## 1 Introduction  
The growing need for fuels and high-added-value chemicals by the global community has boosted environmental issues, mainly because of the abusive consumption of non-renewable sources for these concerns. This fact not only leads to a notable decrease in fossil fuels, but also to increasing prices and greenhouse emissions. In this context, lignocellulosic biomass and agricultural waste have recently emerged as a renewable, sustainable, widely available alternative source for the development of fine chemicals [1; 2; 3]. The winemaking sector is one of the most productive worldwide, involving the generation of millions of tons of waste. As a result, several studies have focused their attention on the reuse of grape by-products in the food sector, as ingredients for making functional foods, since they are a good source of bioactive compounds [4; 5; 6]. Grapes are indeed one of the main agroeconomic crops around the world, with \(>\)77.8 million tons produced annually, with ca. 57% being utilized in the wine-making process [7; 8]. The main vinicification by-products are represented by grape stalks (GS) that account for 2.5-7.5 wt% of the overall grapes and are obtained after destemming process. GS are not considered a hazardous waste, but their great content of organic matter and their seasonality contribute to potential pollution troubles. Therefore, GS valorization has become a challenging field of investigation for minimizing their environmental impact by the conversion of their main components (cellulose, hemicellulose, and lignin) into new value-added compounds. As reported by [PERSON] et al., several GS valorization processes--such as acid and enzymatic hydrolysis of polysaccharides and further fermentation--have been investigated and optimized in recent years [9]. However, utilizing this process exclusively to produce ethanol from GS sugars is unattractive in terms of cost balance. Therefore, other valued streams must be exploited for feasibility to use these wastes in an integrated manner, which increases profits and attracts stakeholders to discover these by-product resources. In the last few years, biomass upgrading has been achieved using process intensification techniques, leading to higher efficiency and sustainability [1; 2; 3; 4]. In this context, the cavitation phenomena in ultrasound (US)-assisted extraction (UAE) favors the matrix solvation and cell wall rupture [10]. The formation, growth, and collapse of gaseous microbubbles in a liquid phase may dramatically improve mass transfer [10]. The powerful local effects of these microbubbles provide harsh conditions in the reaction medium, as immense localized heating and pressure [11], because of the generation of microjet streams, shock waves, turbulences, or shear forces. This phenomenon generates mechanical effects, such as the breakdown of tissues in small fragments, and intensive mixing and heating [12], in contrast to conventional methods. Therefore, cavitation has a remarkable impact on accelerating chemical reactions, increasing conversions and yields, or enhancing selectivity in both homogeneous and heterogeneous systems [11]. UAE is a well-known process applied for the recovery of high-added-value metabolites from different lignocellulosic matrices [13; 14; 15]. Along with US, microwave (MW) heating shows a positive effect in terms of reaction control, residence times, and energy efficiency, thus being considered an environmentally friendly technology. The interaction of MW with the matter originates by selective absorption of radiation by polar molecules (together with ionic conduction), leading to a volumetric heating in contrast to conventional treatments, where the heat is transferred by conduction from the flask wall [16; 17]. These phenomena provide high efficiency for each process, minimizing side reactions and undesired by-products, as well as higher yields at shorter residence times (energy saving).