The kinetics of colour degradation, chlorophylls and xanthophylls loss in pistachio nuts during roasting process

Abstract

Objectives: Colour is amongst the parameter which is used for process control during roasting. Materials and Methods: In this study, the effects of hot air roasting temperature (120, 130, 145, 160, and 170°C) and hot air velocity (0.6, 1.3, and 2 m/s) on colour change kinetics of pistachio nuts were investigated by employing image analysis and simultaneously chlorophylls and xanthophylls (lutein and β-carotenoid) concentration were determined by spectrophotometric measurement method. Results: We found that roasting temperature and hot air velocity had significant effect on colour changes. There is a correlation between a- and b-value with chlorophylls and xanthophylls concentration, respectively. The roasting temperature was found to be the main factor affecting colour development. The variations in the pigments concentration and colour parameters of pistachio nuts were adequately simulated by quadratic and cubic polynomials. The changes in L-, b-values, and xanthophylls degradation were well-fitted to the first-order kinetic model while a-value and chlorophylls degradation followed the zero-order kinetic. The activation energy was determined at 113.9, 116.7, and 117.2 kJ/mol with R2 ≥ 99.9 and 191, 195, and 163.2 kJ/mol with R2 ≥ 99.5 and 73.7, 71.3, and 81.6 kJ/mol with R2 ≥ 99.9 for L-, b-, and a-value in hot air velocity of 0.6, 1.3, and 2 m/s, respectively. Conclusions: Activation energy of chlorophylls and xanthophylls degradation were in the range of the activation energy for the yellowness (b-value) and redness (a-value) reactions.