18 Microwave processing D.A. E. Ehlermann, Federal Research Centre for Nutrition Germany 18.1 Introduction Generally, the effects of microwave energy can be classified as either 'macro- scopic or 'microscopic. When the energy is used for heating food the effect is macroscopic and results in a specific heating pattern. However, the causes of certain features are due to microscopic effects, i.e. to physics at the atomic level The advantages of the technology are quick and uniform heating and the reduc tion of water usage. Food is a complex system that contains many components such as biological molecules, water and microorganisms. Its structure is determined by such struc tures as cells, membranes, polymers, proteins and lipids. There has always been a suspicion that microwaves might have ' properties that influence unex pected changes in microorganisms, nutrients and living cells but at the time of writing the allegations are unproven and the reported effects could always be attributed to particular heating pattems and insufficient temperature control in the experiments. Representatives of mainstream science have therefore concluded that microwave heating is a safe method of food processing and can be usefully pplied both in the domestic household and in the food industry. Various aspects are discussed in several of the sections that follow Electromagnetic waves in the frequency range of 300 MHz to 300 GHz are usually called'microwaves': this terminology is inappropriate but popular. The prefix 'micro' would lead one to expect that the wavelength is in the micrometer range but in free space the wavelength ranges from decimetre to millimetre and therefore is in agreement with the dimensions of articles of daily use. For example, in radar systems microwaves provide reasonable spatial resolution and considerable radius of action and can be compared with optical imaging systems
18 Microwave processing D. A. E. Ehlermann, Federal Research Centre for Nutrition, Germany 18.1 Introduction Generally, the effects of microwave energy can be classified as either ‘macroscopic’ or ‘microscopic’. When the energy is used for heating food the effect is macroscopic and results in a specific heating pattern. However, the causes of certain features are due to microscopic effects, i.e. to physics at the atomic level. The advantages of the technology are quick and uniform heating and the reduction of water usage. Food is a complex system that contains many components such as biological molecules, water and microorganisms. Its structure is determined by such structures as cells, membranes, polymers, proteins and lipids. There has always been a suspicion that microwaves might have ‘athermal’ properties that influence unexpected changes in microorganisms, nutrients and living cells but at the time of writing the allegations are unproven and the reported effects could always be attributed to particular heating patterns and insufficient temperature control in the experiments. Representatives of mainstream science have therefore concluded that microwave heating is a safe method of food processing and can be usefully applied both in the domestic household and in the food industry. Various aspects are discussed in several of the sections that follow. Electromagnetic waves in the frequency range of 300 MHz to 300 GHz are usually called ‘microwaves’; this terminology is inappropriate but popular. The prefix ‘micro’ would lead one to expect that the wavelength is in the micrometer range but in free space the wavelength ranges from decimetre to millimetre and therefore is in agreement with the dimensions of articles of daily use. For example, in radar systems microwaves provide reasonable spatial resolution and considerable radius of action and can be compared with optical imaging systems
Microwave processing 39 using electromagnetic waves in the frequency range of THz(10 2Hz), equivalent to a wavelength of um(micrometre). Photons of microwaves correspond to an energy range of l ueV to l me V and are incapable of ionisation(binding energies of electrons to the atom are 4ev or above). Applications of microwaves are mostly in radiocommunication, radar and heating Electromagnetic radiation was discovered by Rudolf Hertz in 1888 who did not believe in its practical value nor in any possibilities for its industrial exploita- tion(Hermann, 1988)but today nearly every household uses the microwave oven or the cellular phone. In order not to disturb other uses of microwaves, the appli cation of microwave heating is limited by the International Telecommunication Unions(TU) to a number of frequency bands(Fig. 18.1). For practical purposes only the following frequency bands are exploited 2450 MHz in domestic ovens and industry 970MHz. 915 MHz and 897 MHz in some countries for industrial applications 