Getting out the stink from smoke-tainted wine

Bushfires during summer 2019–2020 decimated entire vineyards in South Australia, Victoria and New South Wales, but smoke, which was far more widespread and insidious, seeped into grapes and into fermenting barrels, yielding unpleasant, unsaleable product.

Grapes exposed to smoke from bushfires, even if the flames don’t come anywhere near the plant, can affect the quality and flavour of the wine made from these smoke-tainted grapes.

With the ongoing threat of bushfires every year, new tools are constantly being developed to help the industry. In 2020, the Australian Government even set up a $5 million grant program to support wine grape producers affected by smoke taint caused by the extreme bushfire season that summer.

Now researchers report in ACS’s Journal of Agricultural and Food Chemistry that they have developed a way to lessen this smoke taint to improve the palatability of the wine using molecularly imprinted polymers (MIPs).

Science to the rescue

Volatile compounds are responsible for many of the appealing aromas and flavours of food and beverages, including wine. These compounds come from the grapes themselves, as well as during the fermentation and aging processes. But not all volatile compounds present in wine are desirable.

Some, produced by bacteria or yeast, smell like nail polish remover, rotten eggs, burnt rubber or onions. And if grapes absorb volatile compounds from bushfires, the resulting wine can take on a disagreeable smoky, ashy or medicinal character that compromises quality and reduces value.

Producers use a variety of adsorbents to remove these unpleasant compounds from wine. However, these techniques can also remove desirable constituents that contribute to the beverage’s colour, bouquet and flavour. Therefore, Kerry Wilkinson and colleagues decided to investigate whether the volatile compounds responsible for smoke taint could be selectively removed with the aid of molecularly imprinted polymers (MIPs).

MIPs are synthetic materials that can bind to one or more target compound. The researchers hoped to use the polymers to fish out the smoky substances from wine, while leaving the tasty and visually appealing components in place.

The team added beads made of MIPs — either contained in muslin or mesh bags or floating freely in the liquid — and then used chemical analysis, as well as a taste-testing panels of experts, to evaluate the results.

In some cases, the MIPs were regenerated by rinsing them after extraction to remove the smoke compounds, enabling them to be reused.

The results

In terms of improving the wines’ sensory characteristics and removing unpleasant volatile phenols, the researchers found that adding MIPs to wine after fermentation was more effective than adding MIPs during fermentation.

The addition of MIPs to smoke-tainted Pinot Noir wine (for 24 h with stirring) achieved 35–57% removal of guaiacol, 4-methylguaiacol, cresols and phenol, but <10% of volatile phenol glycoconjugates were removed and some wine colour loss occurred. Of the MIP treatments that were subsequently applied to Semillon and Merlot fermentations or wine, MIP addition post-inoculation of yeast yielded the best outcomes, both in terms of volatile phenol removal and wine sensory profiles.

In addition, regenerating the MIPs generally enhanced removal of smoke volatile compounds.

Although the MIPs also adsorbed some desirable compounds associated with aroma, flavour and red wine colour, the researchers say the project showed that the method can be used successfully to reduce the perception of smoke taint.

The authors acknowledge funding and support from the Australian Government as part of a Cooperative Research Centre Project; Wine Australia; Metabolomics South Australia, which is funded through Bioplatforms Australia; the National Collaborative Research Infrastructure Strategy; and the South Australian Government.

Image credit: iStock.com/tibor13