The thermal kinetics of texture change and the analysis of texture variability for raw and roasted hazelnuts

Texture changes of hazelnuts which were dry roasted at temperatures from 120 to 180 degreesC, for durations ranging from 5 up to 60 min, were studied using instrumental analysis. The textural changes in hazelnuts were represented by the fracture force obtained from compression tests. The magnitudes of the parameters for the corresponding texture change model were determined using one step non-linear regression. The order of the reaction was found by plotting isothermal curves of texture response against time; statistical analysis of the data showed that it was best represented by a first-order reaction. The rate constants were assumed to have an Arrhenius-type dependence on temperature. The activation energy and the frequency (pre-exponential) factor at a reference temperature (150 degreesC) were determined to be 39.25 kJ mol(-1) and 0.0421 s(-1), respectively. Young's modulus and the fracture stress for roasted hazelnuts were calculated to be 4.93 and 1.54 MPa, respectively. Both roasting temperature and time had significant effects on the textural changes in hazelnuts, and the variability in the texture decreased during the roasting process. The physical properties of hazelnuts were correlated with the texture response. A possible major factor causing the high variability in texture was the internal cavity present at the core of each hazelnut. The development of a model of the thermal kinetics of texture change over a large roasting temperature and time range should allow optimization studies to be used for determining the best roasting schedule in terms of delivering the required product texture.