Identification and control of volatile compounds responsible for important sensory attributes
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.
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
To improve understanding of the importance and control of the potent ‘tropical fruit’ thiol compounds through common viticultural practices, studies of foliar sprays were completed over several seasons. Such sprays, including basic nitrogen and sulfur formulations, applied in Chardonnay or Shiraz vineyards close to veraison were found to have a major influence on the concentrations of thiols in finished wines.
The 2019 vintage Shiraz wines made from fruit sourced from vines sprayed with a lower dose nitrogen and sulfur formulation had only minor sensory differences compared to the control wines, with a higher ‘cooked vegetal’ attribute, while a higher dose spray gave strong undesirable sulfidic-related attributes including ‘rubber’ and ‘drain’, as well as lower colour intensity. For the Chardonnay wines, both dose rates had a strong effect on ‘tropical fruit’ aroma and flavour in the wines, and also contributed a sulfur compound-related ‘struck flint’ character. Overall, the Chardonnay wines made from treated vines showed more desirable sensory properties than the Shiraz wines, confirming the results from a previous vintage. Wines from a final study assessing a single foliar spray to Chardonnay vines were made during the 2020 vintage. Such an approach, where only one spray application needs to be made per season, may be more practical for growers to adopt.
Riesling flavour: TDN and aged character
Riesling flavour characteristics, especially after some time in bottle, can be affected by the presence of TDN (1,1,6-trimethyl-1,2-dihydronaphthalene), sometimes described as ‘kerosene-like’. If TDN is present early in a Riesling wine’s life it can be considered detrimental. A collaboration with Hochschule Geisenheim University, Germany continued, with a series of investigations into viticultural and other influences on the propensity of Riesling vineyards to give wines with elevated TDN. A viticultural experiment involving shade cloth of different colours found that TDN concentration diminished with the influence of shade cloth; black and green shade cloth gave a greater reduction of TDN than red shade cloth, with few other sensory effects. The colour of the shade cloth significantly affected the composition of the grapes and wine, specifically around the wavelengths that induce changes for chlorophyll and/or carotenoids. This provides a practical means of limiting the contribution of TDN in wine made from grapes grown in warmer climates.
‘Apricot’ flavour in white wine
A sensory-compositional study of the relative effects of region of origin, harvest date and grapevine clone was used to probe ‘apricot’ flavour in Viognier and the monoterpenes related to this flavour found in many white wines. Region of origin was the most important factor, but clone and harvest date were also significant, with one clone giving a higher level of ‘apricot’ aroma and flavour. This result has potential to provide producers with knowledge to apply in decisions on future plantings or harvest timing.
‘Raisin’/’jammy’ flavour in ripe Shiraz
The volatile compounds that cause ‘raisin’ or ‘cooked fruit’ aroma, especially in late-picked Shiraz, are not well understood. Ripe and overripe grape samples and wines were assessed using a sensory panel, detailed quantitative volatile analysis and GC-olfactometry analysis (with individuals acting as ‘detectors’ to smell compounds as they are emitted from a gas chromatograph instrument). These investigations showed that several compounds are implicated in ‘raisin’/‘cooked fruit’ characters. Further work will be conducted to confirm the role of these compounds.
The effect of volatile compounds in Shiraz flavour compared to taste, mouthfeel and texture compounds
Responses to wine flavour involve multisensory perception, with the experience of drinking a wine involving the senses of smell, taste and touch, and also usually the visual sense. To assess the role of volatile compounds in the overall flavour experience, the interactions between the volatile fraction of a Shiraz wine, a tannin/anthocyanin extract and taste-active amino acids were assessed by a trained sensory panel at concentrations matching those of a full-bodied Shiraz wine. Interestingly, volatiles made the greatest contribution to sensory properties such as body or viscosity, while the tannin fraction had little effect on these characteristics. The presence of amino acids, previously largely unrecognised as being important to wine flavour, led to greater sweetness and a suppression of bitter taste, with proline, the major amino acid found in wine, having the largest effect on sweetness. This result could have important implications for better control of so-called ‘fruit sweetness’ in dry red wines.
Assessment of rapid, alternative sensory methods
Quantitative sensory descriptive analysis is a powerful and precise sensory method that allows detailed characterisation of sensory properties across a set of wines, but has the major disadvantage of requiring many days to complete. The results obtained from alternative, much faster methods were compared with those obtained from sensory descriptive analysis. Six Shiraz wines of varying styles were characterised by four different methods:
- sensory descriptive analysis (DA) with a highly trained sensory panel
- the rapid ‘check all that apply’ method using a panel of 35 semi-trained assessors
- the ‘rate all that apply’ (RATA) method using a panel of 35 semi-trained assessors
- the rapid polarised sensory positioning method using the trained panel.
