Science
Diacetyl and the buttery note
A bacterial by-product of malolactic fermentation, and the actual explanation for buttery Chardonnay.
The short answer
Diacetyl is produced by lactic acid bacteria as an intermediate during malolactic fermentation. It is associated with buttery and buttered-popcorn characters, and it is the principal explanation for buttery Chardonnay — not oak.
Overview
This is one of the most useful individual compounds for a drinker to know about, because it settles a very common confusion.
It is the same compound used to flavour cinema popcorn, which is why the descriptor is so immediately recognisable.
How it works
The mechanism, at the scope it has actually been established.
Lactic acid bacteria produce diacetyl from citric acid metabolism during and after malolactic fermentation. It is an intermediate rather than an end product.
Concentration is not fixed by the conversion. Bacteria and yeast can reduce diacetyl further to acetoin and 2,3-butanediol, which have far higher sensory thresholds and are effectively unnoticeable.
Whether it survives therefore depends on conditions: bacterial strain, temperature, oxygen, how long the wine sits on its lees afterwards, and when sulfur dioxide is added. Leaving the wine on lees after conversion is associated with lower final concentrations, because yeast continue to reduce it.
Reported sensory thresholds are in the region of a few milligrams per litre in white wine and higher in red, where other components mask it.
This is why a winemaker can put a wine through full malolactic fermentation and produce very little buttery character, and why "did it go through malo?" is a less precise question than it sounds.
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.
- 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.
Sources
Sources consulted
- The Australian Wine Research Institute — AWRI
- UC Davis Department of Viticulture and Enology — University of California, Davis
- International Code of Oenological Practices — OIV
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