## 1 Introduction  
Phenolic compounds are secondary metabolites present in various parts of plants [1, 2]. They belong to the group of biologically active compounds and their production depends on various enzymes participating in metabolic reactions [3]. The metabolism of these compounds is integrated with the biochemical and morphological regulatory patterns of plants [4]. Phenolic compounds in plants are used as protective substances against various types of stress that can be caused by environmental conditions, pathogens and injuries [1]. They have a key role in plant defense mechanisms [5, 6]. Green walnut, shell, husk, kernel, bark, root and leaves of _Juglans regia_ L. are widely used in the pharmaceutical and cosmetic industries [7]. The research carried out showed that walnuts have a higher antioxidant activity than pistachios and hazelnuts [8]. Walnuts are considered a good source of tocopherols and essential fatty acids [9, 10]. Moreover, walnuts contain other compounds that may be beneficial in promoting human health, including polyphenols, tannins, folate [11], dietary fiber, protein, melatonin [12] and sterols [9, 13]. The leaves and bark of walnuts are used as a natural dye for wool colorization [14]. Extracts prepared with different parts of the walnut have anti-inflammatory, blood purifying, anticancer, cleansing and detergent properties [15]. The water extract made with walnut bark protectsmice against biochemical toxicity caused by cyclophosphamide [16]. It contains several therapeutically active ingredients, especially polyphenols [17; 18]. The bark of _Iuglans regia_ L. contains chemical components such as \(\beta\)-sitosterol, ascorbic acid, juglone, folic acid, gallic acid, regiolone, and quercetin-3-\(O\)-\(\alpha\)-\(l\)-arabinoside [19]. The results of several other studies demonstrated that in most cases the extracts obtained from different parts of the walnut using alcohols, water or a mixture of them demonstrated a higher content of phenols than the extracts of other non-polar solvents such as petroleum ether, chloroform, n-butanol, benzene and cyclohexane. Polar solvents such as methanol, ethanol, water, acetone and ethyl acetate are believed to be the best solvents for polyphenols extraction [20]. The dried bark of walnut contains significant amounts of polyphenols. The extraction of _Iuglans regia_ L. bark using ethyl acetate allowed us to obtain polyphenols in the amount of 34.83 mg GAE/g (dry mass) [21]. Other studies indicated the possibility of obtaining 138.5 mg GAE/g (dry mass) (using ethyl acetate as a solvent), 311.5 mg GAE/g (dry mass) (using diluted acetone), 172.5 mg GAE/g (dry mass) (using methanol extract), 182 mg GAE/g (dry mass) (using water extract) [22]. Subcritical water extraction is one of the most promising, scalable to industrial capacity and environmentally friendly isolation methods of biologically active compounds from plant raw materials [23; 24; 25]. The unique properties of water in a subcritical state are mainly due to a relatively high boiling point, high dielectric constant and high polarity [26]. With the increase in the water temperature, its electric permittivity decreases, diffusion rate increases and viscosity together with surface tension decreases. Consequently, materials with a high polarity and high solubility in water under normal conditions are extracted more efficiently at low temperatures, while low-polar and non-polar compounds require less polar medium to be present at elevated temperatures [1; 27; 28]. [PERSON] et al. [28] described the correlation of an increase in the phenols content in the extract with an increase in the temperature, as a result of the water polarity decrease. An increase in the water temperature increases the diffusion rate and desorption kinetics, and increases the dissociation of compounds. A reduction in viscosity and the surface tension of water, as a result of a temperature increase, improves the quality of contact between the solvent and the raw material.