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

Understanding the role of varietal thiols in red wine
Sulfur compounds such as 3-mercaptohexanol (3-MH) and 3-mercaptophexyl acetate (3-MHA) are important flavour compounds in white wines, most notably in Sauvignon Blanc, where they contribute ‘tropical fruit’, ‘passionfruit’, ‘grapefruit’ and ‘box hedge’ characters. Much less is known about the sensory significance of thiol compounds in red wines.

A sensory quantitative descriptive analysis study was conducted to assess the effects of varietal thiols in red wine flavour, with the compounds added singly or in combination to a Pinot Noir wine. Earlier analytical data had indicated that Pinot Noir wines can have quite high concentrations of 3MH compared to other red varieties. For the sensory addition study, however, little effect was observed, except when the compound 3MHA was added at a high level (double the highest concentration found in a recent red wine compositional survey), where it gave a ‘box hedge’/‘sweaty’ flavour. This study provided evidence that these thiols are unlikely to contribute to fruit-related attributes in Pinot Noir flavour. In recent AWRI sensory-compositional correlative studies there has been some evidence that 3MH can contribute to ‘green’/‘vegetal’ flavour in Shiraz, and this was also noted in wines from the vineyard foliar spray investigations.

Foliar nutrient sprays and changes in varietal thiols
The influence of foliar sprays on the concentration of potent ‘tropical fruit’ thiol compounds in grapes and wine has been studied over several seasons. Chardonnay wines made from an experiment with a sulfur and urea-containing spray formulation applied in a Barossa Valley vineyard in the 2019/2020 season showed a strong increase in ‘passionfruit’, ‘green grass’ and ‘box hedge’ sensory attributes and in thiol compound concentration, even when only a single spray pass was applied. The wines showed less ‘banana’/’confectionery’, ‘stone fruit’ and ‘floral’ characters than the wines made from unsprayed vines (Figure 9). As part of this study, grapes were also harvested from rows that had been sprayed in the previous season. The wines made from this treatment were found to be not significantly different from the untreated control, showing that the foliar spray did not cause a carry-over effect in the following year.

In 2020/2021 a study was completed with Adelaide Hills Chardonnay fruit, combining the effects of the spray protocol with the use of high and low thiol-producing yeast strains. In addition, trials were undertaken with industry partners to assess foliar sprays under commercial vineyard practice conditions, in Chardonnay and Sauvignon Blanc vineyards in Padthaway. Feedback on the effects on wine flavour and the practicality of the spray regime has been positive.

‘Apricot’ flavour in white wine
To investigate the previously identified link between grape-derived monoterpene compounds and ‘apricot’ aroma in white wines, free and glycosidically bound monoterpenes were measured in berries of two clones of Viognier (Entav 1042 and Montpellier 1968) sampled from sun-exposed and canopy-shaded positions at two sites (one warmer [Riverland] and one cooler [Eden Valley]). Of the five free monoterpenes measured, geraniol was found at the highest concentration for each sample (59 to 148 µg/kg). Similar levels of total free monoterpenes were found for all the Eden Valley samples and for Riverland/Entav/sun-exposed grapes (220-280 µg/kg). However, when comparing the site/clone/exposure pairs, 33 to 50% lower concentrations of total free monoterpenes were found in canopy-shaded grapes than sun-exposed grapes in both clones at the Riverland site. For the bound monoterpenes, monoterpene polyglycosides were the most prevalent, with a 1.3 to 2.0-fold higher concentration found in the sun-exposed grapes for each of the four site/clone/exposure pairs. No difference was seen for geraniol glucoside between any site/clone/exposure pair. Overall, the effect of sun exposure in increasing grape monoterpenes in Viognier grapes was more evident in the Riverland than in Eden Valley. This might have been due to the more substantial canopy on the Riverland vines and/or the later ripening and harvest of Eden Valley fruit during heatwave conditions.

‘Raisin’/’jammy’ flavour in ripe Shiraz
The volatile compounds that cause overripe ‘port-like’ or ‘dried fruit’/‘jammy’ aroma in red wines, and especially in Shiraz, are not well characterised. Sensory assessment of late-harvest Shiraz grape berries from a Barossa Valley vineyard showed an increase in ‘dried fruit’ and ‘jammy’/‘cooked fruit’ attributes compared to the commercial harvest timepoint. Gas chromatography-olfactometry analysis (with human assessors acting as ‘detectors’) was completed and showed key odorants (specifically short-chain aldehydes and ketones and longer chain diketones) were present in grapes that had experienced extended hang-time, or were more shrivelled. Based on these key odorants, analytical methods were developed, including a specialised chemical ionisation procedure. These were used to quantify compounds in grape and wine samples with overripe characters to confirm their importance.

Interactions of volatile compounds with taste and mouthfeel compounds in red wine flavour
The experience of drinking and enjoying a wine arises from the combined input from the senses: sight, smell, taste and touch. While little studied, non-volatile compounds such as tannins and amino acids can interact with volatile aroma compounds to give rise to an integrated perception of desirable wine flavour. The abundant amino acid, proline, was shown in previous studies to be involved in ‘fruit sweetness’, playing a role in enhancing perceived viscosity and fruit flavour.

An existing HPLC analytical method used for quantification of amino acids was complemented with a newly developed rapid nuclear magnetic resonance (NMR) method requiring little sample preparation or instrument time, developed in conjunction with Metabolomics SA. A range of commercially produced red wines were analysed for amino acid concentration and subjected to sensory assessment. In line with recent data from another sample set, some amino acid levels were found be surprisingly high, well above their reported taste thresholds. Interestingly, inland warm climate Cabernet Sauvignon wines had some of the highest concentrations.

To investigate the interactive effects of amino acids with volatiles, colour and tannin, a statistically designed blending study was initiated, involving two lots of Riverland Cabernet Sauvignon wines, with higher and lower amino acids respectively, and an alternative variety red wine with low amino acids but high colour, flavour and tannin. The wines were blended according to a ternary design, and the effect of the different components will be examined to determine optimal levels of flavour and mouthfeel attributes and the relationships with wine composition.