The aim of this project was to determine the effect of downy mildew on the aroma profile and ageability of Merlot and Cabernet Sauvignon. The compounds responsible for the atypical aromas perceived, were also identified.
A Merlot and Cabernet Sauvignon vineyard in Bordeaux was selected for the trial.
- Vines were trained on a VSP system and cane pruned.
- The control section followed a normal spray program to protect vines against disease.
- In the other section, no fungicides were applied.
- Healthy and infected grapes were harvested separately.
- Using a standard winemaking protocol, the following wines were made:
- A control wine made from healthy grapes.
- Additional wines simulating 5% – 20% infection were also made.
The study showed that berries that are infected with downy mildew shrivel, and the constituents of the berry is modified, affecting the chemical and sensory profile of the wine.
- Maceration and alcoholic fermentation both delivered wines with cooked fruit (prunes, figs, cooked peaches, bell pepper) and herbaceous aromas.
- Even without maceration, the cooked fruit aromas were evident, indicating that yeast metabolism plays an important role in the formation of cooked fruit aromas in wines made from infected grapes.
- The volatile compounds causing the atypical flavours found in wines made from infected grapes were identified.
- The intensity of the herbaceous/cooked fruit, off-flavours depends on the variety – Merlot being most susceptible from just 5% infection.
- The infection threshold before an effect in the wine was noticeable, was higher in Cabernet Sauvignon.
- The evolution of sensorial and molecular markers was higher in Merlot, compared to Cabernet over time, and increased as infection % increased. This indicates that wines made from downy mildew infected grapes, have a lower ageing potential, more so in Merlot.
SIGNIFICANCE OF THE STUDY:
Grape quality and chemical composition has an impact on the aroma profile, as well as the ageing potential of wines. Downy mildew infection of grapes, will negatively affect wine. This is an important consideration for winemakers who are faced with infected grapes in the cellar.
Image source: P.G. Goussard