The Australian Wine Research InstituteThe Australian Wine Research Institute

When vineyards and grapes are exposed to smoke this can result in wines with undesirable sensory characters, such as smoky, burnt, ashy or medicinal, usually described as ‘smoke tainted’. Consumers have been shown to respond negatively to smoke tainted wines. The compounds in smoke primarily responsible for the taint are the free volatile phenols that are produced when wood is burnt. These can be absorbed directly by grapes and can bind to grape sugars to give glycosides that have no smoky aroma. Often these glycosides are described as smoke taint precursors. During fermentation (and also over time in barrel or bottle) these glycosides can break apart, releasing the volatile phenols into the must or wine, and allowing the smoky flavour to be perceived. These glycosides can also release the volatile phenols in the mouth during the drinking of wine, which may contribute to the perception of smoke taint.

What factors affect smoke taint in the vineyard?

The key factors that determine whether smoke-exposed grapes become tainted are the grapevine growth stage, grape variety, smoke composition and the length of smoke exposure. More details about vineyard risk factors can be found in the following fact sheets.

• Smoke taint – entry into grapes and vineyard risk factors
• Stubble burning – a possible source of smoke taint in grapes
• Minimising the impact of controlled burns on wine-grape production
• I can smell smoke – now what?  – Q&A article published in Aust. & N.Z. Grapegrower & Winemaker

What are options for managing smoke-exposed fruit?

There are a number of steps that can be taken in the vineyard and winery to minimise the sensory impacts of smoke exposure. These include hand harvesting, excluding leaves, keeping fruit cool, separating press fractions, fining and reverse osmosis treatment. More details are available in the following fact sheets:

• Smoke taint – practical management options for grapegrowers and winemakers (AWRI fact sheet)
• Treating smoke-affected grape juice with activated carbon (AWRI fact sheet)
• Treating smoke-affected wine with activated carbon (AWRI fact sheet)
• Remediation of smoke-affected wine by dilution (AWRI fact sheet)
• What can be done to identify and reduce smoke effect in grape and wine production (DAFWA fact sheet)

Assessing grapes and wines affected by smoke exposure for smoke taint

The AWRI recommends assessing the risk of smoke taint via a combination of analytical testing of grapes and sensory assessment of a small-scale ferment made from the same grapes.

Grape samples should be submitted for analysis of volatile phenols and non-volatile smoke precursors. Tips for sampling, packaging and transport of grapes for smoke analysis are here: Grape sampling, processing and transport following vineyard smoke exposure. Smoke taint analysis is available from a number of laboratories in Australia, listed on Wine Australia’s Fire and smoke assessment page. Details of the smoke taint analysis options available from AWRI Commercial Services to customers located within Australia are here: Smoke taint analysis – fact sheet for Australian customers.

Conducting a small-scale ferment of potentially affected grapes allows wineries to conduct sensory assessment of the small-scale wines and gain further information to help determine the potential risk for smoke taint to develop in wine. Refer to this protocol: Small-lot fermentation method and demonstration video: small-lot fermentation video for information on how to conduct a small-lot fermentation.

Customers located outside of Australia can only submit wine samples (not grapes) for smoke analysis, due to biosecurity requirements. Information on submitting samples to AWRI Commercial Services from outside Australia is here: Smoke taint analysis – fact sheet for international customers.

Smoke taint analysis results and interpretation

Smoke taint analysis results will include the volatile compounds guaiacol, methylguaiacol, ortho-, meta- and para-cresol, syringol and methylsyringol, as well as the non-volatile precursor compounds syringol gentiobioside, methylsyringol gentiobioside, phenol rutinoside, cresol rutinoside, guaiacol rutinoside and methylguaiacol rutinoside. More information on what the analytical results mean, as well as the sensory impact on wine, can be found in this article: Smoke taint analysis and interpretation.

To assist with interpretation of analytical results, the AWRI has established a background database of volatile phenols and precursors collected from grape and wine samples that have not been exposed to smoke. This background data can be compared to the results of potentially exposed fruit to determine the likelihood of the fruit or wine containing elevated concentrations of taint compounds. Australian companies can forward their AWRI analytical results to the AWRI helpdesk on helpdesk@awri.com.au  for assistance with interpretation of results.

The background data for Australian grapes is available on pages 39–42 of this Final Report (pdf download). Information useful for understanding smoke taint results for grapes in the USA can be found in this UC Davis article. For free volatile phenols, analytical results from other laboratories can generally be compared to the AWRI’s volatile phenol background data. For glycoside data, however, analytical results produced using a hydrolysis/GCMS methods are not comparable to results from the AWRI Commercial Services’ LCMS method, as demonstrated during a cross-validation of analytical data between Vintessential Laboratories and AWRI Commercial Services. As such, it is not possible to compare glycoside results between laboratories that use the two different methods or to compare results from a hydrolysis/GCMS method to the AWRI’s background database for glycosides.

For wines that have been analysed after oak treatment, results cannot reliably be compared to background data due to extraction of oak volatile compounds from the oak.

Sensory impacts

Both the volatile phenols and the precursor compounds are known to have a sensory impact on wine. The following fact sheet provides sensory thresholds for smoke taint compounds and additional information about the sensory effects of smoke taint in wine: Sensory impact of smoke exposure (AWRI fact sheet). A study investigating consumer acceptance of wine blends with differing proportions of smoke-affected wine was completed in early 2020, with results summarised here: Case study: consumer acceptance of smoke-affected wines (AWRI fact sheet).

Smoke sensory evaluation

AWRI Commercial Services has a specialised smoke sensory panel that can assess the intensity of smoke-related characters in wine. Visit the Sensory services page for more information.

Managing smoke-affected vineyards

If analysis of grapes shows high levels of smoke markers, growers may choose not to harvest affected blocks for winemaking. Once the decision not to pick has been made, there are a number of factors that should be considered in managing the blocks, to maximise their potential for the following season. It’s important to note that there is no risk of carry-over of smoke taint from one season to the next. The following resources provide advice on management practices for vineyards where grapes are not harvested for winemaking, due to smoke impacts:

• Managing smoke-affected vineyards where fruit is not harvested for winemaking (AWRI fact sheet)
• Options and considerations for dealing with unharvested fruit in smoke-affected vineyards (AWRI webinar recording 27 February 2020)

Fire damage to vineyards

If vines are actually burnt during a bushfire, these resources provide advice on managing damaged vines.

• How to manage burnt or fire damaged vines
• Grapevine recovery after fire – AWRI fact sheet
• Grapevine recovery from fire damage – DPI Vic fact sheet
• Information pack – managing fire-damaged grapevines
• Webinar recording  – Assessing and managing fire-damaged grapevines (7 January 2020)
• Wine Australia podcast recording – Bushfire recovery with Greg Horner of Mt Bera Vineyards
  (22 January 2020)
• Wine Australia case study – Bushfire recovery