## 1 Introduction  
Blueberry (_Vaccinium corymbsum_ L.), a fruit from the Ericaceae family, is becoming increasingly popular due to health benefits and pleasant taste. It is a rich source of bioactive compounds, such as vitamins, anthocyanins, phenolic compounds and essential minerals [1, 2, 3]. The most important vitamins found in blueberries include vitamin C, E and K, while anthocyanins, flavonoids and phenolic acids are the main phenolic compounds [4, 5]. Studies on commercial blueberry samples have shown that the vitamin C content in fresh fruit ranges from 3.4 to 9.5 mg per 100 g, anthocyanin content from 25 to 495 mg/100 g and quercetin from 24 mg/100 g. The total phenolic content ranges from 48 to 304 mg/100 g [6; 7]. Regular consumption of blueberries is associated with a number of health benefits, including a reduced risk of cardiovascular diseases, anti-inflammatory effects and protection against neurodegenerative disorders. Anthocyanins in blueberries exhibit strong antioxidant properties, neutralizing free radicals and protecting cells from oxidative damage [8; 9; 10]. These compounds also demonstrate anticancer activity, as confirmed by numerous in vitro and in vivo studies. Fruit processing methods, including those applied to blueberries, can lead to significant losses of nutrients [11]. Traditional thermal treatments such as boiling, baking and pasteurization have been shown to reduce anthocyanin and vitamin content. Boiling blueberries in water causes leaching of antioxidant compounds and the extent of loss increases with longer thermal processing times [12; 13; 14]. One of the fruit processing methods with high potential is extrusion-cooking technology. Extrusion-cooking involves the mechanical processing of food materials under the influence of high pressure, shear forces and controlled temperature [15; 16; 17; 18]. The ability to precisely adjust the temperature during the extrusion-cooking process helps to minimize the degradation of heat-sensitive nutrients and bioactive compounds. As a result, it is possible to produce food with high health-promoting value and improved sensory qualities [19; 20]. Extrusion-cooking is widely used in the production of both functional foods and various snack products [21; 22]. Blueberries can serve as a valuable functional addition to such products. This additive has recently become popular for its health-promoting properties resulted from high content of bioactive compounds such as anthocyanins, phenolic acids and flavonoids. To date, some types of functional food enriched with the fruit was evaluated. Blueberries are added in form of powder, juice, extract, dried fruit etc. Despite significant pro-health activity of the additive, relatively few products with documented by scientists' activity are available on the market. Most of papers are based on quality/quantity analysis and biological activities of selected form of blueberry additive. Example of analysis of functional food product are studies based on exploration the enrichment of extruded snacks made from rice and buckwheat flour with blueberry powder, evaluating their sensory and antioxidant properties. These studies have shown that increasing the amount of blueberry in the formulation leads to a rise in antioxidant activity and an improvement in nutritional profile. Significant decreasing of fat content and starch was also recorded [23]. Interesting studies based on preparation of blueberry juice powders as potential functional food ingredients were also presented [24]. Scientists explicitly indicate that the freeze-dried juice powders can be used as rich source of pro-health secondary plant metabolites as well as for stabilization new assortment of functional food. The resulting powders exhibited favorable properties, including low water activity (below 0.5), high solubility (exceeding 90%), and efficient retention of bioactive compounds--recovering over 70% of total phenolics and more than 60% of total monomeric anthocyanins.