Is buttery aroma perception in wines predictable from the diacetyl concentration?

An important flavour compound associated with malolactic fermentation (MLF) is the volatile di-ketone, diacetyl (2,3-butanedione), which is considered as the major contributor to buttery flavour in wines. Commercial Chardonnay, Cabernet Sauvignon, Merlot and Shiraz wines from eight vintages (1992- 1999), 20 viticultural regions and 28 wine producers were analysed for diacetyl and also the buttery aroma intensity was rated by a trained sensory panel. The wines examined were found to have a wide range of diacetyl concentration, i.e. 0.3 to 0.6 mg/L (mean value 0.4 mg/L) for the 24 Chardonnay wines, and 0.3 to 2.5 mg/L (mean value 1.1 mg/L) for the 43 red wines. The buttery aroma, while significantly correlated with diacetyl concentration in the Chardonnay wines (P < 0.01), was not predicted very well by the regression equation, but the prediction was improved when the free sulfur dioxide concentration of the wines was taken into account (adjusted R2 = 0.43, P < 0.01). There was a weak, although statistically significant, correlation between diacetyl concentration and the buttery rating for the red wine data set, when the sulfur dioxide concentration was taken into account (adjusted R2 = 0.12, P < 0.05). This weak correlation may have been, in part, due to the generally low concentration of diacetyl in relation to the published aroma threshold in red wine. It was also noted that there was a substantial decrease in diacetyl concentration for a subset of red wines, but not for white wines, of various vintages cellared for three years at 15°C.