22 125 MHz. reserved bands for future use. The most relevant application to food is that of heating, both in the household and in the food industry. In industry, a range of practical applications exploit the particular heating patterns achievable by microwaves, which are superior to esf song in general is to deliver more homogeneous heating at a faster rate and Iventional heating in the given circumstances. The aim of microwave proces in particular for pasteurisation and sterilisation. It is therefore necessary to under- stand the physical principles and to achieve an insight into the limitations of potential systems in this respect. For general information the reader is referred to 1010101010101010-010-210-10 6 Mhz= 22 m Visible light 915Mhz=0.33m Ultraviolet 2450Mhz=0.12m X-radiation 22125Mhz=0.014m------ Gamma radiation Cosmic radiation Photon energy [ev Fig 18.1 Range of energies(electromagnetic spectrum): the range of frequencies usefu for microwave heating is marked
using electromagnetic waves in the frequency range of THz (1012Hz), equivalent to a wavelength of mm (micrometre). Photons of microwaves correspond to an energy range of 1meV to 1 meV and are incapable of ionisation (binding energies of electrons to the atom are 4 eV or above). Applications of microwaves are mostly in radiocommunication, radar and heating. Electromagnetic radiation was discovered by Rudolf Hertz in 1888 who did not believe in its practical value nor in any possibilities for its industrial exploitation (Hermann, 1988) but today nearly every household uses the microwave oven or the cellular phone. In order not to disturb other uses of microwaves, the application of microwave heating is limited by the International Telecommunication Unions (ITU) to a number of frequency bands (Fig. 18.1). For practical purposes only the following frequency bands are exploited: • 2450 MHz in domestic ovens and industry. • 970 MHz, 915 MHz and 897 MHz in some countries for industrial applications. • 22 125 MHz, reserved bands for future use. The most relevant application to food is that of heating, both in the household and in the food industry. In industry, a range of practical applications exploit the particular heating patterns achievable by microwaves, which are superior to conventional heating in the given circumstances. The aim of microwave processing in general is to deliver more homogeneous heating at a faster rate and in particular for pasteurisation and sterilisation. It is therefore necessary to understand the physical principles and to achieve an insight into the limitations of potential systems in this respect. For general information the reader is referred to Microwave processing 397 Wavelength [cm] 104 104 106 108 1010 102 100 102 10–2 10–4 10–4 10–6 10–8 10–10 10–2 10–6 10–8 10–10 10–12 10–14 10–16 100 Radio waves Infrared Visible light Ultraviolet X-radiation Gamma radiation Cosmic radiation Photon energy [eV] 13.6 Mhz = 22 m 915 Mhz = 0.33 m 2450 Mhz = 0.12 m 22125 Mhz = 0.014 m Fig. 18.1 Range of energies (electromagnetic spectrum): the range of frequencies useful for microwave heating is marked
398 The nutrition handbook for food processors textbooks and reviews or survey articles(Rosenthal, 1972: Dehne, 1999; Harris and Von Loeseke, 1960: Decareau, 1985: Mullin, 1995) 18.2 The principles of microwave heatin Microwave processing is simply heating by radiation(Kaatze, 1995). As such it is similar to infrared heating: the energy transfer is by radiation and not by con- vection or conduction. However, there are significant differences: in infrared heating the penetration of the radiation into the substance is marginal and the main portion is heated by conduction from the surface into the centre, in microwave processing microwaves penetrate throughout the volume of the sub- stance and the heat-sources'are dissipated inside it. This can contribute to a more uniform heating of the substance. However, the penetration of microwaves is limited and this must be taken into consideration both industrially and domesti- cally. Microwaves are reflected by metals but transmitted by materials such as glass, plastics, paper and ceramics. Microwaves are produced by a vacuum tube device called a magnetron and the power in household ovens ranges from 600 to 1500w while industrial installations use up to 50kw. Energy conversion effi ciency by magnetrons is around 50% and the remaining heat is usually dissipated by air cooling transfer, it is necessary to understand that electromagnetic energy comes in por- tions called 'photons'or ' quanta that are discrete but very small quantities. An impinging photon must match exactly the energy difference between several flowed atomic energy states of the electrons in the treated material; otherwise no energy is absorbed and the object is'transparent to the electromagnetic wave This is the reason that food, which is mainly water with regard to microwave heating, can be heated, but glass or plastic containers remain cold. (A mother heating a babys bottle in a microwave oven and testing the temperature by ensing its surface temperature with her cheek may not realise that the milk is boiling inside. )In addition, the heat capacity of the substance must be consid- ered. It determines the heating effect: some food components such as fat do not absorb the microwave energy as efficiently as water does; but their heat capac ity is much lower than that of water and despite the lower absorbance they are heated much faster.