Project number
WW WdT 20-01
Project leader
Prof Wessel du Toit
Institution
South African Grape and Wine Research Institute, Department of Viticulture and Oenology, Stellenbosch University
Project is co-funded by The Department of Science and Innovation
Project completed
2022
EXECUTIVE SUMMARY
Objectives and Rationale
This study aimed to develop a new phenolic extraction and testing method for grapes that shows better correlation with wine phenolic levels, as existing commercial methods often lack this correlation, particularly for tannins.
Methods
Various grape sample preparation methods were compared, including hand-crushing, machine-crushing, and established methods like Iland and Glories. Phenolic determinations were performed on red grapes and wines, with a focus on Shiraz, using spectrophotometric methods. Experiments also evaluated the Glories index’s suitability for assessing the contribution of seed tannin to wine.
Key Results
A newly developed machine-crushing method showed promise for correlating phenolics in Shiraz grapes and wine, especially for anthocyanins and total phenolics, and demonstrated better repeatability than hand-crushing. The Glories method was found to indicate increased wine tannin contribution from grapes with more seeds but tended to overestimate this contribution when fewer seeds were present, and its validity regarding seed tannin contribution was questioned in the absence of seeds.
Key Conclusion of Discussion
The new machine-crushing grape phenolic analysis method is a potential tool for the South African wine industry to assess Shiraz grape and wine correlations, though further validation for other cultivars is needed. Caution is advised when interpreting the Glories index’s seed contribution to wine tannin levels.
Take Home message for Industry
A new machine-crushed phenolic method can enable correlations between grape and wine phenolics in Shiraz, provided a representative vineyard sample is used. The Glories index should be interpreted carefully regarding seed tannin contribution.