**4. Quarantine**

Dried timber, nuts and fruits are commonly treated by chemical fumigation to control field and storage pests before being shipped to domestic and international markets. Because chemical fumigants such as methyl bromide are no longer available [34], there is a heightened interest in developing non–chemical pest control. An important key to developing successful thermal treatments is to balance the need for complete insect mortality with minimal impact on the product quality. A common difficulty in using conventional hot–air disinfestation is the slow heating rate, non–uniform temperature distribution, and possible heat damage to heat–sensitive commodities [35]. A more promising approach is to heat the commodity rapidly using radio frequency (RF) or microwave dielectric heating to control insects [35, 36].

Interest in controlling insects, using electromagnetic energy, dates back nearly 70 years. Headlee [37, 38], cites one earlier report of experiments determining lethal exposures for several insect species to 12 MHz electric fields and the body temperatures produced in honey bees due to dielectric heating. Nelson [39] has shown that microwaves can kill insects in grain; however one of the challenges for microwave insect control is to differentially heat the insects in preference to their surrounds. Nelson [39] shows that differential heating depends on microwave frequency. It appears that using a 2.45 GHz microwave system, which is the frequency used in domestic microwave ovens, heats the bulk material, which then transfers heat to the insects; however lower frequencies heat the insects without raising the temperature of the surrounding material beyond 50°C [39].

Nzokou et al [40] investigated the use of kiln and microwave heat treatments for the sanitisation of emerald ash borer (*Agrilus planipennis* Fairmaire) infested logs. Their microwave treatment method was conducted in a 2.8 GHz microwave oven (volume: 0.062 m3, power: 1250 W) manufactured by Panasonic (Panasonic Co., Secaucus, New Jersey). Due to the limited volume of the microwave oven, two runs were necessary to treat logs assigned to each microwave treatment temperature. Their results showed that a temperature of 65°C was successful at sanitising the infested logs. Microwave treatment was not as effective as kiln treatment, probably because of the uneven distribution of the microwave fields and temperature inside the treated logs. This uneven temperature distribution is partly due to the nature of microwave heating, but may also be due to their choice of microwave chamber used during their experiments.

In spite of this, with the high costs and level of energy needed to thoroughly heat logs to the desired 65 °C using conventional heating, microwave heating is still a very attractive solution for rapid heat sterilisation of infested wood materials [40]. The problem of ensuring appropriate temperature distribution inside treated materials can be easily overcome by using appropriate microwave applicators rather than a multi-mode cavity [41]. Several options are available including conveyer belt feeds through a long choke tunnel into a purpose built applicator [41], or projecting a very intense but short duration microwave field pulse into the material, using an antenna. Plaza *et al.*[42] have developed a system which employs a circular wave-guide energized by two microwave sources oriented at 90 ° to one another. This orthogonal orientation of the microwave fields ensures that they do not interfere with each other, but provides a high power source from relatively cheap mass produced 1 kW magnetrons. Microwave magnetrons of greater power output than 1 kW are usually one or two orders of magnitude more expensive than the 1 kW versions.

52 The Development and Application of Microwave Heating

microwave dielectric heating to control insects [35, 36].

the temperature of the surrounding material beyond 50°C [39].

used during their experiments.

desirable.

**4. Quarantine** 

methods, especially when rapid drying and high throughputs of moist material are

Dried timber, nuts and fruits are commonly treated by chemical fumigation to control field and storage pests before being shipped to domestic and international markets. Because chemical fumigants such as methyl bromide are no longer available [34], there is a heightened interest in developing non–chemical pest control. An important key to developing successful thermal treatments is to balance the need for complete insect mortality with minimal impact on the product quality. A common difficulty in using conventional hot–air disinfestation is the slow heating rate, non–uniform temperature distribution, and possible heat damage to heat–sensitive commodities [35]. A more promising approach is to heat the commodity rapidly using radio frequency (RF) or

Interest in controlling insects, using electromagnetic energy, dates back nearly 70 years. Headlee [37, 38], cites one earlier report of experiments determining lethal exposures for several insect species to 12 MHz electric fields and the body temperatures produced in honey bees due to dielectric heating. Nelson [39] has shown that microwaves can kill insects in grain; however one of the challenges for microwave insect control is to differentially heat the insects in preference to their surrounds. Nelson [39] shows that differential heating depends on microwave frequency. It appears that using a 2.45 GHz microwave system, which is the frequency used in domestic microwave ovens, heats the bulk material, which then transfers heat to the insects; however lower frequencies heat the insects without raising

Nzokou et al [40] investigated the use of kiln and microwave heat treatments for the sanitisation of emerald ash borer (*Agrilus planipennis* Fairmaire) infested logs. Their microwave treatment method was conducted in a 2.8 GHz microwave oven (volume: 0.062 m3, power: 1250 W) manufactured by Panasonic (Panasonic Co., Secaucus, New Jersey). Due to the limited volume of the microwave oven, two runs were necessary to treat logs assigned to each microwave treatment temperature. Their results showed that a temperature of 65°C was successful at sanitising the infested logs. Microwave treatment was not as effective as kiln treatment, probably because of the uneven distribution of the microwave fields and temperature inside the treated logs. This uneven temperature distribution is partly due to the nature of microwave heating, but may also be due to their choice of microwave chamber

In spite of this, with the high costs and level of energy needed to thoroughly heat logs to the desired 65 °C using conventional heating, microwave heating is still a very attractive solution for rapid heat sterilisation of infested wood materials [40]. The problem of ensuring appropriate temperature distribution inside treated materials can be easily overcome by using appropriate microwave applicators rather than a multi-mode cavity [41]. Several options are available including conveyer belt feeds through a long choke tunnel into a It has been shown earlier that microwave heating in moist materials, such as the body of an insect, induces a very fast moving wave of heat and water vapour [17]. The intensity of this wave is directly linked to the intensity of the microwave fields [43], therefore using very intense microwave fields may rupture the internal organs of insects, due to local steam explosions.

