α-Terpinyl ethyl ether: stereoselective GC × GC confirmation and identification of its precursors in wine

Abstract

Wines exhibit profound chemical complexity which arise from a diverse array of compounds that contribute to its sensory profile. Among these are the terpenes, which are a class of volatile compounds that play a key role in imparting floral, fruity and resinous, aromas in wine, and can undergo complex chemical reactions even after fermentation [1,2]. One of such compounds is α-terpinyl ethyl ether (CAS number: 27153-54-4), which remains poorly characterized with limited hypotheses on its formation, precursors or enantiomeric preference in wine. This study thus presents the analytical confirmation and profiling of α-terpinyl ethyl ether enantiomers in single varietal wines. Reference analytical standards of α-terpinyl ethyl ether enantiomers were prepared in house by an ad hoc synthetic protocol, designed to prevent any possible loss of configuration of the reagent (α-terpineol) into the product (α-terpinyl ethyl ether). The enantiomerically pure, isolated, and purified α-terpinyl ethyl ether isomers were then successfully characterized and identified through flow injection APCI-HRMS, e-GCxGC-EI-MS, and NMR, to confirm their structures and purities [3,4]. To this aim, an enantioselective method based on a γ-cyclodextrins-derivatized GC primary separation phase was optimized. The quantitative method was thoroughly validated for linearity, limit of quantitation, recovery, matrix effect, and robustness. Then, single varietal wines were analysed to determine the concentrations of the enantiomers, along with other closely related terpene enantiomeric species (linalool, α-terpineol, and limonene). The study aimed also at identifying potential direct precursors of α-terpinyl ethyl ether, by preparing stock solutions of pure R-linalool, R-α-terpineol, and R-limonene in a model wine solution. The results showed that α-terpineol in wine undergoes a reversible and slow conversion to α-terpinyl ethyl ether without loss of the initial chiral configuration. On the other hand, linalool is irretrievably converted to both α-terpineol and α-terpinyl ethyl ether, with a predominant loss of the initial chiral configuration. This observed distinction highlights the different chemical behaviour of these terpene compounds and their significance in the wine aroma, particularly over prolonged time. Finally, qualitative sensory analysis further elucidated the distinct olfactory profiles of the enantiomers. Overall, the study contributes to a deeper understanding of α-terpinyl ethyl ether origin in wine.

References

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