Wine industry by-product: Full polyphenolic characterization of grape stalks

In this research work grape stalk samples from the Douro region were analyzed concerning polyphenolic composition. Different solid-liquid extraction conditions were tested for reaching the optimal parameters to obtain higher amounts of polyphenols. In general, the best conditions comprise the use of a mixture of acetone/ethanol/water (1:1:1) at room temperature during 20 min, using a pre-wash treatment with deionized water. The polyphenolic extractions with acetone are generally more effective than without acetone. Total phenolic determination (Folin-Ciocalteu), antioxidant capacity (DPPH/FRAP) and more accurate methods such as HPLC-DAD and HPLC-ESI-MS were performed. An intensive polyphenolic characterization (including a possible identification of a new polyphenol) and compound quantification was achieved. A comparative study between these two analysis approaches was accomplished, which allowed to conclude that the Folin-Ciocalteu and DPPH/FRAP determinations do not always allow a direct correspondence between high phenolic content or antioxidant/reducing capacity and real high polyphenolic content on crude polyphenolic extracts.