Project 3.1.1

Identification and control of volatile compounds responsible for important sensory attributes

Project summary

The flavour of wine is determined to a large extent by volatile compounds that are perceived during consumption by the sense of smell. The overall flavour of a wine, as well as specific flavour notes, is provided by dozens of naturally occurring chemical compounds of widely varying potency and concentration.

Within this project, the formation of, and factors which influence aroma compounds recently found to be responsible for ‘stone fruit’ (apricot-peach), which is common in Chardonnay and other white varieties, will be investigated, with fermentation and vineyard studies and analyses of commercially produced wines.

The role of the capsicum-like methoxypyrazine compounds in Shiraz wines, previously considered not relevant to green flavour in this variety, will be explored through whole bunch fermentation studies, analysis of stalks and leaves, and determination of the genetic basis of the expression and regulation of this metabolite in grape bunches through collaborative work with CSIRO.

Jammy/raisin flavour will be studied in collaboration with Project 4.1.1 (Managing the impact of vintage advancement and compression). The identity of the compound(s) that give overripe jammy/raisin flavour of Shiraz, and more generally the effect of bunch exposure, have not been adequately established. The lack of knowledge of the compound(s) responsible for this flavour is a major gap in setting a measurable target in viticultural projects, especially given increases in growing season temperatures and vintage compression. The effect of bunch exposure on this and other flavour compounds, including TDN which contributes bottle-aged character to Riesling wines, will also be examined in this project.

The role of thiols and other sulfur compounds in red varieties will be determined. Foliar nitrogen and sulfur vineyard sprays have been shown to have the potential to positively affect thiol concentrations in wine, and will be investigated.

Working with the rotundone mapping Project 4.4.4, the compound causing ‘musk’ and (non-pepper) spice in Shiraz will be investigated. In addition, investigations of Shiraz wines sourced from the rotundone (4.4.4) and terroir projects (3.3.1, 4.4.1) will allow determination of new or less understood volatiles that are key to premium wine flavour.

The effect of blending alternative grape varieties with established varieties produced in the Riverina, Riverland and Murray Valley regions will be investigated, to provide enhanced flavour characteristics, acid, colour, and weight to these types of wines. The project will also have a component whereby off-flavours and taints will be identified and studied.

Current practice in sensory evaluation of research wines uses sensory descriptive analysis, and while considered the most powerful and sophisticated method available, requires several weeks of data generation and substantial time for data analysis. In recent years, alternative faster methods have been developed in food science applications. This project will evaluate these protocols to determine their utility in wine studies and for wider industry use. Advantages of using untrained consumers compared to trained panellists will also be assessed. The project will also investigate technology for simulating wine experiences in lifelike environments, to better capture consumer responses.

Latest information

Methoxypyrazines in Shiraz: the role of whole bunch fermentations and concentration in stalks
Previous studies showed that the inclusion of stalks in a Shiraz fermentation can result in elevated concentrations of methoxypyrazines, compounds that give a ‘green’ herbaceous character to wines. These compounds were not previously thought to be important to Shiraz wines. To investigate this further, methoxypyrazines were measured in the rachis (stalk) of Shiraz grape bunches sampled during ripening. Methoxypyrazine concentrations declined post-veraison, but remained relatively high at maturity, and were highest in the rachis from vines that had previously been shown to produce wines with ‘green’ flavour. Grape berries had negligible levels of pyrazines. To assess the influence of whole bunches in winemaking, an experiment was conducted with Shiraz and Pinot Noir grapes from the 2018 vintage, using different ratios of whole bunches to crushed/destemmed fruit.

Understanding the role of varietal thiols in red varieties
Thiol compounds are known to be important flavour compounds in Sauvignon Blanc and other white varieties, where they contribute ‘tropical fruit’, ‘passionfruit’, ‘grapefruit’ and ‘box hedge’ characters. Much less is known about the role of these compounds in red wines. A survey was conducted of the concentration of thiols in 105 commercially produced red wines from across Australia. The wines included Shiraz, Cabernet Sauvignon, Grenache, Merlot and Pinot Noir as well as examples of Malbec, Durif, Tempranillo, Mataro and Petit Verdot. For two of the thiols quantified, none of the wines had a detectable concentration. For the compound 3-mercaptohexanol (3-MH), generally described as having a ‘grapefruit’ aroma, all wines had a concentration higher than the sensory detection threshold measured in model wine, indicating the possible contribution of this compound to red wine flavour. Figure 8 shows a summary of the results for the red wines, and it seems from these data that Pinot Noir wines can have higher concentrations of this compound. While only a small number of Grenache wines were analysed, there was some evidence that the concentration was also higher for this variety, possibly pointing to a possible contribution from 3-MH to the ‘red berry’ character of Pinot Noir and Grenache wines. There were higher levels in some examples of each variety, and further work will investigate causes for these differences. It is known that 3-MH can be susceptible to oxidation, and the formation of its precursors can be influenced during post-harvest grape handling. Figure 8 also gives an indication of the confidence interval for the median concentration for each variety. For Pinot Noir and Mataro, the confidence intervals are large, reflecting the wide variation in 3-MH concentration found for these varieties. Further work is planned to investigate this result.

