Citric acid is a organic acid that occurs naturally in citrus fruits. Fruits like limes and lemons have naturally high levels. 

Citric acid used to solely be produced from citrus fruits until 1917 when a process was developed to produce citric acid from sugar. This is still the most common way citric acid is produced today.

Citric acid is a common additive for food products and is classified under the E number of E330. 

Citric acid is used as a flavoring and preservative in food and beverages, especially soft drinks. It can also be used as a emulsifying agent.