120–150 °C, Maillard reactions can also occur at room temperature. Some of the Maillard end products may be toxic or carcinogenic. One example of such a product is acrylamide, a potentially toxic compound that is formed at temperatures above 180 °C, especially in baked or fried products (French fries). However, studies have also shown that acrylamide may be formed at 100 °C and very high temperatures are not necessary (Mottram and others 2002).
Important physical changes due to heating
Important physical changes include changes in a commonly known parameter called texture. Texture itself is a difficult, multifaceted concept that is within the realm of “psychorheology” and texture should therefore be assessed by taste panels. Textural attributes such as hardness and softness can be related to the engineering quantity known as Young's modulus, but in no straightforward manner. The structural elements in the food that contribute to texture, such as cell walls, colloidal particles, and biopolymer networks, change due to temperature, moisture, and chemical composition, thus changing texture. Some of the most obvious effects of heating on texture are thermal softening due to heating and the increase in crispiness due to moisture loss.
Few Common Food Processes Relevant to Microwave Combination Heating
Of the large number of food processes that are possible, a few common operations are amenable to automation, the ultimate goal of combination heating. Following is a brief overview of some of these: broiling, baking, steaming, and simple reheating. Each type of heating is divided into dry heat and wet heat (Moore and Others 1980).
Broiling and roasting
Broiling is the process of cooking food with high heat applied directly to the food, most commonly from above. Heat transfer to the food is primarily through radiation. The temperature of the oven walls is typically around 500 °F (260 °C) during broiling, but the direct heat flux is perhaps one of the highest possible. The problem associated with broiling is that the surface of the food is heated at a much faster rate (due to radiation) compared to the interior of the food, which is heated by conduction. Greater heating at the food surface leads to the desirable browning. However, it needs to be ensured that the food interior is also heated adequately while broiling.
Roasting is cooking food using dry heat from an open flame, 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 turned frequently during the process. The heat during roasting is transferred to the food mostly by conduction. Roasting causes caramelization on the food surface.
Baking is the process of prolonged cooking of food by dry heat inside an oven. The food during baking is heated due to a combination of radiation from surrounding heating surfaces and convection from the surrounding air. Among other factors such as ratio of ingredients and sequence in which the ingredients should be mixed, the optimal time and temperature for baking have been designated as most critical in the 4 cardinal rules of baking (Masi and Carlos 2007). The typical baking temperatures are above the boiling point of water and in the range of 150 to 260 °C (McGee 1984). Temperature during baking is important as it plays a critical role in caramelization and final moisture content in the product during the process. The high temperature used in baking dehydrates the surface of the food causing browning.
The 5 factors that affect the changes during baking are heat, moisture content, enzyme activity, and changes in starch and protein contents and structure (Yin and Walker 1995). The most important physical and biochemical changes that occur during baking are volume increase, structural fluidity, moisture removal, crumb resilience, and crust formation (Yin and Walker 1995).
Steaming 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 pressurized), substantially lower than in broiling or baking. As a result, the caramelization and Maillard browning reactions mentioned above cannot happen in a steaming process. Condensing steam provides much higher heat fluxes than when using hot water (further discussed later). During steaming, thermal softening occurs and the moisture loss should be low.
Reheating 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 Involved
Various modes of heating are summarized here to help focus attention to similarities and differences between them.
Conventional hot air jet impingement heating
Hot air is perhaps the most common type of heating. Here heating is from the surface of the food, which stays the hottest at any time, compared to any interior location. Figure 4A illustrates the temperature profile in the food during hot air heating, with the highest temperatures at the surface and progressively lower temperatures inside. Rate of heating is decided by the ambient fluid temperature, transfer coefficient at the surface, and the thermal properties of the food. Thermal properties of food include thermal conductivity, density, and specific heat. Thermal conductivity of the food is the lowest for a dry, porous food and highest for a frozen food having large amounts of water. Jet impingement heating, as well described by Wahlby and others (2000), is a special form of hot air heating in which jets of hot air at high velocities impinge on the product for faster heat transfer. An example of heat flux in a jet-impingement oven is shown in Figure 5A.
