Enzymatic synthesis of steryl ferulates

Steryl ferulates are plant sterols esterified to ferulic acid, a common phenolic acid. This esterification leads to sterol esters with improved biological properties, such as antioxidant activity. Commercially available and extracted steryl ferulates from rice bran are often limited in their sterol profiles. For further research and later food applications, a simple enzymatic esterification could address the lack of availability of single steryl ferulates. Whereas several enzymatic procedures for the esterification of steryl fatty acid esters have been published, no fully enzymatic procedure for steryl ferulates has been reported so far. We optimized both direct esterification of beta-sitosterol with ferulic acid as well as transesterification with ethyl ferulate yielding steryl ferulates. The reaction was catalyzed by a lipase from Candida rugosa, which lead to yields of 35 and 55% for the direct esterification and transesterification, respectively. Moreover, both reactions followed a similar time course over incubation. The enzyme activity was rather low, which is probably due to the specificity of the different isoenzymes of C. rugosa lipase. However, successful conditions for a fully enzymatic synthesis of steryl ferulates are reported for the first time.