## 1 Introduction
Some of them are used as a source of livestock feeds or as organic fertilizers; however, nowadays, they are mostly composted or discarded in landfills, causing environmental stress and pollution [24; 25]. In order to minimize their impact on the environment and add value to the food industry, in recent years, more and more studies have focused on finding new directions for using by-products from fruit processing as a functional food additive [26; 27]. Extrusion has also been investigated for exploiting the by-products of food processing [28]. [PERSON] et al. [7] exploited blackcurrant juice press residues in barley flour, oat flour, or oat bran extruded snacks, while [PERSON] et al. [14] studied the effects of cherry, blackcurrant, and chokeberry pomaces as a value-added component on the content of bioactive polyphenols, nutritional value as well as antioxidant capacity in corneal-based extruded snacks. They found that the addition of berry pomaces increased the bioactive compounds and dietary fiber content and improved the color, texture, and sensory properties of the extruded snacks. [PERSON] et al. [29] formulated gluten-free crackers based on brown rice flour and apple pomace. The supplemented crackers contained higher dietary fiber, minerals, and antioxidant activity than control samples but presented lower hardness and fracture force compared with the controls. [PERSON] et al. [30] developed corn extruded snacks supplemented with apple and rosehip pomaces at 10-20% addition levels, while [PERSON] et al. [3] investigated the properties of extruded corn puffs with the addition of 5-20% blackcurrant and chokeberry pressing residues. The addition of pomaces increased the antioxidant properties and bioactive and dietary fiber content; however, it decreased the expansion rate, water absorption, and solubility indices of the extruded corn puffs. The fruits of bilberry (_Vaccinium myrtillus_ L.) are currently harvested from wild bushes or grown in eastern Europe and northern Africa [31]. The dietary intake of these fruits has been linked to several health benefits, such as antioxidant, antibacterial, anti-inflammatory, antiproliferative, hypoglycemic, cardioprotective, antidiabetic, hepato- and neuroprotective activities [26; 32; 33; 34; 35]. Bilberries became identified as superfluuits based on their richness in phytochemicals, antioxidants, and vitamins. However, bilberry popularity comes primarily from the extremely high anthocyanin content of the fruits, dominated by delphinidin, cyanidin, petunidin, peonidin, and malvidin glycosylated with galactose, glucose, and arabinose [36; 37; 38]. The high anthocyanin content has also promoted these fruits as valuable sources of natural pigments for food applications [26]. Industrial bilberry juice processing results in the separation of the pomace, consisting mainly of skins, seeds, and residual pulp, which represent between 20% and 25% of the initial fruit weight, depending on the pressing efficiency but especially on the use of enzyme preparations [14; 39]. High contents of anthocyanins, phenolic acids, flavanols, flavonols, coumarins, procyanidins, tannins, terpenoids, and stilbenes are found in bilberry pomace, several times higher compared to the fruit juice or the whole fruit [40; 41]. Unfortunately, the high moisture content of pomaces makes them extremely susceptible to microbial spoilage; therefore, pomaces need to be dried in view of their use to produce natural colorants, nutraceuticalals, or functional foods [42]. The dark purple powders obtained after the drying and milling of bilberry pomace contain oils rich in n-3 polyunsaturated fatty acids originating from seeds (n-6/n-3 ratio = 0.91) [43] and high amounts of phenolic compounds, especially anthocyanins, that recommends them as a food ingredient in novel functional snacks [44].