## 1 Introduction  
Today there is a trend of returning to the forgotten wild edible fruits that were once successfully used in folk medicine or cuisine. In this study, attention was paid to the fruits of the guelder rose due to its health benefits and nutritious qualities, as well as its desirable and unusual color [1]. In Poland, the _Viburnum opulus_ (VO) plant, i.e., guelder rose, is one of two species (along with _Viburnum lantana_) naturally occurring in Poland. Genus Viburnum, which belongs to the family Caprifoliaceae, consists of approximately 150 species of shrubs and small trees. The guelder rose is a shrub typical of North America, Western and Central Europe, Asia, Caucasus and Asia Minor. It grows up to 3 to 5 m. Its fruits are red, vitreous and round-shaped. In the beginning, the fruits are green and as they ripen they turn red at the end of September, and when fully mature they are red and shiny [2, 3]. The fruits of _V. opulus_ are characterized by an unpleasant bitter taste since they contain saponin glycosides and slightly toxic viburnine. The fruits need to be frozen before they can be eaten since freezing makes the fruits' bitter taste less intense [4, 5]. Edible fruits are used as ingredients for preparing jams, marnalades, juices, drinks and sauces. In Canada, they are used as substitutes for cranberries and in Russia and Ukraine, the fruits are used in 'kalinnikov' pies. In Turkey, there is the commercially available gilaburu juice--a traditional non-alcoholic beverage, which needs to undergo the fermentation process. During this process, fruits are placed in plastic drums with tap water, in a dark place at room temperature for four months to eliminate the sour flavor. The fruits can be used after this time. Their juice is commonly used for medicinal purposes (e.g., to treat colds, liver diseases, diabetes or hypertension) [3]. Numerous clinical trials demonstrated an inverse relationship between the fruits' consumption and diseases. There is much evidence to show that a sufficient quantity of these fruits in the diet reduces diseases such as cancer or cardiovascular and neurodegenerative disease [6; 7; 8]. The fruits are a wealthy source of bioactive substances which may act as antioxidants. The color of the fruits indicates the presence of chemoprotective substances, which have biological activity in the body [7]. In the research, a lot of attention is paid to the antioxidant content of the fruits but their effectiveness in the fight against free radicals is still unknown. The important medicinal effect of antioxidants is the protection against diseases induced by reactive oxygen species. They are capable of donating electrons to oxidants making them harmless to the cell. The natural bioactive compounds (plants' secondary metabolites) proved to have pharmacological activity in terms of lipids regulations, insulin sensitivity and antioxidant activity [9]. VO fruits are characterized by high amounts of polyphenols, phenolic acids, flavonoids, procyanidins, catechins, anthocyanins as well as organic acids (e.g., ascorbic acids and L- malic acids) [3; 9; 10]. Due to these compounds, numerous in vivo studies demonstrated that different morphological parts of VO show anti-microbial, anti-diabetic, anti-obesity, anti-inflammatory and anti-cancer properties. Moreover, a positive effect on the urinary system was observed in animal studies [3]. Despite its sour and bitter taste, the VO fruit, after appropriate freezing, shows great potential for use in juice technology, as a food additive, in cosmetics or pharmaceuticals. On the Polish market, only its bark is available for making infusions while fruits are used at home for making jams and liqueurs [8; 11]. Some are trying to use the fruit as a food additive, e.g., for pear juice with the addition of the VO juice or cakes enriched with a VO fruit pomace. Some studies indicate the possibility of adding VO fruits to meat, e.g., cooked minced turkey [12] or homogenized meat products [13].