Flavour esters

A very important group of flavour molecules is the esters. Many flavour esters can be prepared from organic acids and alcohols from renewable resources. Im-... 

Fig. 23.1 Microbial routes from natural raw materials to and between natural flavour compounds (solid arrows). Natural raw materials are depicted within the ellipse. Raw material fractions are derived from their natural sources by conventional means, such as extraction and hydrolysis (dotted arrows). De novo indicates flavour compounds which arise from microbial cultures by de novo biosynthesis (e.g. on glucose or other carbon sources) and not by biotransformation of an externally added precursor. It should be noted that there are many more flavour compounds accessible by biocatalysis using free enzymes which are not described in this chapter, especially flavour esters by esterification of natural alcohols (e.g. aliphatic or terpene alcohols) with natural acids by free lipases. For the sake of completeness, the C6 aldehydes are also shown although only the formation of the corresponding alcohols involves microbial cells as catalysts. The list of flavour compounds shown is not intended to be all-embracing but focuses on the examples discussed in this chapter... 

Esters are widespread in fruits and especially those with a relatively low molecular weight usually impart a characteristic fruity note to many foods, e.g. fermented beverages [49]. From the industrial viewpoint, esterases and lipases play an important role in synthetic chemistry, especially for stereoselective ester formations and kinetic resolutions of racemic alcohols [78]. These enzymes are very often easily available as cheap bulk reagents and usually remain active in organic reaction media. Therefore they are the preferred biocatalysts for the production of natural flavour esters, e.g. from short-chain aliphatic and terpenyl alcohols [7, 8], but also to provide enantiopure novel flavour and fragrance compounds for analytical and sensory evaluation purposes [12]. Enantioselectivity is an impor-... 

Fig. 23.3 a Ester formation via alkyl-CoA alcoholysis with yeasts (preferably Geotrichum fragrans) according to [85], exemplarily shown for ethyl-2-methylbutanoate and ethyl tiglate. b Some possible flavour esters producible depending on amino acid and alcohol used as substrates... 

Rhizopus arrhizus dead mycelium was found to be very active in organic solvents as a naturally immobilized lipase. Triglycerides hydrolysis and interesterification, esters and glycerides synthesis, natural flavour esters preparation and racemic mixtures resolution in pharmaceutical drugs synthesis are among the successfully designed processes, each of one with a specific reactional medium. 

A typical research example includes the synthesis of a K -channel blocker intermediate (Fig. 7.10). Here, an aminotransferase is used to replace a keto group by an amino function to yield the desired intermediate [20]. Other potential applications include the biosynthesis of flavouring esters. In nature such compoimds are often formed by a specific acyl CoA transferase. For practical purposes, however, the alternative, less complicated route, using lipases and esterases in their synthetic mode, seems more appropriate. 

Gonzalez-Navarro, H. and Braco, L., Lipase-enhanced activity in flavour ester reactions by trapping enzyme conformers in the presence of interfaces, Biotechnol. Bioeng., 59, 122-127, 1998. 

Ma F, Hanna MA (1999) Biodiesel production a review. Biores Technol 70 1-15 Magnusson A, Hull K, Holmquists M (2001) Creation of an enantioselective hydrolase by engineered substrate-assisted catalysis. J Am Chem Soc 123 4354 355 Manjon A, Iborra JL, Arocas A (1991) Short-chain flavour ester synthesis by immobilized lipase in organic media. Biotechnol Lett 13 339-345 Margolin A (1996) Novel crystalline catalysts. TIBTECH 14 223-230... 

Gmndschober F. (1977) Toxicological assessment of flavouring esters. Toxicology 8, 387-390. 

Longland, R.C., Shilling, W.H. Gangolli, S.D. (1977) The hydrolysis of flavouring esters by artificial gastrointestinal juices and rat tissue preparations. Toxicology, 8,197-204. 

A.P. de los Rfos, F.J. Hernandez, F. Tomas-Alonso et al.. Synthesis of flavour esters using free Candida antarctica lipase B in ionic hquids, Elavour Eragrance J. 23 (2008) 319-322. 

Gubicza, L., Kabiri-Badr, A., Keoves, E., Belafi-Bako, K. (2000). Large-scale enzymatic production of natural flavour esters in organic solvent with continuous water removal. Journal of Biotechnology, 84, 193—196. 

Dheeman DS, Henehan GTM, Frias JM (2011) Purification and properties of Amycolatopsis mediterranei DSM 43304 lipase and its potential in flavour ester synthesis. Bioresour Technol 102 3373-3379 Dive V, Yiotakis A, Nicolaou A, Toma F (1990) Inhibition of Clostridium histolyticum collagenases by phosphonamide peptide inhibitors. Eur J Biochem 191 685-693... 

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