Determination and quantitative analysis of the principal polyphenolic compounds present in stem extracts of native Greek islands grape varieties-assessment of their antioxidant activity in Vitro

Grape stems are vinification byproducts with no economic interest which are being discarded. In this work we have studied the principal native grape varieties (Vitis vinifera) that are cultivated in Greek islands, in respect to their content in major soluble polyphenols and the assessment of their antioxidant properties. In this regard, their respective extracts were obtained by an optimized ultrasound extraction procedure and their main polyphenolic substances were determined by HPLC-DAD. In addition, their total polyphenolic content (TPC) was estimated pohotometrically. All extracts were found to contain polyphenols as 46–54% w/w, with flavonoids and stilbenes being the most abundant. It must also be noted that trans-resveratrol and ε-viniferin were assayed in considerably highest concentrations as compared to those reported in respective grape skins (considered as the principal source of trans-resveratrol in grapes). In addition, various resveratrol glucosides were also detected (but not quantified) by LC/MS/MS.

The antioxidant properties of all extracts were evaluated by the DPPH^. radical scavenging and FRAP assays. Those extracts exhibiting the highest TPC, were further evaluated for their ability to inhibit CuSO₄–induced LDL oxidation and to reduce the intracellular reactive oxygen species (ROS) in smooth muscle cell cultures. Most stem extracts examined, were found to possess significant antioxidant activities which are in direct relation to their high polyphenolic content. These results indicate that grape stems could account as a rich source of polyphenolic compounds, and trans-resveratrol in particular. Therefore, their extracts could be used for the production of high added value products by the pharmaceutical and cosmetic industries.