18 microwave processing

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 397 using electromagnetic waves in the frequency range of THz (1012 Hz), equivalent to a wavelength of mm (micrometre) Photons of microwaves correspond to an energy range of meV to meV and are incapable of ionisation (binding energies of electrons to the atom are 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 Wavelength [cm] 104 102 100 10–2 10–4 10–6 10–8 10–10 10–12 10–14 10–16 108 1010 Radio waves Infrared 13.6 Mhz = 22 m Visible light 915 Mhz = 0.33 m Ultraviolet 2450 Mhz = 0.12 m X-radiation 22125 Mhz = 0.014 m Gamma radiation Cosmic radiation 10–10 10–8 10–6 10–4 10–2 100 102 104 106 Photon energy [eV] 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 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 efficiency 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 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 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 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 400 The nutrition handbook for food processors Temperature [°C] 121 120 80 40 0 10 20 30 40 50 60 Time [min] 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 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 Microwave processing 401 Relative absorption 25 Raw beef 20 15 10 f = 2.45 GHz Distilled water –20 20 60 100 140 Temperature [°C] Fig 18.4 Energy absorption, water vs beef in relation to temperature Distilled water and beef are mainly determined by the course of the dielectric properties of water below the freezing point; around the freezing point energy absorption reaches a maximum; at higher temperatures the energy absorption efficiency decreases continuously for water; for raw beef there is an initial decrease and a later increase because of the mobility of ions in aqueous solution within the meat well as such related parameters as dielectric properties, electrical conductivity, heat capacity and thermal conductivity Packaging formats may introduce another difficulty because increased field strength at corners and edges can lead to local over-heating that leaves other portions under-treated 18.3 The effects of microwave radiation on food It is common practice to classify the effects of microwave energy on a medium as ‘thermal’ or ‘non-thermal’ However, this terminology is incorrect The inter- 402 The nutrition handbook for food processors action of matter with the electromagnetic field always results in an energy transfer and therefore in a temperature change However, some effects are specific to the interaction with electromagnetic energy and cannot be achieved by conventional heating Other effects are identical to interaction with thermal energy i.e by conventional conductive or convective heating For microwaves, the optical and geometrical effects during exposure of matter can result in locally high and low power levels and therefore in hot spots and cold spots In many cases, researchers were unaware of such phenomena and therefore their conclusions on microwave-specific effects were incorrect The objects exposed to microwaves can be considered as antennae: if they are geometrically separated and are small compared to the wavelength(s) they will not be ‘seen’ by microwaves, in the same way as a radio or TV antenna must be of correct size for optimal reception As a consequence, many reported observations about biological effects in living cells must be carefully compared to and separated from possible heating effects Literature in this field includes that of many different subjects, and the studies have been carried out from the viewpoint of various disciplines and are mostly inconsistent in terminology The direct effects on such substances as nutrients, enzymes, proteins, microorganisms and cell membranes are of interest when considering the quality of microwaved food ‘There is no substantiated mechanism of interaction of microwaves with atoms, molecules, organisms and microorganisms other than volumetric heating’ (Ponne and Bartels, 1995; Mudgett, 1989) Over the past few decades there has been a heated debate that resurfaced during recent years due to concerns about mobile phones, electrical power lines and electro-smog It has been shown that temperature control in most of these experiments was not possible to the level that was necessary Several papers and patents claimed beneficial ‘athermal’ effects, but it could be shown that identical effects could be reached in conventional heating by simulating the heating patterns expected to occur in microwave heating Kinetic laws lead to a variety of hypotheses: if an intermediate reactant has a long transition life time, electric fields can increase the probability of molecular collision; if the reactant has a short life time, the electric field can re-orient the molecules to more favourable positions for further reaction Another hypothesis assumes extremely high temperatures at locations of atomic or molecular dimensions All those theories still need experimental verification The principal difficulty for such