## 3 Results and Discussion
### Effect of the Presence of Bacteria  
Based on the complete dataset collected for the RF treatments of TSB and BHI using the calibrated setup, all parameters (i.e., temperature, frequency, power consumption) were accepted for further use. To assess whether the presence of bacterial cells in the two media affected the different parameters, the experiments from the previous section were repeated in the presence of bacteria. TSB was inoculated with _S. Typhimurium_, while BHI was inoculated with _L. monocytogenes_, both to a cell density of approximately \(8\log_{10}\text{CFU}/\text{mL}\). Experiments were only conducted for 20 min, since preliminary experiments indicated that the complete bacterial population was already inactivated after this treatment time due to the high temperatures. Figure 6 shows the comparison of the temperature evolution in TSB and BHI both for the case with and without bacterial cells. Both in TSB and BHI, temperatures were generally higher in the presence of bacterial cells, but differences were rarely statistically significant (results not shown). The increased heating rate in the presence of bacterial cells that sometimes occurred was probably caused by the influence of the cells on the dielectric properties of the samples. The effect of the presence of cellular material on the dielectric properties of solutions is well-known and has even led to the development of applications for the real-time estimation of microbial biomass during growth experiments [28; 29; 30]. The  
Figure 5: Power consumption profiles in TSB (**A**) and BHI (**B**) during RF treatments. Individual replicates and the average power consumption profile are shown. exact influencing mechanisms of this phenomenon are quite complex, and hence, a detailed explanation falls outside the scope of this study. In brief, bacterial cell membranes are characterised by a low conductance, exerting a significant influence on the total dielectric properties of cells suspensions, especially at the high cell densities that are applied in this study [31]. The principal mechanism for the dielectric polarisation at frequencies in the MHz range is the accumulation of charges from extra- and intracellular fluids (i.e., the Maxwell-Wagner effect) at the membrane [32; 33]. The effect of bacterial cell presence on the frequency inside both media is illustrated in Figure 7. Both for TSB and BHI, the statistical analysis (results not shown) showed no significant differences between the situations with and without cells. While the frequency in BHI was consistently higher in the presence of \(L\). _monocytogenes_, the maximum difference was rather small, i.e., 0.02-0.04 MHz. Therefore, the presence of the bacterial cells did not seem to influence the frequency of the RF waves. Figure 6: Temperature profiles (average \(\pm\) SD) in TSB (**A**) and BHI (**B**) during RF treatments, both with and without bacterial cells. TSB and BHI were inoculated with \(S\). _Typhimuriumium_ and \(L\). _monocytogenes_, respectively. Finally, Figure 8 shows the power consumption evolution in both media for the cases with and without bacterial cells. For TSB, the power consumption was always higher when cells were present in the broths, even though statistical differences were only observed between 1 and 7 min of treatment (results not shown). For BHI, the power consumption was lower when cells were present after approximately 10 min, although differences were only significant after 19 and 20 min. The different behaviour of the two medium-microorganism combinations was probably caused by the combined action of two effects. Firstly, as mentioned, the power consumption without bacterial cells was higher in BHI than in TSB. Secondly, the influence of the presence of bacterial cells on the dielectric properties is dependent on the type of microorganisms, with the effect being considerably larger in Gram-positive bacteria than in their Gram-negative counterparts due to the different cell envelope organisation [34; 35].