Sulfites are a derivative of sulfur and a natural by-product of fermentation. Most wines contain very low levels of sulfites, which have been used for hundreds of years by winemakers to clean and sterilize equipment and barrels; to kill off bacteria; and to prevent browning and possible spoilage. Sulfur is also sometimes sprayed in a vineyard to prevent disease and pests. Most wines contain very low levels of sulfites even when the winemaker doesn't utilize sulfur anywhere! Under U.S. law, any wine with sulfites higher than 10 ppm must state "contains sulfites" on the label.