Acid-base indicators change color depending on whether they are in acids or bases. Many berry juices contain a substance called anthocyanin that changes color depending on whether mixed with an acid ...

"A screening method for the determination of ascorbic acid in fruit juices and soft drinks". Food Chemistry 116 (2): 509-512, 2009.

The findings indicate that citric acid, the juice additive, may help alleviate recurring shortages of a protein called Factor VIII, which is important for the normal clotting of blood and is ...