The surrounding medium (typically air) is at a high enough temperature near the surface causing rapid evaporation. Pressure-driven flow is small initially from lower internal temperatures due to lack of heat penetration. Thus evaporation at the surface can more than match the water transport due to capillarity and small amounts of pressure-driven flow, 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 increased since pressure-driven flow is still small. Thus conventional heating can nicely contribute to surface crispness, as desired in many baked and fried foods.
Radiative heating uses electromagnetic waves. In the context of food processing, the entire range of infrared electromagnetic waves is typically further divided into near-infrared (0.75–3 μm), mid-infrared (3–25 μm), and far-infrared (25–1000 μm) regions. Infrared heating of 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 reflected, part of it is absorbed, and the remaining, if any, is transmitted. The wavelength of radiation incident on the food depends on the emission characteristics of the source of the radiation. An example of spectral distribution of infrared radiation in a microwave-halogen combination oven is seen in Figure 6, showing much of the radiation in the near-infrared range. As the infrared radiation penetrates, its energy level drops exponentially. Penetration depth, defined as the distance over which the flux has dropped to 37% of its value at the surface, is one of the critical heating parameters as it determines the spatial variation in the rate of infrared heating. Penetration depth depends on the food composition (and its changes during processing) and the source of infrared radiation (see Datta and Almeida 2005 for more information). Penetration depth in foods is typically of the order of 1–10 mm.
When radiant heat does not penetrate sufficiently, its effect is somewhat like hot air heating (Figure 4b), except the heat fluxes are higher. In this case, radiative heating can cause the surface to lose moisture and eventually increase the temperature to the levels where color and flavor developments take place. An example of heat flux from a halogen lamp in a combination oven is shown in Figure 5B. However, when radiative heating has significant penetration, as with halogen, it can mimic microwave heating and thus the rules for microwave heating (without any focusing effect) will start to apply in that moisture accumulation at the food surface might actually increase (Datta and Ni 2000).
Fundamentally, microwave heating is volumetric and nonuniform. Inside the metallic walls of a microwave oven, microwaves form resonant patterns that are regions of high-and-low intensity electric fields, as illustrated in Figure 7 for a particular oven. This is one 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 absorption is nonuniform. The shape, size, and properties of the load, as well as the design of the microwave oven, complicate this scenario for energy absorption, but nonuniformity remains the rule. Its characteristics compare to those of conventional heating as follows:
It's quick, the rates of heating are much higher than in conventional heating.
It's generally more nonuniform than conventional heating.
It's selective, moist areas heat more than the dry areas. Such selectivity is absent in conventional heating.
Unlike conventional heating, significant internal evaporation inside the microwave-heated material leads to additional mechanisms of moisture transport that enhance moisture loss during heating.
It can be turned on or off instantly, unlike conventional heating.
Oven factors that lead to heating rate and nonuniformity of heating include placement inside the oven (Rakesh and others 2010), effect of oven size and geometry, use of turntables (Geedipalli and others 2007), oven power, use of power cycling, use of mode stirrers, and effect of feed location (Zhang and Datta 2003). Food factors that affect the energy absorption are food dielectric properties, food volume, food shape, and food aspect ratio (Zhang and Datta 2001). Nonuniformity of energy absorption is manifested in terms of corner and edge over-heating, focusing effects (enhanced internal heating) due to curved surfaces of the food, and resonance inside the food (where the food itself behaves like a cavity, with regions of high and low electric fields). These factors have led to microwaves often being used only for reheating of food rather than actual cooking. Heating uniformity within a microwave oven chamber can be improved by using a turntable, power cycling, having a mode stirrer in the oven, and active microwave packaging. Note that power on–off cycling is approximately equivalent to using continuous power at an average power level that is given by the fraction of the on-time multiplied by the maximum power.
