Glass transition and reaction rates: Nonenzymatic browning in glassy and liquid systems

Effects of physical state and glass transition on nonenzymatic browning rate in water, glycerol, poly(vinylpyrrolidone) (PVP), and maltodextrin (MD) systems were studied. All systems had the same concentration of reactants, glucose and lysine, in the water phase. The systems, except water, had also comparable water activities (0.33). Sorption isotherms and glass transition temperatures (T(g)) at various water contents for the freeze-dried PVP and MD systems were determined. Nonenzymatic browning rate was determined at several temperatures from optical density at 280 and 420 nm. The PVP and MD systems contained 12.9 and 8.2 g of H(2)O/100 g dry matter and had T(g) values of 67 and 62 degrees C at 33% relative humidity and 24 degrees C, respectively. The liquids exhibited significantly higher browning rates than the concentrated systems, especially below the T(g) values. The browning rate was higher in the PVP than in the MD system, suggesting that a possible phase separation may affect nonenzymatic browning in foods.