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.
Factors influencing texture during winemaking
The amount of time juice spends in contact with grape solids can influence the texture of white wines. In the 2017 vintage, Chardonnay and Sauvignon Blanc wines were produced after the juice had been in contact with grape solids for 24, 48 and 72 hours. Methods were optimised to analyse polysaccharides and phenolics in juice and ferment samples to track the changes in composition during winemaking. Wines produced after longer contact time with solids generally showed higher concentrations of polysaccharides, whereas the concentrations of phenolics remained similar regardless of contact time. These results will be compared to sensory analysis outcomes to assess the impact of juice treatment on wine texture.
Dissolved CO2 was recently found to be a factor influencing the textural properties of white wines. This work was expanded to assess the influence of dissolved CO2 on red wines. Shiraz and Cabernet Sauvignon wines were prepared with different levels of CO2, alcohol, tannins and pH to assess the relative influence of each factor. Sensory analysis indicated that sub-spritz concentrations of dissolved CO2 can have a significant influence on red wine mouth-feel.
Understanding the drivers of bitterness in wine
Sulfonated derivatives of indole have previously been identified as important bitter compounds in wines. Factors influencing the formation of these compounds were investigated with the aim of developing ways of better managing wine bitterness during wine production. Model wine studies involving SO2 addition to the indole derivative, tryptophol, found that the bitter compound developed more readily at lower pH (3.2) and higher temperatures (22°C). In grape juice, the process of sulfonation was similar to that in model wine but concentrations were lower. Further studies investigated the influence of pH and temperature on the evolution of phenolics and indole derivatives during white and red winemaking. Results indicated that the concentration of indole derivatives increased during fermentation. Future work will confirm whether sulfonation of phenolics occurs after post-ferment additions of SO2 and subsequent storage.
Sensory analysis using Napping was also conducted on a commercially produced sulfonated indole derivative and a structurally related synthesised compound to assess any sensory effects of the compounds’ differing chemical structures. Both compounds were found to have sensory properties similar to those of other known bitter compounds.
Concentrations of bitter compounds in wine may not have a direct relationship with overall wine bitterness due to potential interactions between bitter compounds and other wine components, particularly macromolecules. Methods were developed to assess the interactions between macromolecules and bitter compounds using nanoparticle tracking analysis (NTA). The particle sizes of isolated polysaccharides in contact with sulfonated tryptophol were consistently smaller than those of the isolated polysaccharides alone. This suggested that the polysaccharides were contracting in size upon interacting with the bitter compound, which in turn suggests that wine bitterness may be influenced by the concentration of polysaccharides.