One factor known to affect thiol concentration in wines is nitrogen fertilisation, either in the soil or through foliar sprays. A project investigating the effect of foliar application of nitrogen and sulfur on Shiraz and Chardonnay grapevines is underway, with two doses applied. The sprays have been shown to have been taken up by the vines, with increased concentrations of amino acids and subsequently, yeast assimilable nitrogen. Finished wines will be assessed to determine any effects on varietal thiol concentrations and amino acid profiles.

Riesling flavour: TDN and aged character
The compound TDN (1,1,6-trimethyl-1,2-dihydronaphthalene) is important to the flavour of bottle-aged Riesling wines, but can be considered a negative character in young wines. The formation of TDN was studied in 2016, assessing the effects of bunch zone sun exposure and vineyard site temperature in the Barossa and Eden Valleys. Accelerated ageing experiments were also conducted with Riesling wine, investigating both sensory and chemical effects. Results shaped a vineyard study conducted in 2018, looking at modulating light quality at the bunch zone. Wines made from sun-exposed Riesling berries had much higher concentrations of TDN than those made from shaded berries. Wines have also been produced by collaborators in Germany, using winemaking protocols aligned with those in Australia, to assess whether viticultural or winemaking influences might be of greater importance to TDN development in wine during bottle ageing.

‘Stone fruit’ flavour in white wine
Following earlier work where several terpene compounds were shown to be involved in ‘apricot’ character in white wines, studies have been carried out to determine the formation of these compounds in grapes of four Viognier clones during ripening. Samples were taken from veraison to after harvest in an Eden Valley vineyard. Two clones showed similar monoterpene concentration profiles throughout ripening but the other two clones were substantially lower in monoterpene concentration. Winemaking studies will be undertaken to investigate how these berry compositional differences relate to wine composition and sensory properties.

‘Raisin’/’jammy’ flavour in ripe Shiraz
While the propensity of Shiraz grapes to shrivel under periods of high temperatures late in the season is well known, the chemical compounds related to ‘overripe’, ‘raisin’ or ‘jammy’ characters are not established. Work has started to better understand heat-related reactions that occur to produce compounds that may contribute to these sensory attributes. Model systems have been studied, as well as heated grape material. Working with collaborators at SARDI, overripe berries from both sun-exposed and shaded bunches have been subjected to sensory analysis, with ranking tests showing clear differences in ‘jammy’ flavour among the samples, which will be related to gas chromatography-sniffing experiments to identify compounds involved in the sensory differences.

Assessment of rapid, alternative sensory methods
The technique known as ‘Pivot© Profile’ has scope for use in industry as well as research. It is a free description method, where judges use a ‘pivot’ wine as a reference to assess a set of samples. For each wine, judges note descriptive terms that are more or less intense than the pivot. A simple count of terms allows interpretation of how the wines in the set differ, and a map can be created to easily visualise the results.

In 2017, Wine Australia invited 50 eminent sommeliers to Australia for a series of events. This provided a chance to assess the descriptors used by international wine professionals for Shiraz wines using the Pivot© Profile method. In addition, it was an excellent opportunity to measure the preferences of this group, and learn more about their thoughts on Australian Shiraz. A wide range of ultra-premium Australian Shiraz wines, up to $250 retail price and sourced from multiple regions, were tasted by this group, as well as, in a later session, a group of Australian winemakers.

The two groups of judges who used the Pivot© Profile method (international sommeliers and Australian winemakers) generated highly similar groupings of the wines, showing that the method is robust, and not dependent on judge background. The Pivot© Profile method also related closely to the conventional sensory descriptive analysis method applied at the AWRI, although this method, which involves rating the intensities of defined attributes, gave more detailed and statistically analysable data.

Further sets of wines have been examined using Pivot© Profile, and there have been similar successful results. The great advantage of this method is its rapidity, requiring only one session to complete, compared to descriptive analysis which can require six or more sessions. The main challenge with the method is the data entry and the relatively complex data analysis. Other rapid methods, such as Napping or Rate All That Apply, have also been investigated. Napping (also called projective mapping) involves sorting and grouping wines on a grid, and seems to give a somewhat different insight into sensory differences, emphasising certain attributes at the expense of others. Further work is planned to evaluate Napping in more detail.