eating of the fruit of the knowledge of good and evil
Some have thought that original sin is sex. Others have argued that original sin is the knowledge of good and evil. The final school of thought I will bring up is not only is original sin, or the eating of the forbidden fruit, but that good and evil is shades of grey and to be determined by who consumes the apple, particpates in the world.
Brian is taking a bite of orginal sin, our baser instincts and using those to fuel himself. He uses knowledge to provide for himself, and others around him. He has taken sin, knowledge of good and evil, applied reason, logic, self interest, and imagination and redefined right and wrong for himself.
He comes from a deeply religious mother, and a hipocrotical family structure. He sees the concept of original sin as false. Staying away from the tree was to protect Adam and Eve. Staying asway from knowledge should protect. It was only be applying knowledge and creating his own sense of right and wrong that Brian was able to survive and build his own empire.
Every time he takes a bite of that apple, he is taking a bite of what once hurt and lied to him, and is saying I create my own sin, as well as everyone else does. We create or own worlds, our own right and wrong.
The apple represents Brian saying fuck you and I own my own sin, sell you sin, and I will thrive off it as well.