Smoke Effect in Grapes and Wine

Managing quality in a changing climate

1. Introduction (what)

The on-going presence of atmospheric smoke from bushfires through Victoria has caused concern in a number of different wine grape growing regions throughout south-eastern Australia, in particular Victoria.

This factsheet has been produced in collaboration with the Grape and Wine Research and Development Corporation (GWRDC), Department of Primary Industries (DPI) Victoria, The Australian Wine Research Institute (AWRI) and the Victorian Wine Industry Association (VWIA) to provide information for those grapegrowers and wineries considering sampling grapes, wine or leaves to assess the likelihood of being negatively impacted upon by the smoke.

2. Assessing potential impact (why)

Since 2003, a number of research programs have been initiated across Australia to better understand how atmospheric bushfire smoke is absorbed by the vine and how this ultimately impacts on reducing overall wine quality. Research in 2007 has suggested that the various smoke compounds, of which there are over 3000, appear to enter the vine through the leaves, and are actively translocated to the fruit (Krstic and Whiting 2007).

The chemical composition of bushfire smoke varies greatly depending on the types of native vegetation and actual burning conditions, but guaiacol and 4-methylguaiacol appear to be good ‘indicator’compounds in both grapes and wine of potential smoke exposure and relate in a negative way to the likely impact on overall wine quality. Guaiacol (2-methoxyphenol) has aromas described as smoky, phenolic, aromatic, burnt, and burnt bacon and 4-methylguaiacol has smoky, leather, and spicy aromas.

However, these are only two of the many potential compounds within the bushfire smoke and current research is investigating other potential ‘indicator compounds’. It should also be noted that guaiacol and 4-methylguaiacol are compounds normally imparted to wine from oak and this needs to be taken into account when wine samples are being collected for analysis.

How much exposure to smoke is required to create a smoke taint effect in grapes and wine?

Although research is ongoing in this area, it is interesting to note that a single heavy exposure of smoke to grapevines is sufficient to result in smoke taint in wine, assuming that the exposure occurs sometime between fruitset and harvest. Vines and grapes are highly sensitive to uptake of smoke from veraison to harvest.

Research has also shown that repeated smoke applications to the same vines has resulted in a cumulative effect on the levels of smoke compounds and smoke aromas in wine (Kennison et al. 2008). Therefore, grapes that have repeated smoke exposures or smoke exposures for a long period of time produce wines with an accumulation of smoke aromas and compounds.

3. When and where to sample

Samples can be collected from grapes, leaves, unfermented juice or wine and submitted for testing. What samples are submitted for testing depends on the point in time in the growing season when samples are collected, and the particular party interested in assessing likely smoke impact.

Grapevine leaf and fruit samples should be collected in the morning prior to 10 am. This ensures that samples are collected at times of low temperature and moisture stress, and ensures consistency between different sampling dates.

The most effective time to test levels of guaiacol and 4-methylguaiacol in grapes is as close to harvest date as possible. It should be noted, however, that a finding of low concentrations in grapes at harvest is not a guarantee that any wine made from those grapes will be free from smoke taint. Guaiacol can be released from precursors and bound forms during fermentation.

Research is currently underway to identify these precursors and to develop diagnostic or predictive assays to quantify the potential for release of guaiacol during fermentation, however such assays are not currently available.

For growers and wineries with a number of vineyards that may require sampling and testing, a strategic approach should be adopted. All vineyards do not need to be sampled at the same time. Sample those varieties which ripen the earliest first (e.g. Pinot Noir and Chardonnay). It is important to note that for lighter style varieties such as Pinot Noir, smoke taint characters might be perceived as more pronounced than full bodied styles such as Cabernet Sauvignon and Shiraz. It is important to sample in a procedure that provides the most useful information first.

For example for a number of vineyards of the same grape variety, sample the vines considered to be first exposed to smoke . This is likely to provide an idea of the worst case scenario.

In terms of the different types of samples being submitted for analysis, the following is provided as a guide:

3.1 Grapes:

Previous research has indicated that the variability of guaiacol and 4-methylguaiacol smoke effect ‘indicator’ compounds in an individual vineyard is high. Therefore is important to ensure that a representative sample be collected from the entire vineyard. It is recommended that a random 30 bunch sample from across the entire vineyard is collected and only 1 bunch collected per vine. Once all 30 bunches have been collected, strip approximately half the berries off each bunch and place into a large container. Mix the berries and from this container collect, bag and label a sample of berries weighing approximately 500 grams.

3.2 Leaves and canes:

While entry of these smoke compounds appears to be via the leaves of a grapevine, it is thought that measurement of levels of guaiacol and 4-methylguaiacol in leaves might provide a better indicator of smoke taint potential in both grapes and wine. This is still a research hypothesis being tested this season. However, for those who wish to submit leaf samples for analysis please collect at least 100 grams of approximately 50 leaves (blade and petiole, select fully functioning leaves from the middle of the shoot, avoid old basal and young distal leaves). Ensure that samples are collected from across the entire vineyard and only collect 1 leaf per vine). Collect, bag and label a sample and submit for analysis. Cane samples can also be analysed if required. Please discuss any leaf or cane sampling with experts from the AWRI.

3.3 Unfermented juice:

A 50 mL centrifigued or settled juice can be submitted in labeled plastic containers. Juice samples will need to be frozen prior to despatch and it is advisable to add a protective amount of SO2 (~200 mg/L) to prevent fermentation.

Image (c) The Australian Wine Research Institute. Image supplied courtesy of the AWRI Online Image Collection.

3.4 Wine:

A 50 mL sample of finished wine in either a plastic or glass labeled container can be submitted for analysis. Also, in response to issues associated with the release of bound forms of guaiacol and 4-methylguaiacol during winemaking, a number of winemakers are opting to conduct laboratory-scale ferments in an attempt to better understand the actual impact on final wine. This could give a good indication of a presence of taint and its likely severity. The AWRI has proposed the following protocol. A basic fermentation protocol has been proposed by the AWRI in the 2009 season. This includes:

(i) Sampling

This procedure should be followed for both white and red grapes. A sub-sample of 1 kg of loose berries from the 30 bunch grape sample should be used.

(ii) Crushing, pressing and fermentation

Crush grapes by hand in a clean sealable container using a potato masher. Add potassium metabisulphite (PMS) 50 ppm, Pectic enzyme at twice the recommended rate and diammonium phosphate (DAP) 200 ppm. Prepare and add an active dried wine yeast such as EC1118, 300 mg/L, ferment at 20-25°C, mixing few times per day. It is ideal to maintain a management regime that matches actual large-scale production conditions and ensure the fermentation does not stick or develop reduced characters. Aim for a five day ferment if possible. The majority of guaiacol is usually released during the first four days of fermentation or within the first 8ºBé.

(iii) Post fermentation analysis

Dryness of fermentation can be verified using clinitest tablets. Hand press through a straine

r and aim to extract at least >550 mLs of finished wine. It is preferable to add 60 ppm PMS, cold settle overnight or centrifuge. Submit wine to an experienced sensory panel that is familiar with the identification of smoke taint characters. A 50 mL sample of finished wine in either a plastic of glass labeled container can be submitted for analysis. Wines that are originating from a Phylloxera Infested Zone (PIZ)/Phylloxera Exclusion Zone (PEZ) areas do not need a plant health declaration and can be shipped direct. Use a clean plastic drink size bottle if wines have a high C02 content.