Sulfites (also commonly called sulfiting agents) are sulfur-containing preservatives which prevent food spoilage and discoloration by browning.
The sulfiting agents used by the food industry are sulfur dioxide, sodium sulfite, sodium bisulfite, potassium bisulfite, sodium metabisulfite and potassium metabisulfite. They are used to preserve foods such as processed potatoes, dried fruits, dehydrated vegetables, shellfish and other seafood, as a bleaching agent for food starches. Sulfites are also used in wine because they prevent bacterial growth but do not harm the yeast needed for fermentation (sulfites concentrations are generally higher in whites than in reds because the red wines contain more natural antibacterial flavonoids). These chemicals are also used in medicines (both prescription and over-the-counter) to help keep them stable and potent for a longer period of time.
Unfortunately, sulfites can cause human health problems. The FDA has banned the use of sulfites in foods that are important sources of vitamin B1 because the chemicals will destroy that vitamin. The agency has ruled that any prepared food that contains more than 10 parts per million of sulfites must declare the presence of the chemical on its label.
Furthermore, a sizeable minority of people are sensitive to sulfites. The chemicals can cause allergic reactions and asthma attacks in sensitive people. Other effects in susceptible people range from headaches, stomach cramps, and hives to life-threatening anaphylactic shock.
From the science dictionary at http://biotech.icmb.utexas.edu/