The interaction of electromagnetic energy with matter is determined by the dielectric properties of the material. The permittivity of a material can be expressed as a complex quantity, the real part (') of which is associated with the capability of the material for storing energy in the electric field of the electromagnetic wave, and the imaginary part (") is associated with the conversion of electromagnetic energy to heat inside the material [39]. This is the phenomenon commonly referred to as dielectric heating. The dielectric properties also determine the reflectivity of a material.

The power dissipated per unit volume in a nonmagnetic, uniform material exposed to radio frequency (RF) or microwave fields can be expressed as:

$$P = \left(\pi E\right)^2 \sigma \ = \left. 55.63 \times 10^{-12} \text{ f } \left(\pi E\right)^2 \mathbf{x}^{\prime} \tag{11}$$

Therefore in a system composed of two or more materials, there will be preferential heating in favour of the material with the least reflectivity and higher dielectric loss factor. The rate of temperature increase also depends on the density and thermal capacity of the heated material [35, 36]:

$$\frac{dT}{dt} = \frac{P}{\rho \mathcal{C}}\tag{12}$$

Termites are a good example of insects that infest economically important products. For example, in the United States, the annual cost of treating damage caused by the Formosan termite (*Coptotermes formosanus*) exceeds \$US 1 billion [44]. The radar cross section of some insect species, including termites, has been modelled by treating them as drops of water of equivalent size and shape [45].

Liquid water exhibits dielectric relaxation at around 22 GHz [46] (Figure 4). There are higher dielectric relaxations in water at about 280 GHz [46], 4.5 THz and 15.4 THz [47]. The dielectric properties of grains, soil and wood also depend on their moisture content [48] (Figure 5).

**Figure 4.** Dielectric properties of pure water as a function of frequency and temperature (calculated using equations and data from literature [46])

**Figure 5.** Dielectric properties of wood (density = 500 kg m-3) as a function of frequency and moisture content varying between 0 % and 100 % on a dry wood basis (calculated using equations and data from literature [48])

Dry wood-in-service is in hydro-thermodynamic equilibrium with its surroundings. This condition is known as the equilibrium moisture content [49]. Depending on the atmospheric conditions, equilibrium moisture content is usually about 12% moisture on a dry wood weight for weight basis. When termites invade wood, they often import moisture into the structure to maintain a suitable microclimate for their foraging activities. The maximum moisture content that wood can attain before free water begins to form is known as fibre saturation. This occurs at about 25 - 30 % moisture content [49], depending on the wood species. Fibre saturation refers to the state when all the cells are free of water and only bound water is found within the cell walls. Usually termites do not increase the moisture content beyond fibre saturation. The dielectric properties of termites (modelled as water) and wood at fibre saturation are significantly different from each other (Figure 6).

54 The Development and Application of Microwave Heating

using equations and data from literature [46])

literature [48])

**Figure 4.** Dielectric properties of pure water as a function of frequency and temperature (calculated

**Figure 5.** Dielectric properties of wood (density = 500 kg m-3) as a function of frequency and moisture content varying between 0 % and 100 % on a dry wood basis (calculated using equations and data from

**Figure 6.** Comparison of the dielectric properties of wood (density = 500 kg m-3) at fibre saturation with water

Treatment of termite infestations using microwave energy, at 2.45 GHz, has been available for some time [50, 51]. This technique does not directly heat the termites, but heats the surrounding wood to more than 55°C [52], which then causes termite mortality. Unfortunately, the combination of high reflectivity and low dielectric losses for water in the lower microwave frequency band (2.45 GHz) means that there is virtually no differential heating between the termites and wood that is at fibre saturation; however significant differential heating should occur once the frequency increases above 20 GHz (Figure 7). Research in the field of ultra-high frequencies (>20 GHz) indicates that these frequencies may selectively heat insect pests in favour of the materials they infest [52, 53]. Therefore

research into ultra-high frequency microwave based insect control should yield some valuable insights over the coming decades [52].

**Figure 7.** Relative dielectric heating and wood at fibre saturation moisture content, calculated using equation (12) and the dielectric properties of water and wood [48]

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 treatment.

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 small-scale poultry mortality event. They inoculated the turkey carcasses with *Bacillus atrophaeus* spores and *Salmonella enterica* before inserting them into a purpose built portable microwave unit (Sanitec Industries), along with other organic waste. The units are designed to treat in excess of 250 kg/per hour of waste. The system has been designed so that the waste is transported through the microwave fields along a screw so that the final exposure time and temperature profile is a minimum of 30 minutes at 95 °C. The system generated a seven-log reduction in the microbial load of Salmonella and a five-log reduction in Bacillus spores. These results illustrate the potential of using microwave radiation for quarantine procedures. The following sections will illustrate more specifically how microwave energy can manage pests in agricultural and forestry systems.
