Chemistry
More than 800 aroma compounds have been identified in wine, some of them at the level of nanograms per litre.23 A few are tied to a single grape or a single cellar process, and some work at tiny doses: for about 80% of a tasting panel, rotundone was detectable at 16 ng/L in red wine.1
Odour thresholds reported in the studies, on a logarithmic scale. Each step to the right is ten times more.
C₁₅H₂₂O · PubChem 5321003
black pepper
Rotundone, a sesquiterpene, was identified in 2008 as an aroma compound with a strong spicy, peppercorn aroma, and its concentration tracked the black pepper intensity rated by panels in Shiraz grapes and wines.1 Most panellists detected it at 16 ng/L in red wine, yet about 20% could not smell it even at 4000 ng/L in water.1 Black and white peppercorns contain roughly 10,000 times more of it than a very peppery wine.1
C₉H₁₄N₂O · PubChem 32594
green bell pepper
IBMP gives grapes a bell pepper aroma and has a reported threshold of about 2 ng/L in water.2 Of 29 grape cultivars analysed at UC Davis, quantifiable levels were found only in Cabernet Franc, Cabernet Sauvignon, Merlot, Sauvignon Blanc and Sémillon.2 In Sauvignon Blanc the pea pod, capsicum character of methoxypyrazines, of which IBMP is the major one in grape berries, balances the passion fruit and grapefruit of varietal thiols, while excessive amounts reduce consumer acceptance, particularly in red wines.3
C₁₃H₁₆ · PubChem 121677
kerosene, petrol
TDN is well known for the petrol aroma of aged Riesling.4 In young wines Riesling held 6.4 µg/L on average against 1.3 µg/L in other varieties, and 27 of 28 Rieslings exceeded the 2 µg/L detection threshold measured by a Cornell team.4 A Geisenheim study put the recognition threshold in young Riesling at 10–12 µg/L and found that serving the wine cooler, at about 11 °C, made the kerosene note easier to identify.5
C₆H₁₄OS · PubChem 521348
grapefruit, passion fruit
3MH is one of the volatile thiols behind the varietal aroma of Sauvignon Blanc; the free thiol is not present in the juice and is released by yeast during alcoholic fermentation.6 Its two mirror-image forms have similar thresholds of 50 and 60 ng/L in model solution, but the R form recalls grapefruit and the S form passion fruit.7 It is also found in Sémillon wines, including sweet wines from grapes with noble rot.7
C₆H₁₂OS · PubChem 88290
blackcurrant
4MMP has an odour threshold of 0.8 ng/L in wine, and in the Sauvignon Blanc wines analysed its levels were far above that threshold.8 Grape juice holds it as a non-volatile cysteine-bound precursor, which is cleaved during alcoholic fermentation.9 In red wines with a strong blackcurrant aroma, the concentration of 4MMP correlated very well with the intensity of that note.10
C₁₀H₁₈O · PubChem 6549
floral
The floral and citrus notes of many white varieties are attributed to monoterpenes and monoterpene alcohols such as linalool.11 In Muscat Blanc à Petits Grains grapes linalool was the major aroma contributor.12 Monoterpenes matter in Muscat, Gewürztraminer and Riesling, where much of them sits in the berry as odourless glycosides that can be released during ripening, winemaking or ageing.13,11
C₁₀H₁₈O · PubChem 1712087
lychee
Comparing lychee fruit with Gewürztraminer wine, Cornell researchers found that cis-rose oxide had the highest odour activity among their shared odorants and concluded that these shared odorants, above all cis-rose oxide, were responsible for the lychee aroma of the wine.14 It was not detected in the Chardonnay and Riesling control wines.14 In Gewürztraminer grapes, cis-rose oxide and geraniol were the major contributors to aroma.12
C₄H₆O₂ · PubChem 650
butter
Diacetyl, a diketone that imparts a buttery aroma, is a major flavour metabolite of lactic acid bacteria; in wine its metabolism takes place during malolactic fermentation by Oenococcus oeni.15 Its formation depends on citric acid metabolism, and its final level on the bacterial strain, pH, temperature, sulphur dioxide, aeration and yeast lees.15 In Pinotage wines from Stellenbosch University trials, diacetyl correlated with the buttery character of wines that went through malolactic fermentation.16
C₉H₁₆O₂ · PubChem 41285
coconut, wood
The detection threshold of cis-whisky lactone in 12% alcohol solution is 20 µg/L, against 130 µg/L for the trans form, so at average concentrations in oak-aged red wines the cis form is above threshold and the trans form below it.17 In tests by a Bordeaux team on a reconstituted red-wine fruity aroma in model solution, both forms dulled red berry notes and strengthened blackberry and spicy ones.17 Whisky lactones give coconut-like odour impressions, and red wines aged with a large dose of oak chips scored higher for woody and coconut notes.18,19
C₆H₈O₃ · PubChem 62835
curry, walnut
Sotolon, a lactone, gives powerful Madeira-like, oxidised, curry and walnut notes and has been much studied in Jura flor wines, aged Roussillon sweet wines and old Port.20 In Jura yellow wines aged more than six years in barrel, its key role was confirmed at 112–387 µg/kg.21 The specific metabolism of flor yeasts, which grow as a film on the wine surface, explains the acetaldehyde and sotolon content of these wines.22