Grape By-products as a Source of Polyphenols: Development and Optimization of Green Extraction Methods

The wine industry is a major global sector, generating large volumes of by-products such as grape pomace and stalks, which can pose environmental challenges [1]. These residues are known to be rich in natural polyphenols, compounds with well-documented health benefits, prompting growing interest in their extraction [2].
This work explores the antioxidant potential of wine production by-products. Three eco-friendly extraction methods, ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), and subcritical water extraction (SWE), were evaluated and compared with a traditional extraction technique (CE), using grape by-products from two varieties: Tinta Miúda and Cerceal Branco. The resulting extracts were analyzed for total phenolic content (TPC) and antioxidant activity using the FRAP and ABTS assays.
Among the techniques studied, UAE (at 70% amplitude for 20 minutes) yielded the highest polyphenol amount. Extracts from the Tinta Miúda variety demonstrated the greatest TPC (78.9 ± 1.8 mg GAE/g dry weight (dw)), as well as superior antioxidant activity (FRAP = 106 ± 3 mg AAE/g dw; ABTS = 123 ± 2 mg AAE/g dw). Current efforts are directed toward identifying the specific phenolic compounds responsible for these effects using high-performance liquid chromatography coupled with diode array detection.
These results highlight the potential of grape by-products as sustainable sources of natural antioxidants. The use of green extraction technologies supports circular economy principles, promoting the valorization of winemaking residues into high-value functional ingredients for the food industry.