Distribution of heat inside a food is highly dependent on the size and dielectric properties of the food as well as characteristics of the oven. Two extremes of temperature profiles in microwave heating are mostly surface heating and highly focused internal heating (Figure 4C and D, respectively). Compositional and temperature variation in the food, present initially or developed during the heating, can contribute to spatially nonuniform energy absorption. For example, in microwave thawing, the outside layer will thaw 1st since the outside typically absorbs more energy. Once the material thaws, its energy absorption (dielectric loss) increases tremendously. This thawed outer layer essentially shields much of the microwave energy, and heating rates inside drop significantly. Nonuniformity of heating due to temperature variation during heating is also evident in salty foods where the microwave absorption (a dielectric property) increases with temperature. In a drying process, typically more moisture is lost from the outer regions. The microwave absorption in the drier outer regions will reduce subsequently, and the microwaves will be preferentially absorbed in the wetter regions, leading to more efficient evaporation of the moisture (faster drying)—a very desirable situation.
In microwave heating, the surrounding medium (air) is at room temperature 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-driven flow is small. Therefore, during the early heating process, the surface can lose some water and there is insignificant moisture accumulation 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 increased pressure-driven flow. Since the moisture removal capacity of the surrounding unheated air is small, it is eventually overpowered by the moisture transport to the surface from inside due to increased pressure-driven flow, and moisture accumulates at the food surface.
Induction heating is part of electromagnetic heating. Unlike microwave heating in induction heating, it is the cookware that is heated 1st. The cookware needs to be made of magnetic material (steel, iron, nickel, or various alloys). In one oven model where induction is combined with microwave heating (discussed later), below the smooth glass, coils produce a high-frequency alternating magnetic field, which heats the cookware placed on the glass. Molecules in the cookware are excited by the alternating magnetic field, causing the cookware to become hot and cook the food. The cooktop's glass–ceramic surface is unaffected by the magnetic field since it contains no magnetic material, only the heat of the cookware warms the glass heating surface.
Addition 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) temperatures involved are likely to be lower than in dry heating (with hot air) or the other cooking techniques mentioned above. Heat flux due to steam heating is not readily available (Braud and Others 2001). A range of heat transfer coefficients due to film condensation of water vapor is provided (Michailidis and others 2009): 5000–6700 . Heat flux, obtained from (see, for example, Sa-adchom and others 2011) where q″is flux, hc is the heat transfer coefficient due to condensation, Tsat is the saturation temperature of water, and Ts is the surface temperature; and that would put flux values perhaps in the vicinity of 100000 W / m2, a value significantly higher than infrared or hot air.
Combination Heating Appliances
Microwave combination heating appliances currently in use can be grouped into:
microwave with infrared (halogen lamp or grill),
microwave with hot air (jet or otherwise),
microwave with steam,
microwave with induction heating,
various combinations of the above.
In microwaves with infrared heating, a source of infrared heat is provided inside the oven, using halogen lamps or heated rods, as shown in Figure 8A and C, respectively. In microwaves with hot air, typically forced hot air is provided that emulates simultaneous hot air heating. Note that airflow in a typical microwave oven without forced air is low, leading to very low rates of heat and moisture transfer (Verboven and others 2003). In jet impingement technology, air jets of much higher velocities impinge on a food surface, increasing the rates of heat and moisture transport (see Figure 8B). Microwaves with grill or a source of infrared are sometimes combined with hot air as well, an example of which is shown in Figure 8C. Microwaves with steam is a very recent feature—here steam is generated and fed to the oven cavity (Figure 8D). In one of the few models available with microwaves and steam, as of this writing, steam appears to be used not simultaneously with microwaves, but as a mode available in the same appliance. It should be possible to design combinations that sequence microwave and steam in such an appliance. As no known scientific study exists, combination of microwaves with steam will not be discussed further. However, producing secondary steam with heat from microwaves within a package or a food container placed inside a microwave oven is well known and several patents exist in this area (Matsuba 2011; Unwin 2011). This also has not been reported in the research literature. Finally, a microwave oven in combination with an induction-heating cooker has been reported in the patent literature (Figure 8e). A shielding plate is mounted on the bottom surface and an induction coil is provided below the shielding plate in order to selectively choose between microwave and/or induction cooking (transmit a high-frequency magnetic field). As no known commercially available unit or its scientific study exists, this combination will also not be discussed further.
