## 1 Introduction  
Today, more than ever, there is a growing interest in sustainability, including, but not limited to crop by-products. Consequently, adding value to biowaste is more important than ever [1]. Considering and treating biowaste as a source of bioactive compounds can result in significant benefits in terms of sustainability, as well as major benefits to the food and beverage industries [2]. Grape stems are widely known materials that are left over from grapes and wine-making. Every year, millions of tons of waste are produced from the cultivation of grapes with only a small percentage of them (<4%) remaining for the feeding of animals. Grape stems represent about 5% of the waste from the process of wine-making, and in general, the production of grapes [3]. Regarding the grape stems, researchers worldwide have successfully identified their composition [4; 5]. They mainly consist of water [6], cellulose and hemicellulose, lignin, proteins, acids, sugars, as well as polyphenic compounds, particularly those derived from red grapes. The possibility of creating value from the by-products of the abovementioned process has been studied by several researchers [7; 8; 9; 10]. For instance, [PERSON] et al. [11] studied the antiproliferative effect of grape stem extracts against three cancer cell lines. Similarly, [PERSON] et al. [7] studied the antioxidant, antimicrobial, anti-inflammatory, and anti-aging properties of the grape stem extracts. Of all their components, phenolic compounds are of great importance owing to their antioxidant, antibacterial, and antifungal properties [12; 13; 14; 15]. To this end, multiple extraction techniques have been employed to prepare grape stem extracts, including solid-liquid extraction [16], ultrasonication (US) [17], and pressurized liquid extraction [9]. However, to the best of our knowledge, the pulsed electric field has not been employed to date. The pulsed electric field (PEF) is a green extraction technique that has been used in recent years to optimize the extraction process of precious substances [18; 19; 20]. PEF is an extraction technique, based on electro-permeabilization, aiming at the creation of pores in the cellular membranes, and thus, assist in the extraction of compounds. Owing to its inherent advantages, PEF has been successfully employed for the improved extraction of polyphenols from citrus fruits [21], potato peels [22], _Sideritis raiseri_[23], Merlot grapes [24], etc. Moreover, it has been shown that the use of PEF in combination with ultrasound can increase the extraction of phenolics from plant by-products [25]. The PEF is typically applied prior to the ultrasound-assisted extraction as a pretreatment step. However, this step does not exclude the release of ingredients from the raw materials during PEF treatment. In particular, short-duration pulses between 100 ns-1 ms are used with a voltage between 1 and 3 kV.  
The present study aimed at investigating whether the use of PEF prior to the ultrasound-based extraction of polyphenols from grape stems can enhance their extraction, and to what extent. In addition, volatile compounds contained in the extracts were examined, since they are valuable molecules widely used in the food industry. To this end, grape stem extracts were prepared, using the ultrasound treatment. Moreover, PEF pretreated samples (with various solvents) were subjected to the ultrasound treatment and compared.