## 3 Results and Discussion
### Bacteriophage K5 Capsules Powder and Its Characteristics
Other encapsulation techniques, such as emulsion or extrusion, can hardly compete with the capsules obtained by spray drying. With the help of a similar capsule matrix of 1.6% alginate and 10% whey protein isolate, [PERSON] et al., were able to produce uniformly spherical but significantly larger capsules than the present study, with diameters of 1-2 mm [33]. Capsules produced by emulsion and enzymatically triggered gelation of milk protein for _Lactococcus lactis_ P008 bacteriophages were considerably smaller at 132 um, although not in the size range of spray-dried capsules [33]. By optimizing the polymer/protein ratio of a suspension with sodium alginate (1.53%), sodium caseinate (2.72%), \begin{table}
\begin{tabular}{c c c} \hline \hline
**Capsules Characteristics** & **Alginate + Micellar** & **Alginate + Whey Protein** \\  & **Casein Capsules** & **Isolate Capsules** \\ \hline Humidity (\%) & 8.70 \(\pm\) 0.86 & 9.43 \(\pm\) 0.32 \\ Water activity (–) & 0.15 \(\pm\) 0.03 & n.d. * \\ Particle diameter d\({}_{10,3}\) (μm) & 3.6 \(\pm\) 1.2 & 5.1 \(\pm\) 0.4 \\ Particle diameter d\({}_{50,3}\) (μm) & 8.9 \(\pm\) 0.7 & 12.0 \(\pm\) 1.9 \\ Particle diameter d\({}_{90,3}\) (μm) & 17.5 \(\pm\) 4.4 & 46.2 \(\pm\) 2.9 \\ Encapsulation efficiency (\%) & 4.2 \(\pm\) 2.1 ** & 3.6 \(\pm\) 2.1 *** \\ Bacteriophage titer (pfu g\({}^{-1}\)) & (3.4 \(\pm\) 1.7) \(\times\) 10\({}^{7}\) & (1.6 \(\pm\) 0.9) \(\times\) 10\({}^{7}\) \\ \hline \hline \end{tabular}  
* n.d.—not determined. ** Values inserted into Equation (3): \(N_{\rm S}\) = (3.6 \(\pm\) 0.3) \(\times\) 10\({}^{7}\) pfu mL\({}^{-1}\), \(N_{\rm C}\)*\(=(1.5\pm 0.7)\times 10^{6}\) pfu mL\({}^{-1}\). *** Values inserted into Equation (3): \(N_{\rm S}\) = (2.0 \(\pm\) 0.1) \(\times\) 10\({}^{7}\) pfu mL\({}^{-1}\), \(N_{\rm C}\)*\(=(7.3\pm 4.1)\times 10^{5}\) pfu mL\({}^{-1}\).  
\end{table} 
Table 2: Analysis of the bacteriophage K5 capsules regarding physical and biological parameters. Capsules were mainly composed of sodium alginate and milk protein (either micellar casein or whey protein isolate). and _Salmonella_ bacteriophages Felix O1 using vibrational nozzle technique, minimum particle diameters of 0.16 mm were achieved after extrusion [36]. The most important quality parameter of the capsules is the encapsulation efficiency, which was roughly 4% in this study (Table 2) (calculated with absolute bacteriophage titers; calculation with logarithmized bacteriophage titers would correspond to an encapsulation efficiency of around 80%). In addition to forcing low humidity and low particle diameter, it should be considered that bacteriophages can be affected by heat, drying, and mechanical stress. A compromise should, therefore, be made between these conflicting concerns. As already shown and discussed in Section 3.2, bacteriophage K5 is heat sensitive and easily inactivated. However, in spray drying, the critical factors are the wet-bulp temperature and the outlet temperature (not the inlet temperature), which were 35 \({}^{\circ}\)C and 53-59 \({}^{\circ}\)C, respectively, in this study, which means that no higher temperatures affect the bacteriophages. Since sensitivity towards heat differs among bacteriophages, results cannot be directly compared with other literature. To the best of our knowledge, there is no data regarding encapsulation of K5 or other T7-like bacteriophages. However, bacteriophage-specific heat sensitivity was also shown by [PERSON] et al., since they observed encapsulation efficiencies in a broad range of 73-97% for several _Bacillus subtilis_ and _Salmonella_ spp. bacteriophages encapsulated at the same conditions [53]. In general, encapsulation matrices can be modified by adding substances known to have a protective effect on bacteriophage survival during heat treatment and spray drying. The use of proteins in spray drying processes of bacteria, lipids, and volatile components has already delivered promising results on several occasions [81, 82, 83, 84].