The main problem: Modern research in the development of fermented milk products is focused on increasing the bioavailability of milk components, as well as the use of bacterial components of sourdough, which increase health properties. The use of the fermentation process of skimmed milk with a combined starter consisting of traditional for cottage cheese and starter cultures of probiotic cultures immobilized in a gel of biopolymers is very important. Purpose: The purpose of this study is to determine the optimal amount of starter, consisting of an association of probiotic cultures immobilized in a gel of biopolymers (membranes) to be added to fermented skimmed milk in order to enrich it with functional ingredients. Methods: A one-factor experiment was used. The culture association Propionibacterium freudenreichii subsp. was used as a regulatory factor. Shermanii, Bifidobacterium lactis and Streptococcus thermophilus, immobilized in a biopolymer gel, added to skimmed milk in the form of membranes (determined as a percentage of the mass of fermented milk). Controlled factors are the main indicators characterizing the efficiency of the skim milk fermentation process; these are active acidity, the logarithm of the number of viable cells of bifidobacteria, the logarithm of the number of viable cells of propionic acid bacteria, and organoleptic evaluation. Results and their significance: Based on the results of a mathematical analysis of the totality of values of controlled factors depending on the amount of starter cultures of probiotic cultures, mathematical models were built to determine the degree of influence of the starter on the quality indicators of the product, using the Table Curve 3D-v4 mathematical computer program.