NONENZYMATIC BROWNING-INDUCED WATER PLASTICIZATION - GLASS-TRANSITIONTEMPERATURE DEPRESSION AND REACTION-KINETICS DETERMINATION USING DSC

An exotherm, observed in differential scanning calorimetry (DSC) scans of amorphous food materials above their glass transition temperature, T-g, may occur due to sugar crystallization, nonenzymatic browning, o r both. In the present study, this exothermal phenomenon in initially anhydrous skim milk and lactose-hydrolyzed skim milk was considered to occur due to browning during isothermal holding at various temperatur es above the initial T-g. The nonenzymatic, Maillard browning reaction produces water that in amorphous foods, may plasticize the material a nd reduce T-g. The assumption was that quantification of formation of water from the T-g depression, which should not be observed as a resul t of crystallization under anhydrous conditions, can be used to determ ine kinetics of the nonenzymatic browning reaction. The formation of w ater was found to be substantial, and the amount formed could be quant ified from the T-g measured after isothermal treatment at various temp eratures using DSC. The rate of water formation followed zero-order ki netics, and its temperature dependence well above T-g was Arrhenius-ty pe. Although water plasticization of the material occurred during the reaction, and there was a dynamic change ill the temperature differenc e T-T-g, the browning reaction was probably diffusion-controlled in an hydrous skim milk in the vicinity of the T-g of lactose. This could be observed from a significant increase in activation energy. The kineti cs and temperature dependence of the Maillard reaction in skim milk an d lactose-hydrolyzed skim milk were of similar type well above the ini tial T-g. The difference in temperature dependence in the T-g region o f lactose, but above that of lactose-hydrolyzed skim milk, became sign ificant, as the rate in skim milk, but not in lactose-hydrolyzed skim milk, became diffusion-controlled. The results showed that rates of di ffusion-controlled reactions may follow the Williams-Landel-Ferry (WLF ) equation, as kinetic restrictions become apparent within amorphous m aterials in reactions exhibiting high rates at the same temperature un der non-diffusion-controlled conditions.