Molecular drivers of wine texture and taste
This project continues key elements of current research and will realise opportunities for identifying compounds that lead to positive and negative taste and texture outcomes, throughout wine production. Such negative characters can occur through different stages of the wine production process, from grapegrowing (temperature and exposure impacts), throughout processing, and also post-bottling.
Increasingly the premiumisation of red and white wine is inseparable from the concept of texture as it defines style and typicality (the interaction between terroir and winemaking practice). It has been proposed that in-mouth texture defines the ‘typicality’ of many of the most valuable commercial wines of the world, for example the creaminess of barrel fermented white Burgundy, the oily texture of Alsatian Pinot Gris made from high solids juices, and the oily and drying nature of Viognier made with skin contact from the Northern Rhone, or the rich full-bodied expression of Shiraz produced in the Barossa. It could also be argued that the high value placed on these wines by consumers is the result of a perception of uniqueness of some sensory property, whether it be flavour or texture, associated with a particular region or vineyard site. In terms of taste, many European and new Australian styles of red wines, are positively characterised by a savouriness, but despite knowledge of molecular drivers of savoury (e.g. umami) flavours in foods, similar compounds have not yet been characterised or their functions defined in wines. Compounds described by ‘mouthfulness’, or ‘kokumi’ have also been characterised in foods but not in wine, but evidence exists that such compounds may be present in wines.
Understanding the drivers of negative wine characters
Formation of the bitter/‘hard’ tasting compound tryptophol sulfonate was investigated post-bottling in Chardonnay, Riesling and Gewurztra¬miner wines, and its concentration was found to stabilise after approximately nine months. The results further confirmed that wine SO2 concentration during bottle storage (50 vs 150 mg/L) was a significant driver of tryptophol sulfonate formation. Threshold testing revealed a heterogeneous distribution across tasters, with only around 1 in 10 tasters being able to taste tryptophol sulfonate at the concentrations found in the white wines at 18 months post-bottling. Subsequent formal sensory analysis of the white wines using an unscreened panel found no significant differences in bitterness among the white wines with different tryptophol sulfonate concentrations.
A follow-up study was conducted with Shiraz fruit fermented using the same high tryptophol-producing yeast as the white wine study and adjusted to two SO2 concentrations (40 and 110 mg/L) and two pH values (3.4 and 3.7) prior to bottling. The study showed that as for the white wines, little tryptophol sulfonate was formed during winemaking, but in contrast to the white wine study, the compound did not increase in the red wines post-bottling, regardless of SO2 concentration or pH. This is most likely due to binding of SO2 by red wine components.
Proteinaceous fining agents were previously found to be ineffective in removing tryptophol sulfonate from wine, so as an alternative, the potential for increased concentrations of wine polysaccharides to mitigate the bitter sensory impact of tryptophol sulfonate was explored. Such an increase of polysaccharide concentration in wine can be achieved by the addition of commercial polysaccharide additives or by fermentation on solids or by yeast lees contact. To test this possibility, isothermal calorimetry was used to explore interactions between tryptophol sulfonate and three polysaccharide fractions taken from white wine – a high molecular weight mannoprotein fraction, a small molecular weight fraction containing rhamnogalacturonans, and a medium molecular weight polysaccharide fraction containing arabinogalactan proteins and low molecular weight mannoproteins. The medium molecular weight polysaccharide fraction is known to affect taste and perception of hotness. The results of the trial suggested that tryptophol sulfonate did not bind with these polysaccharides at the concentrations found in wine. The difficulty of removing tryptophol sulfonate from wine once it has formed suggests that prevention strategies, including selection of low tryptophol-producing yeast, and/or judicious application of SO2 pre-bottling should be employed to manage its formation.
Discovery of a potentially new bitter compound in white wine
Phenolic fractions were isolated from a bitter-tasting white wine prepared from hard pressings. A non-targeted metabolomics approach was used to analyse the composition of these fractions and identify compounds that correlated with their perceived bitterness. A hexose ester of coumaric acid was identified as a suspected bitterant. The molecule was synthesised, and the synthesis process scaled up to produce sufficient quantities for sensory assessment, with its identity and purity validated by NMR. A tasting of the compound in model wine by experienced assessors indicated that it was a potential bitterant. Subject to further sensory assessment, the preparative-scale synthesis methodology will be applied to produce similar compounds of the same class for further bitterness assessment.
Towards an understanding of ‘savoury’ character in wine
The term ‘savoury’ is synonymous with complex, high-quality wines. While the molecular drivers of ‘savoury’ character in wine are uncertain, the amino acid glutamic acid, succinic acid, and sodium and potassium salts are likely to contribute to ‘savouriness’ in wine either directly or most likely by interaction. The concentration ranges of these compounds (Figure 10) were determined based on a survey of Australian and international wines, and the information gained will be incorporated into future sensory experimental plans.
Impact of dissolved CO2 in still wines
Still wines contain sub‐saturated concentrations of carbon dioxide (CO2) which are modified by winemakers prior to bottling to achieve a desired level, depending on wine type and style. Previous sensory investigations on the interactions of dissolved carbon dioxide (DCO2) with the wine matrix in still wine showed that DCO2 predominantly contributes to wine texture by adding a ‘spritz’ character. To further explore the perception of DCO2, a model mouth system that mimicked the hydrophilicity of mouth surfaces was developed to determine CO2 ingress through different thicknesses of human saliva. Thicker salivary layers were more effective in reducing CO2 ingress when DCO2 levels were high (1.5 g/L). At CO2 concentrations more typical of still red and white table wine (0.375 g/L) salivary layer thickness did not influence CO2 ingress, suggesting that individual differences in CO2 perception would be more pronounced in wines that are semi-sparkling/’spritzy’ than in still table wines.
Richard Gawel, Keren Bindon