Reducing sugars effect on available lysine loss of casein by moderate heat treatment

In order to investigate the effect of various reducing sugars on the available lysine loss by Maillard reaction, four model systems were prepared by mixing casein with glucose, fructose, lactose or maltose, followed by storage at 37 °C, 50 °C and 60 °C. The available lysine contents were monitored periodically. Highest and lowest reaction rates were observed in the model systems containing glucose and fructose, respectively, at the three temperatures. The two disaccharides behaved very similarly, with reaction rates between those of the monosaccharides studied. The activation energies of glucose, lactose and maltose systems were similar (116–132 kJ mol−1), while that of fructose was somewhat higher (166kJ mol−1). This difference was supposed to be in part due to the different mechanism of tautomerization of fructose, highly dependent on temperature. Therefore, though the potential nutritional damage at moderate temperatures is lower when fructose is used instead of other reducing sugars, its higher activation energy can reverse the effect at higher temperatures.