**5. Effect of microwave heating on seeds and plants**

56 The Development and Application of Microwave Heating

valuable insights over the coming decades [52].

research into ultra-high frequency microwave based insect control should yield some

**Figure 7.** Relative dielectric heating and wood at fibre saturation moisture content, calculated using

Park*, et al.*[54] studied the survival of microorganisms after heating in a conventional microwave oven. Kitchen sponges, scrubbing pads, and syringes were deliberately contaminated with wastewater and subsequently exposed to microwave radiation. The heterotrophic plate count of the wastewater was reduced by more than 99 percent within 1 to 2 minutes of microwave heating. Coliform and E. coli in kitchen sponges were completely inactivated after 30 seconds of microwave heating. Bacterial phage MS2 was totally inactivated within 1 to 2 minutes, but spores of *Bacillus cereus* were more resistant than the other microorganisms tested, requiring 4 minutes of irradiation for complete eradication. Similar inactivation rates were obtained in wastewater-contaminated scrubbing pads; however microorganisms attached to plastic syringes were more resistant to microwave irradiation than those associated with kitchen sponges or scrubbing pads. It took 10 minutes for total inactivation of the heterotrophic plate count and 4 minutes of treatment for total inactivation of total coliform and E. coli. A 4-log reduction of phage MS2 was obtained after 2 minutes of treatment with 97.4 percent reductions after 12 minutes of microwave

Devine et al. [55] conducted a trial in which microwave radiation, coupled with steam heat, was used to treat organic waste (1,136 kg of culled turkey carcasses), designed to simulate a

equation (12) and the dielectric properties of water and wood [48]

treatment.

In 2006, the cost of weed management and loss of production to Australian agricultural industries was estimated to be about \$4 billion annually [56]. Depletion of the weed seed bank is critically important to overcoming infestations of various weed species [57]. Mechanical and chemical controls are the most common methods of weed management in cropping systems [58, 59]. The success of these methods usually depends on destroying the highest number of plants during their early growth stages [58] before they interfere with crop production and subsequently set further seed. These strategies must be employed continually to deplete the weed seed bank.

Interest in the effects of high frequency electromagnetic waves on biological materials dates back to the late 19th century, while interest in the effect of high frequency waves on plant material began in the 1920's [60]. In many cases, short exposure of seeds to radio frequency and microwave radiation resulted in increased germination and vigour of the emerging seedlings [61, 62]; however, long exposure usually resulted in seed death [59].

Davis *et al.*[63, 64] were among the first to study the lethal effect of microwave heating on seeds. They treated seeds, with and without any soil, in a microwave oven and showed that seed damage was mostly influenced by a combination of seed moisture content and the energy absorbed per seed. Other findings from their studies suggested that both the specific mass and specific volume of the seeds were strongly related to seed mortality [64]. This could be due to the "*radar cross-section*" [65] presented by seeds to propagating microwaves. Large radar cross-sections allow the seeds to intercept, and therefore absorb, more microwave energy. The geometry of many seeds can be regarded as ellipsoids or even spheres, so the microwave fields are focused into the centre of the seed (see equation 8). Therefore larger seeds focus more energy into their core, which results in higher temperatures at the centre of the seed (see equation 9), leading to higher mortality rates. Seeds whose geometry can be approximated as being cylindrical will also focus more energy into their core as their dimensions increase (see equations 6 and 7).

Barker and Craker [66] investigated the use of microwave heating in soils of varying moisture content (10-280 g water/kg of dry soil) to kill 'Ogle' Oats (*Avena sativa*) seeds and an undefined number of naturalised weed seeds present in their soil samples. Their results

demonstrated that a seed's susceptibility to microwave treatment is entirely temperature dependent. When the soil temperature rose to 75°C there was a sharp decline in both oat seed and naturalised weed seed germination. When the soil temperature rose above 80°C, seed germination in all species was totally inhibited.

