- Monoterpenes are important volatile compounds to the aroma of many white wines. Many monoterpenes are chiral and found in wine as different isomers. These isomers (also known as enantiomers) are non-superimposable mirror images of each other, maintain the same molecular structure and in many instances possess different sensory characteristics. Much research in wine has investigated the contents of monoterpenes, but has largely overlooked the monoterpene enantiomer profiles and important roles of the different monoterpene enantiomers in wine.
Monoterpene enantiomer profiles in grapes and final wine are due to many factors, including genotype (grape variety), climate, temperature, soil type and winemaking technique. These compounds are of great interest to wine as they are related to the unique sensory identity of varietal wine. The aim of the project was to: firstly develop a robust and valid method for monoterpene enantiomer identification and quantitation, secondly investigate the effect of grape variety, growing region and wine style on the profiles of monoterpene enantiomers, thirdly determine the important role of monoterpene isomer profiles in wine matrix and evaluate the effect of interactions between monoterpenes and other wine components on odor perception of wine.
The study presented the comprehensive exploration of monoterpene enantiomers (S-(-)-limonene, R-(+)-limonene, (2R,4S)-(+)-cis-rose oxide, (2S,4R)-(-)-cis-rose oxide, (2R,4R)-(-)-trans-rose oxide, (2S,4S)-(+)-trans-rose oxide, (2R,5R)-(+)-trans-linalool oxide, (2R,5S)-(-)-cis-linalool oxide, (2S,5S)-(-)-trans-linalool oxide, (2S,5R)-(+)-cis-linalool oxide, R-(-)-linalool, S-(+)-linalool, S-(-)-α-terpineol, R-(+)-α-terpineol, and R-(+)-β-citronellol) in white wines from different grape varieties using head-space solid phase micro-extraction-multidimensional GC-MS (HS-SPME-MDGC-MS). In addition, it presented for the first time the varietal differences from monoterpene enantiomers and enantiomer fractions which were calculated by dividing the first eluting enantiomer by the total enantiomers of each compound (enantiomer pairs). Despite heterogeneity in vintages, regions, and styles, varietal wines could be clearly differentiated. Results have expanded upon the current knowledge of varietal distinctiveness based on isomer profiles and enantiomeric fraction.
Riesling wines possess very diverse flavors as the composition of grapes, can be altered by environmental characteristics and viticultural practices. Moreover, Riesling wines carry more stylistic variability in terms of residual sugar content, than any other major international white grape variety. Our study found that chiral monoterpene profiles and enantiomer fractions could be important factors to classify Riesling wines according to their geographical origin and style.
Sensory discrimination tests were performed to elucidate the interactions between monoterpene isomers at low concentrations and other wine components. The results imply that a combination of enhancing and suppression effects occur and impact the perception of monoterpene isomer profiles in Pinot gris wines. Furthermore, the effects of same monoterpene profiles differed according to the matrices, displaying the many interactions that occur and affect sensory perception.
The study on chiral monoterpenes in white wine not only expands our knowledge on monoterpene isomer analysis and distribution, but it provides information for varietal quality, by offering an objective measure of flavor quality. Additionally we have shown that when investigating sensory perception it is important to use a matrix as close to the original product and we further support that aroma compounds found at concentrations below their known perception thresholds can effect aroma perception.