Nowhere in the Bible does it say that Adam and Eve had bitten into an apple. The apple nevertheless became the symbol of original sin, but it is never mentioned anywhere in Genesis. It simply speaks of the 'fruit of the tree which is in the garden.
According to certain interpretations, it could be an apple, a pear, a pomegranate or a fig because after eating, Adam and Eve covered themselves with a fig leaf. The error concerning the apple came about in the 2nd century when Aquila of Sinope, also known as Aquila of Pontus translated into Greek the Hebrew text of the Song of Solomon. In the Hebrew text, It went: "I resurrected you under the tree where your mother was corrupted", and when translated to Greek it came out as "I raised you under the apple tree that's where you were corrupt.". Obviously, he saw a hint of the forbidden fruit.
St Jerome in his Latin translation of the Old Testament interpreted the passage in the same way, so this probable misinterpretation is why the apple is still commonly believed as the forbidden fruit.
- Source 1: Etymonline Online Etymology Dictionary