Several patents dealing with microwave treatment of weeds and their seeds have been registered [67-69]; however none of these systems appear to have been commercially developed. This may be due to concerns about the energy requirements to manage weed seeds in the soil using microwave energy. In a theoretical argument based on the dielectric and density properties of seeds and soils, Nelson [70] demonstrated that using microwaves to selectively heat seeds in the soil "*can not be expected*". He also concluded that seed susceptibility to damage from microwave treatment is a purely thermal effect, resulting from soil heating and thermal conduction from the soil into the seeds. This has been confirmed experimentally by Brodie *et al.*[71].

Microwaves can kill a range of weed seeds in the soil [63, 64, 72], however fewer studies have considered the efficacy of using microwave energy to manage already emerged weed plants. Davis et al. [63] considered the effect of microwave energy on bean (*Phaseolus vulgaris*) and Honey Mesquite (*Prosopis glandulosa*) seedlings. They discovered that plant aging had little effect on the susceptibility of bean plants to microwave damage, but honey mesquite's resistance to microwave damage increased with aging. They also discovered that bean plants were more susceptible to microwave treatment than honey mesquite plants.

Brodie et al. [73] studied the effect of microwave treatment on Marshmallow (*Malva parviflora*) seedlings, using a prototype microwave system based on a modified microwave oven. The prototype system, energised from the magnetron of the microwave oven operating at 2.45 GHz, has an 86 mm by 43 mm rectangular wave-guide channelling the microwaves from the oven's magnetron to a horn antenna outside of the oven. This allowed the oven's timing circuitry to control the activity of the magnetron.

Horn antennas (Figure 8), like the design used in the experimental prototype are very popular for microwave communication systems [74]. The vertical plane of the horn antenna is usually referred to as the E-plane, because of the orientation of the electrical field (or Efield) in the antenna's aperture. The horizontal plane is referred to as the H-plane, because of the orientation of the magnetic field (or H-field) of the microwave energy. The H-plane electric field distribution in the aperture of a horn antenna, fed from a wave-guide propagating in the TE10 mode, is approximated by:

$$E = E\_o \cos\left(\frac{\pi}{a}x\right) \tag{13}$$

In the case of a cylindrical object, such as a plant stem, the microwave's electric field distribution created by a horn antenna [21] can be described by:

$$E = \text{\textpi}E\_o \frac{I\_o \left(\text{\textdegree} r\right)}{I\_o \left(\text{\textdegree} r\_o\right)} \cdot \text{Cost} \left(\frac{\pi}{a} \text{\textdegree}\right) \tag{14}$$

**Figure 8.** A typical horn antenna showing the orientation of the electrical field component of the microwave energy in the antenna's aperture

The resulting temperature distribution can be described by [75] :

58 The Development and Application of Microwave Heating

seed germination in all species was totally inhibited.

confirmed experimentally by Brodie *et al.*[71].

demonstrated that a seed's susceptibility to microwave treatment is entirely temperature dependent. When the soil temperature rose to 75°C there was a sharp decline in both oat seed and naturalised weed seed germination. When the soil temperature rose above 80°C,

Several patents dealing with microwave treatment of weeds and their seeds have been registered [67-69]; however none of these systems appear to have been commercially developed. This may be due to concerns about the energy requirements to manage weed seeds in the soil using microwave energy. In a theoretical argument based on the dielectric and density properties of seeds and soils, Nelson [70] demonstrated that using microwaves to selectively heat seeds in the soil "*can not be expected*". He also concluded that seed susceptibility to damage from microwave treatment is a purely thermal effect, resulting from soil heating and thermal conduction from the soil into the seeds. This has been

Microwaves can kill a range of weed seeds in the soil [63, 64, 72], however fewer studies have considered the efficacy of using microwave energy to manage already emerged weed plants. Davis et al. [63] considered the effect of microwave energy on bean (*Phaseolus vulgaris*) and Honey Mesquite (*Prosopis glandulosa*) seedlings. They discovered that plant aging had little effect on the susceptibility of bean plants to microwave damage, but honey mesquite's resistance to microwave damage increased with aging. They also discovered that bean plants were more susceptible to microwave treatment than honey mesquite plants.