experiments can be understood by recognising that ‘temperature’ is defined at the macroscopic level by the average of the molecular oscillations in the volume under consideration and this does not apply for a point at the atomic level How could the temperature of an atom or inside a microorganism be measured experimentally? From the viewpoint of the food technologist it is possible to conclude that the strong electric and magnetic fields have an effect on cell membranes These effects result in a change of permeability, in functional disturbances and even in rupture, but not change the nutritional value and not affect the technology of microwave heating Some theories also predict a direct effect of microwaves on enzymatic activity, but there is not yet any experimental proof The effect of Microwave processing 403 microwave chemistry on reaction rate and specificity of reaction needs to be investigated In conclusion, the microwave heating of food only confers properties to food that are conferred through conventional heating Lethal effects to microorganisms, parasites and other organisms are exclusively due to heat Speed and homogeneity of heating are influenced by the composition of the food and the mass in relation to power of the oven, as well as by the technical features of the appliance Because cold spots may occur care must be taken when anticipating the reduction of microorganisms Pathogen microorganisms might survive in one area of the food while other parts show elevated temperatures Standing time after cooking may be used to achieve more even temperatures by heat exchange through conduction 18.4 The safety of microwave-heated food Unknown hazards and mysterious phenomena in microwave heating are intimidating to most people and, in contrast to conventional heating, there is no visible heat source Inside the oven the food is heated up to cooking temperature so the obvious query is whether radiation leaking from the device can heat anyone standing close to it For this reason, the permissible energy density at the surface of any microwave facility is limited to 10 mW/cm2 for an unlimited length of time of exposure in the USA and European countries In some eastern European countries and the former Soviet Union the upper limit for long-term exposure to microwaves has even been set to 10 mW/cm2 This very low limit was justified because of supposed non-thermal effects attributed to interaction of microwaves with living organisms The alleged health problems included nervousness, hormonal imbalance, malformations and anomalous brain activity The experimental evidence, however, for such phenomena is unsubstantiated (Foster, 1992; Hileman, 1993) The practical and achievable limit of 10 mW/cm2 is justified because the microwaves that are used are identical with those used in therapy (For therapeutic effects the energy density must be well above a level of 100 mW/cm2.) There are numerous studies to determine damage thresholds and it has been observed that no permanent effects occur at levels below 100 mW/cm2 For a critical organ, the eye, it was observed that cataract formation may occur at 150 mW/cm2 when the microwaves are applied for more than 90 minutes Within certain limits the body can absorb energy including microwaves and compensate for the temperature increase easily by removing excessive heat by means of blood flow There are certain avascular structures in the body that may have a relatively poor heat exchange; this is possibly true for testicles and temporary sterility has been reported after microwave exposure The energy flow from the sun may be considered for comparison: on a sunny day in summer the infrared portion of the spectrum may carry as much as 100 mW/cm2, which is not responsible for sunburn Hence, introducing a safety factor of 10 is considered sufficient and the 404 The nutrition handbook for food processors limiting energy density of 10 mW/cm2 is widely accepted However, in recognising concerns about cell phones, broadcast antennas and satellites a lowering of this limit by another factor of 10 is being considered To enhance radiological safety of microwave appliances several features are implemented Safety latches cut off the power as soon as the door is opened and the user is not exposed to spurious radiation Further chokes and absorbing strips are attached to the doors as seals to eliminate any leakage of microwave radiation to the outside Industrial facilities are equipped with energy trapping devices at the conveyor and system openings for product entry and exit Such facilities for continuous treatment have to be operated ‘open door’, whereas the household oven is used batchwise and the door must always be closed The majority of household appliances and industrial facilities that are built and installed today are well shielded At the time of purchase, for a domestic appliance the energy leakage is usually considerably less than mW/cm2 at cm from the door as presently regulated in most countries Microwave appliances have been regularly used in households for several years and no