## 1 Introduction
Their role in food products is the formation of texture, water and oil binding, as well as stabilization [18]. Against this background, more attention has been paid to the by-products and co-products of flaxseed (_Linum usitatissimum_ L.) [19]. Due to its high content of bioactive compounds, it has been categorized as a functional or bioactive food [20]. The mucilage obtained from flaxseed seeds contains L-galactose, D-xylose, L-arabinose, L-Hammose and D-galacturonic acid, and it demonstrates a high ability to hold water and bind oil, which is important when stabilizing the food systems [13; 21; 22; 23; 24; 25]. The polysaccharides that make up the mucilage are responsible for the formation of a multi-form structure and better resistance to environmental stresses. From a nutritional perspective, flaxseed mucilage is a fine source of dietary fiber and has many health benefits, such as the prevention of diabetes, obesity or colon cancer, decreasing blood cholesterol levels and improving insulin sensitivity. Flaxseed polysaccharides are called flaxseed gum (FG), as the main component of soluble dietary fiber in flaxseed (3-9 wt% of the flaxseed) [16; 22]. FG is composed of 80% neutral and acidic polysaccharides and 9% protein. Neutral monosaccharides consist of xylose, glucose, arabinose and galactose, while acidic monosaccharides consist of rhamn, galactose, fucose and galacturonic acid [16; 20; 25]. The protein in FG is predominantly conlinin [23]. FG is most commonly used as a thickener, stabilizer, gelling agent and emulsifier [16; 22; 23; 25]. From a functional point of view, FG more closely resembles gum Arabic or guar gums than other commonly used gums and can be used to replace most non-gelling gums in food and non-food applications due to its \"weak gel\"-like properties and remarkable ability to retain water [16; 20; 25; 26; 27; 28; 29]. FG is a promising new dairy ingredient, which acts, for instance, as a natural stabilizer in stirred yogurts [24]. However, very little is understood regarding its effect when added to dairy by-products. Moreover, FG also has the advantage of relatively low cost compared with most commercial gums [28]. Hence, based on the technological characteristics of FG, it can be considered that it can potentially be used as a stabilizer and thickener for acid whey. It was reported that FG can be obtained from flaxseed oil cake (FOCE)--a cheap by-product from flaxseed oil production [25; 30]. This is particularly meaningful, since a large amount of press cakes and residues are available, and their management meets the criteria of a \"zero waste\" economy [21]. To the best of our knowledge, there are no reports of studies on the use of FG as a natural stabilizer for fermented beverages based on acid whey. We hypothesized that the application of FG as a natural stabilizer would allow obtaining new products with high added value. Hence, the goal of the research was to produce fermented beverages from acid whey with various FG concentrations and evaluate their microbiological, physicochemical properties, as well as bioactivity and sensory acceptance during refrigerated storage (5 \(\pm\) 1 \({}^{\circ}\)C) for 28 days.