Brodie et al. [73] studied the effect of microwave treatment on Marshmallow (*Malva parviflora*) seedlings, using a prototype microwave system based on a modified microwave oven. The prototype system, energised from the magnetron of the microwave oven operating at 2.45 GHz, has an 86 mm by 43 mm rectangular wave-guide channelling the microwaves from the oven's magnetron to a horn antenna outside of the oven. This allowed

Horn antennas (Figure 8), like the design used in the experimental prototype are very popular for microwave communication systems [74]. The vertical plane of the horn antenna is usually referred to as the E-plane, because of the orientation of the electrical field (or Efield) in the antenna's aperture. The horizontal plane is referred to as the H-plane, because of the orientation of the magnetic field (or H-field) of the microwave energy. The H-plane electric field distribution in the aperture of a horn antenna, fed from a wave-guide

> 

*a*

(13)

(14)

cos *<sup>o</sup> EE x*

In the case of a cylindrical object, such as a plant stem, the microwave's electric field

 

 *o o o o I r E E Cos x I r a*

the oven's timing circuitry to control the activity of the magnetron.

distribution created by a horn antenna [21] can be described by:

propagating in the TE10 mode, is approximated by:

$$\begin{split} T &= \frac{n \alpha c\_o \kappa^{\prime\prime} \pi^2 L\_o^2 \left( e^{4\beta^2 \gamma t} - 1 \right)}{4 k \beta^2 I\_o \left( 2 \mathfrak{R} r\_o \right)} \Bigg[ \frac{4 \alpha \gamma t}{\left[ I\_o \left( \alpha r\_o \right) I\_o \left( \mathfrak{R} r\_o \right) \right]^2} e^{\frac{-r^2}{4 \gamma t}} + I\_o \left( 2 \mathfrak{R} r \right) \\ &+ \left\{ 2 \mathfrak{R} I\_1 \left( 2 \mathfrak{R} r\_o \right) + \frac{h}{k} I\_o \left( 2 \mathfrak{R} r\_o \right) \right\} \Bigg( r\_o - r \right) e^{\frac{-\left( r\_o - r \right)^2}{4 \gamma t}} \Bigg] \cdot \operatorname{Cov} \left( \frac{\pi}{a} x \right) \end{split} \tag{15}$$

Three horn applicators, with varying aperture dimension (180mm by 90 mm; 130 mm by 43 mm; and 86 mm by 20 mm), were developed and tested during various experiments [59, 73, 76, 77]. Aperture size of the horn applicator profoundly affects the treatment time needed to kill plants, with the smaller aperture needing much less time to provide a lethal dose; however the total energy density needed to kill the plants (microwave output power density multiplied by treatment time) was the same irrespective of the horn aperture size. The resulting lethal dose, which was sufficient to kill all the test species, was 350 J cm-2[75] for each plant.

Because energy rather than treatment time is the key factor in plant mortality, two options for using microwave energy to manage weeds become evident; either a prolonged exposure to very diffuse microwave fields or a strategic application of an intensely focused microwave pulse is sufficient to kill plants. Bigu-Del-Blanco*, et al.*[78] exposed 48 hour old seedlings of *Zea mays* (var. Golden Bantam) to 9 GHz radiation for 22 to 24 hours. The power density levels were between 10 and 30 mW cm-2 at the point of exposure. Temperature increases of only 4 °C, when compared with control seedlings, were measured in the microwave treated specimens. The authors concluded that the long exposure to microwave radiation, even at very low power densities, was sufficient to dehydrate the seedlings and inhibit their development. On the other hand, recent studies on fleabane (*Conyza bonariensis*) and paddy melon (*Cucumis myriocarpus*)[75, 77] have revealed that a very short (less than 5 second) pulse of microwave energy, focused onto the plant stem, was sufficient to kill these plants. In both cases, rapid dehydration of the plant tissue appears to be the cause of death. This is because microwave heating results in rapid diffusion of moisture [29] through the plant stem as suggested earlier.

