Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
email: Office@practicalwinery.com
1 · 2 · 3 · 4
MAY/JUNE 2010
WINEMAKING
BY
Paul K. Bowyer (Technical
Manager, Laffort Australia),
Marie-Laure Murat (Lab
Manager, Sarco, France),
Virginie Moine-Ledoux
(Research Director, Laffort)
A
ll winemakers are concerned with quality, to greater or lesser extents andwithin budget constraints. In white winemaking, appearance of the wine is secondary to the nose of the wine. It is critical that a wine of quality be expressive of both varietal character and terroir. For aromatic wines such as Sauvignon Blanc, which typically do not improvewith bottle age, saleability relies heavily on freshness and maintenance of the wine’s aroma profile.
Wine aroma changes over time. The first aromatic compounds to be lost are those that are most susceptible to oxidation, such as the volatile thiols in Sauvignon Blanc (and other varieties such as Riesling, Colombard, Gros Manseng, Verdelho) including 4MMP, 3MH and, particularly, 3MHA(Figure 1).1
Methods of preserving wine aroma include several approaches, from reductive (protective) handling of fruit and juice, to addition of tannin, although all preservation methods have drawbacks. For example, reductive handling requires the use of inert gases, while tannin additions, if done excessively, can alter the palate of the final wine.
Glutathione
A more recent approach to aroma preservation is the use of the protective power of glutathione.2 Glutathione (GSH) is a
plant-derived natural antioxidant, which is superior in action to both ascorbic acid and sulfur dioxide. It has been known for many years that GSH is a more potent antioxidant than ascorbic acid, and that it is thus preferentially consumed ahead of ascorbic acid under oxidative conditions.3
Figure 5: Progression of sulfur through a series of oxidation states, from reduced (H2S) to elemental sulfur (S8), then sulfur dioxide (SO2) and sulfite (SO32–), finishing with the (SO42–) form.
It is thought that one function of GSH in plant systems is to maintain ascorbic acid in its reduced form for improved physiological activity.4 In grapes, GSH accumulates during ripening5 with levels varying considerably, ranging from 17 to 114 ppm,6 whereas in yeast, GSH constitutes about 1% of the dry mass.7,8
The glutathione content of wine is known to correlate with the yeast available nitrogen (YAN) content of the juice,8 for the simple reason that if juice YAN is low then GSH is consumed by the yeast as a nitrogen source.
For example, in Figure 2, when juice YAN is sufficient, the GSH content of juice and finished wine is approximately equal. When juice YAN is less than 100 ppm, very little GSH remains in the corresponding wine. Additionally, yeasts assimilate GSH in the early stages of fermentation and release it in the latter stages, particularly during autolysis (Figure 3), although this is a slow process.
Given the potency of GSH, winemakers can use this compound in winemaking as a natural antioxidant for preservation of wine aromas that are susceptible to oxidation, and concurrently delay the appearance of developed characters. By using the protective power of glutathione, winemakers are now able to preserve wine freshness and prolong consumer appeal in a way that has not previously been possible.
Previously, we have demonstrated the large reduction in GSH levels observed during the oxidative handling of juice, and also that selected fining practices do not remove GSH from juice.10
Top · Next