Sour tasting white powder.
Ascorbic acid (Vitamin C) is an essential nutrient that the human body cannot manufacture from other compounds.
It is needed for the formation of collagen, the protein that makes up connective tissue, and is essential to muscles, bones, cartilage, and blood vessels.
Ascorbic acid is also a good anti-oxidant, preventing damage from oxygen free radicals.
Ascorbic acid is added to many foods for its nutritive value, but is also used extensively as an anti-oxidant, to prevent flavors and colors from being damaged by oxidation. It is often used in canned or frozen fruits to prevent the browning that accompanies oxidation.
While not as powerful an anti-oxidant as sodium bisulfite, it has a better reputation, by virtue of being a vitamin.
An isomer of ascorbic acid (a molecule with the same number and type of atoms, but in a different arrangement) called erythorbic acid is often used as a cheaper anti-oxidant than ascorbic acid. It has little or no effect as a vitamin, but it has the same anti-oxidant properties.
To make ascorbic acid soluble in fats, it is reacted with fatty acids such as palmitic acid to form ascorbyl palmitate, used to prevent oxidation in fats and oils.