WINETECH Technical Yearbook 2019

Limited data is known about the influence of ellagitannins on the oxidative stability of white wines. Oxidative instability was first observed during the early 1990s, and since then winemakers have acted precautionary to prevent early maturation. Icon white wines have, irrespective of their sensory quality, also a significant shelf life. The management of barrel maturation plays a decisive role in the making of premium white wines. Apart from the role in micro- oxygenation and the increase in phenolic and flavour compounds, oak wood also plays an important role in the oxidative status of white wines. It influences the redox potential and consequently the oxidative stability of wine. The anti-oxidative capacity of oak wood depends on its ellagitannin concentration. Early research indicated that the ellagitannin concentration increases rapidly during the first three months of barrel maturation. Different physico-chemical factors explain its subsequent decrease in concentration.

The high reactivity of ellagitannins towards oxygen is one of these reasons. Research was done at the University of Burgundy to obtain more data about the stabilising capacity of compounds extracted from wood, especially about the oxidation of white wines. The first step was to differentiate between wine that was matured in barrels with different toasting levels and tannin potential. Molecular analyses were also executed on ellagitannins, grape phenolic compounds and glutathione to identify the wood compounds, which influence the oxidative stability of wines. Chardonnay, Sauvignon blanc and Semillon wines from 2015 and 2016 were investigated over a period of eight months barrel maturation. The following conclusions were made after all the experiments: ● Barrel maturation improves the oxidative stability of white wines.

● A positive correlation exists between the tannin potential of barrels and the oxidative stability of wines. ● Different toasting levels do not have an effect on the oxidative stability of wines. ● A positive correlation exists between the tannin potential of the barrel and the ellagitannin concentration in the wines. ● As confirmed previously, a considerable increase in the ellagitannin concentration of wine occurs during the first three months of barrel maturation. ● The duration of barrel maturation requ i red to obta i n the max imum ellagitannin concentration in wine is rather determined by the composition of the wine than the tannin potential of the barrels. Factors like pH, alcohol concentration and temperature can influence the rate of ellagitannin extraction from the wood.

● The evolution of the known anti- oxidant, glutathione, according to the tannin potential of the wood was also monitored. During barrel maturation the glutathione concentration of the wine decreased progressively, depending on the barrel and the wine (Nikolantonaki et al. , 2018). REFERENCES Nikolantonaki, M., Coelho, C., Diaz-Rubio, M-E., Rozier, J., Haroun, C., Zbirko, A., Gougeon, R., Badet-Murat, M-L. & Vicard, J-C., 2018. Impact of oak tannin and barrel toasting on the oxidative stability of dry white wine. Australian and New Zealand Grapegrower and Winemaker , August 2018: 63-68. Vivas, N. & Glores, Y., 1996. Research note: Role of wood ellagitannins in the oxidation process of red wines during aging. American Journal of Enology and Viticulture 47(1): 103-107.