Harvesting grapes from vines that have been exposed to bushfire smoke can result in wines containing smoke taint, described as “smoky, burnt and ash” aromas and “cigarette, ash tray, acrid and burnt” flavours. Aroma compounds enter the vine by absorbing onto the protective waxy cuticle layer or moving into the stomata on leaves and thereafter finding their way through the phloem into grapes. Smoke taint can be attributed to the various volatile phenols (VPs) absorbed, with guaiacol and 4-methylguaiacol (4MG) being the most predominant compounds, and several other VPs contributing. These compounds can be found in grape juice and wine in free and glycosylated (linked to sugars) forms. The free form of a VP is odorous.
Glycolysates are non-odorous but have ashy palate effects as VPs are released by in-mouth enzymes. VPs are not all bad. They can contribute positive characteristics like vanilla, woodiness, and spiciness to the aroma (for example, when they are extracted from oakwood). Guaiacol can also be found naturally at very low levels in non-smoke exposed, non-oaked wines (notably Shiraz). The perception of smokiness from guaiacol is dependent on the aromatic intensity of the wine, with sparkling base showing smoky character at 6-10 μg/L of guaiacol, medium bodied red at 15-25 μg/L and full-bodied Shiraz at 30-40 μg/L. Smoke taint can thus be masked by the presence of other volatiles (e.g., from addition of oak extracts), which depends on wine style.
Research conducted over the past few years has led to the development of guidelines on how to handle smoke tainted fruit. Making wine to minimise volatile extraction and trying to minimise extraction of glycolysates at the same time, for early release or blending, is a sensible strategy. An alternative winemaking strategy is to maximise release of VPs from glycolysates and remove them from the wine to avoid later release. Treatments that have been shown to significantly remove VPs are reverse osmosis (relatively expensive), activated charcoal (strips out a lot of other components including colour), and yeast hulls (not as efficient at removal but has shown some interesting results).
Download guidelines to minimise smoke taint