Wednesday, June 26, 2013

Earthiness in wine is unrelated to the soil

I have previously reported on Alex Maltman's view of the inapplicability of geologic terms as wine descriptors because geologic minerals have not been scientifically shown to transit from the soil into the grape berry and the resulting wine. I have also shown that "chalky minerality" is a tannin effect rather than a "mineral taste." In this post I will explore the source of another putative minerality term -- earthiness.

Joe Janish (wineweekly.com, Wine Term: Earthy) describes earthy wines as having " aromas and flavors of soil, minerals, vegetation, and or wet leaves." Voicing a widely held belief, Joe states "... it is believed that the earthiness comes from the soil in which the vine is planted."

Wine chemists beg to differ with this characterization. They see the earthiness in wine as an off-odor caused by geosmin (trans-1, 10-dimethyl-trans-9-decalal), a secondary metabolite (compound produced by an organism that is not required by that organism for growth, development, or reproduction) produced by the fungus species Penicillium expansum (M.H. Siddique, Study of the biosynthesis pathway of the geosmin in Penicillum expansum, Doctoral Thesis submitted to the University of Toulouse, 5/11/12; Lisanti et al., Oenological treatments for the removal of geosmin, responsible for earthy off-flavors, in wine, Paper prepared for the XXX1st World Symposium of Vine and Wine, Verona (Italy), 15-20 June, 2008; Florence Kennel, Bordeaux Boffin solves geosmin connundrum, Decanter.com, 1/14/05; R.R.M. Paterson et al., Why do food and drink smell like earth, Communicating Current Research and Education Topics and Trends in Applied Microbiology, 2007) or geosmin in combination with another earthy/musty-smelling secondary metabolite 2-MIB ( methyl-isoborneol) which is produced by the fungus Botrytis cinera (Siddique, Laguerche in Kennel, and Paterson et al.).

Geosmin, the name is a combination of two Greek words which translate to earth odor, exists as both (+) and (-) isomers and is responsible for a freshly plowed-earth odor in a wide variety of foodstuff to include drinking water, bottom-feeding fish, grape juice, and wine. Humans have a very low odor and taste sensitivity to the compound and, while deemd desirable at low levels, it is viewed as off-putting at higher concentrations. Siddique perceives the presence of geosmin to be "highly detrimental to the aromatic quality of wines due to its low olfactory perception threshold and stability during aging." Higher concentrations of the compound are generally attributed to the (-) isomer which is 10 times more potent than its (+) counterpart.

The source of geosmin appears to be the fungus Penicillum expansum which is found on rotten grapes. In a gas chromotography-mass spectrometry study of rotten Bordeaux grapes, 48 Penicillum-related species were examined and all of the geosmin-producing strains belonged to the P. expansum species (Siddique). The presence of geosmin in juice obtained from rotting grapes would seem to indicate that the P. expansum that developed on the grapes contributed to the geosmin in the wine (Siddique). According to Kennel, two to five clusters in a hundred are enough to contaminate a wine, requiring long-term barrel maturation (two years in the case of Pinot Noir) for dispersal. Lisanti et al., examined six treatments for decreasing the concentrattion of geosmin and found that treatment with grape seed oil appeared to be effective in reducing the earthy flavors in both red and white wines.

In many of the studies that have been done to date, geosmin is found in association with MIB, a product of Botrytis cinera. It is not clear exactyly how they work together but scientists are willing to concede that the earthy smell could be a result of their collaborative work (Siddique, Laguerche in Kennel, and Paterson et al).

So, when confronted with odors such as mushroom, camphor, mossy, moldy, or earthy, think of grape selection, or judicious application of grape seed oil. Do not think soil.

©Wine -- Mise en abyme

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