Article pubs.acs.org/JAFC
Distribution and Organoleptic Impact of Ethyl 2‑Methylbutanoate Enantiomers in Wine Georgia Lytra,†,‡ Sophie Tempere,†,‡ Gilles de Revel,†,‡ and Jean-Christophe Barbe*,†,‡ †
Univ. Bordeaux, ISVV, EA 4577, Unité de recherche OENOLOGIE, 33882 Villenave d’Ornon, France INRA, ISVV, USC 1366 OENOLOGIE, 33882 Villenave d’Ornon, France
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ABSTRACT: The enantiomers of ethyl 2-methylbutanoate were assayed in several wines using chiral gas chromatography (βcyclodextrin). Analyses of 37 commercial red wines from various vintages and origins revealed the almost exclusive presence of the S-enantiomeric form. The average concentration was ∼50 μg/L, but the oldest samples were found to contain higher ethyl 2methylbutanoate levels than the youngest wines. The olfactory threshold of a racemic mixture of ethyl (2R)-2-methylbutanoate and ethyl (2S)-2-methylbutanoate (50:50, m/m) in dilute alcohol solution was 2.60 μg/L, almost twice that of the S-form, which was 1.53 μg/L. Ethyl (2S)-2-methylbutanoate and the racemic mixture of ethyl (2R)-2-methylbutanoate and ethyl (2S)-2methylbutanoate had different aromatic nuances: the former was mainly defined by fruity descriptors, such as green apple (Granny Smith) and strawberry, whereas the latter had an unspecific, caustic, fruity, solvent odor. Sensory analysis revealed an enhancing effect of ethyl (2S)-2-methylbutanoate on the perception of fruity aromas in the matrices studied: the “olfactory threshold” of the fruity pool, consisting of esters found in red wines, in dilute alcohol solution alone was higher than that of the same mixture supplemented with 50 μg/L ethyl (2S)-2-methylbutanoate. The sensory profiles of these aromatic reconstitutions highlighted the contribution of ethyl (2S)-2-methylbutanoate to black-berry-fruit descriptors. KEYWORDS: ethyl 2-methylbutanoate, enantiomers, chiral GC, red wine, fruity aroma, perceptive interactions, enhancing effect
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isomer considered,30,31 it was important to separate the two enantiomers to obtain an accurate assessment of their organoleptic impact. The goal of this work was to separate and assay 1 enantiomers, ethyl (2R)-2-methylbutanoate (1a) and ethyl (2S)-2-methylbutanoate (1b), in wines from various vintages and origins and evaluate their organoleptic impact on red wines.
INTRODUCTION Ethyl 2-methylbutanoate (1) occurs widely in the aroma volatiles of fruit and other foodstuffs.1 Its olfactory threshold in water was established at 0.1 μg/L by Flath et al.1 Paillard2 described this compound as conferring an intense odor, characterized as “apple-like”, “green”, “fruity”, and “giving an impression of ripeness”. 1 has been identified as a key aromatic compound in several fruits, particularly as a major contributor to the aroma of many apple cultivars3,4 and essences,2 pineapples,5 Asian pear (Pyrus serotina),6 blueberry,7 tropical southeast African durian fruits,8 Amazonian cupuaçu fruits,9 virgin olive oils,10 Muscadine grape juice,11 grapefruit,12 and orange,13,14 as well as strawberry fruit and juice.15 1 has also been identified in alcoholic beverages, such as palm wine,16 Chinese “Yanghe Daqu” liquors,17 and several types of wine. This compound was first characterized in aged Riesling wines.18 Using Charm analysis, Chrisholm et al.19 and Schlich and Moio20 evaluated 1 as one of the most potent odorants in Chardonnay and Riesling wines. 1 was identified as an important contributor to the aroma of white Scheurebe and Gewürztraminer wines,21 as well as red Grenache,22 Cabernet Sauvignon, and Merlot wines.23 When gas chromatography−olfactometry (GC-O) was applied to wine extracts, 1 was mainly described as “fruity apple”,23 “green apple”,24 “apple, sweet”,25 and “red fruits”.26 1 has one asymmetrical carbon atom, indicating the possibility of two different enantiomers. Previous works investigated this enantiomeric distribution in several fruits,6,10,14,27,28 as well as in wine, particularly in Dornfelder red wine.29 As the olfactory threshold and descriptors of an odoriferous compound may differ according to the stereo© 2014 American Chemical Society
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MATERIALS AND METHODS
Chemicals and Odorant Stimuli. Absolute ethanol (analytical grade, 99.97%, Scharlau Chemie S.A, Barcelona, Spain) was distilled before use. Sodium sulfate (99%) was provided by Scharlau Chemie S.A. Microfiltered water was obtained using a Milli-Q Plus water system (resistivity = 18.2 MΩ cm, Millipore, Saint-Quentin-enYvelines, France). Standard grade compounds were obtained from commercial sources as follows: ethyl propanoate, ethyl 2-methylpropanoate, ethyl butanoate, ethyl 2-methylbutanoate (racemic mixture 50:50, m/m), ethyl hexanoate, ethyl octanoate, ethyl 3hydroxybutanoate, 2-methylpropyl acetate, butyl acetate, and hexyl acetate from Sigma-Aldrich, Saint-Quentin-Fallavier, France; ethyl-d5 butanoate from Cluzeau, Sainte Foy la Grande, France; 3-methylbutyl acetate from VWR-Prolabo, Fontenay-sous-bois, France. Ethyl (2R)-2hydroxy-4-methylpentanoate, ethyl (2S)-2-hydroxy-4-methylpentanoate, and ethyl (2S)-2-methylbutanoate (1b) were synthesized by Hangzhou Imaginechem Co., Ltd., Hangzhou, China. Samples. Ethyl 2-methylbutanoate was assayed in 37 red wines from several vintages and origins (vintages 1982−2010). All analyses were carried out in February 2010. Dilute alcohol solution was prepared with double-distilled ethanol and microfiltered water (12%, Received: Revised: Accepted: Published: 5005
February 7, 2014 May 9, 2014 May 11, 2014 May 20, 2014 dx.doi.org/10.1021/jf500670z | J. Agric. Food Chem. 2014, 62, 5005−5010
Journal of Agricultural and Food Chemistry
Article
ions: m/z 102 as quantifier and m/z 57, 85, 87, 74, 115, and 130 as qualifiers. 1a and 1b were characterized by comparing their linear retention indices and mass spectra with those of standards. Calibration curves were evaluated in dilute alcohol solution (12%, v/v), using a representative range of average ester concentrations found in wines (linear range = 0.2−400 μg/L). The calibration curves were plotted as the relative peak areas (analyte versus the internal standard) as a function of the compound concentration. The functions are linear over the concentration range, presenting correlation coefficients of 0.9932 and 0.9955 for 1a and 1b, respectively. Repeatability was evaluated by relative standard deviation of 10 independent assays performed under the same analytical conditions over a short period of time. The relative standard deviation of the area ratios was
