## 1 Introduction  
Contemporary society is experiencing dynamic shifts in dietary preferences and lifestyles, leading to a growing interest in alternative forms of nutrition [1, 2]. One significant trend gaining popularity is the plant-based diet, characterized by exclusive or predominantly plant-based consumption. One of the primary drivers of this increased popularity is the response to society's diverse dietary needs and preferences. Many individuals grapple with lactose intolerance or milk protein allergies, while others increasingly adopt plant-based diets for ethical and environmental considerations. In addressing the growing interest in plant-based diets and consumer expectations, the food industry offers an expanding array of plant-based beverages, such as yogurts [3]. By utilizing diverse raw materials, manufacturers can produce products with appealing flavors and textures, as well as beneficial health properties [4]. Although fermented dairy and plant-based beverages have been extensively studied, previous research has primarily focused on their basic production and probiotictic potential. However, limited studies have systematically compared the effects of different lactic acid bacteria cultures on both dairy and plant-based matrices, particularly in terms of texture, syneresis, and bacterial viability during storage. Additionally, the long-term stability and structural properties of mixed dairy-plant beverages remain underexplored. In this context, the fermentation of plant-based beverages is emerging as a fascinating area of research, bridging tradition with modern technologies [5; 6]. In the food industry, fermented beverages constitute a significant product category, as they are valued for their sensory attributes and potential health benefits [7; 8]. Traditionally, lactic acid fermentation is widely employed in the production of dairy beverages, such as yogurts and kefirs, where lactic acid bacteria (LAB) play a pivotal role in converting lactose to lactic acid, thereby influencing the product's texture, flavor, and shelf life. Despite the growing popularity of fermented plant-based beverages, significant challenges remain regarding their fermentation efficiency and stability. Unlike dairy matrices, plant-based substrates often lack essential nutrients that support bacterial growth and fermentation, leading to variations in texture, acidity, and probiotictic viability. The effect of specific lactic acid bacteria strains on these properties, particularly in mixed dairy-plant beverages, has not been thoroughly investigated. Both the growing interest in plant-based diets and the search for alternatives to dairy products have contributed to the development of plant-based fermented beverages, including oat-based beverages. The fermentation of plant-based beverages, such as those made from nuts, cereal seeds, or legumes, offers an alternative for individuals seeking flavor diversity and the health and functional benefits associated with this process [5; 9]. Plant-based beverages with probiotictic properties represent an interesting alternative to dairy products; however, their refinement requires further research. Blending raw materials allows for the creation of products with interesting flavors and textures. Meanwhile, oat-based products are particularly recommended for individuals suffering from diabetes and hypertension [10]. Oats are fiber-rich, positively affecting digestive health and lowering cholesterol levels. They are also a source of beta-glucans, which have immunomodulatory effects. Combining milk and oats can increase the nutritional value of the beverage, namely combining the protein and calcium from milk with the fiber and other nutrients from oats [11]. Studies indicate the possibility of enriching milk-oat beverages with cultures of potentially probiotic lactic acid bacteria strains [12; 13]. Using probiotics for fermentation can improve the digestibility of beverages by breaking down complex carbohydrates and proteins into simpler compounds.