# Inactivation of Contaminated Fungi in Rice Grains by Dielectric Heating
## 3 Results and Discussion  
### Optimization of Frequency Band under Dielectric Heating System  
The dielectric properties [dielectric constant (\(\epsilon^{\prime}\)) and loss (\(\epsilon^{\prime\prime}\))] of _Aspergillus_ sp. BP17 and rice powder samples were investigated at frequency ranges as 40 to 8500 MHz as shown in Figure 5. The result showed that dielectric constants of both _Aspergillus_ sp. BP17 and rice powder were decreased when frequency increased. In accordance with [PERSON] et al. (2017) reported that the electric constant is generally decreases when frequency increases for all types of rice due to the presence of polar molecules such as glucose and fructose [40]. At low frequencies (<200 MHz), the dielectric constants of _Aspergillus_ sp. BP17 rapidly decreased from 31.5 to 21.8 at frequencies between 40 to 200 MHz (Figure 5a, upper line). A previous study reported that the dielectric constants of fungi were higher than 4.0 at frequencies lower than 200 MHz including _Botrytis cinerea_, _Alternaria solani_, _Ceratocystis radiicola_, _Rhizoctonia solani_, _Lasiodiplodia thebromae_, _Fusarium oxysporum_ and _F. solani_[39]. However, the dielectric constants of rice powder slowly reduced from 5.06 to 4.63 at frequencies between 40 to 200 MHz (Figure 5a, bottom line). In comparison between fungi and rice samples, _Aspergillus_ sp. BP17 showed higher dielectric constant than the rice sample due to it being associated with cell membrane polarization [46].  
The dielectric loss of _Aspergillus_ sp. BP17 rapidly decreased from 48.92 to 13.13 at frequencies between 40 to 200 MHz (Figure 5b, upper line). After that, the values were slowly increased when frequencies were increased between 2000 and 8500 MHz. In contrast, the dielectric loss of rice was 0.26 and 0.86 at frequency of 40-200 and 8500 MHz, respectively (Figure 5b, bottom line). _Aspergillus_ sp. BP17 and rice powder showed a different dielectric loss because the change of dielectric loss was caused by the relative effects of ionic conductivity and free water relaxation [40; 41; 47]. Moreover, the dielectric loss behavior of _Aspergillus_ sp. BP17 was similar to the results reported in edible fungi [47]  
Figure 5: Dielectric properties of _Aspergillus_ sp. BP17 and rice powder: (**a**) Dielectric constant (\(\epsilon^{\prime}\)) and (**b**) Dielectric loss (\(\epsilon^{\prime\prime}\)).  
and the dielectric loss behavior of rice in this study was also similar to Jasmine rice 105, Hompathum rice, Phitsanulok rice, Chinart rice, and Gorkor 43 rice [48].  
According to the US Federal Communication Commission (FCC), it has been reported that the optimal specific frequencies for ISM uses 13.56, 27.12, and 40.68 MHz of RF and 915 and 2,450 MHz of MW [44]. So, three ISM frequency bands, including 40.68, 915, and 2450 MHz, were selected and calculated the ratio of dielectric constant and loss in fungal and rice powder samples (Table 1). The result showed that optimal frequency was 40.68 MHz based on the highest of ratio (194.9 folds) between dielectric loss of _Aspergillus_ sp. BP17 and rice powder, which is the same result with Figure 5b. Therefore, at a frequency with the high difference of dielectric loss between fungi and rice can be applied for fungal inactivation using dielectric heating applications based on the \"thermal runaway\" phenomenon as described [33]. Therefore, the frequency of 40.68 MHz was selected for fungal control using dielectric heating system in the next experiment.