Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
email: Office@practicalwinery.com
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MAY/JUNE 2010
WINEMAKING
Below, we will demonstrate how manipulation of GSH during fermentation can increase aroma longevity in aromatic wines.
Figure 1: Comparison of aroma profiles of a wine fermented with Zymaflore X5 rehydrated with Dynastart®, analyzed at two time points (June 2007 and January 2008) separated by 7 months with cellaring at 15°C under screwcap. Aroma intensity is expressed as Concentration/Perception Threshold (c)/PT), where a value of ≥1 indicates contribution to wine aroma. 4MMP = 4-methoxymercaptopentanone (broom/boxtree); 3MH = 3-mercaptohexanol (grapefruit); 3MHA = 3-mercaptohexyl acetate (passion fruit); IA = isoamyl acetate (banana); PE = phenyl ethanol (rose); PEA = phenyl ethyl acetate (tea); HA = hexyl acetate (pear).
Figure 2: Correlation between juice YAN and subsequent GSH content of wine.
Antioxidants
Antioxidants behave in a manner similar to electrochemically-active species, because they respond to the potential for oxidation or reduction in their environment. For example, examining a series of chemicals that are thought of as antioxidants (Figure 4), one can see that tannins are more effective antioxidants than SO2.
In white and rosé wines, no significant amounts of tannin are present, and the next most effective antioxidant in such wines is glutathione. Clearly, maximization of GSH will lead to better wine preservation. The goal of winemakers should thus be to maximize the GSH concentration in these wines.
What makes GSH an effective antioxidant? It is the presence of a thiol group in the molecule. Thiols provide many of the strongest fruity characters found in certain wines (such as Sauvignon Blanc), and these aromas can be susceptible to oxidation precisely because they are good antioxidants, like GSH. The problem is that when they react, as antioxidants, their contribution to wine aroma is lost or altered, hence wine quality deteriorates.
The thiol group (–SH) is a reduced form of sulfur (S). Sulfur is able to react with oxygen to form oxides (as in SO2), sulfites (SO32–2–) and, ultimately, sulfate (SO42–). Since an informal definition of oxidation is “combination with oxygen,” the progression of sulfur from –SH to SO42– constitutes the behavior of an antioxidant (Figure 5).
With glutathione, the situation can be slightly more complicated in that, for the more chemically-minded, GSH acts as a nucleophile and regenerates hydroquinone and quinone species, which are oxidized forms of phenols (see Figure 6). The end result is typical antioxidant behavior, since the oxidized form of the molecule is reduced to the original phenolic form, with glutathione now bound to the phenolic core to form the “grape reaction product.”
Preserving wine aroma
Some simple practical adaptations can be made to preserve the glutathione content of wines. Maintenance of the SO2 level
Figure 3: Concentration of GSH during fermentation and lees ageing with sufficient YAN.
Figure 4: Comparison of reduction potential of various antioxidants.
is essential, remembering that it has both anti-microbial and antioxidant roles. Lees ageing, provided the lees came from a healthy fermentation, are clean and are free of contaminating organisms, can clearly improve wine GSH content, since GSH is released by yeast during the start of autolysis.
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