Here is a list of possible science fair project titles using fruits and acid:

* The Effect of Different Acids on the Deterioration of Fruits

* How Does the pH of Fruit Juice Affect Its Taste?

* The Effects of Acids on the Color and Texture of Fruits

* How the Amount of Acid in a Fruit Affects its Ripening Process

* Investigating the Relationship Between the Acid Content of Fruits and Their Nutritional Value

* The Effects of Acid on the Shelf Life of Different Types of Fruit

* How Does the Acid Content of Fruits Affect the Activity of Digestive Enzymes?

* Using Acids to Preserve Fruits and Vegetables