Ultrasound irradiation promoted lipase-catalyzed synthesis of flavonoid esters with unsaturated fatty acids

This study was focused on the enzymatic esterification of rutin and naringin with different free unsaturated fatty acids to produce the corresponding flavonoid esters catalyzed by Novozym 435 under ultrasound pretreatment. An ultrasonic frequency of 25 kHz, power of 200 W for rutin and 150 W for naringin and time of 1 h was determined to guarantee satisfactory degree of esterification and lipase activity. The influence of substrates concentration and molar ratio was investigated subsequently. The optimum production was achieved in component solvent of acetone and 2-methyl-2-butanol for rutin and pure 2-methyl-2-butanol for naringin at 50 °C with flavonoid concentration of 40 mM and flavonoid to fatty acid molar ratio of 1:4, resulting in a flavonoid esters conversion of above 83.2% in relative short reaction time (48–72 h). In optimum conditions, the overall esterification reaction time using the ultrasonic pretreatment process was 24 h shorter than that of mechanical stirring process without damaging the lipase activity.