## 3 Results and Discussion
### Good Survival during Storage at 0 and \(-\)20 \({}^{\circ}\)C
Successful Application of Bacteriophage K5 Capsules in Semi-Solid Cereal Matrices and Unsuitable Application in Milk Products  
As an alternative to pure bacteriophage products, i.e., as powder or tablets, the produced capsules should be applied in different food matrices. Initially, the bacteriophages were added to a milk product as \"bacteriophage-biotics\", similar to probiotic products, which already exist on the market. However, the preliminary studies (data not shown) indicated that the bacteriophage capsules from this study are not stable in liquid and almost neutral milk products (pH value of milk: \(\sim 6.6\)[99]). Instead, dry and semi-solid products were then considered. Potential advantages of applying the capsules in semi-solid products include the lower water content, minor influence of the pH value, and decreased diffusion processes. Furthermore, dry storage of the capsules themselves was already promising (see Section 3.5). Cereal cookies, composed of oat flakes and glucose syrup, were used as a model matrix. The stability of the bacteriophage K5 titer over a period of 28 days is shown in Figure 7.  
The bacteriophage capsules that were incorporated into the cereal cookie were not visible after mixing and drying. After the drying process, the rather bulky structure of oat flakes was firmly held together by the glucose syrup and the surface of the oat flakes appeared slightly glossy. The texture of the cookie remained firm and dry throughout the storage period of 28 days, with a water activity of 0.59 \(\pm\) 0.02. Figure 7: Stability of bacteriophage K5 titer in a cereal cookie at 23 \({}^{\circ}\)C (). The limit of quantification was 2.3 \(\times\) 10\({}^{2}\) pfu (related to one cookie with a mass of 2.505 g). A photograph of the cereal cookie is presented in the lower right corner of the picture. The presented total titer of bacteriophage K5 (Figure 7) is referred to one cookie that contained 0.05 g encapsulated bacteriophage K5 in 1.5 g of oat flakes and 1.0 g of glucose syrup and was dissolved in 20 mL MBS for quantification. Over 28 days of storage at 23 \({}^{\circ}\)C, there is a slight tendency of a bacteriophage K5 titer reduction by less than 1 log unit. The fluctuations between the values result from the more difficult release of the bacteriophages from the capsules from the oat flakes/glucose matrix. To achieve more constant values, the incubation time in MBS could be increased. Losses are also conceivable due to potential attachment of bacteriophages to the oats during the centrifugation step. In addition, it should be noted that the determination was only twofold. However, what is quite clear is the difference of bacteriophage reduction between storing the pure capsules (Section 3.5) and storing the capsules in cereal cookies, both at 23 \({}^{\circ}\)C. After 30 days, the bacteriophage titer was reduced by more than 2 log units in the pure capsules versus less than 1 log unit in the cereal cookies. When free bacteriophages are placed in a highly concentrated sugar solution, they normally suffer osmotic shock [100,101]. An opposite effect is seen for encapsulated bacteriophages. A strong and restricted water binding of alginate and micellar casein could have counteracted osmotic forces originating from the high glucose concentration in the surrounding syrup. In a study by [PERSON] et al., extruded microcapsules composed of alginate, whey proteins and bacteriophage K were soaked in corn syrup before drying. The procedure was found to maintain bacteriophage titer in capsules during the subsequent drying step [90]. Preservation of bacteriophage capsid protein structure by hydrogen interaction with saccharides was proposed as an underlying mechanism [90,102]. To the best of our knowledge, no studies applying (encapsulated) bacteriophages in cereal products are available. However, encapsulated probiotics were studied for their application in cereal bars. [PERSON] et al., incorporated encapsulated _Lactobacillus acidophilus_ and _Bifidobacterium animals_ ssp.