## 3 Results and Discussion
### RF Treatment of Nutrient Broths without Bacteria
#### 3.2.1 Temperature  
Concerning temperature, the objective was to develop an RF setup capable of providing treatments with a maximum temperature difference of 5 \({}^{\circ}\)C between different replicates for each nutrient broth, i.e., TSB and BHI. Figure 3 shows the temperature profiles during the high-frequency treatment of TSB and BHI media for various repetitions. After 15 min, temperatures of 71-76 and 71-75 \({}^{\circ}\)C were reached in TSB and BHI, respectively, hence satisfying the prerequisite of a maximum 5 \({}^{\circ}\)C temperature variation over the different replicates. However, the observed temperature variations would exert a significant influence on microbial inactivation rates during thermal inactivation treatments. If temperature variations cannot be further reduced, temperatures should be monitored in future studies  
\begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value** \\ \hline Primary coil & 1.09 mm wire diameter \\  & 4 turns of diameter 6.88 mm \\ Secondary coil & 0.20 mm wire diameter \\  & 100 turns of diameter 6.10 mm \\ Frequency LC resonance & 9.84 MHz \\  & 33.22 MHz \\  & 120 pF \\  & 29.15 \(\pm\) 0.1 MHz \\  & 28.93 MHz \\  & \\  & \\  & \\  & \\ \end{tabular}
\end{table}
Table 1: Optimised initial parameters and design specifications of the RF setup without a sample load.  
during each inactivation trial (i.e., rather than relying on an assumed average temperature profile) in order to allow correct data interpretation.  
The temperatures of 71-76 \({}^{\circ}\)C reached after 15 min are sufficiently severe to acquire the thermal inactivation of both \(L\). _monocytogenes_ and _S. Typhimuriumium_ in liquid food products such as milk [23], and the relatively slow heating rate enables frequent sampling to study the inactivation kinetics. In future studies, the input voltage could also be controlled once a certain target temperature is reached in order not to expose bacteria to excessive temperatures. After 30 min, temperatures of 81-88 \({}^{\circ}\)C and 86-90 \({}^{\circ}\)C were reached in TSB and BHI, respectively. Consequently, the prerequisite of a maximum temperature variation of 5 \({}^{\circ}\)C was not always satisfied for the relatively high temperatures reached during the 30 min treatment. Since such high temperatures are generally not required for the inactivation of common vegetative foodborne pathogens, this limitation is not a major disadvantage for the developed RF setup. In addition, the high temperatures reached during the 30 min treatments indicate that microorganisms that are more heat resistant than \(L\). _monocytogenes_ and _S. Typhimuriumium_ can potentially also be inactivated with this RF setup, barring the further improvement of the repeatability of the obtained temperature profiles.  
It should be noted that temperature measurements were in this study only conducted at one location in the sample. Therefore, the measured temperature is not representative  
Figure 3: Temperature profiles in TSB (**A**) and BHI (**B**) during RF treatments. Individual replicates and the average temperature profile are shown.  
for the entire sample volume, as non-uniform heating and the resulting heterogeneous temperature distributions that lead to varying effects on bacterial cells are a well-known challenge for RF treatments [24]. However, due to the small sample sizes used (i.e., 3 mL) and the relatively fast convection in liquid samples, temperature heterogeneities in the samples are limited compared to commercial RF setups. In future studies, it is possible to measure the temperature at different locations in the sample, using infrared temperature sensors or optical fibre temperature sensors, which do not interact with the electromagnetic field. Theoretically, it is also possible to use computer simulations to model the temperature distribution in the samples during the RF treatment, which is based on measured temperatures and sample properties (e.g., dielectric properties, rheological properties, density, geometrical parameters) [25; 26].