The extent of the Maillard reaction was studied by measuring furosine and color formation in infant and enteral formula-resembling model systems prepared by mixing calcium caseinate, laboratory-obtained or commercial whey protein with lactose or dextrinomaltose (ingredients similar to those used in infant and enteral formula manufacture) and heating the mixture at 100, 120, or 140 degrees C for 0-30 min. The furosine determination was performed by HPLC and the color determination by measuring colorimetric parameters L, a, and b in a reflection photometer. The first steps of the Maillard reaction could be followed by furosine determination when initial ingredients had low thermal damage. Hence, furosine may be an indicator of low thermal damage in ingredients with <100 mg/100 g of protein. At the concentrations used in these model systems, similar to those in infant and enteral formulas, furosine values (indirect measure of lysine losses) were higher in lactose than in dextrinomaltose systems, in which only glucose, maltose, maltotriose, and maltotetraose among all of the sugars present showed reactivity with casein. Finally, the advanced steps could be followed by color determination when the initial ingredients had high thermal damage or the model systems were heated at high temperature or for a long time. Among the parameters assayed, b was the most sensitive.