Well first I want to point out to you that it does not say that they ate an apple. I don t known when that idea started that it was an apple. I am not even sure they grow apples in the Middle East. It is almost certain it was not an apple. If you saw the TV special on Genesis in November it pictured the fruit of this tree more like a type of berry - it looked like a blackberry, which of course does not grow on trees but on bushes.
Hebrew tradition says the fruit was an apricot.
Another ancient tradition says it was a pomegranate. Many Bible scholars think it was probably a fig, because it is the only fruit mentioned in this context. In verse 7 when Adam and Eve make coverings for themselves they make it of fig leaves. They eat the fruit, realize they are naked and immediately grab leavens from the tree to cove themselves, and they happen to be fig leaves. The truth is the text does not tell us the type of fruit.