Citric acid is a naturally occurring weak organic acid found in citrus fruits like lemons and oranges. It's commonly used as a food additive for several reasons:

* Acidity regulator: Citric acid helps adjust the pH of food, making it more acidic or less acidic, depending on the desired effect.

* Flavoring agent: It adds a tart, sour flavor to foods and beverages.

* Preservative: It inhibits the growth of bacteria and yeast, extending the shelf life of food.

* Antioxidant: Citric acid can help protect food from oxidation and spoilage.

Safety: Citric acid is generally considered safe for human consumption and is found naturally in many foods. However, some people may be sensitive to it and experience side effects such as stomach upset, nausea, or diarrhea.

Common Uses:

* Soft drinks

* Candy

* Jams and jellies

* Sauces and dressings

* Baked goods

* Frozen foods

Note: The E number system is a standardized way of identifying food additives in Europe.