Science
Methoxypyrazines and green character
The compounds behind green pepper in Cabernet and grassiness in Sauvignon Blanc — and why sunlight on the fruit matters more than heat.
The short answer
Methoxypyrazines are grape-derived compounds perceptible at a few nanograms per litre, associated with green pepper, blackcurrant leaf and grassy characters. Their concentration falls with sunlight exposure on the fruit during ripening.
Overview
These compounds are the mechanism behind one of the most consistent differences between cool and warm expressions of the same variety.
They are also among the most potent odorants known in wine, detectable at concentrations that are difficult to intuit — parts per trillion.
How it works
The mechanism, at the scope it has actually been established.
The principal compound in grapes is 2-methoxy-3-isobutylpyrazine, usually abbreviated IBMP, alongside related isopropyl and secondary-butyl forms.
Concentration is highest in the weeks before véraison and declines during ripening. The decline is associated most strongly with light exposure on the fruit itself rather than with ambient temperature, which is why leaf removal in the fruit zone is the standard viticultural response.
Consequently, canopy density, row orientation, training system and crop level all bear on the final concentration — the grower has direct influence here in a way they do not with many aroma compounds.
Varieties in the Cabernet family carry them characteristically: Cabernet Sauvignon, Cabernet Franc, Merlot and Carménère, alongside Sauvignon Blanc, their shared relative.
A separate source is worth knowing about: the multicoloured Asian lady beetle can introduce a related pyrazine at harvest if it is present in significant numbers, which has been a real problem in parts of North America.
Why it matters in the glass
It is a large part of why a Marlborough Sauvignon Blanc and a Sancerre differ, and why Bordeaux in a cool vintage tastes noticeably different from a warm one.
What people get wrong
1 widely repeated claim this page corrects.
Compounds named on this page
Where a compound comes from is chemistry; what it smells like is perception. These are kept apart deliberately.
- 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.
Sources
Sources consulted
- Research on methoxypyrazines and green character in wine — Peer-reviewed oenological literature
- The Australian Wine Research Institute — AWRI
- UC Davis Department of Viticulture and Enology — University of California, Davis
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