## 1 Introduction  
Diet plays a crucial role in health and well-being, leading to increased interest in functional foods. The definition of such foods includes, among others, milk and dairy products containing probiotic bacteria cultures and enriched with additional ingredients that significantly increase the nutritional properties of the final product [1, 2]. Microalgae can become such an additive to improve the nutritional value. Over the last decade, we have observed a growing interest in food supplementation with microalgae and extracts or other ingredients derived from microalgae biomass. This is mainly due to the high content of protein and valuable vitamins and minerals. The most popular microalgae used in industry are _Spirulina_ (_Arthrospira platensis_) and _Chlorella_ (_Chlorella pyrenoidosa_), which are obtained in mass cultures in photobioreactors. Currently, due to their chemical composition, they play a key role in aquaculture, as a component of feed [3]. Fish fed with microalgae show faster growth and are characterized by an increased content of valuable nutrients in their meat [4]. In addition, microalgae are used in the production of cosmetics [5] and in the production of biofuels [6]. In the case of the food industry, microalgae are currently mainly used as a food dye--directly or extracted from them, e.g., chlorophylls and carotenoids [7; 8]. These beneficial features of microalgae have not yet found their full application, and they can be relatively easily used as a valuable food additive [5]. Moreover, microalgae such as _Spirulina_ have been historically consumed in the Americas (Mexico) and Africa (Chad) for a long time [9]. After drying, they can be taken in the form of a powder added to food and drinks or as tablets swallowed during a meal [10]. The group of numerous substances with bioactive potential, occurring in significant quantities in microalgae of the _Arthospira_ and _Chorella_ species, includes, among others, carotenoids, fatty acids, vitamins (mainly from the B group), micro- and macroelements (zinc, iodine, bromine, iron, copper, phosphorus, and magnesium) as well as high-value protein, which can constitute 60-70% of the dry mass [11]. The high protein content in microalgae is mainly due to proteins participating in photosynthesis. In cyanobacteria such as _Arthospira_, it is mainly phycocyanin, which can constitute 15-20% of the dry mass of algae. This protein has a blue color and is responsible for the absorption of energy from orange and red light, which it then transfers to chlorophyll. Phycocyanin has strong antioxidant properties, inhibits the growth of cancer cells in vitro, and can bind metals, which is why it is often used as a component of various types of preparations in the cosmetic and pharmaceutical industries [12; 13]. It is worth noting, however, that within the same species of microalgae, their chemical composition is not constant and depends on factors that determine growth in the culture, such as temperature, pH, lighting, mineral content in the medium, and carbon dioxide concentration [3]. Therefore, algal preparations obtained from the same species of microalgae, but from different manufacturers, will significantly differ in the content of nutrients, e.g., protein or iron. The food industry has explored the use of microalgae in various products, such as bread [14] and cookies [15]. Research by [PERSON] et al. (2019) [16] found that adding Spirulina to cookies increased their protein content; cookies with a 15% Spirulina addition had the highest protein and mineral levels. Dairy products are a promising base for microalgae enrichment. Kefir is a valued dairy product due to its probiotic properties and rich nutritional composition. Its nutritional value and the bioavailability of its constituents are typically higher than that of milk, thanks to the fermentation process.