I use fresh avocado, carrots and cucumber. I do not know why they don't rot, but know they last for years. I gel my soap so maybe the high temperature helps?
My soap is not perfect, it is made by hand by me. It weighs what the package states. I do sell partially gelled soap, there is nothing wrong with it, most times people think it looks cool. I do explain why it looks like it does, if asked.