Physicochemical characterisation of iron oxides and hydroxides applied as food additive E 172

Within the European Union, iron oxides and hydroxides with specifications defined in Commission Regulation No 231/2012 are allowed as food additive E 172 in numerous food categories. In this work, a detailed physicochemical characterisation of twelve commercially available E 172 iron oxides was performed. Based on material colour, crystal structure and particle shape, four main forms were identified by electron microscopy and X-ray diffraction: red spheroidal hematite; red elongated hematite; yellow elongated goethite; black spheroidal magnetite. Each type contained a fraction of nanoparticles. Raman spectroscopy and X-ray photoelectron spectroscopy identified maghemite at the surface of magnetite particles. Time-of-flight secondary ions mass spectrometry showed differences in surface chemistry and trace contaminants between E 172 forms. Inorganic impurity levels measured by inductively coupled plasma mass spectrometry remained below EU regulatory limits. The results provide insight into the distinct forms that are applied as E 172, and address current data gaps on physicochemical characterisation.