Many question exactly what is being added to wine beyond the expected yeast and grapes. The additives used in wine production, similar to food production, are strictly limited by regulations that vary from country to country. Most wines remain stable after bottling and don’t need many additives other than those to deter oxidation and microorganisms. And from a food safety angle, most will find it comforting to know most bacteria cannot survive in wine because of the alcohol and natural acidity.
One of the most common and confusing additives is sulfur dioxide.
Many use the term “sulfuring” for this process, but it’s misleading because it is not sulfur but sulfur dioxide being added. This confusion is most likely from the technique used to sterilize empty casks. In the past, winemakers lowered a piece of burning sulfur into the cask. The sulfur dioxide produced in this reaction dissolved into the residual water in the cask, thus sterilizing it. Candles are used today but generally a powder or gas form is added.
As with most chemicals used in food production, sulfur dioxide is harmless when used in the correct levels. It is also used in fruit juices, dried fruit, cured meats and sausages, pickles and molasses. It is a very useful additive but does have disadvantages; it is an irritant/allergen for some consumers. This is the reason for the U.S. labeling statement: “This wine contains sulfites.”
Sulfur dioxide helps prevent some wine faults. The most common is oxidation (browning). It is key that oxidation be kept to a minimum in order to maintain freshness. Oxygen sometimes dissolves in wine, causing browning and creating an off flavor. Sulfur dioxide is used to prevent this reaction. It is also used for antiseptic purposes. Acetobacter may be the most common form of bacteria attacking wine and turning it into vinegar. This bacteria needs oxygen to flourish, but a dose of sulfur dioxide followed by filtration will rid the wine of the invader.
It can also be used for correcting a wine after oxidation. It may be added to freshen “tired” wines that may have suffered a slight degree of oxidation. This usually occurs due to poor production and handling. It’s important to remember this addition isn’t a magic cure, because it only helps maintain a quality factor but can’t improve quality that didn’t already exist.