Kinetics of Colour Development of Molten Glucose, Fructose and Sucrose at High Temperatures

Pure sugar crystals melt at high temperature and liquids progressively change from colourless to dark brown, attracting much attention as a possible source of caramel or as a cooking media. The aim of this study was to evaluate colour development in pure sugar melts at temperatures above 160 A degrees C as a function of time. Crystalline glucose, fructose and sucrose (12 g) were placed in test tubes and subjected to temperatures of 160, 170, 180, 190 and 200 A degrees C (+/- 1 A degrees C). Colour was determined quantitatively with a computer vision system and evaluated as lightness/darkening (L/L-0) and total colour difference (a dagger E). A modified and reparameterised Gompertz model was successfully used to explain the kinetic behavior of L/L-0 and a dagger E and the Arrhenius type dependency was found to relate the temperature effect. Fructose had the highest reaction rates for L/L-0 and a dagger E at all temperatures. The activation energy values for a dagger E were 56.1, 44.5 and 126.8 kJ/mol for glucose, fructose and sucrose, respectively. Crystalline sucrose does not brown but the colour develops at surfaces which become noncrystalline and sticky before the colour appears. Glucose had the lowest reaction rates for L/L-0, proving that is more appropriate to use in products with extensive thermal treatments, making it more suitable to manage non-enzymatic browning reactions and obtain products with different characteristics.