The rapid methods gave similar overall results to the time-consuming DA, with RATA showing the greatest promise as a rapid alternative, being more amenable to conventional, shorter statistical analysis and giving the most detailed results. Polarised sensory positioning, which involves assessors rating the degree of difference of each wine against three reference wines, showed promise as a sensitive means of assessing overall sensory differences using a relatively small panel in a single session, although with less informative outcomes regarding attributes that differed.
Artist in residence: Elizabeth Willing
In 2019/2020 the AWRI hosted an artist in residence as part of an Australian Network for Art and Technology program, which aims to bring art and science together in a mutually beneficial collaboration. The visiting artist, Elizabeth Willing, was interested in exploring cross-sensory perceptions of colours and shapes related to wine sensory properties. Elizabeth, based in Brisbane, has a background in using food and beverages in her art practice. A projective mapping procedure was completed with the six Shiraz wines used in the rapid sensory methodology evaluations discussed above, to assess associations of wine flavour with colours and shapes. While all wines in the study were Shiraz, the colours that tasters associated with the wines when tasted in black glasses were surprisingly well-defined. Figure 1 shows that the Canberra District wine, which was relatively acidic, was associated with red colours, while the ‘vegetal’/‘stalky’ Yarra Valley wine was mapped in the green part of the spectrum. The high ‘dark fruit’ flavoured McLaren Vale wine was associated with deeper blue/purple colours, while the Shiraz-Grenache blend with ‘floral’ and ‘confection’ characters was related to yellow shades. The panel also separately marked each wine on a scale for associations with smooth, curving shapes versus angular, spiky shapes, and here the wines with higher astringency were rated as more spiky, while the softer, slightly sweeter wines were rated as more curved.
The outcomes from this type of assessment could be used by producers to align packaging and marketing cues to match wine sensory perception, with a view to enhancing the tasting experience. A small preference study (n=38 assessors) was also conducted with three of the wines tasted under three conditions: white sensory booth, booth decorated with a graphic matching one of the wines, and booth with a graphic matching another of the wines. The results suggest that the graphics affected preference, providing insight into options for differentially increasing enjoyment for in-store tastings or cellar door settings.
Elizabeth developed artworks relating to the wines studied, some of which feature on the cover of this report. She also worked with a Brisbane hospitality venue to showcase her wine-related art and intends to develop a larger exhibition stemming from her experiences at the AWRI.
Investigating compounds responsible for ‘plastic’ and ‘mousy’ off-flavours
Further understanding has been gained about the compounds responsible for two off-flavours in wine – described respectively as ‘popcorn’/‘mousy’ and ‘plastic’/‘mothball’/’jonquil’ – through evaluating historical data and by analysing wines with these characters. Analysis of data from 108 sparkling ferments revealed a clear link between indole concentration and the sensory panel response for the ‘mothball’ character, with sparkling wines classified as affected by this character having significantly higher concentrations of indole than those where the off-flavour was not evident.
The role of o-aminoacetophenone and other similar compounds in indole-related off-flavour was also assessed, with this compound quantified in 16 wines representing a range of intensities of indole-like off-flavour. No detectable levels were found in any of the wines, demonstrating that this volatile compound was not a contributor to the off-flavour. Indole itself, however, was clearly implicated in the off-flavour. From further GC-olfactometry of three affected wines, there was an indication of other compounds contributing indole-like aroma, with identification of these compounds underway. As part of this work, an improved and efficient analytical method for o-aminoacetophenone was developed and validated, with the analyte included in the method used for ‘mousy’ compounds.
In related work, three white wines shown to have ‘mousy’ off-flavour based on sensory data were analysed for compounds associated with this off-flavour after a period of exposure to air, to investigate the possibility of an oxidative pathway for their formation. Acetyl tetrahydropyridine (ACTPY) was found to be increased in two of the three wines. A small survey of nine fortified wines (a style of wine sometimes affected by ‘mousy’ characters) was also completed, where not detected or negligible concentrations of ACPY (acetyl pyrroline) and AP (acetyl pyridine) were found. Evidence from the last 12 months indicates that ACTPY is the main compound responsible for ‘mousy’ off-flavour.