Based on energy calculations for plants and seeds on the surface of sandy soil [72, 76], the energy needed to kill dry seeds is an order of magnitude higher than the energy needed to kill already emerged plants. The microwave energy dose needed to kill a paddy melon or fleabane plant was approximately 350 J cm-2 (or 35 GJ ha-1) [77]. This is an order of magnitude higher than the embodied energy (2.2 – 3.0 GJ ha-1) associated with chemical weed management [79-82]; however the real microwave energy requirements on a large scale will depend on the plant density and spatial distribution. Therefore the microwave energy requirements may be greatly reduced if appropriate techniques, such as weed seeker systems [83], are employed to only turn on the microwave unit when the system encounters a weed in the field. The growing problems of herbicide resistance [84] also warrants ongoing research and development of microwave weed control technologies. Other strategies may also reduce this energy requirement even further.

It has been well documented that the dielectric properties of most materials are temperature and moisture dependent [22, 85-88]. Ulaby and El-Rayes [89, 90] studied the dielectric properties of plant materials at microwave frequencies. Plants with high moisture content have higher dielectric constants and will therefore interact more with the microwave fields, rendering them more susceptible to microwave damage.

Equation (15) was used in an iterative calculation, where the new dielectric properties for plant based materials were recalculated after every second of microwave heating, based on the changes in temperature and moisture content of the plant during that interval of the microwave heating progresses. This results in non-linear heating responses and sudden jumps in temperature when there is no change in the applied microwave field strength (Figure 9). The sudden jump in temperature for the 15 mm diameter stem is the result of "thermal runaway". The onset of thermal runaway is also dependent on the microwave field intensity (Figure 10) and the heat transfer properties of the heated material (Figure 11). Under the influence of simultaneous heat and moisture diffusion during microwave heating, the effective thermal conductivity of a microwave heated material can be many times the normal value for the plant tissue [17] (see equation (3)).

In most cases, thermal runaway is a problem during microwave heating. It usually leads to undesirable charring of the microwave heated material (see Figure 2) [91]; however it has been very effectively used in some applications such as the development of a microwave drill [92, 93] and preconditioning of wood for further preservative treatment and drying [94, 95].

Vriezinga has concluded that thermal runaway in moist materials, and water in general, is caused by: the specific characteristic of the dielectric properties of water, which decrease with increasing temperature [87] (Figure 4); and resonance of the electromagnetic waves within the irradiated medium due to changes in the dielectric properties of the material during heating [87, 88]. Resonance will only occur when the object's dimensions are some multiple of the wave length of the microwave fields inside the object. That is why thermal runaway only becomes evident in the 15 mm diameter stem (Figure 9), while the smaller stems are too small to allow internal field resonance. Internal steam pressure, induced by

also reduce this energy requirement even further.

rendering them more susceptible to microwave damage.

times the normal value for the plant tissue [17] (see equation (3)).

95].

Based on energy calculations for plants and seeds on the surface of sandy soil [72, 76], the energy needed to kill dry seeds is an order of magnitude higher than the energy needed to kill already emerged plants. The microwave energy dose needed to kill a paddy melon or fleabane plant was approximately 350 J cm-2 (or 35 GJ ha-1) [77]. This is an order of magnitude higher than the embodied energy (2.2 – 3.0 GJ ha-1) associated with chemical weed management [79-82]; however the real microwave energy requirements on a large scale will depend on the plant density and spatial distribution. Therefore the microwave energy requirements may be greatly reduced if appropriate techniques, such as weed seeker systems [83], are employed to only turn on the microwave unit when the system encounters a weed in the field. The growing problems of herbicide resistance [84] also warrants ongoing research and development of microwave weed control technologies. Other strategies may

It has been well documented that the dielectric properties of most materials are temperature and moisture dependent [22, 85-88]. Ulaby and El-Rayes [89, 90] studied the dielectric properties of plant materials at microwave frequencies. Plants with high moisture content have higher dielectric constants and will therefore interact more with the microwave fields,

