In summary:
• Trained panels may recognise guaiacol at lower thresholds than untrained panels.
• The taste thresholds for guaiacol, 4-methylguaiacol and 2,6- dimethoxyphenol are lower than the aroma thresholds.
• The taste threshold may have a significant retronasal component.
• The guaiacol threshold in water is lower than the threshold in apple juice or wine.
• There is a large range in detection thresholds between sensory panellists.
• Perceived aromas of single compounds may change depending on the concentration.
• The recognition threshold (correctly identifying the compound) is greater than the detection threshold (e.g. 2µg/L compared to 0.57 µg/L for guaiacol – Siegmund and Pollinger-Zierler 2006)
• Thresholds have been determined in different ways so it is difficult to compare one with another.
Aroma and taste thresholds described here are for individual compounds and there is good evidence that when these compounds are present in mixtures with other compounds, they can be detected when present below these thresholds. Compounds above threshold concentrations can mask the detection of other compounds (Francis and Newton 2005).
Anecdotal reports from winemakers
In a dry white wine, a level of 2-3 µg/L of guaiacol seems to reflect the threshold for detection of smoke taint. At 6 µg/L guaiacol, almost anyone could smell and taste the ‘smoke’ related effect. The conclusion they drew was that guaiacol was generally a good indicator of smoke taint.
Summary of regional seminars in Victoria and the 2006-07 fire situation
A series of regional seminars were conducted in Victoria in April 2007. Presentations were made by a group of researchers from Western Australia who are working in a cooperative project with several agencies, and an update on the Victorian work was also delivered. The Knoxfield seminar attracted 50 attendees, Oxley 34 and Nagambie 19. In addition to the regional seminars a technical group meeting was held at Milawa. This was attended by 26 industry and agency personnel. A summary of the presentations and the feedback from industry is presented below.
Ongoing work being conducted in WA
The research group from Western Australia who attended the meetings in Victoria included Assoc. Prof. Mark Gibberd and Dr Kerry Wilkinson from Curtin University of Technology, and Ms Kristen Kennison and Mr Glynn Ward from the Department of Agriculture and Food.
Smoke taint in grapes and wine, as a consequence of grapevine exposure to smoke compounds, has resulted in financial losses and decline in product quality on numerous occasions for several wine producers throughout Western Australia (WA), Australia and the world. Wine grape regions within WA, such as Manjimup and Pemberton, have increased susceptibility to smoke taint damage due to their proximity to forested areas. Indiscriminate bushfire events have created smoke taint damage to grapevines on several occasions and have affected wine grapes grown in the Perth Hills, Swan Districts, Blackwood Valley, Great Southern, Margaret River, Manjimup and Pemberton regions. Incidence of bushfire generated smoke taint is rising as vineyards are increasingly planted in bushfire susceptible regions.
The Department of Agriculture and Food, WA (DAFWA) in conjunction with Curtin University of Technology, Margaret River, initiated research to investigate the issue of smoke taint in grapes in late 2005. Research was supported through funding by the Grape and Wine Research and Development Corporation (GWRDC). The key objectives of the smoke taint research program in WA were:
.Identify and isolate the specific volatile organic compounds in smoke that produce sensory smoke taint characteristics in grapes and wine by use of quantitative chemical analysis.
.Identify possible key periods during grapevine growth and development when smoke may be of negative effect producing ‘smoky’ aromas in wine.
.Identify grapevine mechanisms of smoke compound assimilation, translocation and storage within organelles with an emphasis on grape berries.
. Determine the effect of smoke applied at various phenological stages of grapevine growth and development.
.Determine the effect of variation in smoke concentration and exposure durations on grapevines.
. Evaluate seasonal and yearly effects of smoke exposure on grapevinegrowth, function and development.
.Investigate possible field based amelioration and protective treatments.
Several experiments have been established to investigate these objectives.
Smoke effects on grape bunches and organoleptic perceptions in wine Research was initiated to demonstrate the effect smoke exposure of grapes had on the chemical composition and sensory characteristics of wine in early 2006. Post harvest wine grape bunches were utilized in this experiment and exposed to smoke in purpose built smoking facilities at Kings Park and Botanic Gardens, Perth (WA). Two wine treatments were made from smoked and un-smoked grapes (1) free-run juice treatment (2) free-run juice on skins treatment. Quantitative chemical analysis of wines for guaiacol, 4- methylguaiacol, 4-ethylguaiacol, 4-ethylphenol, eugenol, furfural, 5- methylfurfural and vanillin has been conducted by the Australian Wine Research Institute (AWRI). Sensory wine analysis of difference tests and best estimate thresholds were also performed. Wines vinified from smoked grapes exhibited ‘smoky’, ‘dirt’, ‘earthy’, ‘burnt’ and ‘smoked meat’ aroma characters.
Effect of smoke exposure and duration on field-grown grapevines and subsequent wine quality
Experiments performed in early 2006 focused on the field application of smoke to grapevines from the period of veraison to harvest. Smoke was applied in both single and successive field applications to grapevines to measure the chemical composition and sensory characteristics of resultant wines. Prior to this experiment, smoke had not been applied in a field situation to grapevines to measure the effects on chemical compounds in grapes and wine. Furthermore, the effect of timing and duration of smoke exposure to grapevines had not been reported with concentration of associated smoke compounds in wines unknown.
Field based treatments applied smoke to Merlot vines for (1) successive smoke applications applied to the same vines two times per week from veraison to harvest and (2) single smoke application applied to previously unsmoked vines two times per week from veraison to harvest. In each treatment, smoke was applied at veraison then at 3, 7, 10, 15, 18, 21 and 24 days post veraison. Quantitative chemical analysis of wines for guaiacol, 4- methylguaiacol, 4-ethylguaiacol, 4-ethylphenol, furfural, eugenol, vanillin and 5-methylfurfural was conducted by the AWRI. Sensory analysis of wine with a trained sensory panel is currently in progress.
Smoke application to grapevines at various phenological stages of growth and development
Field based applications of smoke were applied to separate Merlot grapevine plots at key stages of grapevine growth and development including leaves 10cm, flowering, berries pea sized, beginning of bunch closure, beginning of veraison, berries full colour and at harvest. Smoke was applied to grapevines for 30 min per day on two successive days. Sensory and quantitative chemical analysis of field experiment wines will be conducted in 2007.
Effect of smoke timing and duration on grape berries
A rapid model for the application of smoke to grape berries has been developed for future experimentation. One such experiment initiated in 2006-07 growing season investigates the influence of smoke on waxed versus un-waxed grape berries. Smoke was applied to both waxed and unwaxed berries at two times prior to veraison and two times post veraison.
Smoke treatments were applied to berries for durations for 5, 10, 20, 40 and 80 minutes. Quantitative chemical analysis of samples from this trial is currently being performed by AWRI.
