Insights into the multiple positive effects of natural deep eutectic solvents for highly efficient synthesis of diacylglycerol through enzymatic esterification

Diacylglycerol (DAG) has extensive applications in many fields due to its health benefits and amphiphilic property. Herein, enzymatic esterification of oleic acid and glycerol in natural deep eutectic solvents (NADESs) was employed to synthesize DAG. Firstly, Novozym 435 and choline chloride: glycerol (ChCl:Gly) system were selected as the most appropriate immobilized lipase and NADES for DAG synthesis. Subsequently, the reaction conditions of Novozym 435-catalyzed esterification in ChCl:Gly system were optimized, and the content of DAG reached as high as 67.08 wt % ± 0.79 wt % under the optimal conditions (molar ratio of oleic acid to glycerol of 1:4, reaction temperature of 50 °C, enzyme dosage of 4 wt % with respect to the total weight of substrates, reaction time of 7 h). At last, the reasons why ChCl:Gly could induce a higher DAG content were explored from thermodynamics, water activity and reaction interface. The results showed that ChCl:Gly system could absorb the generated water during esterification and increase the reaction interface, thus beneficial to DAG synthesis. Moreover, the activation energy of DAG production (55.36 kJ/mol) was significantly lower than that of monoacylglycerol (MAG) production (62.60 kJ/mol), leading to the priority of DAG production. Briefly, the multiple beneficial effects of NADESs on lipase-catalyzed esterification collectively contributed to an enhancement of DAG content in esterification products.