Biosecurity is critical to protect ecological and agricultural productions systems around the planet. Biosecurity applies various procedures or measures, which are designed to protect the population and environment against the introduction, establishment and spread of harmful biological organisms or substances. The fumigant methyl bromide is an important biosecurity tool for disinfesting internationally traded commodities against quarantine pathogens, pests, and weeds. However, methyl bromide is implicated in the degradation of stratospheric ozone and non-quarantine and pre-shipment uses are being regulated and phased out under the 1992 Montreal Protocol [1]. Quarantine uses of methyl bromide are not currently regulated under the Protocol and are defined as “treatments to prevent the introduction, establishment and/or spread of quarantine pests (including diseases), or to ensure their official control”. Similarly, pre-shipment uses of methyl bromide are currently not regulated and are defined as “non-quarantine applications applied within 21 days prior to export to meet the official requirements of the importing country or existing official requirements of the exporting country” (Decisions VI/11, VII/5 and XI/12 of the Montreal Protocol). Currently, the official worldwide use of methyl bromide for quarantine and pre-shipment purposes as reported by Parties to the Montreal Protocol under Article 7 averages 10,000 tons per year (see link to reference). Therefore, alternative biosecurity technology must be explored.

Among the non-chemical concepts for commodity disinfestation is thermal treatment using solar energy and steam [2]. An advantage of these approaches is there is little risk of resistance developing in pest and pathogen populations. However, these conventional thermal treatments are very time and energy intensive, and this often makes their adoption uneconomical. In contrast, microwave energy is a very fast and efficient method that can be used for directly heating various materials [3]. An advantage of the microwave is that it can heat the commodity and not the container or the environment and this makes the process very efficient. It is even possible to efficiently heat, melt, and process metals using specially designed industrial microwave ovens [4]. Microwave energy can also be used to ionize various gasses, including air, to create plasma ^[5][6][7][5,6,7], which can be used to sterilize surfaces.

2. Thermal Disinfestation

Over the past 50 years, the use of electromagnetic energy, especially in the radio and microwave frequency range, has been proposed as a method of controlling pests in soils, grains, and timber products [8]. Initially, most research was focused on thermal treatment of the commodity ^[8][9][8,9]; however, electromagnetic energy can be used in other ways to disinfest or disinfect materials. Focusing firstly on the thermal effects of electromagnetic treatment, the fatal impacts of high temperatures on botanical and zoological specimens have been studied in detail for over a century [10]. An empirical relationship between lethal temperature and temperature holding time has been developed by Lepeschkin [11]: where T is the lethal temperature (°C), and Z is the lethal temperature holding time, in minutes [10]. Individual relationships for different species of plants and pathogens ^[11][12][13][11,12,13] have been developed over time. Ultimately, heat can provide similar lethal effects to chemicals and therefore has been used in soil, timber, hay, and grain sanitation processes [14] for some time. Various heat sources have been considered for sanitation applications. These include, solarization, solar heating, flaming, steam heating, and electromagnetic heating. The following section highlights how electromagnetic heating, especially using microwave energy, has been used for thermal sanitization.

3. Microwave Heat Sanitization

Many studies have considered microwave heating as a method of sanitization. Table 1 provides an overview of some key research. One of the main areas of study has been the deactivation of weed seeds in the soil. Davis, et al. [15] demonstrated the efficacy of microwave energy for weed management. They developed a prototype system, called the “Zapper” [16], which could treat the soil in situ, using a variant on a horn antenna to apply the microwave energy to the soil’s surface. To obtain consistent pre-emergent control of both broadleaved weeds and grasses, it was necessary to apply at least 183 J cm⁻². Brodie, et al. [17] later confirmed that 185 J cm⁻² of microwave energy, when applied to moist soil, could effectively kill various Lolium spp. (ryegrasses) seeds to a depth of 5 cm. Treating seeds in dry soil required over 550 J cm⁻² of microwave energy to kill seeds to a depth of only 2–3 cm [17]. The energy required to control emerged weeds using a horn antenna is quite variable (77–500 J cm⁻²) ^[15][18][15,18], depending in the species and the height of the horn antenna above the ground. Recent experiments using a 15 cm wide slow-wave applicator, connected to a 5-kW microwave source, and being towed at an equivalent speed of approximately 0.6 km h⁻¹ (17 cm s⁻¹), has demonstrated that applying 20 J cm⁻² of microwave energy can kill most emerged weeds (unpublished).

