## 1 Introduction  
Yellow onion skins (_Allium cepa_ L.) are considered valuable raw material for the extraction of bioactives because they are rich, economical, sustainable, ubiquitous, and available worldwide; additionally, they are not considered a component of the global food supply. A significant amount of onion is cultivated annually, making it one of the most grown vegetables around the world, with an estimated total production of around 66-85.7 million tons per year [1]. Industrial processing leads to a significant amount of onion by-products, estimated to be around 500,000 tons [2], which is considered an important source of polyphenols. Furthermore, onions also represent a considerable amount of dietary phytochemicals such as organosulfur compounds, phenolic acids, flavonoids, thiosulfinates, and anthocyanins [3].  
Flavonoids are well-known antioxidant compounds owing to their high redox potential, which allows them to act as reducing agents and singlet oxygen quenchers [4]. It hasbeen reported that flavonoid levels in the edible portions of onions vary from 0.03 to 1 g/kg, whereas onion skins contain significantly higher levels of flavonoids at 2-10 g/kg [5].  
Therefore, onion skins represent a valuable resource for flavonoid extraction. In most cases, solid-liquid conventional and assisted extraction methods are the most exploited technologies for the extraction of flavonoids from vegetables, but these techniques employ high volumes of solvent and energy [6]. However, high extraction yield, with its energetic feasibility and usage of environmentally friendly solvents, should be considered as an extraction technology. Hence, new solvents are being studied in order to carry out more selective extractions of flavonoid compounds from plants [7; 8], while considering the reduction of extraction time, solvent, and energy.  
When extracted from their natural matrices, polyphenols are highly unstable such that they undergo various chemical reactions because of the presence of unsaturated bonds in their molecular structure and are affected by oxidants, heat, light, and enzymatic reactions during storage [9]. Therefore, protective technologies should be applied to improve polyphenol stability and safeguard them against chemical damage before their industrial application. This can be achieved by applying suitable techniques such as microencapsulation [10].  
It has been suggested that the use of microencapsulated flavonoids instead of the free compounds can overcome the drawbacks of their instability, improve their bioavailability as well as their shelf life [11], thus contributing to the widening of their industrial applications in food and in related fields such as the biopharmaceutical and cosmeceutical industries [12].  
Therefore, the objective of the present study was to test the use of a green, low-cost solvent to extract flavonoids from yellow onion skins. In order to provide a more stable, multifunctional, and safer powder--with multiple uses such as food ingredients, in biopharmaceutical or in cosmeceuticals--a unique combination of biopolymeric matrices such as whey protein isolates (WPI), whey protein hydrolyses (WPH), maltodextrin (MD), and pectin (P) were dissolved in the flavonoid-enriched aqueous extract, followed by freeze-drying. The powders were evaluated for their functional properties, in terms of the flavonoids' entrapping efficiency, total flavonoid and polyphenol contents, antioxidant activity, their microstructural pattern by confocal microscopy, in vitro digestion as well as the thermal and pH stability of the phytochemicals. The cytocompatibility of the powders was tested on mouse fibroblasts to evaluate the release of possible toxic compounds and their proliferative effect. The results obtained in this study could bring certain benefits in terms of exploiting the bioactive potential of yellow onion skin flavonoids or for the development of ingredients with health benefits, thus upgrading their functional characteristics.