Beyond the human and community impacts of bushfires, grape and wine businesses are affected in two key ways – through fire damage to vineyards, buildings, plant and equipment, and through smoke impacts on wine-grapes and the resulting wine. This project evaluated a range of possible preventative and remedial treatments for dealing with smoke-affected grapes and wine. A range of commercially available activated carbons was demonstrated to reduce smoke taint in wine, especially when added to grape juice prior to fermentation. Uptake of findings from this project in the context of the 2019/20 bushfires has already been significant.
The objectives of this project were to:
(i) evaluate a range of possible preventative vineyard mitigation options to reduce uptake of smoke taint compounds in the vine and grapes
(ii) evaluate remedial management options for dealing with smoke-affected grapes and wine
(iii) evaluate the impact of treatments on the sensory characteristics of the wines produced
(iv) evaluate consumer preferences for smoke-affected and treated wine
(v) improve our knowledge on background concentrations of volatile phenols and phenolic glycosides in non-smoke-exposed fruit.
Preventative vineyard mitigation options and strategies to reduce uptake of smoke taint compounds
A model system was developed to evaluate under controlled conditions the effectiveness of a range of existing agrochemicals and other materials for reducing/preventing the uptake of volatile phenols from smoke and their subsequent conversion to phenolic glycosides. Grapes were coated with the materials to be tested and then exposed to gaseous phenols in a closed glass container. Treatments typically did not provide any significant protection and many resulted in higher concentrations of volatile phenols and phenolic glycosides in the grapes. The highest concentrations of volatile phenols and their glycosides were observed after application of oily/hydrophobic materials, suggesting that these materials may enhance the adsorption or transfer of volatile phenols into grape berries.
Remedial management options for dealing with smoke-affected grapes and wine
Fifteen commercially-available activated carbon products were evaluated for their ability to remove smoke volatile phenols and phenolic glycosides from smoke-affected juice and wine. The effectiveness of carbon products in removing smoke taint compounds was highly dependent on the type of carbon product used, the matrix (i.e. juice versus wine; red versus white) and the dose applied. Some carbons were better at targeting phenolic glycosides, while others exhibited selectivity for removing volatile phenols. Generally, activated carbon products were more effective at removing phenolic glycosides from juice than wine. The best-performing activated carbons were able at high doses to remove 90% or more of phenolic glycosides from grape juice but also desirable compounds. These results emphasised the importance of selecting the right carbon product for the desired application.
As activated carbons also remove colour from red must or wine, red wine is better suited to treatment with glycosidases. As such, five commercially available glycosidases were evaluated for their ability to degrade smoke taint related glycosides. All were observed to preferentially, and effectively, cleave some phenolic gentiobiosides in wine. However, many other phenolic glycosides were not cleaved.
Sensory impact of treatments
For sensory evaluation of activated carbon treatment, two products were evaluated in small-scale winemaking trials with smoke-affected Chardonnay and Pinot Noir rosé juices treated at three doses (1, 2 or 4 g/L) prior to fermentation. Sensory evaluation of the resulting wines confirmed that smoke-affected juices can be effectively treated with activated carbon products to reduce the perception of smoke taint in wine. However, the carbon dose must be carefully optimised as the highest dose treatments with 4 g/L activated carbon also showed significant decreases in many of the positive ‘fruit/floral’ attributes. Notably, the Pinot Noir wines where the juices had been treated with 1 g/L of activated carbon had enhanced ‘strawberry’ aroma and flavour attributes compared to the control, while also having a significant reduction in smoke aroma and flavour.
A glycosidase was chosen for treatment of a full-bodied Pinot Noir wine, in conjunction with two activated carbon products to reduce the volatile phenols released in the wine due to cleavage of glycosides. However, combining enzyme treatment of wine with carbon fining did not improve the sensory characteristics compared to fining the wine with activated carbon alone. In this particular study, enzyme treatment of the wine led to increases in the rating of ‘smoky’ characters.
Sensory studies performed on two smoke-affected Pinot Noir wines confirmed that blending can be an effective option for remediation of smoke-affected wine. For both experiments, a blend of 25% or less of the smoke-affected wine with unaffected wine of a similar style was sufficient to lower the smoke taint intensity to be not significantly different from the unaffected wine.
Consumer preferences for smoke-affected and treated wine
In a follow-up study to the blending experiments described above, 82 consumers were asked to describe their liking for a smoke-affected Pinot Noir rosé-style wine in comparison to the tainted wine blended with varying proportions of an unaffected wine. Overall, the least liked wine was the smoke-affected wine and liking scores decreased with greater proportion of smoke-affected wine in the blend. Cluster analysis identified three groups of consumers with significantly different liking scores within each of the clusters. The results from the total sample set, and for the largest cluster group (46% of consumers), demonstrated that even a small proportion of smoke-affected wine in a blend can significantly affect consumer liking. Notably, one cluster consisting of 21% of consumers liked all the wines, suggesting that they may not be sensitive to smoke flavours or might even enjoy such characters.
Baseline concentrations of volatile phenols and phenolic glycosides in non-smoke-exposed fruit
Since volatile phenols and phenolic glycosides occur naturally in grapes at low concentrations, it is important to know background concentrations when assessing if samples have been exposed to smoke. A small database of background concentrations already exists and through this project, an additional 325 non-smoke-exposed grape samples were collected from 23 regions across Western Australia, South Australia, Victoria, New South Wales and Tasmania. This allowed baseline concentration data of smoke taint compounds in both grapes and wine to be expanded to 12 of the main winegrape varieties grown in Australia.
Vineyard variability among grapes exposed to smoke was assessed across two blocks, one Chardonnay, one Pinot Noir, after a bushfire in 2018. Field variability of volatile phenols and smoke glycosides was typically within 20%. Importantly all samples, independent of their location within the vineyard, had post-exposure concentrations of smoke taint markers above concentrations for ‘clean’ grapes as indicated from comparison to baseline data.
This project was funded by the Commonwealth Department of Agriculture, Water and the Environment Rural R&D for Profit program through Wine Australia with co-investment from The Australian Wine Research Institute.