3.1. Effects on Soil Biota

It has been demonstrated that microwave soil heating has an immediate impact on soil microbial communities [19]. The populations of some species are significantly reduced [20]; however, other species, including nitrifying bacteria and archaea, are relatively unaffected, except at extremely high doses of microwave energy ^[21][22][23][21,22,23]. Soil bacterial and fungal community compositions change significantly due to microwave soil treatment and recovery of biological diversity takes more than 4–5 weeks [22]. Recent experiments have demonstrated that microwave soil treatments, with the similar intensity necessary to kill weed seeds, significantly reduce the number of soil-borne fungal pathogens, including Fusarium spp., Macrophomina phaseolina, and Thielaviopsis basicola (unpublished).

3.2. Implications for Cropping Systems

The combination of removal of weed competition and soil disinfestation provided by microwave soil treatment results in significant crop yield increases. In field experiments, increases in crop yield of between 18% and 84%, compared with the untreated or hand-weeded controls, have been observed ^[24][25][24,25]. Pot experiments have demonstrated that a single microwave soil treatment can provide significant crop yield increases over several seasons, with the longest observations spanning three years, so far [26]. While disinfestation of soil has been a key focus of microwave research, other disinfestation and disinfestation studies have been undertaken. Table 2 lists some of the key findings from several examples. Most microwave treatments are focused on the temperature range below 100 °C. This is partly because of the very high latent heat of vaporization for water; however, some studies have shown that microwave heating can achieve much higher temperatures. For example, Falciglia and Vagliasindi [48] achieved temperatures of up to 260 °C, when 180 MJ kg⁻¹ of microwave heat was applied to soil at 12% moisture content (dry weight basis). These temperatures are high enough to kill most pathogens, including anthrax spores. Microwave assisted pyrolysis at temperatures above 700 °C have been achieved ^[49][50][49,50]. At these temperatures, organic molecules are decomposed to produce syngas, bio-oil, and biochar. These products can be regarded as sterile.

3.3. Microwave Treatment of Animal Fodder

Weed seeds, pests (e.g., fire ants), and pathogens are often transported in hay [51]. Fumigation can manage these problems; however, like with soil disinfestation, it is possible for heat to overcome these problems as well [52]. In accordance with the temperature–time response of all living things, the mortality of insects, seeds, and pathogens increases with temperature and exposure time [53]. Dong, et al. [54] discovered that the organic matter degradability of wheat straw in the rumen of yaks was increased by around 20% after 4 min of treatment in a 750 W, 2.54 GHz, microwave oven. Sadeghi and Shawrang [55] showed that microwave treatment of canola meal increased in vitro dry matter disappearance, including substances that were deemed undegradable in the rumen. Sadeghi and Shawrang [56] also showed that microwave treatment reduced the rumen degradable starch fraction of corn grain and decreased crude protein degradation of the soya bean meal compared with untreated samples. Small scale in vitro pepsin-cellulase digestion experiments [57] demonstrated that microwave treatment: increased dry matter percentage with increasing microwave treatment time; increased in vitro dry matter disappearance with increasing microwave treatment time; but had no significant effect on post-digestion crude protein content. Live animal trials, using microwave-treated lucerne hay showed an 8.1% higher increase in body weight, compared with animals fed with untreated hay [57].