major accident has been reported Survey studies have confirmed the radiological safety of the appliances that are on the market However, there have been certain consumer demands regarding larger door openings and bigger transparent windows at the front that have led the industry to be more rigorous regarding the regulated leakage limits of mW/cm2 at cm The statistics from such surveys have shown that an increasing number of appliances is manufactured as close as possible to the limits but this should not cause concern because the energy limits already include a reasonable safety factor The presence of metallic conductors in the human body, especially if they are heart pacemakers, can lead to complex effects and locally high electrical currents as does a piece of metal left in food while being heated in a microwave oven This may lead to unforeseen interactions and so a particular note of caution must be given to people with pacemakers Experiments with volunteers have shown that the most critical frequency is around GHz which is used in radar facilities; at those frequencies commonly used for heating purposes, 2450 MHz, only small changes of the heart rhythm have been observed (the maximum energy density in such experiments was limited to 25 mW/cm2) However, such warnings are irrelevant in the case of household appliances as the regulated leakage limit is usually mW/cm2 at cm distance Some precautions may be advisable where microwave facilities are installed on a food production line in industry However, radiological safety at open-door facilities has also been generally established Prejudice and lack of technical understanding are causes of unfounded allegations that food heated by microwave might become toxic and exotic chemical compounds could be formed However, microwave radiation cannot break chemical bonds and cannot cause ionisation nor create free radicals Here there is an essential difference from ionising radiation which has a quantum or photon energy that is larger by several orders of magnitude For this reason the possibility of induced chemical reactions other than thermal ones is nonexistent This has been confirmed by many studies and it has been possible to attribute any par- Microwave processing 405 ticular chemical effect to uncontrolled heating patterns in various experiments Microwave heating is newer than traditional methods such as broiling, roasting, frying, smoking and barbecuing so it is less understood and accepted by the public However, in practice, microwave ovens are used in a large and increasing number of households with no concerns Variations in food shape, size or composition can also result in underdone products, and this is another risk in microwave cooking Lack of understanding has resulted in illness from surviving pathogen microorganisms or the formation of microbial toxins; however, this is no different from undercooking food by traditional methods In the same way, local overcooking can lead to the formation of unacceptable chemical changes as it does in traditional heating methods Because there are no substantiated, special mechanisms of the interaction of microwaves with atoms, molecules, organisms and microorganisms the toxicological concerns regarding the consumption of food that is heated by microwave have no foundation in reality 18.5 The nutritional adequacy of microwave heated food Since the discussion above has shown that chemical, biological, microbiological, radiological and toxicological implications for microwave heated food give no cause for concern, it should be expected that there are no particular nutritional problems associated with the procedure Any difference in the nutritional value of microwaved food compared with food treated by conventional means must be attributed to the different heating regimes Microwaves bring the advantage of fast heating throughout the food with a lower temperature burden because of reduced cooking time As a consequence, the loss in heat-sensitive vitamins is minimised In addition, the extent of chemical reactions, such as the Maillard reaction, may be reduced and the retention of nutrients be enhanced In microwave heating, less water is needed so that less extraction of valuable nutrients including minerals occurs The quantification of such advantageous effects is largely dependent on the way the appliance or industrial facility is set up However, in industrial applications the amount of water may be tailored to achieve the optimal effect between heating, microorganism reduction and retention of nutrients 18.6 Future trends Because of its ease and convenience microwave cooking has been established considerably more in the domestic situation than it has in the food industry Many homes today have a microwave oven and have done since the beginning of the 1970s and it may be expected that the food industry will now learn about the advantages Even the formulation of complex recipes and composite dishes can be adapted to technical needs of microwave heating and some conventional recipes are easy to tailor (George, 