Mitigation of climate change impacts on the national wine industry by reduction in losses from controlled burns and wildfires and improvement in public land management
This is a collaborative project with partners including Wine Australia, the Australian Government Department of Agriculture and Water Resources, Agriculture Victoria, La Trobe University, Wine Victoria and the AWRI. The AWRI’s role is to evaluate a range of possible remedial management options and processing tools for dealing with smoke-affected grapes and wine. This is especially important as smoke taint is known to develop during bottle ageing of wine through continued conversion of non-volatile glycosides from contaminated grapes into their free volatile forms. In addition, the AWRI is collaborating with Agriculture Victoria/La Trobe to evaluate vineyard-based monitoring, preventative and remedial management options for dealing with the variable composition of atmospheric smoke and associated risk of smoke taint in wine.
This project is supported by Wine Australia, through funding from the Australian Government Department of Agriculture and Water Resources as part of its Rural R&D for Profit program and the Australian Wine Research Institute.
Smoke taint mitigation studies
The effectiveness of 14 activated carbon products in fining smoke-affected wine and juice was studied for a range of varieties. Previous studies had been largely limited to red juice and wine due to limited availability of smoke-affected white juice and wine. This year, additional studies were able to be undertaken on ‘real’ smoke-affected Sauvignon Blanc, Chardonnay, Riesling and Pinot Noir juices and wines sourced in 2018 and 2019. Carbon fining removed up to 80% of smoke glycosides from the Sauvignon Blanc juice; however, removal from wine was more challenging, but found to be more effective in white than red wines (e.g. 40-60% removal compared to 0-10% when dosed at 2 g/L carbon). The effectiveness of carbon fining in removing smoke glycosides in the Sauvignon Blanc juice was also evaluated at various dose rates (0.5 to 2.0 g/L) in the presence of caffeic acid, tannins and volatile phenols and at different pH levels. Caffeic acid and tannin were found to hinder removal of smoke glycosides, but changes to juice pH or the presence of volatile phenols did not have an impact on glycoside removal.
In preparation for winemaking trials, three activated carbon products were evaluated at 4°C and in the absence of mechanical mixing, with neither of those parameters affecting removal rates. The ability of carbons to remove a higher concentration of glycosides in a white grape juice was also explored, as well as their effectiveness in additional juices with varying degrees of smoke taint. Results indicated that the adsorption of smoke glycosides by activated carbon is concentration-dependent. Batch to batch variation for three carbon products shortlisted for winemaking trials was investigated. While there was some variation (i.e. up to 10%) in performance, overall the carbon products still showed consistent removal of smoke glycosides. As a result of the benchtop studies, two carbon products were selected for 2019 winemaking trials.
Glycosidase experiments were undertaken on additional wines. Five commercially available glycosidases were evaluated and found to cleave 60-70% of the gentiobiosides contained within two smoke-affected Cabernet Sauvignon wines, one Sauvignon Blanc and one Pinot Noir wine. Less cleavage occurred for other smoke glycosides. These results mirror other studies where gentiobiosides were also preferentially cleaved. Since the impact on wine sensory properties was not known, a smoke-affected Pinot Noir wine was treated with glycosidases and the resulting wine presented to the quality panel. Glycosidase treatment did not reduce the perception of smoke taint, suggesting that for this to be a feasible remediation option it may need to be performed in conjunction with carbon treatment to remove free volatile phenols. Industry samples of an enzyme-treated smoke-affected Pinot Noir wine were also analysed and similar trends were observed. Two novel glycosidases, (i) Halothermothrix orenii (Hal) β-glycosidase; and (ii) Alicyclobacillus acidiphilus (Aci) β-glycosidase were evaluated for their abilities to cleave smoke glycosides in two smoke-affected Pinot Noir wines. However, these glycosidases were found not to cleave smoke glycosides in real wines, perhaps a consequence of enzyme inhibition at natural wine pH. Additional enzyme studies have now commenced with smoke-tainted Pinot Noir and Chardonnay wines, with the aim of undertaking larger-scale trials for sensory analysis.
Masking smoke taint
When a suspected tainted Sauvignon Blanc wine with elevated residual sugar (14.4 g/L) was presented to the AWRI’s technical quality panel, the panel did not assess the wine as smoke tainted. This raised the question as to whether sugar can mask the perception of smoke taint. To assess this, sugar (14 g/L, 50% glucose, 50% fructose) was added to a smoke-affected Pinot Noir wine which was then presented to the AWRI’s technical quality panel. However, in this instance sugar addition did not mask the perception of smoke taint, with the panel finding the wine to be affected by smoke characters.
Vineyard variability among grapes exposed to smoke during a 2018 bushfire was assessed across two blocks (one Chardonnay, one Pinot Noir). For the majority of grape samples (90% for Pinot Noir and 100% for Chardonnay), the levels of volatile phenols and smoke glycosides were relatively consistent (within 20% RSD of the mean) and all samples had concentrations of exposure markers above concentrations observed for ‘clean’ grapes in previous baseline studies.
Smoking experiments using Chardonnay grapes (1.6 tonnes) and smoking tents were undertaken in February 2019, yielding approximately 1,000 litres of juice for winery-scale remediation treatment trials. One hundred litres of the artificially smoked juice were fermented into wine, with the remaining juice being held in frozen storage. Fires in February 2019 provided an opportunity to source smoke-affected Pinot Noir and Chardonnay grapes, juices and wines. Winemaking trials were performed with the 2019 smoke-affected Chardonnay and Pinot Noir (rosé style) juices using two activated carbon products at three dose rates (1.0, 2.0 and 4.0 g/L) in duplicate (total of 28 x 50 L ferments including controls). Juices were treated with the carbon products prior to fermentation, with bentonite added to assist in settling after a contact time of 48 hours. The percentages of smoke glycosides remaining in each of the juices after carbon treatment and bentonite settling are shown in Figures 23 and 24. For certain dose rates, carbon product number 14 was slightly more effective at removing the smoke glycosides but it also removed more colour. Furthermore, both carbon products were more effective at removing smoke glycosides from the Chardonnay juice than the Pinot Noir juice; however, the latter did contain higher initial concentrations of total smoke glycosides. Sensory impacts of the treatments on the wines made from the treated juices will be assessed. Larger-scale (200 L) ferments were also undertaken on these juices to produce wines for additional remediation studies.
The AWRI is collaborating with the University of Adelaide’s Industrial Transformation Training Centre on smoke taint research. Assistance was provided for two PhD projects, one investigating whether in-canopy misting can reduce the uptake of smoke molecules in grapes and a second investigating the use of cyclodextrins for mitigating smoke taint in juice and wine.
A collaboration with the University of Adelaide, PIRSA, Grains Producers SA, SA Grains Industry Trust, and La Trobe University has commenced to assess the potential impact of smoke from stubble burns on grapes and wine, with funding obtained from the South Australian Wine Industry Development Scheme.