Soap berries or soap nuts come from the Sapindus mukorossi tree. They contain saponin, which is a glucoside with foaming characteristics ie. lathers like soap. Structurally saponin’s have both hydrophilic and lipophilic functionality, but are weaker than soap. Water soluble saponin’s are toxic to fish.
The problem with soap berries is that they don’t actually clean anything well or adequately remove soiling. Tests have shown that washing with alternative washing methods like wash balls or soap nuts do not remove soiling better than a wash with pure water. Water alone in a front loader received a rating of 43%, soap nuts received 42%.