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 degrees C water activity a(w): 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 a(w) 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. (c) 2006 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.