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 taste and texture during winemaking
The evolution of polysaccharides during the entire process of white winemaking was determined for the first time. In Sauvignon Blanc and Chardonnay (Figure 10, Sauvignon Blanc shown), the concentration of medium molecular weight (MW) polysaccharides previously shown to contribute positively to white wine texture by reducing hotness and increasing perceived viscosity was reduced during settling off grape solids and significantly reduced from the middle stages of fermentation to pre-bottling. The concentration of high MW polysaccharides (consisting mainly of mannoproteins) that assist in protein and cold stability was stable during juice settling but increased throughout fermentation and through to the pre-bottling stage. The concentration of low MW polysaccharides thought to be extracted from skin fragments increased steadily throughout the winemaking process.
Understanding the drivers of negative wine characters
Recent investigations indicated that tryptophol sulfonate contributes negatively to white wine by adding bitterness and hotness. Knowledge of its formation and interfering factors will therefore assist in developing strategies to reduce its impact. To facilitate this, a simpler, cheaper, faster and more sensitive method to measure tryptophol sulfonates and related compounds in red and white wines using HPLC with fluorescence detection was developed.
Factors influencing tryptophol sulfonation were explored using Gewürztraminer and Chardonnay wines. After fermentation using a newly developed high tryptophol-producing ‘rose’ yeast, the effects of wine pH and SO2 were investigated by monitoring the samples over time. Sulfonated tryptophol formed more readily at high pH (3.6) compared to low pH (3.2) and more so in Gewürztraminer (200% increase at high pH compared to the control) than in Chardonnay (30% increase). Importantly, sulfonation of tryptophol mostly occurred post-ferment following SO2 addition.
A further investigation was conducted with large-scale ferments to assess the potential impact of sulfonation on sensory profiles. Riesling, Gewürztraminer and Chardonnay grapes from 2019 were fermented in single-batch, large-scale ferments using the high tryptophol-producing ‘rose’ yeast. After ferment, wines were split into triplicate 20 L batches and different concentrations of SO2 and acid were added to give wines with high and low doses of SO2 at high and low pH. Wines will be monitored to assess the formation of tryptophol sulfonate until sensory analysis at 12 months post-bottling.
Wine polysaccharides may influence the perception of bitter molecules in wine due to interactions that prevent the molecules from interacting with oral receptors. To assess this possibility, nanoparticle tracking analysis was used to assess the interaction between tryptophol sulfonate and characterised polysaccharides previously derived from white wine. The particle size distribution of medium and low molecular weight polysaccharides was generally shifted upward when tryptophol sulfonate was added, suggesting that the taste-active tryptophol sulfonate interacts with wine polysaccharides, potentially reducing its bitter impact. Further work to confirm the exact nature of this interaction will be conducted.
The protein casein used to remove phenolic-induced bitterness from white wine was trialled as a potential fining agent for tryptophol sulfonates. Using typical fining rates, casein was partially effective in removing low levels of tryptophol sulfonates, but it was ineffective at removing tryptophol sulfonates at levels typically found in Australian wines. Moreover, the discovery that tryptophol sulfonates mostly form late in the winemaking process practically rules out the use of traditional amelioration steps including protein fining.