###### Abstract  
The aim of this study was to evaluate the effect of the addition of various milk protein powder preparations (MPC-80, WPC-70, sweet whey, non-demineralized or demineralized whey, whey permeate, rennet casein, butermilk) to selected quality parameters and digestibilitybility of milk proteins in kefir. Kefir samples analyzed in the study were prepared under laboratory conditions with three industrial bacterial starter cultures. They were examined microbiologically (lactobacilli, lactococci, yeast) and for pH, total protein content, hardness, adhesiveness, water-holding capacity (WHC), and protein digestibility (using two in vitro methods along with the determination of the content of available lysine and glycine). The counts of lactococci and lactobacilli were estimated at levels above 7 and 6 log(CFU/mL), respectively. Yeast was not found in the kefir samples. The pH value of the samples was 4.4-5.2. The pH, hardness, adhesiveness, and WHC were dependent on the type of kefir starter culture and the type of milk protein powder added. Each protein preparation added increased the amount of available lysine and glycine. However, when converted for 1 g of kefir protein, the changes in the content of available lysine and glycine were variable and depended on both the milk protein powder and starter culture used.  
Keywords:fermented mills; kefir starter culture; hardness; adhesiveness; milk proteins digestibility; milk protein; whey protein; casein; butermilk +
Footnote †: journal: Medical  
milk base. Each of these additives produces a different effect on the quality and nutritional characteristics of fermented milk. For example, WPC has a higher biological value than SMP and stimulates the growth of several strains of lactic acid bacteria [4].  
Kefir contains essential amino acids and easily digestible proteins, which when digested help the body to maintain its functions [5]. In recent decades, several innovative kefir drinks have been introduced, including those enriched with ingredients such as milk proteins which render the drink with specific and valuable properties [6; 7; 8]. The nutritional characteristics of kefir are related to the bioavailability of its ingredients. Bioavailability refers to the degree to which a nutrient has been released and absorbed from the digestive system and used by the human body. Digestibility in turn is related to bioavailability which refers to the susceptibility of products or nutrients to the activity of digestive enzymes. The digestibility of a food product is determined by its ingredients, including proteins, carbohydrates, and fats, or the food as a whole [9; 10]. It is assessed by calculating the coefficients of the digestive process of a human or animal organism--digestion in vivo--or by testing the level of the enzymatic hydrolysis of meals under laboratory conditions--digestion in vitro [9; 10; 11; 12; 13; 14].  
This study aimed to evaluate the effects of the addition of various forms of milk proteins, including MPC, WPC, demineralized or non-demineralized whey, sweet whey, whey permeate, rennet casein, and buttermilk powders, on selected quality parameters and the digestibility of milk proteins in kefir. In addition, the study compared two methods applied for determining the digestibility of milk proteins using in vitro enzymatic digestion.