Citric acid

Citric acid is a weak organic acid, and is triprotic. It is a natural preservative and is also used to add an acidic, or sour, taste to foods and soft drinks. In biochemistry, it is important as an intermediate in the citric acid cycle and therefore occurs in the metabolism of almost all living things. It also serves as an environmentally benign cleaning agent and acts as an antioxidant and a lubricant.
Citric acid exists in a variety of fruits and vegetables, most notably citrrus fruits. Lemons and limes have particularly high concentrations of the acid; it can constitute as much as 8% of the dry weight of these fruits. The concentrations of citric acid in citrus fruits range from .005 mol/L for oranges and grapefruits to .030 mol/L in lemons and limes. These values will vary depending on the circumstances in which the fruit was grown.
At room temperature, citric acid is a white crystalline powder. It can exist either in an anhydrous (water-free) form or as a monohydrate. The anhydrous form crystallizes from hot water, whereas the monohydrate forms when citric acid is crystallized from cold water.
The monohydrate can be converted to the anhydrous form by heating it above 74 °C. Citric acid also dissolves in absolute (anhydrous) ethanol (76 parts of citric acid per 100 parts of ethanol) at 15 degrees Celsius.