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.
Impact of dissolved CO2 in still wines
Still wines are the only alcoholic beverages that contain significant but sub‐saturated concentrations of carbon dioxide (CO2) – concentrations that are routinely adjusted by winemakers using gas exchange prior to bottling. A previous investigation of the direct impact of dissolved carbon dioxide (DCO2) and how it interacts with the wine matrix to influence the properties of still white wine was furthered by:
- conducting a study on the impact of DCO2 on the sensory properties of red wines
- exploring a consumer perspective by quantifying the dynamics of CO2 loss from the glass during serving and consumption.
As with the previous findings for white wines, higher DCO2 concentrations in red wines decreased bitterness and astringency and increased sweetness while having no significant effect on viscosity, hotness or overall flavour. Importantly, DCO2 did not show consistent interactions with key aspects of the wine matrix in either white (ethanol, acidity) or red wine (ethanol, tannin), implying that the intensity of the spritz sensation elicited by DCO2 has a compelling effect on the tastes and textures of both wine styles.
A previously developed ‘in wine glass’ measuring system was used to assess wine DCO2 as it would be experienced by wine consumers. A study quantified the losses of DCO2 in white wines of varying composition from the time of opening, through pouring and when standing in the glass prior to being consumed. A single white wine was adjusted to two levels of tartaric acid (H2T), ethanol and fructose, creating eight wines of differing composition. For each wine, 150 mL at 7oC was poured into restaurant-style glasses and allowed to stand at room temperature for 15 minutes while regularly undergoing DCO2 analysis. Pouring a wine into the glass resulted in immediate significant reductions in DCO2 compared with the concentration in the bottle, and the DCO2 of the wine in the glass continued to decline in a linear fashion during standing. Increased ethanol in the wine resulted in a significantly higher rate of decrease in DCO2 (Figure 1). However, it is unlikely that most consumers would notice a difference in ‘spritz’ sensation, as even under the highest rate of decrease, it was only after 15 minutes of standing that the decrease in DCO2 was greater than the reported difference threshold of DCO2 (i.e. the concentration difference in CO2 in white wines that is perceptible as a difference in ‘spritz’ by a trained sensory panel).
Does using grape seed powder as a bentonite alternative affect wine sensory properties?
Powdered grape seed has been proposed as a natural, grape-derived alternative to bentonite for producing protein-stable wines, but it is not yet known whether its use affects wine sensory attributes. Since wines fined using grape seed powder (GSP) contain significantly higher concentrations of phenolics, it was considered that this might impart some undesirable sensory traits. To investigate the effect of GSP on wine sensory attributes, two doses, 7.5 g/L (low) and 15 g/L (high) of GSP were applied during the fermentation of Sauvignon Blanc and Semillon juices. Protein concentration in the finished wine was reduced by 4-6% by the low GSP dose and by 37-57% by the high GSP dose. Wines that received the highest dose still had a residual protein concentration greater than that generally associated with heat stability; however, surprisingly, heat stability was significantly improved. This suggests there may be some kind of protective factor in the grape seed preparation contributing to the improved heat stability. Along with higher phenolic concentrations, GSP-treated wines were more yellow in appearance. The Sauvignon Blanc wines treated with GSP were slightly more bitter than the untreated control, but interestingly, were rated higher in viscosity. The GSP-treated Semillon wines were similar to the untreated control in terms of astringency, but the wine that received the highest GSP dose had slightly higher bitterness. With the exception of yellow colour, the overall effects of GSP addition on sensory attributes were generally minor.
Understanding the drivers of negative wine characters
Small-scale white winemaking investigations of the formation of the ‘bitter’/‘hard’ tasting compound tryptophol sulfonate indicated that the compound mostly forms after fermentation and that SO2 concentration and pH influence its formation. Riesling, Gewürztraminer and Chardonnay juices were fermented in single-batch large-scale fermenters using the high tryptophol-producing ‘rose’ yeast. Just prior to bottling, the wines were split and SO2 was added at either 50 or 150 mg/L, and then split again to receive either no addition or a 2 g/L addition of tartaric acid to adjust the pH. Tryptophol sulfonate concentration was found to increase significantly from its pre-bottling levels and was continuing to increase at the latest sampling point nine months post-bottling, with SO2 concentration at bottling being a primary driver of the rate of increase (Figure 2).
As previous work in model white wine had shown that neither the proteinaceous fining agent casein, nor bentonite were effective in removing tryptophol sulfonate, other possible options for removing it from wine were investigated. A medium molecular weight polysaccharide fraction derived from a Chardonnay wine made up of grape-derived arabinogalactans and small mannoproteins (most likely arising from yeast lees contact) was found using nanoparticle tracking analysis to introduce changes in the particle size distribution of tryptophol sulfonate under various ethanol concentrations and pH levels typical of white wines. Based on this information, follow-up work using isothermal titration calorimetry will seek to determine whether this polysaccharide fraction can bind to tryptophol sulfonate, potentially reducing its negative sensory impact in wine. A winemaking trial to assess the factors affecting the development of tryptophol sulfonate in red winemaking and bottle storage, with an additional emphasis on the role of tannins, was also instigated during vintage 2020.
Towards an understanding of ‘savoury’ character in wine
The term ‘savoury’ is often used by wine tasters and consumers to describe complex, high-quality wines. However, to date the molecular drivers and possible origins of ‘savoury’ character in wine are unknown. An initial investigation into the source of the ‘savoury’ character in wine began by searching for the contributors to ‘savoury’ characters in other foods and beverages. The search led to the amino acid glutamic acid, succinic acid and salt as possible contributors to ‘savoury’ character in wine. One hundred Australian wines were surveyed and analysed to establish concentration ranges of glutamic and succinic acid (Figure 3). Salt concentrations were based on the recent Australian literature. Glutamic acid has recently been found to occur above threshold levels in Australian Shiraz wines described by the AWRI sensory panel as ‘umami’-like (described as tasting like broth, meat stock or monosodium glutamate). Tastings of mixtures of these compounds by trained winemakers at wine-like concentrations found that succinic acid elicited a complex sensory profile including ‘bitterness’/’hardness’ and acidity, while all combinations that included glutamic acid were considered ‘savoury’.
Figure 3. Distribution of glutamic acid and succinic acid concentrations in commercially available Australian red and white wines (n=100)
Richard Gawel, Keren Bindon