###### Abstract  
Many diet trends have emerged over the last few years, including plant-based diets with soya as an important component, while fermented milk beverages have been an integral part of the human diet for thousands of years. However, there is little research on the potential of using propionic bacteria for dairy or plant-based food fermentation. The aim of this study was to evaluate the effects of soy beverage addition and propionic bacterium application on the quality of dairy yoghurt products. Three variants of the products--based on cow's milk, soya beverages, and mixtures of both--were prepared and then fermented with yoghurt bacteria, propionic bacteria or a mixture thereof. It was found that it is possible to obtain functional yoghurt products based on cow's milk, as well as milk-soya blends, using a mixture of yoghurt and propionic cultures. The resulting milk, milk-soya, and soya yoghurt products displayed pH values in the correct range and a stable bacterial population during refrigerated storage for 21 days. The refrigerated storage time and the type of culture starter used significantly determined the quality characteristics of the milk, milk-soya and soya yoghurt products, such as their firmness, adhesiveness, and water-holding capacity. v +
Footnote †: journal: Applied sciences  
product obtained by acidifying and coagulating milk with yoghurt bacteria, such as _Streptococcus thermophilus_ or _Lactobacillus delbrueckii_ subsp. _bulgarcius_, among others [4]. In new-generation yoghurts, the use of other bacterial cultures is permitted [4]. The content of viable bacterial cells in the final product should be a minimum of \(10^{7}\) per 1 mL or 1 g [5]. Fermented products are the main natural source of beneficial microflora in the human diet. Fermented milk products have been an integral part of the human diet for thousands of years. Since their beneficial effects on human health were first studied over 100 years ago, there is a growing interest in both the production and consumption of yoghurt [6]. However, over the last few years, many diet trends have emerged. Some of these have been short-lived, while others have caught on so well that they have become popular worldwide. One such example is the plant-based diet. A wide range of probiotic fermented dairy-free beverages are produced worldwide using a variety of plant matrices [7]. It is believed that a plant-based diet has more health benefits for the human body than a diet containing animal products [8; 9; 10]. This results in the restriction or total exclusion of animal products, with the aim of obtaining all the nutrients necessary for the body to function properly [11; 12; 13]. These diets employ legumes, particularly soya. Soya beverages contain high-quality proteins, isoflavones, unsaturated fatty acids, as well as carbohydrates, which, on the one hand, may cause some digestive discomfort, but on the other hand, are increasingly recognized as prebiotic substances [14; 15; 16; 17; 18; 19; 20]. They can also provide protection against many diseases, including cardiovascular disease, diabetes, and cancer [21; 22]. The aim of this research was to analyze the effect of the addition of a soya beverage to cow's milk in different proportions, and to evaluate selected quality parameters of the obtained yoghurt products. Research concerning the fermentation of soya beverages or their addition to a milk matrix is not advanced [23; 24; 25]. To date, there is a lack of scientific data on the fermentation of such matrices using propionic bacteria, either alone or in a mixture with standard lactic acid bacteria starters, such as yoghurt bacteria. Equally little information is available in the literature on the use of propionic bacteria in plant food matrices. _Propionibacteriumum freudeneichii_ is a beneficial bacterium that modulates intestinal microflora, enhances motility, and reduces inflammation [26]. _P.