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Lipase-Catalyzed Synthesis of Sucrose Fatty Acid Ester and the Mechanism of Ultrasonic Promoting Esterification Reaction in Non-Aqueous Media

The preparation of sucrose fatty acid ester (SFAE) by lipase-catalyzing reaction using Candida antarctica lipase B (CalB) and its immobilized form Novozym 435 was reported in this work. The preparation was characterized in non-aqueous media with and without ultrasound irradiation treatment. A conversion rate of SFAE up to 49.60% was achieved using Novozym 435 under the optimal conditions (45.4°C; mole ratio of methyl oleate to sucrose = 6.0:1; 4.0 mL acetone; 4.0 mg/mL Novozym 435; and 24.6 h of reaction). Under optimal ultrasound conditions (50 kHz, 0.15 W/cm2, 166.55 min), reaction time decreased by 75% approximately, compared with the control without ultrasonic irradiation, but the ultrasound irradiation treatment did not affect the SFAE yield catalyzed by Novozym 435. In the CalB-catalyzed preparation of SFAE under the same optimal reaction conditions, ultrasonic irradiation enhanced the activity of CalB during early time points and inhibited its activity after a long period of treatment. Moreover, CalB was further examined using Far-UV circular dichroism (CD) spectroscopy and scanning electron microscopy (SEM) to study the conformation and micro-morphology of CalB structural variations in various ultrasound irradiation treatments. CD results indicated that α-helical regions were increased and random coil regions remained at a similar level of proportion. SEM images showed small holes appeared on the surface of irradiated CalB. Therefore, we conclude that proper ultrasound irradiation could change the secondary structure and the surface morphology of the CalB in molecular level, and could accelerate the esterification reaction process.