Citric acid is found in lemon juice. It is a naturally occurring acid that is also found in other citrus fruits, such as limes. Citric acid is known for imparting a sour taste to these fruits.
It is not economical or efficient to extract citric acid from fruit for commercial use. Instead, the acid is produced on an industrial scale by cultivating an organism called Aspergillus niger. This organism produces citric acid as a byproduct of its metabolism of sugars.
Citric acid is widely used as a flavoring for drinks, such as soft drinks and juices. Its chemical properties also make it an effective preservative, as many bacteria are unable to grow in acidic environments.