3.4. Microwave Treatment of Timber for Pest Control

Non-chemical treatments for infested timber have been reported for some time [58]. The efficacy of excessive heat or cold, electrocution and microwave energy have all been investigated. Heating of the timber using high temperature kilns resulted in 90 to 96% mortality of Western Drywood Termites (Isoptera: Kalotermitidae), three days after treatment [58]. It was interesting to note that mortality rose to 98% after 4 weeks [58]. When exposed to radiofrequencies/microwaves the first reaction of insects is an attempt to escape; this is followed by loss of motor coordination, stiffening, immobility and, after a certain time interval, death [59]. Lewis [60] demonstrated that Western Drywood Termites could be controlled by microwave heating, with 100% mortality being achieved at a microwave dose density of 65 J cm⁻². Massa, Panariello, Pinchera, Schettino, Caprio, Griffo and Migliore [59] investigated the efficacy of microwave treatment of standing live palm trees for the control of Red Palm Weevil (Rhynchophorus ferrugineus Olivier). Their objective was to thermally control the weevil without killing the host plant. Differences in susceptibility of the insect were found between development stages within the species. Their results showed that the adult insects are much more sensitive to heat than the larger larvae with 20 min at 50 °C and only 4 min at 80 °C causing adult death. Lethal time for the larvae varies with weight and the most resistant were those weighting between 4 and 6 g, requiring 30 min at 50 °C [59]. Based on their successful experimental work, a semi-commercial prototype has been developed to treat palm trees, in situ, using a ring applicator to apply the microwave energy to the tree.

3.5. Scale-Up of Microwave Heating Systems

Scale-up of microwave heating systems has been explored for over 40 years [61]. Many efficient industrial microwave heating plants have been established in several industries, including: the food industry, the rubber processing industry, the timber industry, several waste management industries, and for general product drying [61]; however, scale-up of most microwave technologies still requires much more investigation and refinement. Most research is completed at laboratory scale, where the field densities in small samples are high. Achieving similar field densities when processing large volumes of material can be challenging. The two important parameters that limit field density inside the material are: the penetration depth of the microwave fields into the processed material and available power. Penetration depth depends on the dielectric properties of the material and the operating frequency of the microwave system ^{[62][63][64][65]}[62,63,64,65]. Penetration depth is directly dependent on microwave frequency, so the first step in the scale-up process usually implies the use of lower industrial, scientific, and medical (ISM) frequencies. For example, many microwave heating experiments are performed at 2450 MHz, which is the frequency used in domestic microwave ovens; however, industrial-scale systems usually operate at 915 MHz (or 868 MHz or 922 MHz, depending on how this ISM band is defined in different parts of the world), which is the next lower ISM frequency [66]. Adopting the lower frequency increases the penetration depth of the microwave fields. The trade-off of increasing penetration depth is that the volume of material which is absorbing microwave energy significantly increases and therefore the resulting field density in the larger volume is significantly reduced. The field density can be increased by increasing the available microwave power. Microwave generators, varying from a few hundred watts up to 100 kW can be used individually [3] or in multiple generator designs ^[67][68][67,68] for microwave heating applications. Although they only seem to have been used for research and military applications, single relativistic cavity magnetrons have been shown to produce in excess of 4 GW [69]. Multiple phase-locked sources output of 3 GW total output; repetitive, near-GW peak powers at pulse repetition rates up to 1 kHz, and long-pulse operation with energy per pulse of about 1 kJ have also been achieved [69]. Installations vary in design, depending on the commodity being processed. Continuous processing can be achieved using tunnel applicators [3]; however, careful electromagnetic design of the in-feed and out-feed ports is needed to prevent radiation leakage into the environment. Microwave cavities ^[3][70][71][3,70,71] can better prevent microwave field leakage; however, they must be used for batch processing. Therefore, the major challenges of microwave heating scale-up are material handling and achieving similar field densities inside the commodity as was achieved during laboratory-scale experiments.