Lactose and casein react to give a complex mixture of sugars and acids from which lactulose, D-galactose, D-tagatose, 5-hydroxymethylfurfural, lactic acid, glycollic acid, phosphoric acid, and the lactones of 2-deoxy-D-glycero-tetronic and 3-deoxy-D-erythro-pentonic acids have been isolated and identified.