1993, Ramaswamy and van de Voort, 1990) 406 The nutrition handbook for food processors Uncritical enthusiasm about the potential of microwave processing should not disguise the facts and technological conditions Discerning scientists and engineers point to technical difficulties which still exist and to potential problems of unknown dimensions in large-scale industrial exploitation of the technology Some present techniques and operational designs may be inadequate in dealing with microwave technology and future requirements Any limitations should be recognised and be compared with the achievable advantages, otherwise elevated expectations may be deflated by realities However, it is hoped that scientists and engineers will find solutions to the problems associated with microwave applications as they have done with other problems in the past One aspect of such future developments is the choice of appropriate containers and the development of more suitable materials with which to make them Intensive research and development efforts have already led to new materials adapted for unique features in microwave heating because the container is an active component for a microwavable product; the browning pan is an example Fractions of the microwave energy are absorbed in the wall material, or reflected and transmitted to the product in the container 18.7 References decareau r v (1985), Microwaves in the Food Industry, Orlando, Academic Press dehne l i (1999), Bibliography on Microwave Heating of Food, BgVV-Hefte 04/1999, Berlin, Bundesinstitut für gesundheitlichen Verbraucherschutz und Veterinärmedizin foster k r (1992), ‘Health effects of low-level electromagnetic fields: phantom or notso-phantom risk?’, Health Phys 62, 429–35 george r m (1993), ‘Recent progress in product, package and process design for microwavable foods’, Trends Food Sci Technol 4, 390–4 harris r s and von loesecke h (1960), Nutritional Evaluation of Food Processing, New York, Wiley hermann a (1988), ‘Heinrich Hertz und seine große Entdeckung’ (Heinrich Hertz and his great discovery), Naturwissenschaften 75, 219–24 hileman b (1993), ‘Health effects of electromagnetic fields remain unresolved’, C&EN (Nov 8, 1993), 15–29 kaatze u (1995), ‘Fundamentals of microwaves’, Radiat Phys Chem 45, 539–48 mudgett r e (1989), ‘Microwave food processing Scientific status summary by the Institute of Food Technologists’ Expert Panel on Food Safety and Nutrition’, Food Technol 43(1), 117–26 mullin j (1995), ‘Microwave processing’ in Gould G W (ed.) New Methods of Food Preservation, London, Blackie Academic & Professional ponne c t and bartels p v (1995), ‘Interaction of electromagnetic energy with biological material – relation to food processing’, Radiat Phys Chem 45, 591–607 ramaswamy h and van de voort f r (1990), ‘Microwave applications in food processing’, Can Inst Food Sci Technol 33(1), 17–21 rosenthal i (1972), Electromagnetic Radiations in Food Science (Advanced Series in Agricultural Sciences 19), Berlin, Springer [...]... 43(1), 117–26 mullin j (1995), Microwave processing in Gould G W (ed.) New Methods of Food Preservation, London, Blackie Academic & Professional ponne c t and bartels p v (1995), ‘Interaction of electromagnetic energy with biological material – relation to food processing , Radiat Phys Chem 45, 591–607 ramaswamy h and van de voort f r (1990), Microwave applications in food processing , Can Inst Food... Evaluation of Food Processing, New York, Wiley hermann a (1988), ‘Heinrich Hertz und seine große Entdeckung’ (Heinrich Hertz and his great discovery), Naturwissenschaften 75, 219–24 hileman b (1993), ‘Health effects of electromagnetic fields remain unresolved’, C&EN (Nov 8, 1993), 15–29 kaatze u (1995), ‘Fundamentals of microwaves’, Radiat Phys Chem 45, 539–48 mudgett r e (1989), Microwave food processing. .. component for a microwavable product; the browning pan is an example Fractions of the microwave energy are absorbed in the wall material, or reflected and transmitted to the product in the container 18. 7 References decareau r v (1985), Microwaves in the Food Industry, Orlando, Academic Press dehne l i (1999), Bibliography on Microwave Heating of Food, BgVV-Hefte 04/1999, Berlin, Bundesinstitut für gesundheitlichen... enthusiasm about the potential of microwave processing should not disguise the facts and technological conditions Discerning scientists and engineers point to technical difficulties which still exist and to potential problems of unknown dimensions in large-scale industrial exploitation of the technology Some present techniques and operational designs may be inadequate in dealing with microwave technology and... solutions to the problems associated with microwave applications as they have done with other problems in the past One aspect of such future developments is the choice of appropriate containers and the development of more suitable materials with which to make them Intensive research and development efforts have already led to new materials adapted for unique features in microwave heating because the container