The winemaking sector is one of the most important worldwide, which translates into the production of huge amounts of by-products, such as grape pomace and stalks, with high environmental impacts [1]. These grapevine by-products represent potential sources of natural polyphenols, and due to their recognized health-promoting properties, several studies have focused their efforts on their efficient extraction [2].

In this study, three environmental-friendly extraction techniques, namely ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), and subcritical water extraction (SWE), were tested and compared with a conventional extraction (CE) for different winery products from two varieties (Tinta Miúda and Cerceal Branco). The total phenolic and tannin contents (TPC and TTC, respectively) and antioxidant/antiradical activities (through FRAP and ABTS assays) were screened. For all the extraction techniques tested, SWE allowed to recover the highest amount of polyphenols. Regarding the samples analysed, the extract of grape stalk from Cerceal Branco variety obtained using SWE at 150 °C had the highest TPC (17.0 ± 0.2 mg GAE/g fresh weight (fw)). In contrast, the extract of grape stalk from Tinta Miúda variety presented the highest antioxidant activity (FRAP = 15.9 ± 0.3 mg AAE/g fw and ABTS (20.9 ± 1.0 mg AAE/g fw). Work is in progress to identify the individual phenolic compounds of the obtained extracts, which can contribute to the presented antioxidant properties, using high-performance liquid chromatography with diode array detection.

The presented results demonstrated that SWE can be an efficient and green extraction technique for obtaining phenolic compounds from different grapevine by-products, which can be further safely applied to food or cosmetic industries, creating an added value to this residue.