Struck flint aroma in Chardonnay wines: what causes it and how much is too much?

Abstract

Struck flint/struck match/gun smoke/mineral aroma is considered desirable in some styles of wines, with this character sometimes evident in wines such as Burgundian Chablis and cooler climate barrel-fermented Australian Chardonnay. Phenylmethanethiol (benzyl mercaptan) is a potent sulfur-containing volatile aroma compound and is thought to be responsible for struck flint character in wine. However, few studies targeting this character have been done. To address this, over 70 commercially available white wines, mostly Chardonnay, were chemically analysed to establish the variability of phenylmethanethiol, and the wines were assessed by a sensory panel to indicate whether there might be a transition from struck flint aroma to a less pleasant sulfurous/burnt aroma. Interestingly, another potent sulfur-containing aroma compound, 2-furylmethanethiol (furfuryl thiol), was also found in the wines and was at particularly high concentration in wines suggested as having high struck flint aroma. 2-Furylmethanethiol has previously been shown to form in white wines during alcoholic fermentation in the barrel from the furan-2-carbaldehyde (furfural) released by toasted oak staves reacting with the hydrogen sulfide produced by yeast. This survey highlighted that both phenylmethanethiol and 2-furylmethanethiol are linked to struck flint aroma but when higher levels of 2-furylmethanethiol are present, the character might tend toward sulfurous/burnt.

 There was also no information available on the effects of winemaking techniques and commonly used winemaking additives on the formation of phenylmethanethiol. Further investigations in model fermentations of its potential precursors benzaldehyde and hydrogen sulfide were conducted. Wine yeast strains that produced high concentrations of hydrogen sulfide resulted in higher concentrations of phenylmethanethiol during fermentation of a synthetic grape must and increasing concentrations of ammonia (YAN) promoted the formation of phenylmethanethiol by yeast during fermentation. Thus, different winemaking parameters could be used to modulate the concentrations of phenylmethanethiol in wine.