Sulfur vs Sulfur Dioxide vs Sulfides

Sulfur vs Sulfur Dioxide vs Sulfides

There is a lot of misunderstanding and misuse of some of the terminology concerning sulfur compounds in wine. A current trend is to use little or no sulfur in winemaking, but what are the consequences of this decision?

Sulfur is the element and is used as a fungicide in the vineyard. Sulfur Dioxide is used as a preservative in wine. Sulfides are a group of volatile sulfur compounds associated with reductive winemaking, the struck match note found in some wines. Elemental sulfur is used in the vineyard as a spray to prevent powdery and downy mildew. Mildew spores can become resistant to synthetic fungicides, however mildew has never developed a resistance to sulfur. Also sulfur is permitted for use in organic and biodynamic agriculture. Yes, even biodynamic vineyards!
The timing of sulfur sprays in the vineyard is crucial to ensure disease free vineyards. From budburst to veraison vineyards are typically sprayed every few weeks, although more often in wet or humid vineyards. Applications need to cease well before the fruit is picked to ensure that too much sulfur isn't taken into the winery on the skins of the grapes. Although a small amount can be very beneficial to some winemaking styles (more on that later) too much can raise the risk of Hydrogen Sulfide (rotten egg gas) in ferments which can be disastrous. 

Sulfur dioxide (SO2) is composed of one sulfur molecule and two oxygen molecules, and is a completely different chemical from elemental sulfur. It can be added as a liquid, a gas or in solid form. Sulfur dioxide is antimicrobial, anti-enzymatic, a preservative and an antioxidant. It is colorless and, unlike elemental sulfur, has a strong odor. SO2 may be added to grapes, to fermenting juice, right through to finished wine prior to bottling.
Too much sulfur dioxide can result in muted or even off flavours in the wine. Using too little or none at all runs the risk of tainted or spoiled wine.
One important property of SO2 is its ability to bind with acetaldehyde. This compound has an unpleasant smell of bruised apple or sherry, and is produced when wines are exposed to oxygen. When the SO2 and acetaldehyde molecules bind to each other, the resultant substance is odourless. The SO2 effectively strips the wine of its oxidative character. This character is all too common in some of the newer, trendy, 'natural' wines.

Sulfur dioxide is very nasty stuff to handle, anyone who has been exposed to it will not soon forget. But in winemaking we are talking about a few parts per million, these miniscule quantities will not affect the vast majority of the population. There are dozens of other household foods that contain far greater amounts without issue. If you do believe you are sensitive to SO2 there are products you can get to help. Little eye dropper bottles filled with a solution of dilute food grade hydrogen peroxide. Just a drop or two will bind up the sulfur dioxide. But many find it isn't in fact sulfur dioxide that is the cause of their sensitivities.

Sulfites (sulfur dioxide) are oxidized sulfur compounds, sulfides are reduced sulfur compounds. Reduction happens in winemaking when the juice or wine isn't exposed to enough oxygen. This is often considered a fault but when done with care and subtlety it can give complexity to a wine. Reductive winemaking techniques combined with small amounts of elemental sulfur that has come in on the grapes from the vineyard can result in very desirable flavours. One compound, benzenemethanethiol, can give wines a struck match or gunflint character.
This type of winemaking has become more common in the past two decades, especially in grapes such as chardonnay. Winemakers will keep more solids in their ferments which means more sulfur from the vineyard is kept, then the wines are handled in a way to minimise their exposure to oxygen. In expert hands these wines can be amazing, the sulfides making a good wine even better. But there is always the risk that things can get out of control and you end up with a very faulty wine.
- Chris, Albury -