## 1 Introduction  
The production and consumption of fruits, mainly berries, grapes, apples and citrus, has been increasing in the last years, generating consequently a rising number of by-products. From a circular economy and a sustainable point of view, it is crucial to find ways to recycle fruit by-products, most of them considered as wastes and commonly discarded, which could be exploited in novel products or applications. Actually, a great effort has been devoted to the exploitation of agricultural and agri-industry by-products and wastes, on the one hand, valorizing them, for example, as a source of bioactive compounds for food or non-food applications, thus obtaining value from the whole biomass and, on the other hand, reducing the negative impacts of one of the major environmental problems of fruit processing industries [1, 2]. In a processing plant, about one-third of the fruit will likely end up as pomace, which comprises peels, skin, pulp, seeds, leaves and stalks. In Europe alone, it is estimated that these by-products and wastes can account for up to 1.3 billion tons per year [3; 4]. These by-products have been studied as a possible source of bioactive compounds such as phenolic compounds, fatty acids, polysaccharides, and dietary fibers, among other health-promoting nutrients [1; 2]. Pomaces have been reported to have, in some cases, higher amounts of these compounds of interest when compared with corresponding fruit juices themselves [1]. A growing trend in using agricultural and agri-industry by-products as a source of bioactive compounds has been observed lately [4; 5], with a major interest in apple [6; 7; 8; 9; 10], grape [11; 12; 13], citrus [14], tomato [15; 16; 17] and olive [18; 19] pomaces but also in other fruit by-products such as from almond [20], mango [21] or coffee [22] processing, just to name a few. Due to the high volume produced worldwide, grape by-products have been among one of the most studied, where stalks are also included. Grape stalks have been addressed as a promising source of phenolic compounds presenting antimicrobial activity [23]. Additionally, this lignocellulosic biomass, presenting a high content of cellulose (12-40%), hemicellulose (12-35%) and lignin (15-47%), has also been noted as a possible source to produce activated carbon [24] or fermentable sugars after hydrolysis [25; 26; 27], which could be used for bioethanol production, for instance [28]. The applications of the lignocellulosic fraction may be perfectly integrated with bioactive compounds valorization. The consumption of healthy foods has been a trend in the past years, leading to increasing demand in the search for natural products. _Sambucus nigra_\(\mathrm{L}\) is a deciduous shrub whose berries and flowers have been used since ancient Rome in formulations for areas as diverse as folk medicine or food industries [29]. Elderberries, due to their high content in anthocyanins and polyphenols and their peculiar sensorial characteristics, are being used in jams, jellies, pies, syrups, beverages and also as concentrates and infusions [30; 31]. Elderberries consumption has been associated with the prevention and therapy of a variety of diseases, such as cardiovascular diseases, diabetes, and obesity. Elderberries extracts have also shown promising antiviral, antibacterial, antifungal and antidepressant activities, as well as a significant impact on the immune system [32; 33]. The plantation of \(S\). _nigra_ has been growing since the 1980s throughout all Europe, though an increased interest has been noticed in the last 20 years [31] and an elderberries production of 1500-2000 tons per year is estimated in Portugal alone, specifically in the Varoas Valley, located in northern Portugal [34]. Elderberries industrial processing originates by-products, with elderberries stalks the most abundant, accounting for more than 10% of the initial elderberries weight [34]. Currently, these stalks, as most food by-products [4], are disposed of, used for composting, or used for heating purposes [34].