Anyone who has read a wine label in the United States has noticed the somewhat ominous statement “Contains Sulfites”. When I first noticed this warning, I thought to myself what are sulfites doing in my wine and why do they require a warning on the label? This Arts & Sciences will provide a brief look at why wines contain sulfites and why a warning label is needed.

         Sulfites are a group of chemicals that contain a sulfur atom with several oxygen atoms attached to it (See the figure). Sulfur dioxide or SO2 when dissolved in water forms sulfurous acid, and sulfur trioxide SO3 when dissolved in water forms the very corrosive H2SO4 or sulfuric acid.

         Sulfites both occur naturally and unnaturally in wine as a product of the fermentation process and as additives by the wine maker during the wine making process as a preservative. Sulfur especially in its organic form (ie. sulfur containing amino acids) is essential for life. For example sulfur plays an important role in protein structure through the formation of disulfide bridges. These bridges link different parts of proteins together to make them into the proper shape to perform their function. The activity of many enzymes and hormones such as insulin rely on disulfide bridges. Disulfide bridges also put the curl in your hair. Hair straightening products break these bridges to straighten hair out. However the point is that sulfur is an important part of living things and during fermentation of grape juice or cheese, sulfites are also produced. During fermentation of grape juice up to 15 parts per million sulfites are naturally produced.

         Winemakers also add sulfur dioxide to the freshly pressed grape juice or must to prevent oxidation, stabilize the wine, and inhibit natural yeast activity and bacterial growth. In France many winemakers still sterilize their barrels by burning sulfur in them. Also, a solution of potassium metabisulfite in water serves as a great disinfectant for winemaking equipment. The antioxidant qualities of sulfites protect wine from aging too quickly. If you have ever had a very old bottle of red wine, the first thing you often see is that the purple and red pigment have all oxidized to brown and amber. Sulfites in wine slow the aging of the pigments. Sulfur can also be used to block malolactic fermentation. Organic wines do not contain sulfites beyond the natural products of fermentation and tend to age more rapidly than wines with sulfites. In the US, wines that bear the organic seal must have less than 90 and 100 parts per million sulfites for the red or white and sparkling wines respectively.

         So, if winemakers use sulfites to make their wine, why is there a warning on wine labels? Studies have shown that sulfite sensitive individuals can suffer adverse effects from consuming sulfite containing foods and beverages. Asthma can be triggered by different stimuli such as dust, pollen, moulds and sulfites. Although food triggered asthma is not common among asthmatics—6-11% of children with asthma and less than 2% of adults with asthma—the United States Food and Drug Administration (FDA) moved in the ‘80s and ‘90s to reduce the risk of sulfite sensitive individuals or unknowingly ingesting sulfited foods and beverages. The FDA prohibited the use of sulfites on fresh fruits and vegetables and required the labeling of foods and beverages that have detectable levels of sulfites. US law requires a “Contains Sulfites” label when the wine contains greater than 10 parts per million of sulfites. Soon the European Union and Canada will also require sulfite labeling on wine sold in those countries. According to a study from the Center for Food Safety and Applied Nutrition in the US the labeling for sulfites has significantly reduced the number of reports of adverse effects from consuming undeclared sulfites.

         The label appears to be working and saving sulfite sensitive individuals from potentially serious adverse effects. As for those not sensitive to sulfites the only downside is the over use of sulfites can impart the burnt match smell that is not pleasant. So if you are having a dinner party with a know asthmatic you may want to have at least one bottle of organic wine on hand as a courtesy.