## 3 Results and Discussion
### Bacteriophage K5 Capsules Powder and Its Characteristics  
Bacteriophage capsules were produced according to the in situ microencapsulation method described in Section 2.2. Independent of the type of milk protein added to the matrix (either micellar casein or whey protein isolate), a white fine non-adhesive powder was obtained, representatively shown for the variant with micellar casein in Figure 5a. The smaller visible aggregates were very easy to break up with a spatula. Figure 5b shows the topography of the capsules, which consisted of a majority of smaller but also larger particles with a pulled-in surface. Roughly measured, the diameters of the capsules were between 2.5 and 10 \(\upmu\)m.  
Additionally, Table 2 contains characteristic parameters of the bacteriophage K5 capsules, such as humidity, water activity, particle size, bacteriophage titer, and encapsulation efficiency. In general, both variants did not differ greatly between each other. Only the values of the particle size measurement were slightly higher in the variant with whey protein than in the variant with micellar casein. Figure 5: Bacteriophage K5 capsules, exemplary shown for the variant with micellar casein. (**a**) White powdery product after spray drying. (**b**) Topography recorded using scanning electron microscopy and a magnification of 1.500\(\times\). Both products had a low humidity content of around 9% and a low water activity (Table 2). Water activity was only measured for the product containing micellar casein, but it must be similar for the product with whey protein isolate. A low humidity is a quality parameter and particularly advantageous for future large-scale production, storage, and application. In this context, attention should be paid to the storage conditions and a dry environment as the particles can otherwise easily absorb water [73]. The humidity of the capsules in this study is in accordance with other literature data. According to [PERSON] et al., a residual moisture content of 5% or more is the rule for spray drying, since the drying process does not vaporize all the liquid and moisture remains in the capsules [74,75]. [PERSON] et al., produced _Salmonella_ bacteriophage Felix O1 microcapsules with Eudragit S100(r) and trehalose, with a final humidity of 9% [76]. [PERSON] et al., obtained 5.5-8.5% humidity in sodium alginate capsules when spray dried at different temperatures [77]. However, it is not the absolute humidity itself that matters, but the storage stability, which is discussed in more detail in Section 3.5. The particle size (d\({}_{50,3}\)) was around 10 um for both matrices and in general no particle fractions were above 50 um. This fits to our overall aim of producing small capsules that are not sensorially recognizable in food products. According to [PERSON] et al., particle diameters should not exceed 100 um in order not to influence the sensory properties of yoghurt [78]. The biggest particles with diameters of up to \(\sim\) 46 um, included in the d\({}_{90}\) values, could represent aggregates consisting of several smaller particles. However, these aggregates can be easily broken up, for example, by ultrasonic treatment [49]. Although the composition of the matrix, the solids concentration of the feed, the gas volume flow, the nozzle of the spray dryer, and other equipment parameters have an influence on the size of spray-dried particles, the size magnitude itself stays constant. Compared with the present data, many other studies found similar particle sizes, i.e., in the lower um range, for spray-dried capsules. [PERSON] et al., measured a particle size (d\({}_{50}\)) of 6 um for their polymer saccharide capsules [76]. [PERSON] et al., and [PERSON] et al., obtained diameters below 5 um in a powdery product mainly made of trehalose and amino acids [79,80]. Sodium alginate capsules containing _Bacillus subtilis_ or _Salmonella_ spp. bacteriophages resulted in particle sizes (d\({}_{50}\)) between 9 and 24 um [53].