Photooxidation browning mechanism of small α-dicarbonyl compounds on natural mineral particle in the presence of methylamine/ammonia

•Kaolinite can form mixing particles with α-dicarbonyl.•The browning of α-dicarbonyl on kaolinite is initiated by photooxidation.•Methylamine is more readily to yield imine than ammonia.•Ammonia is more readily to form diimine than methylamine after imine formation.