Modelling of the kinetics of colour change in hazelnuts during air roasting

The colour changes that occur in hazelnuts during the dry roasting process were experimentally studied in order to determine the magnitudes of the parameters for a corresponding colour change model. Roasting times varying from 5 to 1900 min and a number of roasting temperatures in the range from 100 to 180 °C were analysed. The order of the reaction was found by plotting isothermal curves of colour change against time at a number of different roasting temperatures. Statistical analysis of the data demonstrated that the colour parameter, L was best represented by a first-order process. The rate constant was assumed to have an Arrhenius-type dependence on temperature. The activation energy (E_a) and the frequency (pre-exponential) term (k_∞) were determined as 64.5 kJmol^-1 and 33416.5 s^-1 respectively for the conditions studied. Theoretical equations, based on the exponential integral function, to predict first-order quality changes when product temperature is varying exponentially were developed. These equations were validated by comparison with the experimental colour data extracted from the non-isothermal region of the roasting process.