Analysis of wine thiols and precursors
Background
Volatile thiols are widely recognised as important contributors to the aroma of some wine varieties, impacting on both the style and quality of a wine. Volatile thiols can be extremely potent aroma compounds, and are perceivable at the part per trillion (ppt) level. Wine thiols responsible for desirable tropical fruit scents are not present in grape juice as free thiols, but are released from non-volatile precursors during alcoholic fermentation. Precursors to these thiols, such as S-cysteinyl conjugates, have been identified in Sauvignon Blanc juice.
In order to further our understanding of the presence and role of thiol precursors in grapes and the impact of fermentation on the release of desirable thiols, we are developing analytical methods for the detection and quantitation of thiols and their precursors in juices and wines. Typically these methods utilise HPLC-MS or GC-MS and some isolation and concentration step preceding the analysis. The low sensory threshold of these compounds necessitates the development of extremely sensitive analytical methods, and this remains the focus of our activities.
Progress
Various thiol precursors and labelled analogues have been prepared by synthetic means to provide pure reference compounds and internal standards. Availability of these compounds is vitally important to the identification and quantitation work being undertaken.
Methods are being optimised for the determination of important wine thiols at wine relevant concentrations, using either GC-MS or HPLC-MS. The methods utilise derivatisation of the reactive free thiols and detection of the derivatives for quantitation.
Hydrolytic studies are underway using model wines of differing pH to assess the rate of hydrolysis of yeast-derived thiols. This work will give an indication of the rate at which thiols are converted into derivatives with different sensory properties during storage.
Isolation and HPLC-MS methods are being investigated for the determination of wine thiol precursors in grape juice and wine. Recently, a new potential precursor to 4-MMP was identified for the first time in a Sauvignon Blanc juice extract using HPLC-MS experiments.
Highlights
- Numerous reference compounds have been prepared synthetically
- Analytical methods are being optimised for determination of volatile thiols important to wine aroma
- Degradation studies are in progress to assess the fate of volatile wine thiols
- A new potential precursor to 4-MMP has been identified in Sauvignon Blanc juice for the first time
- Novel techniques for the extraction of volatile thiols from wine are being investigated
- Analytical methods are being explored for determination of thiol precursors in grapes and wine
Project leader: Dr David Jeffery
Project team members:
Publications:
811 Howell, K.S.; Swiegers, J.H.; Elsey, G.M.; Siebert, T.E.; Bartowsky, E.J.; Fleet, G.H.; Pretorius, I.S.; de Barros Lopes, M.A. Variation in 4-mercapto-4-methyl-pentan-2-one release by Saccharomyces cerevisiae commercial wine strains. FEMS Microbiol. Lett. 240: 125–129; 2004 (click here to order).
917 Swiegers, J.H.; Francis, I.L.; Herderich, M.J.; Pretorius, I.S. Meeting consumer expectations through management in the vineyard and winery: the choice of yeast for fermentation offers great potential to adjust the aroma of Sauvignon Blanc wine. Aust. N.Z. Wine Ind. J. 21: 34–42; 2006 (click here to order).
997 Swiegers, J.H.; Capone, D.L.; Pardon, K.H.; Elsey, G.M.; Sefton, M.A.; Francis, I.L.; Pretorius, I.S. Engineering volatile thiol release in Saccharomyces cerevisiae for improved wine aroma. Yeast 24: 561–574; 2007 (click here to order).
1062 Pardon, K. H.; Graney, S. D.; Capone, D. L.; Swiegers, J. H.; Sefton, M. A.; Elsey, G. M. (2008) Synthesis of the individual diastereomers of the cysteine conjugate of 3-mercaptohexanol (3-MH). J. Agric. Food Chem. 56: 3758-3763 (click here to order).
1115 Swiegers, J. H.; Kievit, R. L.; Siebert, T.; Lattey, K. A.; Bramley, B. R.; Francis, I. L.; King, E. S.; Pretorius, I. S. (2009) The influence of yeast on the aroma of Sauvignon Blanc wine. Food Microbiol. 26: 204-211.
1134 Fedrizzi, B., Pardon, K.H., Sefton, M.A., Elsey, G.M., Jeffery, D.W. (2009) First Identification of 4-S-Glutathionyl-4-methylpentan-2-one, a Potential Precursor of 4-Mercapto-4-methylpentan-2-one, in Sauvignon Blanc Juice. J. Agric. Food Chem. 57: 991-995.