Natural Flavor Ester Synthesis Catalyzed by Lipases

In spite of the fact that several flavors and fragrances are obtained either by chemical synthesis or by extraction from plants, the application of biocatalysis for the sake of a safe and productive pathway by more sustainable chemical processes is the main alternative. The current study focuses on the synthesis of three flavor esters, namely cis-3-hexen-1-yl acetate, 2-phenylethyl acetate, and methyl phenylacetate, via transesterification and esterification of the natural corresponding alcohols or acids. The impact of optimizing variables that influence this lipase-catalyzed synthesis, such as enzyme formulations, solvent-free media, and acetylating agents, is crucial to achieving higher conversions. The enzymatic transesterification using Novozym 435 afforded cis-3-hexen-1-yl, and 2-phenylethyl acetates with high yields (>90%) in green solvents. Similar results were obtained in solvent-free system, which is more economic for the scaling up of this synthesis. In the case of esterification reactions, the removal of water, formed as a by-product, with the use of Aquasorb enhanced the conversions as in the case of methyl phenylacetate attained with a conversion of 89% in the presence of Novozym 435. However, this effect was not observed in the larger scale reaction (with 0.85 mol of cis-3-hexen-1-ol). Instead, the efficient strategy of gradual addition of acetic acid has proven to significantly improve the yield of cis-3-hexen-1-yl acetate (from 2% up to about 80% in the case of the preparative reaction with 100 mL substrate). Box-Behnken analysis was also performed to identify the lowest amount of acetylating agent and the shorter time to obtain the highest conversion ratio. This analysis showed that a triacetin/alcohol molar ratio of 1 and 1.75 can be sufficient to obtain a conversion >90% and up to 95%, for cis-3-hexen-1-yl acetate and 2-phenylethyl acetate, respectively, similarly to what is obtained with higher triacetin/alcohol molar ratios and comparable reaction time.