Elsevier

Food Chemistry

Volume 145, 15 February 2014, Pages 796-801
Food Chemistry

Synthesis and evaluation of odour-active methionyl esters of fatty acids via esterification and transesterification of butter oil

https://doi.org/10.1016/j.foodchem.2013.08.124Get rights and content

Highlights

  • Flavour active methionyl esters were prepared from butter oil.

  • Enzymatic method is superior to chemical method for synthesis of methionyl esters.

  • Odour activity of methionyl esters is correlated to fatty acid carbon chain length.

Abstract

Methionol-derived fatty acid esters were synthesised by both chemical and lipase catalysed esterification between fatty acids and methionol. Beneficial effects of both methods were compared qualitatively and quantitatively by GC–MS/GC-FID results. And the high acid and heat stability of our designed methionyl esters meet the requirement of the food industry. Most importantly, the sensory test showed that fatty acid carbon-chain length had an important effect on the flavour attributes of methionyl esters. Moreover, through Lipozyme TL IM-mediated transesterification, valuable methionol-derived esters were synthesised from the readily available natural material butter oil as the fatty acid source. The conversion of methionol and yield of each methionyl ester were also elucidated by GC–MS-FID.

Introduction

Esters are important flavour compounds because of their occurrence in a wide range of natural sources, various odour characteristics and their wide range of uses in flavourings. They are naturally present in fruits, mostly at concentrations between 1 and 100 ppm. They are employed in fruit-flavoured products (i.e., beverages, candies, jellies, and jams), baked goods, wines, and dairy products (i.e., cultured butter, sour cream, yogurt, and cheese) (Janssens, De Pooter, Schamp, & Vandamme, 1992).

The sensory notes of esters are related to the organic acid and alcohol from which they are derived. Moreover, the odour intensity of esters decreases with the increase of molecular weight (Belitz, Grosch, & Schieberle, 2009); however, the flavour industry needs esters with strong odours and high molecular weight to prolong the duration of the odour of flavourings. Esters containing sulfur can be synthesised to yield flavour substances with strong odour and high molecular weight, as flavour compounds containing sulfur exhibit very low sensory threshold levels and very good odour characteristics (Liu & Crow, 2010). The sulfur-containing esters can be divided into two classes; one in which the sulfur atom belongs to the organic acids, such as methyl 3-(methylthio)propionate, and the other in which the sulfur atom is from an alcohol or mercaptan (AOM), such as methionol (Boustany, 1966).

Methionol is an alcohol with low odour threshold ranging from about 1 to 3 ppm and imparts a powerful odour, described as soup-like, meaty, boiled potato-like, vegetable-like, savoury or toasted cheese-like. Although methionol has been regarded as an off-flavour compound in beer and wine (Hill & Smith, 2000), it is considered as an important constituent of the overall aroma profiles in cheeses, particularly premium quality Cheddar and Camembert cheese (Landaud, Helinck, & Bonnarme, 2008).

Among the methionyl esters, methyl and ethyl 3-(methylthio)propanoate dominated in the vacuum headspace extract (VHS) of yellow passion fruits. Both compounds have previously been found in pineapple juice. 3-(Methylthio)propyl acetate (methionyl acetate) possesses herbaceous odour impressions and a typical vegetable-like character, as described in the literature in numerous flavour systems. In addition, methionyl butanoate (sulfury, cheese-like, mushroom-like) and methionyl hexanoate (tropical fruit note, methional-like, canned pineapple), which have threshold concentrations of 10–20 ppb and 500 ppb respectively, were also identified in the VHS extract obtained from the juice of the yellow variety (Nijssen, Visscher, Maarse, Willemsens, & Boelens, 1996). Werkhoff, Güntert, Krammer, Sommer, and Kaulen (1998) revealed the presence of 3-(methylthio) esters of propanoic acid in yellow passion fruit. The aroma properties of the 3-(methylthio) esters of propanoic acid are not very interesting, with the exception of the hexyl derivative. In general, the 3-(methylthio)propanoic acid esters have a sulfury, vegetable-like odour, and only hexyl 3-(methylthio)propanoate with its fruity and geranium-like odour note may contribute to the overall olfactory impression of the passion fruits. Overall, these research results indicated that methionol derived organic compounds usually have a basic meaty flavour. This presumption can be used to direct the development of new meaty aromas and promote research and development of meaty flavour.

