Transforming winemaking waste: grape pomace as a sustainable source of bioactive compounds

The valorisation of winemaking by-products has emerged as a sustainable approach to mitigate environmental challenges and promote resource efficiency within the wine industry. Grape pomace (GP), the primary solid residue generated during vinification, retains a substantial portion of the grape’s polyphenols, representing a valuable source of natural antioxidants. Transforming this by-product into high-value bioactive ingredients holds significant potential for applications across the nutraceutical, food, and cosmetic industries. This study investigated the phenolic composition and antioxidant capacity of GP from different grape varieties in the Rhône Valley area of France, including Alicante, Syrah, Mourvèdre, and Grenache. Total polyphenol content (TPC) was measured using the Folin–Ciocalteu method, while phenolic profiling—covering phenolic acids, flavonoids (flavan-3-ols, anthocyanins, flavonols) and stilbenes—was conducted using HPLC-DAD-ESI-QQQ. Antioxidant capacity was evaluated through DPPH, ABTS, FRAP, and ORAC assays. The results revealed significant retention of phenolic compounds in GP following vinification, with concentrations ranging from 5–35 mg GAE/g DW depending on the sample type (GP skins or seeds). Fermented GP seeds, particularly from Syrah and Alicante varieties, displayed the highest phenolic content, rich in flavan-3-ol monomers and procyanidins, and exhibited superior antioxidant activity compared to GP skins across all vintages. Correlations between phenolic composition and antioxidant activity were explored to further elucidate the functional properties of bioactive compounds and identify the most promising GP extracts for further valorisation. These findings highlight the potential of grape pomace as a readily available, low-cost source of polyphenolic compounds, supporting its conversion into bioactive extracts for a range of industrial applications.