Chemical and enzymatic synthesis of a library of 2-phenethyl esters and their sensory attributes
Introduction
Flavour esters are important and versatile components of flavours and fragrances. These esters have been widely used in the food, beverage, fine cosmetic, and pharmaceutical industries (Surburg & Panten, 2006, chap. 2). 2-Phenethyl alcohol is a colourless liquid with a pleasant floral odour and a mild, warm, rose-honey-like note of moderate to poor tenacity. It has been found in a variety of essential oils, such as rose, carnation, hyacinth, anise, Aleppo pine, orange, blossom, ylang ylang, geranium, neroli and champaca (Colibaba et al., 2012). 2-Phenethyl alcohol is therefore used as an ingredient in many flavour and fragrance formulations, particularly when the smell of rose is desired (Scognamiglio, Jones, Letizia, & Api, 2012).
2-Phenethyl esters derived from 2-phenethyl alcohol are members of the aroma-active structural group aryl alkyl alcohol simple acid esters (AAASAE), such as 2-phenethyl acetate, 2-phenethyl butyrate, 2-phenethyl isobutyrate and 2-phenethyl propionate. Previous research indicated that 2-phenethyl alcohol-derived compounds usually have a basic rose-like flavour and are used in cosmetics and non-cosmetic industries (Yadav & Bokade, 1996). This presumption can be used to direct the development of new floral and rose-like aromas and avoid aimlessness in screening aromas.
Traditionally, these 2-phenethyl esters have been either isolated from natural sources or produced by chemical synthesis (Welsh, Murray, Williams, & Katz, 1989) but the extraction or synthetic yield is low and the cost is high. Thus, it is important to develop an effective or efficient method to generate 2-phenethyl esters. Herein, we report both chemical and enzymatic methods to synthesise 2-phenethyl esters by esterification. In the chemical synthesis, Steglich esterification was used to produce 2-phenethyl esters (Neises & Steglich, 1978). The second esterification reaction is enzyme-mediated, in which lipase TL IM was employed as the biocatalyst (Soumanou and Bornscheuer, 2003a, Soumanou and Bornscheuer, 2003b, Wang et al., 2008).
Fats and oils are important inexpensive, readily available and renewable natural resources in the industry for the production of chemically pure compounds and biodiesels (Metzger & Bornscheuer, 2006). In addition, fats and oils can also be applied in the food industry as starting materials for the synthesis of valuable fatty acid esters. To convert natural fats and oils to esters, enzyme-catalysed bioconversion is a promising approach, which normally uses lipases as the biocatalysts to obtain higher selectivity, specificity and predictability (Schrader, Etschmann, Sell, Hilmer, & Rabenhorst, 2004). In addition to the esterification reaction, which needs prior isolation of free fatty acids, transesterification allows the synthesis of various valuable fatty acid esters directly from fats or oils and alcohols, which makes the process more effective and efficient (Soumanou & Bornscheuer, 2003b).
The butter oil investigated here contains a considerable amount of diverse fatty acids (4–18 carbons) which makes it an attractive dairy fats alternative for the synthesis of fatty acid esters (Chen, Schwartz, & Spanos, 1992). A variety of lipases have been used as the biocatalysts to synthesise flavour esters (Su et al., 2010, Yahya et al., 1998). However, in these studies, environmentally unfriendly chemical acids were used and thus, the esters generated cannot be deemed as natural.
Relatively little research has been done on the utilisation of butter oil in the production of natural flavour esters by enzyme-mediated transesterification (Hill et al., 2001, Malcata et al., 1991). Therefore, transesterification of butter oil to valuable fatty acid esters using immobilised lipase was studied here in the hope of providing a new means for natural phenethyl esters as cost-effective flavourings.
The objectives of the present study were to synthesise 2-phenethyl alcohol-derived flavour esters by both chemical and enzyme-catalysed esterification of fatty acids with 2-phenethyl alcohol. Moreover, we aimed to convert butter oil and 2-phenethyl alcohol into valuable 2-phenethyl alcohol-derived esters through Lipozyme TL IM-mediated solvent-free transesterification and to understand the influence of some synthetic parameters (temperature, substrate molar ratio, enzyme loading, shaking speed and time) in order to achieve a higher conversion of 2-phenethyl alcohol to esters.
Section snippets
Materials and reagents
Hydrogen chloride solution (2 M) in diethyl ether, 4-dimethylaminopyridine (DMAP), N,N′-dicyclohexylcarbodiimide (DCC) were purchased from Sigma–Aldrich Chemical Company (Singapore). 2-Phenethyl alcohol was bought from Merck Schuchardt OHG (Hohenbrunn, Germany). Fatty acids (C4, C6, C8, C10, C12, C14, C16 and C18) were obtained from Firmenich Asia Private Ltd. (Singapore). The solvents were all analytical grade. Butter oil which contained no free fatty acids (analysed by the SPME–GC–MS method)
Qualitative and quantitative evaluation of two methods in preparing 2-phenethyl fatty acid esters
We compared both methods for generating 2-phenethyl esters and determined the conversion of the reactions (Table 1). The conversion of the enzymatic reaction was comparable or even better than that of the chemical method. This may be explained by product loss in the purification steps in the chemical processing which may reduce the yields of the desired compound. Chemical synthesis has other limitations, such as the need to remove impurities and by-products from the final product, and harsh
Conclusion
Both chemical and enzyme-catalysed esterification of fatty acids with 2-phenethyl alcohol can effectively generate 2-phenethyl alcohol-derived esters. Fatty acid carbon chain length had an effect on the flavour attributes of 2-phenethyl esters. Moreover, through Lipozyme TL IM-mediated transesterification, valuable 2-phenethyl alcohol derived esters were synthesised from natural butter oil. The influence of some synthetic parameters (temperature, substrate molar ratio, enzyme loading, shaking
Acknowledgements
The sensory evaluation was supported by flavourists from Firmenich Asia Private Ltd (Singapore).
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