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%.
Another issue is this species have been assessed as posing a high risk of becoming weeds in Australia and are prohibited for entry by legislation without treatment . It is prohibited in Australia to grow Sapindus mukorossi trees. The berries must be treated by AQIS (Australian Quarantine and Inspection Service) on importation, treatments are heat, fumigation or gamma irradiation. But unfortunately they germinate better after being exposed to high heat , so if a seed is found, burn or completely grind it up to prevent it from growing and becoming a noxious weed.