Coupling of Physics in Combination Heating
Whether intended or not, temperature increases due to heating lead to other physical changes in the food material, such as moisture loss, and together they result in changed dielectric, thermal, or moisture transport properties (see Figure 9). These changes, in turn, influence heat and moisture transport due to any of the individual heating modes. During combination heating, not only are the heat transfer, moisture transfer, and electromagnetics coupled together, contributions from individual modes of heating are also coupled. For example, a microwave oven provides volume-heating, whereas a hot air oven heats the food surface. One important aspect of the coupling of physics is whether the coupling exists as one-way or both ways. An example of one-way coupling can be microwave heating where the dielectric properties do not change with temperature, for example, in heating of raw potatoes over smaller changes in temperature; whereas electromagnetic heating leads to temperature changes and, therefore, is coupled with heat transfer; if the dielectric properties are unchanged, temperature changes do not affect the electromagnetics. Another example of one-way coupling can be biochemical and microbiological changes (quality aspects) that are affected by temperature but these changes do not influence the heat transfer process. On the other hand, if the dielectric properties change significantly with temperature or moisture, the electromagnetic aspect is affected by heat and mass transfer and the coupling is two-way, as during thawing.
Studying Combination Heating
Although a number of experimental studies of combination heating exist (Riva and others 1991; Li and Walker 1996; Ren and Chen 2000; McMinn and Others 2005; Sumnu and Others 2005, 2007), they primarily focus on point measurements for temperature and overall moisture loss measurements. Many studies also exist that focus on the final food quality in microwave combination heating, without the details of temperature and moisture information (Yin and Walker 1995; Sakiyan and Others 2011). Obtaining profiles (spatial variations) for temperature is difficult but possible with an infrared camera, while for moisture, obtaining profiles has been nearly impossible, except using the complex and very involved process using magnetic resonance imaging (MRI). An example of temperature profiles using MRI is shown in Figure 10.
Complexities of mathematical models to study combination heating range from simple transport models that assume simple distribution 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 using Maxwell's equations (Marra and others 2010) to combined solution using Maxwell's equations for electromagnetics and heat transfer (Wappling-Raaholt and Others 2002) to perhaps the most complex model to date of multiphase transport in a porous medium with complete solution using the Maxwell's equations in a microwave cavity with two-way coupling between electromagnetics and heat and moisture transfer (Rakesh and Others 2012). Infrared heating inside an oven has also been modeled comprehensively, providing infrared heat flux on a food surface (Dhall and Others 2009), but such a model has not been coupled with microwave heating, with the exception of perhaps the model by Haala and Wisebeck (2000). Some models exist for heating with superheated steam alone (Sa-adchom and others 2011) but not coupled with microwave heating. Detailed models are the primary means by which one can obtain distributions of temperature and moisture and how change occurs with time (see the “Principles of Combining Modes of Heating” section): transient measurements of inside temperature and moisture are extremely difficult. Also, the detailed models that can separate the contributions from various modes of heating are ideal for relating the precise effects of various modes of heating—something not possible with the simpler models that lump various transport processes. The focus of this article is the synthesis of what has been learned from detailed models and experiments. Due to the review nature of this article, the experiments or the models will not be elaborated on further, but the reader will be referred to the detailed studies at appropriate places.
Principles of Combining Modes of Heating
As combination heating is affected by all the food-related and equipment related factors (Figure 2) that affect individual modes of heating, illumination of all the principles will not come easy. Even the principles of heating in a microwave oven only are hard to come by (Dodson 2001). This article is an attempt, using the dominant characteristics of each type of heating, to develop some guidelines for combining various heating modes.
Goals (when we say that we have reached the right combination) are not defined in engineering terms but whatever leads to the right “quality,” as has been mentioned earlier. In more complex processes, such as microwave puffing, goals are even harder to describe in terms of temperature and moisture profiles. In general, coupling, discussed in relation to Figure 9, increases complexities in microwave combination heating. In heating multicomponent foods, such as a multicompartment dinner (Zhang and Datta 2003), 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 in combining modes of heating that cover a large number of uses for combination heating:
Goal A: achieve rapid heating.
Goal B: achieve desired temperature profile.
Goal C: achieve desired moisture profile.
