## 1 Introduction
The nutritional, health-promoting, and sensory values of fermented products are also important. The increase in nutritional value is related, among others, mainly to the production of amino acids and different bioactive compounds [22; 23]. The health benefits associated with consuming fermented foods and beverages include increased digestibility, antimicrobial [24], antihypertensive, antioxidant [25], and even immunostaining [26] properties. Some lactic acid bacteria (LAB) strains also produce exopolysaccharides (EPS), which increase the viscosity of the fermented product [27; 28]. An example of using lactic acid fermentation to reuse bread waste is research conducted by [PERSON] et al. [29]. The authors assessed the potential of two LAB strains (_Weissella confusa_ A16 and _Pediococcus claussenii_ E-032355T) to transform bakery waste into an EPS-enriched bread slurry dedicated as an ingredient in the production of baked goods. It is worth emphasizing that fermented products are considered functional foods, which constitutes a valuable part of the food market, especially in highly developed countries. A constant development and increase in the value of this branch of the food industry is expected around the world [30]. There are many definitions of functional food in the literature, but generally this product category includes unmodified (natural) food as well as food in which ingredients have been modified (including those increasing their bioavailability), added or removed using cultivation conditions or technological/biotechnological processes, which make it possible to obtain specific health-promoting effects or improve general well-being after consumption [31]. Beverages are one of the most popular products in the functional food category mainly due to the convenience of consumption as well as easier distribution and storage [32]. A wide range of traditional, non-dairy fermented drinks are produced around the world [33]. Examples of such products are _Boza_ made from wheat, rye, millet, maize, and other cereals, known in Bulgaria, Albania, Turkey, and Romania [34], _Bushera_ from sorghum consumed in Uganda [35], or _Togwa_ prepared by fermentation of maize flour in Africa [36]. In the case of fermented beverages with potential probiotic properties, it is important to maintain the cell count at an appropriate level throughout the shelf life of the product [37] as well as during the passage through the digestive system [38]. One of the basic procedures aimed at increasing the survival of LAB in a product is the addition of various types of products/substances with prebiotic effects [39]. An interesting example is ground flaxseed, which, in addition to its high content of dietary fiber (prebiotic effect), is also a valuable source of substances such as \(\alpha\)-linolenic acid, omega-3 acid, proteins, and lignan, making it an ingredient with functional potential [40]. The literature contains research results showing the positive effect of ground flaxseed on the stability of LAB in various types of products. For example, [PERSON] et al. [41] (2012) found a positive effect of adding ground flaxseed to the matrix on the increased viability of _Lacticaseibacillus rhamn_ GG during storage for a period of 14 months. A positive effect of flaxseed on LAB survival was also reported by [PERSON] et al. [42] (2013), who found a significantly higher number of LAB in kefir with the addition of flaxseed muciligate after 28 days of storage at 4 \({}^{\circ}\)C compared to unfortified kefir. In turn, [PERSON] et al. [43] (2018) reported a higher number of _Lactokacillus acidophilus_ CCDM 151 in milk with the addition of ground flaxseed (by 0.8 log CFU/mL) after 16 h of fermentation compared to milk without the addition of flaxseed. Despite studies confirming the positive impact of LAB fermentation on food waste reuse, it should be emphasized that products obtained through spontaneous fermentation are not repeatable and may differ in composition and sensory characteristics.