## 3 Results and Discussion
### Heat Sensitivity of Bacteriophage K5  
Pure bacteriophage K5 lysate was used for the thermal inactivation experiments. Depending on whether K5 appears to be resistant against heat or not, an appropriate encapsulation method can be chosen or process parameters can be adjusted. In addition, the data may be relevant for the later application of bacteriophage capsules in food matrices and their production (e.g., admixture before or after heating steps). Figure 3 shows the survival behavior at temperatures of 68-75 \({}^{\circ}\)C.  
The highest bacteriophage K5 reduction took place in each case within the one-minute heating phase, with reductions of 1.5, 2.5, 5.9, and 8.1 log units for 68, 70, 72, and 75 \({}^{\circ}\)C, respectively. Within the following 300 to 600 s, they were reduced by 3.8, 4.3, and 1.9 log units at the temperatures of 68, 70, and 72 \({}^{\circ}\)C, respectively. However, no further reduction was observed at 75 \({}^{\circ}\)C, as the titers were close to the limit of quantification (10\({}^{1}\) pfu mL\({}^{-1}\)). Modeling of the data was performed using the Arrhenius-based model. Thereby, non-linear curve progressions with tailing were observed (except for the data obtained at 75 \({}^{\circ}\)C). The order of reaction \(n\) was 1.42 \(\pm\) 0.06, the death rate \(k_{\textit{ref}}\) was (2.8 \(\pm\) 1.0) \(\times\) 10\({}^{-3}\) s\({}^{-1}\) (pfu mL\({}^{-1}\))\({}^{(1-n)}\), and the activation energy \(E_{a}\) was (9.57 \(\pm\) 1.38) \(\times\) 10\({}^{2}\) kJ mol\({}^{-1}\) (all parameters given with standard error \(s\)). As a reference temperature \(T_{\textit{ref}}\), 345.15 K was defined. The decimal reduction times \(D_{\theta}\) were calculated (again, except for the data obtained at 75 \({}^{\circ}\)C) to be \(D_{68}\)\({}^{\circ}\)C = 0.52 min (\(r^{2}\) = 0.91), \(D_{70}\)\({}^{\circ}\)C = 0.74 min (\(r^{2}\) = 0.98), and \(D_{72}\)\({}^{\circ}\)C = 0.42 min (\(r^{2}\) = 0.97). The \(D_{\theta}\)-values were similar and decreased slightly with increasing temperature (not significant). This effect is also visible in the curve progressions themselves in Figure 3, which appear nearly parallel. Therefore, determination of another kinetic parameter, the \(z_{\theta}\)-value (which represents the temperature increase that is necessary to achieve the same reduction effect in 1/10 of the time), was omitted. The curve progressions, as well as the \(D_{\theta}\)-values, at the different temperatures were different than expected. The strong reduction within the heating phase (steep slope) in connection with a short linear phase (flat slope) and a long tailing range suggests that rather the \"real\" linear phase of the experimental course was not recorded. Reasons for this could be a too low initial titer (N*) and a too great inactivation within the heating phase in which no data points were recorded. However, in order to ensure even temperature distribution in the stainless steel tubes, the heating-up phase cannot or can only be slightly shortened, as previously stated by [PERSON] et al. [46]. To obtain better data for kinetic evaluation in the future, pure bacteriophage lysate with a higher titer should be produced and used for the inactivation experiments. For example, the CsCl ultracentrifugation method according to [PERSON] and [PERSON] results in titers of up to 10\({}^{13}\) pfu mL\({}^{-1}\) depending on the type of bacteriophage [59]. Nevertheless, because of these observations it can be stated that the bacteriophage K5 belongs to the rather heat sensitive bacteriophages. To the best of our knowledge, there is no other study that ever investigated the bacteriophage K5 behavior against heat. However, there is some information available regarding other _Escherichia coli_ bacteriophages or bacteriophages of other bacterial species. Atomic force microscopy demonstrated that heat treatment of the _Escherichia coli_ bacteriophage T7 at 65 \({}^{\circ}\)C led to destabilized particles with loss of capsid tails and DNA.