Wineries tackle smoke taint threat

With the fire burning for several days, grapes in vineyards around the region may have been exposed to damaging levels of smoke, leading to widespread concern about the potential for smoke taint in the resulting vintage of wine. Tainted wine can possess unpleasant smoky, spicy, plastic or even medicinal aromas or flavours.

Illustrating the level of concern in the community, a Woodside seminar on January 20 detailing the impact of bushfires on vines, organised by the Adelaide Hills Wine Region, attracted more than 100 registered attendees. Growers, viticulturists and winemakers were all well represented, with many hoping to separate fact from myth to not only minimise wineries crushing unusable fruit, but to also ensure growers' fruit is not unnecessarily rejected by wineries.

The industry members who packed into Bird in Hand's barrel room heard from AHWR technical committee member and viticulturist Richard Hamilton and Australian Wine Research Institute Victorian node manager Mark Krstic.

While the location of vineyards in and around the 12,000ha fireground had helped firefighters, with vines acting as a fire break thanks to their large green canopies and low dry fuel load, their owners - and many more across the region - now had to consider the potential for smoke taint in their grapes.

"In some ways we are fortunate that this was an early fire in the sense that most varieties are just going into veraison," Dr Hamilton said. "It's still a question of whether we'll actually see smoke taint - it's right on the edge of when we'd expect it to occur."

Assessing the risk of smoke taint is difficult, with damage varying depending on the density of the smoke and length of exposure. Visual recounts are often not accurate enough to tell the full story.

Dr Krstic said smoke tainted berries in two ways - either through direct absorption into the berries, or absorption by the leaves and then that being translocated to the fruit.

The susceptibility of berries to smoke taint is variable - between low and medium - from when they are pea-sized through to the onset of veraison. From seven days post-veraison until harvest, the fruit is highly susceptible to smoke damage, meaning the likelihood of damage across the Adelaide Hills could vary greatly depending on the maturity of the fruit at the time of the fire.

Smoke consists of thousands of different compounds, with only a few likely to taint wine. The compounds that cause the unpleasant flavours and aromas are at their highest concentration around the berry skins, meaning red varieties, which are fermented on their skins, are most likely to be tainted. These compounds include guaiacol (smoke, sweet smoke, smoky bacon aromas and flavours), 4-methylguaiacol (smoky, spicy), syringol (smoky, charry), o-Cresol (plastic), m-Cresol (smoky, bandaid) and p-Cresol (faecal, medicinal).

One major problem facing winemakers is that these compounds often exist as bound glycosylated conjugates, and slowly break down as the wine ages. This means that taint may be undetectable in a young wine, but may emerge as the wine gets older.

Dr Krstic advised winemakers to conduct a small-scale ferment of potentially damaged fruit before harvest to check for smoky aromas. This can be commenced any time from when the fruit reaches 8-9 Baume. He urged winemakers to send samples to the AWRI for analysis to determine the level of free and bound smoke taint-causing compounds.

"If you've had smoke for a number of hours in your vineyard, even at this stage of growth, I'd still be going through all the risk assessment methods that are at hand," he said.

He recommended hand harvesting white varieties to minimise breaking the skins for as long as possible, and ensuring that leaf matter is excluded from the press. Fruit should also be kept cool - around 10 degrees Celcius - if possible.