There are two methods to synthesise esters by esterification. Fatty acid or butter oil and methionol were used as the starting materials in both methods. In the chemical method, 4-dimethylaminopyridine (DMAP) was applied as the chemical catalyst and N,N′-dicyclohexylcarbodiimide (DCC) was used as an esterification agent by removing water and driving the reaction to completion (Farshori, Banday, Zahoor, & Rauf, 2010). The advantage of the DMAP/DCC system is that it does not require a toxic metal catalyst. The second method is the enzymatic method, in which lipase TL IM was employed as the biocatalyst (Soumanou and Bornscheuer, 2003a, Soumanou and Bornscheuer, 2003b, Wang et al., 2008).

The biocatalytic conversion of a structurally related precursor molecule is often a superior strategy, which allows the accumulation of a desired flavour product to be significantly enhanced. As a prerequisite for this strategy, the precursor must be present in nature, and its isolation in sufficient amounts from the natural source must be easily feasible in an economically viable fashion (Dhake, Thakare, & Bhanage, 2013). Inexpensive, readily available, and renewable natural precursors, such as fatty acids which were used as the starting materials in our study, can be converted to more highly valued flavours (Sun et al., 2012).

The use of enzymes for synthesis of flavour compounds is also of great importance, due to the characteristics of enzymatic reactions such as high substrate specificity, high reaction specificity, mild reaction conditions, and reduction of waste product formation. The transesterification of butter oil with methionol in a solvent-free system is particularly attractive as it meets all these requirements plus it is a food grade processing method. In the food industry, the solvent-free system is preferred because of safety concerns when solvent is used (Sun, Yu, Curran, & Liu, 2012). The increase in consumer demand for nutritious and flavourful food supply has led to an increased demand for flavouring materials that may be considered natural. The use of specific enzymes in biosynthetic processes presents great potential to meet this demand.

The objectives of the present study were to synthesise new methionol-derived flavours by chemical-catalysed and enzyme-catalysed esterification of fatty acid with methionol for the first time, since there is no report on the enzymatic esterification of fatty acid with methionol. Moreover, the two methods were compared qualitatively and quantitatively. The sensory description and threshold of each ester were also measured and it was seen that fatty acid carbon chain length has an effect on the flavour attributes of methionyl esters. In addition, we aimed to convert butter oil and methionol to valuable methionol-derived esters through Lipozyme TL IM-mediated solvent-free transesterification.

Section snippets

Materials and reagents

Hydrogen chloride solution (2 M) in diethyl ether, 4-dimethylaminopyridine (DMAP), N,N′-dicyclohexylcarbodiimide (DCC), and methionol were purchased from Sigma–Aldrich Chemical Company (Singapore) and used directly. Fatty acids (C4, C6, C8, C10, C12 and C14) were obtained from Firmenich Asia Private Ltd (Singapore). The solvents were all of analytical grade and used as received.

Butter oil which contained no free fatty acids (analysed by SPME/GC–MS method) was purchased from PGEO Edible Oils Sdn.

Qualitative and quantitative analysis of methionol fatty acid esters

Fatty acid esters can be synthesised with high yields via many chemical methods with low costs and can also be prepared with lipase as catalyst. Here we compared both methods for generating methionyl esters and determined the conversion of the reaction using GC-FID (Table 1). The data showed that the conversions of the enzymatic reaction were comparable to that of the chemical method. Typically chemical methods require less expensive catalysts. However, when we take into consideration other

Acknowledgment

The sensory evaluation was supported by flavourists from Firmenich Asia Private Ltd (Singapore).

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