A kinetic model for browning in the baking of biscuits: Effects of water activity and temperature

We describe here a rigorous and fundamental approach to the extraction of kinetic parameters when solid foods undergo colour change, and provide a generic basis for other studies on foods. Freeze-dried ‘digestive’ biscuit dough samples were used for kinetic studies of browning during the baking process. Browning data were recorded (temperature T: 105–135 °C; water activity aw: 0.04–0.15; t=0–26 min) as colorimeter R, G, B values (reflectance), in order to obtain rates of browning for numerical integration within the kinetic model. Reflectances were weighted using the Kubelka–Munk relationship so as to provide data which are linear with respect to concentration. It was postulated that the rate of formation of coloured product changes according to a first-order kinetic process but is largely zero order within the observation period. An activation energy of 105 kJ/mol was obtained but the rate of browning was independent of aw under the conditions investigated. It was thus possible to model the R, G, and B values very accurately within a kinetic run, and to extract kinetic parameters for doughs with varying sugar concentration. These parameters can now be used to predict the final colour (as R, G, B or L, a, b) for any given temperature–time profile.