Effect of ultrasonic waves on the stability of all-trans lutein and its degradation kinetics

Ultrasound treatment has been widely applied in the extraction of biologically active compounds including carotenoids. However, there are few reports on their effects on the stability of these compounds. In the present study, the stability of all-trans lutein, one of the carotenoids, was investigated under the action of ultrasound. Results showed that ultrasound induced the isomerization of all-trans lutein to its isomers, namely to 13-cis lutein, 13′-cis lutein, 9-cis lutein and 9′-cis lutein as analyzed by HPLC coupled with DAD and LC-MS; and the percentage of the isomerization increased with increasing both ultrasonic frequency and power. The stability of all-trans lutein in dichloromethane was worst among multiple kinds of solvents. Interestingly, the retention rate of all-trans lutein improved as the temperature increased, which runs counter to the Arrhenius law. Under ultrasound irradiation, the degradation mechanism might be different with various temperatures, the degradation of all-trans lutein followed first-order kinetics at 20 °C, while second-order kinetics was followed at 30-50 °C. As the ultrasonic reaction time prolonged, lutein epoxidation nearly occurred. Those results presented here emphasized that UAE techniques should be carefully used in the extraction of all-trans lutein.