Identifying vineyard and winery management practices that impact on tannin extraction

Abstract

The effect of harvest date, fermentation temperature and canopy management on tannin extraction was investigated to assist in viticultural and winemaking decisions to manage tannin levels in wine. Tannin extractability was determined in Shiraz grapes harvested at six different times during ripening in two seasons and fermented at three different temperatures. The effects of thirteen vineyard floor and irrigation canopy management treatments on tannin extractability were also determined on seven different sites

Summary

This project sought to investigate aspects of tannin extraction from grapes into wine that would provide knowledge upon which to base viticultural and winemaking decisions to influence predictably the concentrations of tannin in the final wine. Specific aspects investigated were the effect of harvest date, canopy management and fermentation temperature on grape tannin extraction; all within a context of evaluating the efficacy of grape tannin measurement metrics.
To investigate the effect of harvest date, Shiraz grapes were harvested on six different occasions during maturation from a Barossa vineyard and from a Sunraysia vineyard in two seasons. Wines were made from grapes harvested at the different dates to determine tannin extractability. In addition, grapes from each harvest date were fermented at three temperatures to investigate any interactive effect between maturity and fermentation temperature on tannin extraction. Grape composition factors influencing tannin extraction were determined including total and wine-extractable grape tannin, grape anthocyanin and the amount of skin cell wall material. The same grape composition factors were determined in Shiraz grapes from a canopy management trial with up to thirteen treatments applied at seven vineyard sites across two seasons. Wine was made from a selection of treatments from two sites in the first season.
Tannin extraction was found to vary by site and season during maturation. While there was a trend for tannin extractability to increase with maturity in Shiraz grapes from the Barossa, tannin extraction decreased at the later harvest dates in Sunraysia. Higher fermentation temperatures increased tannin extraction with little effect on the volatile profile of wines. Wines made at later harvest dates and higher fermentation temperatures had more stable polymeric pigments and wine colour density, and these wine quality metrics were maintained as the wines aged. Fermentation temperature may be one additional approach for managing tannin extraction