## 1 Introduction  
Both barley and oat are valuable resources in several countries and belong to the commonly cultivated crops worldwide, ranking fourth and seventh among cereal grains. According to FAO statistics, among the cereal crops grown in Europe, wheat production is on the top, followed by barley, with oat in fourth place. The top producers of oat in 2018 were Spain, Poland and Finland, whereas the biggest producers of barley were France, Germany and Spain [1]. The use of barley and oat for human nutrition is particularly important, since both are a source of starch, proteins, lipids, dietary fiber, B vitamins and phytochemicals with antioxidant activities, such as polyphenols, vitamin E and carotenoids [2; 3; 4]. In addition, oat grains contain avenanthramides, also known for their antioxidant activity [4]. Oat and barley are consumed in lower quantities compared to rice or wheat, but they are also rich sources of fiber, minerals, vitamins and other bioactive compounds, which may provide health benefits, such as a reduced risk of chronic diseases such as coronary heart disease, type 2 diabetes or cancers [2]. Among bioactive compounds occurring in oat and barley, phenolics, in that phenolic acids and flavonoids, gain significant attention, mainly due to their antiradical activities [5]. Phenolics are plant secondary metabolites, playing essential roles in the reproduction and growth of the plants and having important functions in defense mechanisms against pathogens and pests. Some of them also contribute to the color of plants [2; 6]. In addition to their roles in plants, phenolic compounds in human diet may also provide significant health benefits. They show the ability to inhibit cancer development due to various mechanisms (antioxidant activity involving reactive oxygen species, gene expression, enzyme modulation and apoptosis). Furthermore, polyphenols have protective effects against heart diseases, by protecting LDL against oxidation [7]. From the group of phenolic acids, cereal grains contain hydroxybenzoic acids (\(p\)-hydroxybenzoic, vanillic, salicylic, gallic, ellagic and protocatechuic) and hydroxycinnamic acids (caffeic, ferulic, sinapic and \(p\)-coumaric). Ferulic and \(p\)-coumaric acid are the predominant phenolic acids in cereal grains. As reported in [8], ferulic acid is the main contributor to the antioxidant capacity of grains, which has been confirmed in the in vitro study showing that antioxidant potential of cereals is correlated with their ferulic acid content. The highest concentrations of the phenolic acids in cereals are found in whole grains and bran [6; 8]. Flavonoids are present in very small quantities in cereal grains. They are found mainly in the seed coat, aleurone cells, hull (oat) and sometimes the embryo. Barley contains higher amounts of flavonoids than oat. Flavonoids are compounds of diverse structure. They show a strong antioxidant effect due to the ability to bind metal ions [9]. The content of both phenolic acids and flavonoids in cereal grains varies and depends on climatic and agrotechnical conditions, harvesting time, maturity and storage conditions. Genetic factors and species diversity of plants as well as part of the grain belong also to the very important factors [6]. The phytochemicals responsible for the characteristic yellow color of cereal grains are carotenoids. In plants, carotenoids have various functions related to the photosynthesis, photoprotection and they are precursors of phytohormones [10]. They act as powerful antioxidants in many biological systems, due to their ability to react with free radicals. Carotenoids have powerful activity against singlet oxygen generated from lipid peroxidation or radiation. \(\beta\)-carotene, \(\alpha\)-carotene and \(\beta\)-cryptoxanthin show a provvitamin A activity. One of the main pigments of cereal grains, lutein, together with zeaxanthin is accumulated in yellow spot of the retina in human eye, where it absorbs UV radiation and protects the eye from damage [6].