(A piece of meat containing massive fat portions, being cooked in the microwave oven will appear evenly cooked and ready to eat but hen cutting it the hot, liquid fat will spurt from inside!) On the atomic level the following effects determine the energy transfer from the radiation to the food (Fig. 18.2). When an electric field, whether static or alter- nating, is applied the product undergoes polarisation. Polar molecules that carry locally separated charges will orient in the direction of the actual electric field, and water is such a polar molecule. Once oriented the electrical field will stretch the molecule. Other molecules that are normally neutral and nonpolar will become polarised as the electrons are moved to opposite ends of the molecule by
textbooks and reviews or survey articles (Rosenthal, 1972; Dehne, 1999; Harris and Von Loeseke, 1960; Decareau, 1985; Mullin, 1995). 18.2 The principles of microwave heating Microwave processing is simply heating by radiation (Kaatze, 1995). As such it is similar to infrared heating; the energy transfer is by radiation and not by convection or conduction. However, there are significant differences: in infrared heating the penetration of the radiation into the substance is marginal and the main portion is heated by conduction from the surface into the centre, in microwave processing microwaves penetrate throughout the volume of the substance and the ‘heat-sources’ are dissipated inside it. This can contribute to a more uniform heating of the substance. However, the penetration of microwaves is limited and this must be taken into consideration both industrially and domestically. Microwaves are reflected by metals but transmitted by materials such as glass, plastics, paper and ceramics. Microwaves are produced by a vacuum tube device called a magnetron and the power in household ovens ranges from 600 to 1500 W while industrial installations use up to 50 kW. Energy conversion effi- ciency by magnetrons is around 50% and the remaining heat is usually dissipated by air cooling. In order to understand the physical principles behind the radiation energy transfer, it is necessary to understand that electromagnetic energy comes in portions called ‘photons’ or ‘quanta’ that are discrete but very small quantities. An impinging photon must match exactly the energy difference between several allowed atomic energy states of the electrons in the treated material; otherwise, no energy is absorbed and the object is ‘transparent’ to the electromagnetic wave. This is the reason that food, which is mainly water with regard to microwave heating, can be heated, but glass or plastic containers remain cold. (A mother heating a baby’s bottle in a microwave oven and testing the temperature by sensing its surface temperature with her cheek may not realise that the milk is boiling inside.) In addition, the ‘heat capacity’ of the substance must be considered. It determines the heating effect: some food components such as fat do not absorb the microwave energy as efficiently as water does; but their heat capacity is much lower than that of water and despite the lower absorbance they are heated much faster. (A piece of meat containing massive fat portions, being cooked in the microwave oven will appear evenly cooked and ready to eat but when cutting it the hot, liquid fat will spurt from inside!) On the atomic level the following effects determine the energy transfer from the radiation to the food (Fig. 18.2). When an electric field, whether static or alternating, is applied the product undergoes polarisation. Polar molecules that carry locally separated charges will orient in the direction of the actual electric field, and water is such a polar molecule. Once oriented the electrical field will stretch the molecule. Other molecules that are normally neutral and nonpolar will become polarised as the electrons are moved to opposite ends of the molecule by 398 The nutrition handbook for food processors
