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 1500W while industrial installations use up to 50kW. 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 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

  1. 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.
  2. 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]

400 The nutrition handbook for food processors Temperature [°C]

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.

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

Relative absorption

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.

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.

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