## 1 Introduction  
Whey is a by-product of the dairy industry, produced in vast quantities and obtained during the production of remnet cheeses, cuttage and acid curd cheeses (tvorogs) as well as milk protein preparations [1; 2; 3; 4]. It is estimated that the processing of 1 kg of cheese produces ca. 9 L of whey [1; 2; 5; 6; 7]. It is a yellow-greenish liquid, which contains about 45-55% milk solids, including lactose, whey proteins and mineral compounds [6; 7]. The fat content of whey is low, and the casein content, which forms the cheese curd, is negligible. The total solid content in whey is approximately 6-7% [1; 2; 3; 6]. They comprises a significant number of nutritional values and innumerable health benefits, as it is a source of valuable proteins, bioactive peptides, amino acids, vitamins (such as folic acid, cobalamin and riboflavin), as well as minerals [1; 6; 8]. This product has a high biochemical and chemical oxygen demand, so discharging whey with wastewater directly into the water bodies degrades natural ecosystems. It must be disposed of and processed. The processing of whey consists mainly of the use of membrane processes, such as reverse osmosis, nanofiltration and ultrafiltration, and drying processes, in order to obtain the concentrates and isolates of whey proteins, as well as powdered whey, which are used in many branches of the food industry. These products are obtained primarily from the so-called \"sweet whey\" obtained when rennet cheeses are produced. The management of acid whey (AW), which is a by-product in the production of acid curd cheese (tvorg), is much more difficult due to its low pH, lower lactose content, higher mineral content and a more sour and distinct taste compared to sweet whey [1; 2; 6; 9]. AW presents less favorable processing properties compared with rennet whey, and it presents a sustainability challenge to the dairy industry [1; 6; 9; 10]. Unprocessed whey is also not attractive to consumers because of its sensory properties [1; 6]; therefore, it seems promising to use it as a raw material for the production of unfermented and fermented beverages [1; 2; 11]. Various whey-based beverages are being successfully developed around the world, which include fermented, unfermented, alcoholic, non-alcoholic, carbonated, plain and fruit-flavored products [10; 11; 12; 13]. whey fermentation leads to advantages, such as a partial hydrolysis of whey protein (which may cause allergies), increase in shelf life (due to lactic acid production) and the production of volatile compounds that improve the sensory features [2; 8]. On the other hand, the production of whey-based fermented beverages becomes a challenge due to the high water content, and therefore, the low dry matter content, and consequently, the watery consistency of the final product [2; 5; 14; 15]. The texture and mouthfeel of fermented whey beverages (FWB) tend to be weak and watery compared to fermented milk, since liquid whey contains a low percentage of total solids (ca. 6 g/100 g) [16]. Until now, the evaporation, centrifugation or ultrafiltration processes have been used to increase the dry matter of milk intended for the production of fermented milk. An alternative to the above-mentioned processes is adding, among others, milk powder, whey powder, whey protein concentrates, casein and caseinates. The addition of milk powder or whey, however, significantly increases the lactose content in the product, which is not well perceived by consumers who have problems with this sugar intolerance [17]. Additives of non-dairy origin were also used in the past, e.g., modified starch. The current trends in creating new products are directed toward \"clean label\". Therefore, the most commonly used structure-forming compounds are substances of natural origin. This has created a requirement for using exopolysaccharideate-producing starter cultures or the addition of hydrocolloids [12]. Hydrocolloids are defined as hydrophilic molecules, which have a high molecular weight.