Science
Where wine aromas come from
Four origins — the grape, fermentation, oak and ageing — and why a tasting note is not a list of chemicals.
The short answer
Wine aroma compounds have four main origins: some come from the grape itself, some are produced by yeast during fermentation, some are extracted from oak, and some develop slowly in bottle. Most descriptors a taster uses arise from several compounds acting together rather than from any one of them.
Overview
Well over a thousand volatile compounds have been identified in wine, of which perhaps several dozen are present above their sensory thresholds in any given bottle.
The useful organising idea is origin rather than chemistry, because it maps onto decisions someone actually made.
The temptation this section resists is turning tasting notes into chemistry. Naming a compound explains a pattern; it does not license the claim that a particular glass smells of a particular molecule.
How it works
The mechanism, at the scope it has actually been established.
Grape-derived (primary) compounds are laid down in the fruit. Terpenes in Muscat, Riesling and Gewürztraminer; methoxypyrazines in the Cabernet family and Sauvignon Blanc; rotundone in Syrah and a few others. Many exist in the grape as odourless bound precursors and are released during fermentation or ageing.
Fermentation-derived (secondary) compounds are made by yeast and bacteria. Esters account for much of the fruity character in young wine; higher alcohols, diacetyl from malolactic fermentation, and the autolytic compounds from lees contact all belong here.
Oak-derived compounds are extracted from the wood: vanillin, oak lactones, and toast-derived compounds including guaiacol and furfural derivatives.
Ageing-derived (tertiary) compounds develop slowly in bottle through reactions among the wine’s own components, generally in near-absence of oxygen. TDN in Riesling and sotolon in oxidatively aged wines are documented examples.
Perception then complicates everything. Compounds interact, masking and enhancing one another; the same compound reads differently at different concentrations; and individual sensitivity varies enormously, with documented specific anosmias for several important compounds.
Why it matters in the glass
It gives a reader a way to work out what to look for on a label. Fruity esters fade within a few years, so a wine bought for primary fruit should be drunk young; tertiary development is what ageing is for.
It also explains why two people can smell the same glass and disagree in good faith.
Compounds named on this page
Where a compound comes from is chemistry; what it smells like is perception. These are kept apart deliberately.
- Esters
- Fermentation-derived compounds responsible for much of the fruitiness in young wine. Associated with banana, pear drop, green apple, strawberry, tropical fruit. Caution: No single ester accounts for a descriptor. Attributing "banana" to isoamyl acetate is a shorthand, not an identification.
- Terpenes
- Grape-derived compounds behind floral and citrus character in aromatic varieties. Associated with rose, lychee, citrus blossom, lime, geranium.
- Methoxypyrazines — 2-methoxy-3-isobutylpyrazine and related compounds
- Grape-derived compounds behind green pepper and grassy character. Associated with green pepper, blackcurrant leaf, cut grass, asparagus. Extremely low — published thresholds for IBMP in wine are in the region of 1 to 2 nanograms per litre, which is parts per trillion. Individual sensitivity varies several-fold around this. Caution: Their presence does not indicate a defect. A degree of herbaceous character is part of the expected identity of several varieties and regions.
- Volatile thiols
- Sulfur compounds released by yeast from odourless grape precursors. Associated with passionfruit, grapefruit, box hedge, blackcurrant bud. Caution: The same word covers compounds that are valued at low concentration and treated as reduction faults at higher ones. Context and quantity decide which is which.
- Rotundone
- The compound responsible for black-pepper aroma — and one many people cannot smell. Associated with black pepper. Around 16 nanograms per litre in red wine in published studies, with the figure varying by matrix. Around a fifth of tasters in those studies could not detect it at any concentration tested. Caution: The best-documented specific anosmia in wine. Two people can disagree about whether a Syrah is peppery and both be reporting accurately.
- TDN — 1,1,6-trimethyl-1,2-dihydronaphthalene
- The compound behind the petrol note in aged Riesling. Associated with petrol, kerosene, paraffin. Caution: Not a spoilage compound and not an indicator of a fault. Preference for it genuinely divides drinkers.
- Vanillin
- An oak-derived compound, chemically identical to the principal flavour compound in vanilla. Associated with vanilla. Caution: Extraction falls sharply with each use of a barrel, so vanilla character is largely a function of new oak rather than of oak as such.
- Oak lactones
- Oak-derived compounds associated with coconut and woody character. Associated with coconut, wood, sweet spice. Caution: Concentration differs between oak species, and American white oak is generally reported higher — a real difference that does not license "American oak tastes of coconut" as a universal.
- Diacetyl — 2,3-butanedione
- A bacterial by-product of malolactic fermentation, associated with buttery character. Associated with butter, buttered popcorn, butterscotch. Reported in the region of a few milligrams per litre in white wine, and considerably higher in red where other components mask it. Caution: Malolactic fermentation does not guarantee buttery character. Yeast and bacteria can reduce diacetyl further to compounds nobody notices.
For most descriptors there is no single responsible compound. "Blackcurrant" in Cabernet is not one molecule, and claims of the form "this aroma is caused by that compound" should be read as shorthand except in the small number of well-characterised cases.
Go shallower or deeper
The beginner page and the chemistry page are both correct, and neither substitutes for the other.
Sources
Sources consulted
- The Australian Wine Research Institute — AWRI
- Research on volatile thiols in Sauvignon Blanc — Peer-reviewed oenological literature
- Research on methoxypyrazines and green character in wine — Peer-reviewed oenological literature
- Research on rotundone, the peppery aroma compound in Syrah — Peer-reviewed oenological literature
More in wine science
Science
Acidity: the acids in wine and what each does
Tartaric, malic, lactic, citric and acetic acid — where they come from, how they differ, and why pH and total acidity are not the same measurement.
Science
Cool climate and warm climate
The single most useful distinction in wine — what it actually changes in the grape, and where the shorthand breaks down.
Science
Minerality: what the evidence actually shows
A word almost every taster uses, no agreed chemical definition, and no demonstrated route from vineyard rock to matching flavour.