Why lemon juice? Can it be organic apple juice instead?

I did my first ever liver/gallbladder cleanse last night. I got good results today (passed a couple of dozen pea-sized stones...)

Everything went okay except for the use of the lemon juice, which gave me an upset stomach half the night. (I swear it was the lemon juice, and not the olive oil). I would prefer to use organic apple juice instead. Much milder on my stomach lining! Anyone know if the lemon juice can be substituted with a milder (on the stomach) type of juice, such as apple or grape? Thanks in advance.