Forgotten fatty acids-Surface properties supply conclusive evidence for including carotenoic acids.

•Conjugated polyunsaturated alkali carotenoates behave like detergents as do alkali carboxylates.•Conjugated polyunsaturated carotenoic acids react, as do saturated carboxylic acids, with diols and triols to form fats.•Polyunsaturated alkali carotenoates are more amphiphilic than alkali carboxylates with the same number of C atoms.•Conjugated polyunsaturated carotenoic acids are rationally related to fatty acids.