Enzymatic synthesis of short‐chain esters in n‐hexane and supercritical carbon dioxide: Effect of the acid chain length

Enzymatic synthesis is the preferred way to produce so‐called “natural products.” Hydrolases have been used for short‐chain ester synthesis. These esters present a pleasant flavor and they have a lot of applications in different industries. Novozym 435 from Candida antarctica (EC 3.1.1.3, triacylglycerol lipase) was used for hexyl ester synthesis in n‐hexane and supercritical carbon dioxide (SCCO₂). Direct esterification provided higher yields than transesterification for the synthesis of esters. Several carboxylic acids of different chain lengths were tested for the esterification reactions: acetic, propionic, butyric, caproic and caprylic acids. The reactions were carried out at 40°C and the amount of enzyme used was 13.8 g/mol alcohol. Substrates were added at equimolar concentrations, with sufficient stirring to avoid external diffusion control. Different substrate concentrations up to 1.5 M were used. The working pressure was 14 MPa in the case of SCCO₂ and atmospheric pressure in the case of organic solvent. The results in both solvents show that the reaction rate increases with the chain length of the acid, but the final yields were similar. However, some of the reactions prove to be faster in SCCO₂, except for hexyl acetate and propionate synthesis, in which acetic and propionic acid presented a lower solubility in SCCO₂ due to its high polarity. Moreover, an acetic acid concentration of 1.5 M brought about a strong inhibition of the enzyme activity.