## 1 Introduction
However, the values reported in the literature may vary depending on the milk used for production (e.g., milk source, season, and feeding method), the specific starter cultures used, and the fermentation and processing conditions ([17; 18]). Typically, the protein content in kefir based on cow's milk ranges from 2.7 to 4 g/100 g, the fat content ranges from 0.5 (for skimmed milk) to 3.5 g/100 g, and carbohydrates range from 3 to 6 g/100 g. Furthermore, the phosphorus content is reported to be in the range of 90 to even 300 mg/100 g, riboflavin from 0.15 to 0.2 mg/100 g, and vitamin B12 from 0.2 to 0.5 ug/100 g ([PERSON] 2017 [18], [PERSON] 2003 [19], [PERSON] 2024 [17], [PERSON] 2024 [20]). All of these aforementioned components are characterized by high bioavailability. Iron, however, is present in small quantities--typically below 0.1 mg/100 g--and its bioavailability is also low due to interactions with milk proteins and calcium, a characteristic shared with other dairy products ([PERSON] 2006 [21]). Kefir, being a fermented product, provides probiotic bacteria beneficial to the digestive system. Enriching such products with additional nutrients significantly improves their nutritional value [22; 23]. Moreover, polysaccharides present in microalgae may exhibit prebiotic activity, positively affecting the intestinal microflora [24]. The complex chemical composition of microalgae may improve the quality of fermented dairy products. Their addition may stimulate the growth and survival of beneficial bacteria, such as _Lactobacillus acidophilus_ and _Lactobacillus bulgarcius_, both during fermentation and storage [10]. In vitro studies on the digestibility and absorption of nutrients from kefir enriched with microalgae may be crucial to understanding how different processing methods affect their availability. During the digestion of algal proteins, active peptides are released, which can act as inhibitors of the angiotensin-converting enzyme ACE [25] or perform other therapeutic functions, e.g., antioxidant or antibacterial. Adding microalgae rich in bioactive compounds to fermented milk drinks could significantly enhance their properties, strengthening their position as functional food. The enrichment of dairy products with algae, however, is associated with a number of limiting factors that pose a significant challenge. At least four key areas of such challenges can be distinguished: sensory properties, technological challenges, consumer attitudes, and safety issues [26; 27]. Depending on the dose, the algal additive can significantly affect the product's sensory properties. Spirulina or Chlorella often impart an intense taste and smell (\"fishy\" or \"grassy\" notes) and can alter the natural color of the dairy product (to green or bluish). Furthermore, they may change its texture by increasing graininess and viscosity. Conversely, many valuable nutrients from microalgae (vitamins, proteins, and unsaturated fatty acids) are sensitive to processing conditions (heat, light, and oxygen) encountered during food production. This sensitivity can lead to the loss of their nutritional or functional value. Moreover, microalgae components can interact with the dairy matrix constituents, thereby affecting product stability and structure. Microalgae are frequently associated with dietary supplements or health foods, yet their widespread introduction into everyday dairy products necessitates a shift in consumer perception. For many consumers, microalgae are perceived as an \"unconventional\" food, which can lead to distrust. In addition, some microalgae may carry the risk of accumulating heavy metals (e.g., cadmium, lead, and arsenic) or producing toxins (e.g., microcystin), which require rigorous quality controls, especially when cultivated in open systems. Finally, there is also a potential risk of allergic reactions to certain components of microalgae. The aim of the conducted research was to enrich kefir with microalgae additives of two species, _Arthrospira platensis_ and _Chlorella pyrenoidosis_.