Application of a fluorescence-based method to evaluate the ripening process and quality of Pinot Blanc grape

Abstract

Context and purpose of the study – The chemical composition of grape berries at harvest is one of the most important factors that should be considered to produce high quality wines. Among the different chemical classes which characterize the grape juice, the polyphenolic compound, such as flavonoids, contribute to the final taste and color of wines. Recently, an innovative non-destructive method, based on chlorophyll fluorescence, was developed to estimate the phenolic maturity of red grape varieties through the evaluation of anthocyanins accumulated in the berry skin. To date, only few data are available about the application of this method on white grape varieties. The aim of the present study was to evaluate the quality and the ripening process of Pinot blanc grape by a non-destructive fluorescence-based sensor.

Material and methods – The study was performed on two vineyards of cv. Pinot blanc located in the Adige Valley (South Tyrol, Bolzano), in two consecutive vintages. The vineyard differed in the row orientation, east-west or north-south, and then on the sun light exposure of the grape-bunches. The grape phenolic maturity was assessed on intact berries by six measurements from bunch closure to harvest time. In each vineyard, 25 grape-bunches per row sides were flashed by Multiplex® 3.6 (Force-A, Orsay, France), for a total of 3 rows and 150 grape-bunches/measurement. The instrument indices of chlorophyll (SFR_R) and flavonols (FLAV_UV) were considered. Standard grape maturity tests were performed to assess total soluble solids (TSS) and total acidity content of the grape juice by spectroscopic method. At maturity the grapes were processed with a standard vinification protocol for white wines. Total polyphenolic content of wines was determined by a spectrophotometric analysis.

Results – A linear decrease of SFR_R index in the berry-skin during the grape ripening period was recorded. Interestingly, SFR_R values negative correlated with the TTS accumulation in Pinot blanc berries. On the other side, positive correlations between SFR_R and titratable acidity, malic acid and tartaric acid content, were observed. The FLAV_UV index showed an increasing linear trend during the grape ripening period. At harvest, significant difference in FLAV_UV index between the two vineyards was observed. Looking more deeply inside the data, the berry-skin FLAV_UV index significantly differed among the four sun-light expositions, with greater values recorded for the grape-bunches located in south and east sides of the vineyard rows. These results are in accordance with the available literature on the role flavonols as sun-burn protection compounds. Interestingly, the total polyphenolic content of the produced wines showed a positive correlation with the final FLAV_UV values measured in the berry-skin. In conclusion, the Multiplex® indices could improve precision viticulture strategies, such as the implementation of precision harvest practices. Indeed, SFR_R index could be used to indirectly evaluate the whole ripening process of white grapes in term of grape sugar content and acidity, while FLAV_UV could provide useful indications to winemakers about taste of final product. Future studies will be necessary to better correlate the berry-skin FLAV_UV values and the flavours of white wine.