Petrichor & Earthy Notes in Wine

Petrichor describes the earthy scent produced when rain falls on dry soil. The word was introduced in a March 1964 paper by CSIRO scientists Isabel Joy Bear and Richard G. Thomas, published in Nature under the title 'Nature of Argillaceous Odour.' It is formed from the Ancient Greek petra (stone or rock) and ichor (the ethereal fluid in the veins of the gods). The scent arises from a combination of geosmin, a volatile compound produced by soil-dwelling Actinomycetes bacteria such as Streptomyces, and plant-derived oils that accumulate in dry soil during droughts and are released when rain disturbs the surface. The term replaced the older descriptor 'argillaceous odour' and has since entered scientific, culinary, and now wine tasting vocabulary.

• Petrichor is a multi-compound scent: geosmin from bacteria, plant oils from dry soil, and sometimes ozone from lightning all contribute
• Geosmin was first isolated in pure form in 1965 by Gerber and Lechevalier from Streptomyces griseus fermentation broth
• Humans may have evolved high sensitivity to geosmin as an evolutionary cue for locating fresh water and fertile soil

Geosmin (trans-1,10-dimethyl-trans-9-decalol) is a bicyclic alcohol with the molecular formula C12H22O, classified as an irregular sesquiterpenoid. It is biosynthesized by a bifunctional enzyme in Streptomyces coelicolor that converts farnesyl diphosphate into geosmin in a two-step reaction, a pathway that was fully elucidated in 2006. The human olfactory system can detect geosmin at concentrations ranging from 5 to 400 parts per trillion, making it one of the most potently detectable volatile compounds in nature. Geosmin is also responsible for the characteristic earthy taste of beetroots, and it causes off-flavors in drinking water and farmed fish. Under acidic conditions, geosmin decomposes into odorless substances, which is why adding citrus juice or vinegar to beets reduces their earthy taste.

• Geosmin is produced by Streptomyces (Actinomycetes), cyanobacteria, myxobacteria, and certain fungi
• Streptomyces produce geosmin to attract springtails, which help disperse the bacteria's spores through soil
• Geosmin decomposes under acidic conditions, forming the odorless compound argosmin — this is why acidity is used in winemaking remediation attempts

In wine, geosmin is officially classified as an off-aroma contaminant, not a terroir marker. It was first formally identified as a wine fault in a landmark 2000 study by Darriet and colleagues, who used gas chromatography-mass spectrometry to identify it in red and white wines exhibiting 'freshly tilled earth' and 'damp cellar' aromas. The primary pathway for geosmin entry into wine is through grape bunches infected with both Botrytis cinerea and Penicillium expansum mold, a situation promoted by wet weather near harvest. Significantly affected vintages from Burgundy, Bordeaux, and the Loire were documented from 2000 onward. The perception threshold in red wine is approximately 60 to 65 ng/L. Long-term barrel aging may reduce geosmin concentrations, though the process can take several years.

• Geosmin contamination is linked to Botrytis cinerea and Penicillium expansum co-infection of grape clusters, promoted by wet harvest conditions
• The earthy off-aroma can persist for years in bottle; only long barrel aging or acidic conditions significantly reduce it
• Geosmin was once mistaken for 'terroir' character before its chemical identity was confirmed — winemakers now monitor for it analytically

While geosmin is one possible source of earthy aromas, the prized 'forest floor,' 'mushroom,' and 'damp earth' notes in aged Burgundy Pinot Noir are complex tertiary aromas that develop through bottle aging and are not solely or primarily attributed to geosmin. These descriptors reflect the full expression of a wine's terroir and aging chemistry. Brettanomyces (Brett) is a separate and important distinction: this spoilage yeast produces volatile phenols including 4-ethylphenol and 4-ethylguaiacol, which deliver barnyard, leather, band-aid, and medicinal characters. These are chemically unrelated to geosmin. Brett is debated as a flaw versus a complexity contributor, particularly in Southern Rhone, Rioja, and some Bordeaux styles. Minerality, another distinct category, reflects acidity-driven perception of slatey, chalky, or stony notes and has its own separate sensory mechanism.

• Tertiary earthy aromas in aged Pinot Noir (forest floor, mushroom, truffle) develop through bottle aging, not from geosmin contamination
• Brett (Brettanomyces) produces barnyard and band-aid characters via volatile phenols, chemically distinct from geosmin's musty earthiness
• Minerality, earthiness, Brett character, and geosmin are four separate sensory phenomena often conflated in wine tasting notes

When tasting for earthy complexity in wine, allow the wine to open in the glass for several minutes, paying attention to secondary and tertiary aromas that emerge beneath the primary fruit. Look for descriptors such as 'forest floor,' 'mushroom,' 'damp leaves,' 'truffle,' 'dried earth,' or 'wet stone.' These are most prominent in aged Old World reds, particularly Burgundy Pinot Noir, Barolo Nebbiolo, and aged Bordeaux. Serve red wines at 55 to 60 degrees Fahrenheit to allow tertiary aromatics to express themselves without alcohol overwhelming the nose. A helpful calibration tool is to compare side-by-side with an intentionally Brett-affected wine to learn the distinction between pleasant earthy complexity and spoilage-derived funk.

• Earthy tertiary notes are most pronounced in Burgundy Pinot Noir, Barolo, and aged Bordeaux with bottle age of five or more years
• Serving temperature matters: cool service at 55 to 60 degrees Fahrenheit brings out earthy notes, while warmer service emphasizes fruit and alcohol
• Blind tasting tip: forest floor and mushroom notes suggest Old World origin, bottle age, and often limestone-rich cool-climate terroir

For WSET Level 2 candidates, earthy and forest floor aromas should be identified as tertiary characteristics in aged Pinot Noir, particularly from Burgundy's Cote de Nuits, using the WSET Systematic Approach to Tasting framework. At Level 3, students should distinguish between the various sources of earthy aromas: tertiary aging chemistry, Brett (volatile phenol contamination), and geosmin (a mold-related fault). At Diploma level, understanding the chemistry of geosmin, its entry pathway through Botrytis and Penicillium co-infection, and its role as a wine fault versus the perceived terroir quality of aged earthy Pinot Noir is essential exam material. For CMS and MW candidates, precision in labeling these sensory categories separately signals genuine mastery of wine faults and quality assessment.

• WSET L2: Recognize forest floor and mushroom as tertiary aromas in aged Burgundy Pinot Noir and other Old World reds
• WSET L3: Distinguish Brett-derived earthiness (barnyard, band-aid) from tertiary aging earthiness (forest floor, mushroom, truffle)
• WSET L4 / MW: Understand geosmin as a mold-derived wine fault linked to Botrytis and Penicillium co-infection, and differentiate it from desirable tertiary complexity