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 where the transient temperature increase in Figure 11C is close to being the summation of increases in Figures 11A and B. The stair stepping of the temperature profile in microwave only heating and in combination heating occurs due to cycling of the microwaves. Figure 12 shows how the speed of heating, defined as the rate of increase of average temperature of the food with time, increases when modes are combined. Over the duration of heating, however, the contribution from individual modes can change (they are still additive) due to the coupling effects of heat and moisture transfer with properties.
Thus, with the microwave mode being typically additive, time to reach the desired temperature profiles will depend on the contribution from each of the modes—power level (or equivalent power level in the case of cycling of the microwaves), hot air temperature (in the case of convection heating), and heat flux (in the case of radiant heating).
Combining modes to achieve desired temperature profiles (Goal B)
Desired temperature profiles in food can be broadly divided into 2 categories:
Uniform temperature, as in most cooking operations,
temperature that increases near the surface, often to obtain reduced surface moisture.
Combination of heating modes can potentially provide desired temperature profiles for specific food processes. This can be seen using conceptual plots of temperature profiles for individual heating modes, as in Figure 4. Assuming contributions due to 2 heating modes are additive, when we combine 2 somewhat complementary temperature profiles, we can predict approximately what the combined temperature profile will look like.
Obtaining uniform temperature
In microwave heating situations where heating concentrations occur inside the food (see “Desired Food Quality as It Relates to Heating” section), hot air or infrared addition can make the resulting temperature profiles more uniform. Figure 10 illustrates this; here the significantly higher internal temperatures near the center when microwaves are 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 that it leads to a colder surface due to unheated surrounding air. This also can be made more uniform by having a small extent of convective heating (air temperature does not have to be too warm) at the surface so the surface is not cold. Usage of a specialized food container (package) to enable steam produced from water heated by the microwaves to surround the food should also reduce the nonuniformity of heating (Matsuba 2011).
Figure 12 illustrates how combining microwaves with hot air can make heating more uniform while significantly increasing the speed of heating. Temperature nonuniformity is quantified using the coefficient of variation (COV) that is the standard deviation temperature divided by the mean temperature. Higher COV values would 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 the heating parameters involved, as noted in Figure 2.
A more detailed example of how power cycling improves the uniformity of heating in a microwave–hot air combination heating process is shown in Figure 13. The microwave power level to be used is a compromise between faster heating and increased uniformity. When the power level is high, heating is rapid but more 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 conditions, such as rate of hot air heating and positioning of food in the oven, can be changed to obtain uniform temperatures during combination heating. For example, in Figure 14, 3 different positions of the sample inside the oven along with 2 different surface heating rates and with a microwave on–off cycle of 10/60 were simulated. The example demonstrates that a judicious combination heating mode should be used to aim for uniform temperatures during combination heating as certain combinations may help in faster heating but may also lead to more nonuniformity in heating.
Obtaining higher surface temperature
To obtain a temperature profile that increases toward the surface, modes that provide increased surface heating need to be present. As illustrated in Figures 4 and 15, these would include infrared and hot air heating. When the goal is to make the surface dry, steam heating, although providing surface heating, is not a choice as it would make the surface moist.
Combining modes to achieve desired moisture profiles (Goal C)
Desired moisture profiles in food can be divided into 3 broad categories:
keep surface crispy, that is, moisture level drops (perhaps sharply) near the surface,
keep moisture level uniform (typically with minimal overall loss),
lose enough moisture overall, as in a drying process or perhaps as a preprocess to applications such as puffing.
Again, the conceptual plots of moisture profiles in Figure 4 can help us navigate in reaching a desired moisture profile by combining modes. It must also be noted that the rate of moisture loss and the moisture profiles obtained after heating depends on the initial moisture content of the material. For example, Figure 16 shows that average temperatures are higher for a drier food heated using the same combinations resulting in greater moisture loss by the drier food for the same heating time.
Obtaining a crispy surface
When the goal is to make the surface crispy (that is, keep the moisture low or bring it down), microwave power would need to be low and combined with a high rate of removal from the surface (such as by use of high surface temperature and/or air flow rate, or the use of infrared, Datta and Ni 2000). This can be seen in Figure 17B, between cycled microwave and full power microwave heating of a high-moisture material, where the full power microwave causes more accumulation of water toward the surface. For a low-moisture material, as shown in Figure 18B, even full-power microwave heating does not lead to moisture accumulation at the surface, which is desirable for obtaining a crispy surface.
