Malic Acid
Malic acid is a natural sour compound found in apples and other fruits.
Think of it this way
When you bite into a green apple, your mouth puckers from the sour taste. That sharp bite comes from malic acid, stored right in the apple's flesh. It acts like tart powder in a bowl — quick to hit your tongue. The more of it in the fruit, the stronger the sour punch.
Explaining malic acid by grade level
Bite into a green apple and taste that sour zing. That flavor comes from malic acid inside the fruit. Ripe apples have less of it, so they taste sweeter. How you store apples changes how sour they stay.