## 1 Introduction  
Nowadays, the prevalence of cow's milk allergies, lactose intolerance, and hypercholesterolemia directs the food industry and the global market to design, supply, and produce novel milk alternatives. This is also supported by emerging consumer trends in the field of plant-based diets. Such substitutes are obtained from plant-based derivatives including cereals, nuts, legumes, and fruits. In particular, the most popular commercial plant-based beverages are obtained with soy, oat, rice, coconut, almond, hazelnut, quinoa,sesame, and hemp. Currently, worldwide, non-dairy plant-based milk alternatives are a fastgrowing segment in the newer food product development category of specialty beverages. Furthermore, Transparency Market Research (TMR) reported that the plant-based beverages market was forecasted to have approximately 8% of its annual growth during the period from 2019 to 2029 [1]. Designing these innovative food products is in line with current trends aiming to reduce animal production and increase plant production, as a solution for cutting carbon emissions and, therefore, caring for the climate. It is worth underlining that the agri-food industry and global economy place a growing emphasis on the idea of sustainable development. International efforts are made to improve the efficiency of the food industry while reducing its harmful impact on the environment [2]. Literature data indicate that future perspectives on plant-based beverages should focus on their fermentation, as these processes benefit the final product parameters. [PERSON] et al., based on compiled data including minerals, vitamins, and phytochemicals, stated that the enrichment of plant-based beverages should also take place for the design of novel products [3]. New products among the plant-based milk alternatives are fermented beverages. The fermentation process enhances the final product's taste, structure, and health-promoting properties. To add further value to the product, the non-dairy fermented beverages are produced using starter cultures with specific probiotic properties. Fermentation with selected lactic acid bacteria (LAB) may improve the nutritional profile of the product mainly due to the production of amino acids and various bioactive compounds [4; 5]. What is important is that due to consumer awareness, fermented plant-based products are gaining popularity as the process of fermentation is one of the oldest, easiest, and most affordable ways to produce and preserve food and beverages. Moreover, this process also enhances the nutritional, sensory, and shelf-life qualities of such products [6]. The consumer's need to live a healthy lifestyle has increased interest in superfoods; therefore, plant-based beverages are being enriched with, among others, nutrients such as proteins and vitamins, as well as plant extracts. A popular group of fermented beverages are products made from cereals such as barley, maize, oats, rye, or wheat [7] due to their properties suitable for fermentation. Non-alcoholic malt beverages, especially from barley, are gaining increasing interest due to their lack of alcohol and their nutritional properties. It is worth underlining that cereals may also constitute the basis for preparing functional probiotic drinks as they are rich in nutrients appropriate for the growth of probiotic strains [8; 9]. Fermented OBBs despite the usage of LAB strains are also considered to be functional foods due to the content of prebiotic fibre \(\beta\)-glucan from the cereal. The advantage concerning using oat matrix for fermented beverages lies within their composition as these cereals are rich in not only fibre but also proteins, dietary lipids, as well as micronutrients such as calcium, zinc, and iron, and essential amino acids [6; 7; 10]. Oat-based fermented beverages appeared first on the Swedish market in 1994, based on the oatmeal gruel fermented with the Lactobacillus plantarum 299 v/L and the addition of malted barley as well as different fruit drinks in an amount of 5%.