Keeping uniform moisture level
When the goal is minimal loss of moisture, namely to keep initial moisture levels, low-power microwaves can be used that would reduce the pressure-driven flow of moisture. In long-term heating, such as cooking, where the goal is not a crispy surface, it seems the goal would be to maintain somewhat uniform temperature over time and, near the end, 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 also keep the moisture level uniform, as has been suggested in patents (Matsuba 2011).
Lose significant moisture
When the goal is to lose water quickly, as in a drying process, high-power microwave heating can produce significant pressure-driven flow 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 material's moisture content, permeability, and so on) without causing explosion or any undesired puffing.
Controls available in a combination heating oven
Controls available in combination heating (also shown in Figure 2) are:
Power levels used for each individual mode in a combination. For hot air heating, for example, this would mean control of air temperature and perhaps air flow (which controls the surface heat transfer coefficient).
Sequence of the combination, such as microwaves followed by 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 illustrated in the bottom portion of Figure 1. Based on the discussion so far, some guidelines on power level history to achieve the goals mentioned earlier can be provided.
To achieve Goal A (rapid heating alone), all power levels can be at their maximum levels and simultaneous, limited by available power at the wall, especially where the supply lines are 110 V. Since microwaves heat the fastest, more microwave power rather than hot air or infrared would be the most appropriate.
For Goal B, when trying to achieve a uniform temperature profile while increasing the rate of heating, a low microwave power 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 surface, primarily surface heating modes (hot air and low penetration infrared) can be added from the start. Provided there is no focusing effect of microwaves for the particular geometry and properties, microwaves can also be added.
For Goal C, to achieve a crispy surface, higher microwave power can be more useful initially to raise the temperature quickly (something not possible with conventional heating) with minimal effect on moisture transport since pressure-driven flow due to microwave heating increases at later times. Microwave power can be decreased with increasing infrared/hot air to increase the moisture removal capacity as an increasing amount of moisture reaches the surface. The same power level history would be pursued when the goal is surface-browning by increasing temperature and reducing moisture. In some drying applications, microwaves can selectively heat areas of higher moisture near the core of a food material—moisture in these areas are normally more difficult to remove in surface-heating than in hot air or infrared heating. In such situations, microwaves are added in the latter part of the drying process, as has been known in the literature for some time (Decareau 1985).
Sensors for humidity and temperature, as are provided in many newer ovens, can provide feedback control capabilities and make it significantly easier to reach the goals. Also, packaging, especially active packaging, can also be used to reach the goals more effectively.
It is clear from these discussions on controls that many factors will affect the decision on power level history needed for a particular product for using combination heating. In addition to the factors just mentioned, for example, power level history would also depend on the product dielectric and thermophysical properties. Although the desired power level history can be theoretically approached as an optimization problem (Balsa-Canto and others 2005), realistically, the exact combination power level history that works for a particular product is obtained through experimentation. The intention in this study was to provide rational and fundamentals-based guidelines to reduce the amount of experimentation, knowing there are a myriad of factors that affect the process. Although preprogrammed in many of today's combination heating ovens (where we only need to press one button that selects a recipe), these histories are generally arrived at through trial and error by the appliance manufacturers and are proprietary information to them (which made it impossible to include an example of a power level history in use).
Summary and the Future
Combination of heating modes is the holy grail of customized quality that can be automated. This article is an attempt to provide 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 will produce different results that are impossible to summarize. On the other hand, this leads to nearly infinite possibilities and thus novel combinations will keep coming out forever. This article is simply an attempt to make this search process a little more methodical. Other aspects not included in this study include product formulation (Shukla and Anantheswaran 2001) and packaging (Bohrer and Brown 2001) that will continue to play a significant role by modifying the food dielectric and thermophysical properties.
This project was supported by National Research Initiative Grant no. 2003–35503—13737 from the USDA Cooperative State Research, Education, and Extension Service Competitive Grants program.