Microwave processing 399 Fig. 18.2 Dipole rotation and ion oscillation or orientation polarisation versus charge polarisation: the horizontal arrows symbolise the electrical field. the llipsoid (left) symbolises a polar molecule with the rotation indicated by arrows: the circles(right) with the charge marked ons with the linear the external field. As soon as this is complete the effects of rotation and stretch ing will also occur for these particular molecules. In addition, aqueous solutions may contain components such as salts that easily dissociate and form electrically charged ions and in the presence of an electrical field such ions move. The microwaves are not static but oscillate regularly; i.e. these effects of polarisation, rotation, stretching and migration repeat at the rate of oscillation and are ideal synchronised. However, in practice there is a frictional effect i.e. interaction with the electrical field of neighbouring molecules. This retards oscillation of the polarised molecules, the molecules always follow the microwave that drives them and heat energy is transferred to the medium. This means that at very low elec tromagnetic frequencies no energy is imparted because the water molecules can rotate and reorientate themselves quickly enough to follow the field of the microwave and at very high frequencies(approaching 1000 GHz) no energy is imparted because the molecules are too inert to follow the field of the microwave Furthermore, ions in an aqueous solution cannot follow the oscillation of the elec trical field and cannot move over significant distances so energy is consumed in keeping such ions oscillating and this also contributes to heat formation in the medium. From this discussion the complexity of the physics of microwave heating is evident. This is true even when cooking a simple item such as mashed potatoes with table salt; the salt content may affect the heating pattern. Because microwave heating of food(Ponne and Bartels, 1995)is dominated by the physical properties of water it is informative to take a closer look. As long as water molecules are well separated, such as in the gaseous phase, there is only marginal coupling between neighbouring molecules and only a very small number of allowed rotational and vibrational transitions exist. this means that water vapour is transparent to microwaves except for photons of very distinct fre quencies which are absorbed. When the water condenses to a liquid, hydrogen bonds between the molecules prevail and the allowable transitions are converted (widened) to a range(or bands)of photon energies. Hence, liquid water can be heated by a range of microwave frequencies. Quite dramatically, when water
the external field. As soon as this is complete the effects of rotation and stretching will also occur for these particular molecules. In addition, aqueous solutions may contain components such as salts that easily dissociate and form electrically charged ions and in the presence of an electrical field such ions move. The microwaves are not static but oscillate regularly; i.e. these effects of polarisation, rotation, stretching and migration repeat at the rate of oscillation and are ideally synchronised. However, in practice there is a frictional effect i.e. interaction with the electrical field of neighbouring molecules. This retards oscillation of the polarised molecules, the molecules always follow the microwave that drives them and heat energy is transferred to the medium. This means that at very low electromagnetic frequencies no energy is imparted because the water molecules can rotate and reorientate themselves quickly enough to follow the field of the microwave and at very high frequencies (approaching 1000 GHz) no energy is imparted because the molecules are too inert to follow the field of the microwave. Furthermore, ions in an aqueous solution cannot follow the oscillation of the electrical field and cannot move over significant distances so energy is consumed in keeping such ions oscillating and this also contributes to heat formation in the medium. From this discussion the complexity of the physics of microwave heating is evident. This is true even when cooking a simple item such as mashed potatoes with table salt; the salt content may affect the heating pattern. Because microwave heating of food (Ponne and Bartels, 1995) is dominated by the physical properties of water it is informative to take a closer look. As long as water molecules are well separated, such as in the gaseous phase, there is only marginal coupling between neighbouring molecules and only a very small number of allowed rotational and vibrational transitions exist. This means that water vapour is transparent to microwaves except for photons of very distinct frequencies which are absorbed. When the water condenses to a liquid, hydrogen bonds between the molecules prevail and the allowable transitions are converted (widened) to a range (or bands) of photon energies. Hence, liquid water can be heated by a range of microwave frequencies. Quite dramatically, when water Microwave processing 399 + – – + Fig. 18.2 Dipole rotation and ion oscillation or orientation polarisation versus space charge polarisation: the horizontal arrows symbolise the alternating electrical field; the ellipsoid (left) symbolises a polar molecule with the alternating rotation indicated by arrows; the circles (right) with the charge marked symbolise ions with the linear oscillation indicated by arrows
