Wine's Metabolic Geography
(July 1st, 2009) A group of wine-loving European researchers with a very large mass spectrometer have succeeded in tracing the geographical origins of the oak barrels used to age a rather good Burgundy red. After genomics (see Lab Times 3-2009: 26-31), wine apparently has also entered the world of metabolomics, reports Jeremy Garwood.
A collaboration between French researchers in the Burgundy region, who had the wine and wine barrels, and a German lab at Munich's Insitute of Ecological Chemistry, which had the big machine, has resulted in a paper in the journal,
PNAS (vol. 106(23):9174-9), in which they describe how their analysis of the 'chemodiversity' of wine has revealed the 'metabologeography' of the oak wood used to make the wine barrels.
Over recent years, the scientific literature has been filled with stories of DNA footprinting to trace the origins of biological specimens. More recently, this has included ambitious 'metagenomic' projects that seek to identify the genomic DNA signatures of the myriad microorganisms in complex environmental samples. However, biochemists and biophysicists have not been inactive. In the meantime, they have been developing their 'metabolomic' response.
Metabolomics is the systematic study of the unique 'chemical fingerprints' left behind by specific cellular processes. Whereas metagenomics looks at the DNA profile of biological samples, metabolomics wants to know what kind of live metabolic activity these living organisms display in particular environments. It identifies the thousands of different biochemicals that are produced by cells as they get on with living, and has discovered that variations in the food supply and environmental conditions under which organisms live can result in large variations in the type and relative amounts of the same species' metabolic by-products.
This has now been elegantly demonstrated by Regis Gougeon
et al. in their paper identifying the chemical fingerprints left by the leeching of wooden barrels during wine maturation (
Proc Natl Acad Sci U S A 106(23): 9174-9). Using high-field ion cyclotron resonance-Fourier transform mass spectrometry (ICR-FT/MS) up to 'considerable' mass ranges, they recorded "electrospray ionization ICR-FT/MS mass spectra of representative steps in the elaboration of wine" -- from vine grape extracts to fully aged wines. In the mass-to-charge (m/z) range studied (150-2000 m/z) they observed thousands of peaks corresponding to metabolite chemicals that were ionized by their electrospray conditions. For example, a very tiny sip (20 microlitres) of Vosne Romanée, the distinguished red Burgundy wine (vintage 1995), yielded a staggering 17,400 peaks! A result all the more astonishing when you realise that only 20% of these compounds can be assigned to know chemical databases. Furthermore, many of the substances known to give wine its aroma weren't even included in this study because with m/z values below 150, they were too small for detection.
Illustrating the power of their technique, Gougeon
et al., compared wine samples based on different pure grape varieties matured in oak barrels from different parts of France. The Burgundy wines were a red Pinot noir -- "Mercurey rouge 1ere cru", and a white Chardonnay -- "Beaune 1ere cru", both vintage 1998. These were compared with two Côtes du Rhone wines (vintage 2002), a red Syrah from "Côtes Roties", and a red Grenache from "Gigondas".
Using 12 times 24 oak trees (5 lots of pendunculate oaks,
Quercus robur; 7 of sessile oaks,
Quercus petraea) selected from 9 French forests, a total of 96 barrels were made, 8 for each forest, 2 of which were used for each of the 4 wines tested. The wines were matured in these barrels for different periods -- 6 months for the Gigondas, 12 for the Mercurey, 13 for the Beaune before bottling.
Analysing the mass spectra from these wine samples allowed them to readily distinguish the different grape varieties and their geographical origin. What was new in this study was the fingerprint left in the wines by their oak barrels. Distinct metabolic profiles led to three sub-groups, for example, grouping wood from the Burgundy forests of Citeaux, Darney, and Tronçais, or identifying the very distinct oaks from the forest of Bitche in Lorraine. And these groups could be further sub-divided to identify each of the nine originating forests.
A possible application for this technology might be in the detection of wine fraud -- there are some very expensive wine vintages out there. Some of the great reds repose peacefully in wine cellars for many years before being tasted - only then can they be appreciated or their pedigree questioned. However, the price tag on the machine used for this ICR-FT/MS currently limits its application to well-endowed academic research studies.
Not that this seems to have prevented the Burgundy researchers from having a good party -- for their studies they only needed to remove 20 microliter wine samples by passing a fine Hamilton through the cork, leaving them all those bottles of fine wine still containing some 749.98 ml left to drink. And their concluding sentence suggests that they did just that: "Our systems oenology approach provides an unprecedented example of metabologeography translated into chemical representations of the way such noble nectar can shape on the papillas of the wine taster some of the outlines of the scene of its birth." Cheers!
