New discoveries about sulfur

At last year's regular assembly of the American Society for Enology and Viticulture (, the introductory presentation on the role of sulfur in preventing oxidation in wine was given by researchers from Cornell and Davis faculties in the USA and private laboratories from England, Gavin Sacks, Patricia A. Howe and John C. Danilewicz.
New discoveries about sulfur

The presentation is very professional, and oenologists would benefit from reading it in its entirety, but the practical advice given at the end is applicable to any winemaker. Oxidation in wine occurs through two processes, iron-phenols and copper-sulfhydryl, of which the first is far more important.

In it:

• Oxygen reacts very quickly with Fe(II) iron and produces the oxidized form of Fe(III) and hydrogen peroxide (H2O2). Immediately afterwards, Fe(III) reacts with phenolic compounds, producing quinones and Fe(II),

• Bisulfite reacts with quinones and returns them to their phenolic form and thus prevents their participation in further oxidative processes,

• Hydrogen peroxide then undergoes a Fenton reaction where it reacts with Fe(II) to produce hydroxyl free aldehydes, which oxidize the alcohol and produce an oxidative odor and color.

Oxidative aromas, therefore, arise from free aldehydes. They can be formed either through the Fenton reaction, or due to the breaking of sulfur and aldehyde bonds that were formed earlier in the process. Those oxidative-smelling aldehydes are now free in the wine. Until now, the prevailing opinion was that almost all oxidation is due to the Fenton reaction. Therefore, 30-40 mg/L of free sulfur in wine is recommended. These studies, however, showed that even a concentration of less than 1mg/L of free sulfur is sufficient for the rapid binding of H2O2 and prevention of the Fenton reaction. Oxidation, in fact, results from the release of aldehydes bound to sulfur. This release is independent of the inflow of new oxygen, as well as of the temperature at which the wine is stored. All wines with a concentration of free sulfur below 10mg/L showed oxidative aromas and browning due to this release. What this means in practice, especially for winemakers who want to use less sulfur in their production process.

1. It is necessary to produce wine using methods that reduce the amount of aldehyde at the end of fermentation. The use of sur lie methods showed a significant reduction of the most potent aldehyde (methional). With a reduced amount of aldehyde, a significantly lower concentration of sulfur is needed to bind them to the extent that would provide non-oxidative aromas even after the release of aldehyde from sulfur bonds.

2. In the case of red wines where oxygen is systematically introduced during aging for the production of acetaldehyde, which later reacts with tannins and anthocyanins gives softness and a darker color, the initial use of sulfur immediately after the end of fermentation should be avoided.

3. It has been shown that some substances in wine, such as tannin hydrolysis products, can also consume added sulfur. They can be separated from the bound sulfur even in the complete absence of new oxygen. Therefore, it is necessary to ensure a sufficient amount of sulfur immediately after fermentation for wines that are kept reductively until bottling.

Branislav Anđelić

Photo: Pixabay

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