Sulfur dioxide is the acrid smelling gas produced when sulfur
is burned in air.
Sulfur dioxide is a reducing bleach, and can counter the effects of
oxidizing bleaches, thus preserving color in fruits dried in the sun.
The combination of fruit acids and ultraviolet light would otherwise
react with the colorful compounds in the fruit, making them pale.
Sulfur dioxide is produced by burning sulfur in air (for preserving
fruit), or by reacting
in the water in a product such as wine.