400 The nutrition handbook for food processors Time[min] Fig. 18.3 Comparison of heating curves: broken line: microwaves can cause a short warming up period, the targeted holding temperature of 121C is perfectly reached for the desired and shorter period of time; solid line: conventional heating, the warming up period is rather long, the targeted holding temperature is reached only after prolonged periods Cooling behaviour is nearly identical in both cases. becomes solid. that is it freezes to ice. rotation of the water molecules becomes impossible and only small vibrations within the crystalline structure are still pos- sible and so ice becomes nearly transparent for microwaves. This effect is the limiting factor in thawing frozen food by microwaves; but there is a practical solution that is discussed below As a practical consequence, the time-course of heating patterns is most beneficial for microwaves compared to conventional methods(Fig. 18.3). The warming up time is much shorter and results in the protection of nutrients from excessive heat damage and leaching. The target temperature (in this example 121C to achieve sterility) is reached nearly instantaneously and held for a well-defined period of time; whereas in conventional heat-sterilisation the centra and direct cooling is not possible and the cooling behaviour of the product is identical for any sterilisation method The other practical consequence concerns the freezing point(Fig. 18.4). Dis- tilled water clearly shows the heat absorption behaviour theoretically expected and, for food such as raw meat, behaviour is similar at the freezing point of water due to the physical properties of water at this temperature. However, with increas- ing temperatures behaviour differs from that of distilled water and is due to sub- stances such as salts dissolved in the cell contents In practice, microwave heating is controlled by a multitude of interwoven actors such as the radiation source, the volume and design of the oven, the com- position of the food (e.g. proportions of water, salts, fats) and its bulk density
becomes solid, that is it freezes to ice, rotation of the water molecules becomes impossible and only small vibrations within the crystalline structure are still possible and so ice becomes nearly transparent for microwaves. This effect is the limiting factor in thawing frozen food by microwaves; but there is a practical solution that is discussed below. As a practical consequence, the time-course of heating patterns is most beneficial for microwaves compared to conventional methods (Fig. 18.3). The warming up time is much shorter and results in the protection of nutrients from excessive heat damage and leaching. The target temperature (in this example 121 °C to achieve sterility) is reached nearly instantaneously and held for a well-defined period of time; whereas in conventional heat-sterilisation the central temperature only approaches the target value asymptotically. Unfortunately, fast and direct cooling is not possible and the cooling behaviour of the product is identical for any sterilisation method. The other practical consequence concerns the freezing point (Fig. 18.4). Distilled water clearly shows the heat absorption behaviour theoretically expected and, for food such as raw meat, behaviour is similar at the freezing point of water due to the physical properties of water at this temperature. However, with increasing temperatures behaviour differs from that of distilled water and is due to substances such as salts dissolved in the cell contents. In practice, microwave heating is controlled by a multitude of interwoven factors such as the radiation source, the volume and design of the oven, the composition of the food (e.g. proportions of water, salts, fats) and its bulk density as 400 The nutrition handbook for food processors 120 80 40 0 0 10 20 30 40 50 60 Time [min] 121 Temperature [°C] Fig. 18.3 Comparison of heating curves: broken line: microwaves can cause a short warming up period, the targeted holding temperature of 121 °C is perfectly reached for the desired and shorter period of time; solid line: conventional heating, the warming up period is rather long, the targeted holding temperature is reached only after prolonged periods. Cooling behaviour is nearly identical in both cases