Phenolic composition of Nebbiolo grapes: impact of solar radiation

Grapes (Vitis vinifera L.) are fruits rich in polyphenols; the phenolic composition of grape is very complex, including monomeric and/or polymeric molecules as anthocyanins, flavan-3-ols, flavonols, dihydroflavonols, various phenolic acids and hydroxystilbenes. Phenolic composition of grapes varies depending on several factors such as the effect of vintage, soil, temperature, luminosity, cultural practices and developmental stage (1). Aim of this work was to characterize the phenolic composition of Nebbiolo grapes from Piedmont, evaluating the influence of solar radiation. The vineyards for the samples collection were selected in four different areas, namely Piobesi d’Alba (CN), Barolo (CN), La Morra (CN) and Ghemme (NO). First, the amount of incident radiation upon the surface of grapevine rows, taking into account their mutual shading, was calculated. For each vineyard, a radiation map highlighting homogeneous radiation areas was produced; the maps were then employed to carry out the sampling in areas characterized by different class of radiation (in each vineyard 2 or 3 points for both highest and lowest radiation area were selected). The phenolic fraction of different grape samples (harvested at the physiological ripeness) was characterized for total polyphenols, anthocyanins and tannins, the antioxidant activity (DPPH method) and the chromatic characteristics. Liquid chromatography (RP-HPLC/DAD) has been also applied for the characterization of individual polyphenols. The composition of grape samples varied depending on the different radiation areas; however, different behaviors were observed also considering samples from different vineyards. In the locality of Ghemme, the highest values for both polyphenols and antioxidant activity were observed in correspondence of the higher radiation area; samples from the Southern Piedmont generally showed an inverse tendency.