## 4 Discussion
Milk powders or milk protein concentrate powders addition, which are usually applied in fermented milks production, gives the proportion of proteins to total solid content and the proportion of whey protein to casein content close to that of native milk. In contrast, whey protein powders and whey protein concentrates are an important source of whey proteins, with a variable content of other dairy ingredients (e.g., mineral salts) depending on the degree of purification of the preparations (e.g., demineralized whey powders contain about half as much ash than non-demineralized whey powders whey powders and salt content can alter the results). Moreover, it must be remembered that the effect of the addition of a milk base on milk proteins is strongly dependent on the heat treatment of milk and the added milk powder [33]. For example, rennet casein powder contains denatured casein proteins, so the role of these proteins in acid clot formation is not significant. This is probably why the hardness as well as the adhesiveness of the kefir samples prepared with milk protein powders depended on the type of milk protein powder added. These factors, individually and in combination with the effects of using different starter cultures, led to a difficult interpretation of the results obtained by us. We have also shown that the tested texture parameters were strongly influenced by the type of kefir starter culture used. It should be assumed that the decisive factor in this case is the qualitative and quantitative microorganism composition of the starter culture. Many scientists have proven in their research that the microflora can produce exopolysaccharides (EPS), which significantly affects the texture of fermented milk [34]. The WHC of kefir refers to its tendency to retain water or its resistance to phase separation. However, it should be remembered that the centrifugation test used in our experiments may give results irrelevant for syneresis under normal storage conditions. This is because the mechanical stability of the protein network is investigated more under the influence of G forces than under normal storage conditions [33]. Many authors [34; 35; 36; 37] reported that the higher the milk protein content, the higher the WHC. We also showed in this study that the addition of milk powders increased the WHC value of kefirs. [PERSON] et al. [34] showed that the use of microflora influenced the structural parameters and balanced the effect of dry matter enrichment on the WHC value. However, the type of protein added to the milk plays an important role. Increasing the concentration of casein enhances the interaction between its molecules. In this case, the casein chains become shorter, the pore size in the gel becomes smaller, and the gel density increases. In addition, an increased concentration of whey proteins added leads to an increase in the WHC value [35; 36; 38; 39]. This was also confirmed by [PERSON] et al. [40; 41] with the addition of WPC to yogurt. Furthermore, [PERSON] and [PERSON] [35] and [PERSON] and [PERSON] [36] confirmed that yogurt pH has a significant effect on WHC. The available lysine content is a measure of the nutritional value of the milk protein [42]. The content of lysine in food and its availability are extremely important because it is an essential amino acid that is often found in relatively small amounts in diet [43; 44; 45]. [PERSON] et al. [24] determined the content of available lysine in kefir samples obtained in laboratory conditions at a level of 41 mg/100 g of product; for comparison, in 100 g of commercial yogurt, there was an average of 113 mg of available lysine (similar to the content achieved in our experiments). The decrease in the available lysine concentration in milk may be due to thermal treatment (e.g., pasteurization) [42]. Lysine is susceptible to modifications occurring during the processing or long-term storage of food samples. For example, it can react with lactose or other substances and form nutritionally inaccessible (nonreactive) derivatives [44; 45].