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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.

Effects of thermal and low intensity ultrasound combined with heat (LIUH) pretreatment prior to microwave vacuum drying on enzyme inactivation, color changes and nutrition quality properties of dried daylilies were investigated. The peroxidase (POD), ascorbic acid oxidase (AAO) and polyphenoloxidase (PPO) thermal and LIUH (0.2 and 0.4W/cm2) inactivation were determined and compared at 70, 80 and 90°C. Significant reduction in the POD, AAO and PPO activity was seen in daylilies after an ambient LIUH pretreatment than thermal pretreatment. POD, AAO and PPO thermal and LIUH inactivation followed the first order kinetics. LIUH pretreatment had a more positive influence on maintaining color of dried daylilies than thermal pretreatment. Furthermore, LIUH pretreatment resulted in a significant increase in chlorophylls, carotenoids (lutein, zeaxanthin and β-carotene), and a decrease in degree of browning and 5-hydroxymethylfurfural (HMF) when compared with thermal pretreatment. The antioxidant activity and contents of several nutritional components of dried daylilies pretreated by LIUH were also higher than that of dried daylilies pretreated by thermal pretreatment. This study provides a basis for the design of LIUH conditions to control vegetables browning and color changes prior to drying processing.

The ultrasound-assisted synthesis of lutein disuccinate from all-trans lutein (AL) and succinic anhydride (SA) was investigated in this study. Triethylamine was used as the catalyst. Based on the single-factor experiments, a 7-level-3-factor uniform design and response surface analysis were further employed to evaluate the effects of the selected variables including molar ratio of SA/AL, reaction time and ultrasonic power on the yield of lutein disuccinate. The results indicated that the data were adequately fitted into a second-order polynomial model; the molar ratio of SA/AL significantly affected the synthesis of lutein disuccinate, whereas reaction time and ultrasonic power did not. Based on ridge max analysis, the optimum condition for lutein disuccinate synthesis was predicted to be the molar ratio of SA/AL 265.3:1, ultrasonic power 300 W and reaction time 131.6 min with the lutein disuccinate yield of 80.53 ± 0.18%, which give a 43.8% increase compared with the traditional method, and also significantly shorten the reaction time.