insoluble soap

in·sol·u·ble soap

soap made with a fatty acid and an earthy or metallic base (iron or calcium salts of fatty acids).
References in periodicals archive ?
5) Free palmitic acid tends to complex with minerals and form insoluble soaps with calcium ions that cannot be absorbed in the small intestine and are thus excreted in the faeces (Figure 3).
These insoluble soaps are then excreted, preventing bile salts from harming the colon.