# Impact of Deep Eutectic Solvents on Extraction of Polyphenols from Grape Seeds and Skin
## 1 Introduction  
Grape (_Vitis Vinifera_ L. ssp sativa) is generally cultivated in moderate-warm climate zones, and it is of worldwide interest for nutritional and medical purposes [1]. This fruit, as one of the main natural dietary sources of polyphenols, is associated with numerous health benefits. It is importantto emphasize that phenolic composition is affected by the grape variety, degree of ripeness, as well as agronomic and environmental conditions [2]. Moreover, there is a significant difference between parts of fruits (seeds, skin, and stems) considering not only the number of phenolic compounds, but also their concentrations [3]. There are three main classes of polyphenols bounding in grapes: phenolic acids, flavonoids, and stilbenes, each with their own family members. Numerous studies suggest that polyphenols work in more than one way, expressing antioxidant activity and influencing cell communications that affect important biological processes. In particular, grape polyphenols could contribute to a healthy heart by promoting the relaxation of blood vessels to help maintain healthy blood flow and function [4]. Extensive information is available on grape antioxidant activity, concerning the prevention of the degenerative pathophysiological state developed in healthy adults [5]. Approximately 75 million tons of grapes are produced every year worldwide, whereby almost 80% of this quantity is utilized in wine production [6]. Progressively increasing amounts of wine waste represents a serious environmental pollution problem [7; 8]. Amid wine generation, most of the valuable health promoting compounds from grape berries are extracted into juice or wine, but a noteworthy sum remains caught within the pomace (skin and seeds). An increasing body of evidence has recently appeared that the revalorization of wine by-products is conceivable. Grape pomace, such as value-added products containing plenty of active compounds, have been proposed for enrichment of food, pharmaceutical, or even cosmetic items [9]. Extraction of valuable and health-promoting constituents was usually performed using petrochemical and volatile organic solvents. Most of these solvents are of serious concern, since they are flammable, low biodegradable, toxic, and volatile. Along with the growing recognition of the human impact on the environment, green extraction has become an optimal and sustainable way for raw material utilization [10]. Nowadays, several types of solvents, including natural deep eutectic solvents (NADES), have been suggested as an alternative for conventional solvents. NADES are prepared by mixing quaternary ammonium salts (e.g., choline chloride) and naturally derived hydrogen bond donors (e.g., sugars, alcohols, amines) [11]. Commonly, NADES components are non-toxic, cheap, and readily available [12]. Due to their unique physicochemical properties, such as adjustable viscosity, wide polarity range, and high solubilization strength for a broad range of compounds variety, NADES have a great potential for different health related purposes [13]. They were confirmed as excellent solvents for sustainable and environmentally friendly extraction. Choline chloride based natural deep eutectic solvents were demonstrated to have high stabilizing ability for phenolic compounds, which can be correlated with the strong hydrogen bonding interactions between solutes and solvent molecules [14]. Besides the extraction, they have been used in enzyme stability and enzymatic reactions [15; 16; 17]. Furthermore, the literature indicates that natural deep eutectic solvents could be designed with specific biological activity. They were reported to play an important role in enhancing antioxidative activities of plant extracts by possessing this activity itself [12]. Serbia has a long viticulture tradition (it is one of the oldest grapevine growing areas in Europe) due to its favorable geological position.