Equation (15) was used in an iterative calculation, where the new dielectric properties for plant based materials were recalculated after every second of microwave heating, based on the changes in temperature and moisture content of the plant during that interval of the microwave heating progresses. This results in non-linear heating responses and sudden jumps in temperature when there is no change in the applied microwave field strength (Figure 9). The sudden jump in temperature for the 15 mm diameter stem is the result of "thermal runaway". The onset of thermal runaway is also dependent on the microwave field intensity (Figure 10) and the heat transfer properties of the heated material (Figure 11). Under the influence of simultaneous heat and moisture diffusion during microwave heating, the effective thermal conductivity of a microwave heated material can be many

In most cases, thermal runaway is a problem during microwave heating. It usually leads to undesirable charring of the microwave heated material (see Figure 2) [91]; however it has been very effectively used in some applications such as the development of a microwave drill [92, 93] and preconditioning of wood for further preservative treatment and drying [94,

Vriezinga has concluded that thermal runaway in moist materials, and water in general, is caused by: the specific characteristic of the dielectric properties of water, which decrease with increasing temperature [87] (Figure 4); and resonance of the electromagnetic waves within the irradiated medium due to changes in the dielectric properties of the material during heating [87, 88]. Resonance will only occur when the object's dimensions are some multiple of the wave length of the microwave fields inside the object. That is why thermal runaway only becomes evident in the 15 mm diameter stem (Figure 9), while the smaller stems are too small to allow internal field resonance. Internal steam pressure, induced by

**Figure 9.** Temperature response, at constant microwave power density at a frequency of 2.45 GHz, in the centre of a plant stem as a function of plant stem diameter, calculated using equation (17) and assuming moisture content loss (MC = 0.87 to 0.10) described by equation (10)

thermal runaway, may cause stem rupture, if sufficient microwave field intensity can be focused onto the plants (Figure 10).

**Figure 10.** Temperature response in the centre of a 15 mm diameter plant stem as a function of applied microwave field intensity, calculated using equation (17) and assuming moisture content loss (MC = 0.87 to 0.10) described by equation (10)

**Figure 11.** Temperature response in the centre of a 15 mm diameter plant stem as a function of tissue thermal conductivity, calculated using equation (17) and assuming moisture content loss (MC = 0.87 to 0.10) described by equation (10)

Moriwaki et al [96] studied the dehydrochlorination of polyvinyl chloride (PVC) by microwave irradiation using an optical fibre thermo-sensor to investigate the relationship between temperature and microwave absorption onto PVC. Their observations were that: at the beginning of microwave irradiation, the temperature rose in direct proportion to the strength of the incident microwave power and irradiation time; after exceeding a critical condition, the temperature rose quickly (thermal runaway); higher incident microwave power led to thermal runaway starting earlier in the heating process; and higher pre-heating temperatures also led to a faster onset of thermal runaway conditions. These findings are consistent with the modelling displayed in Figures 9, 10, and 11.

Total treatment time, and therefore total applied microwave energy, could be significantly reduced, if thermal runaway can be induced in weed plants during microwave treatment. For example, extrapolating the data presented in Figure 9, it takes approximately 200 to 250 seconds for the 10 mm diameter stem to reach 40 °C; however the 15 mm diameter stem reaches 40 °C in 25 seconds under the same applied microwave power. Therefore the energy required to achieve this temperature rise in the 15 mm diameter stem is only 10 % of the energy needed to heat the 10 mm diameter stem.

Based on existing data, phenomena such as thermal runaway, and the nonlinear temperature/ microwave field strength relationships, it is difficult to discuss "scale up" from small laboratory studies and modelling exercises such as have been used here; however if thermal runaway can be induced in plant tissues, treatment time, and the associated treatment energy, may be drastically reduced; resulting in comparable energy needs to those associated with conventional chemical weed control. This scenario can only be explored by further research into the microwave heating of living plants and plant based materials.

In weed control, microwave radiation is not affected by wind, which extends the application periods compared with conventional herbicide spraying. Energy can also be focused onto individual plants without affecting adjacent plants [75]. This would be very useful for incrop or spot weed control activities. Microwave energy can also kill the roots and seeds that are buried to a depth of several centimetres in the soil [73, 97].
