Managing wine extraction, retention, clarity and stability for defined styles and efficient production
The project will investigate:
- the role of macromolecules such as tannins, polysaccharides, proteins and their aggregate colloids, and their impact on stability, clarity, filtration and fouling;
- the impact of other wine matrix components on macromolecule extraction, retention and function;
- the source of these molecules or their precursors in grapes and yeast, and the impact of winemaking processes such as clarification, flotation, vinification and filtration on their retention and/or transformation;
- the impact of fouling of surfaces by macromolecules leading to production inefficiencies;
- alternative strategies for achieving protein stability and cold stability, for example, through use of novel additives and/or processing techniques;
- practical methods for wineries to determine likely extractability of macromolecules during winemaking and the factors that affect extraction and retention (e.g. enzymes, water additions and heat treatments).
Bentonite alternatives for the heat stabilisation of white wines
This year, for the first time, the use of natural zeolite as an alternative to bentonite to remove the principal proteins responsible for white wine haze was evaluated. Zeolite has been widely used as a catalyst, separator, water softener and desiccant, but importantly from a wine production standpoint it meets the requirements for protein adsorption. Zeolite is also easy to use and cheap compared to many other types of commercially available adsorbents. Figure 11 demonstrates the likely mechanism of protein adsorption by zeolite.
Previous studies have shown that many bentonite alternatives require high doses to be effective. For zeolite, a lack of swelling behaviour can reduce lees production relative to a number of commercial bentonites (Figure 12). In terms of effectiveness, the results showed that treatment with zeolite in the size range of 20-50 microns with an exposure time of three hours was sufficient to achieve complete heat stability. Three white wines were tested, and the results are shown in Figure 13. The Semillon wine was fully stabilised by applying 4 g/L of zeolite, while the Sauvignon Blanc and Chardonnay wines required a 6 g/L dose. Furthermore, it was found that zeolite selectively removed potassium (more than 30%), presenting an opportunity to further study the application of zeolite for improved cold stability. The results of this study show that zeolite may offer a promising alternative to bentonite.
Understanding the role of wine macromolecules in cold stability
Red wines are known to be more difficult to cold stabilise through induced crystallisation than white wines, and this has long been understood to be due to the presence of higher concentrations of rhamnogalacturonan II (RGII) and polyphenols such as tannins (Gerbaud et al. 1997). These macromolecules can cooperatively inhibit crystal growth through fouling of the crystal surface.
Interestingly, although yeast mannoprotein products are marketed as cold stabilisation additives for wine, early research (Gerbaud et al. 1997) actually showed that they were less effective than RGII as crystallisation inhibitors. Since RGII naturally occurs in wine and can be modified by winemaking techniques, this project has revisited this earlier work to better understand the relevance of RGII as a natural cold stabilising agent. A study was performed on two cold-unstable white wines which had not received bentonite addition. It was found that the addition of RGII to white wine at wine-like concentrations improved cold stability from a level 3 fail in the three-day cold test (visible crystals to the naked eye) to a pass for one of the wines (Sauvignon Blanc) but was not as successful for the other wine studied (Semillon) which only improved to a level 2 fail (>10 small and/or larger crystals). Both wines were compositionally very similar in terms of polysaccharides, protein, organic acids and potassium. Future research will aim to understand the conditions under which RGII can successfully inhibit potassium bitartrate crystallisation.
A further study aimed to understand the role of tannin structure in red wine cold stability, taking note of the anecdotal evidence that some red wines are initially cold stable, but may lose this stability during processing, bottling and ageing. A key factor which changes during the life of a red wine is the incorporation of anthocyanins as part of the tannin structure to form polymeric pigments. As a result, tannins become less ‘grape-like’, more cross-linked and potentially less reactive. The three-day cold test was conducted on a cold-unstable white wine after the addition of 0.5 or 1 g/L of grape tannin or wine tannin (sourced from a young wine or an aged wine). It was found that grape tannin added at only 0.5 g/L could significantly reduce crystallisation during the cold test. Wine tannin could also markedly reduce crystallisation, but only at higher concentrations. The age of the wine tannin did not change the response (Figure 14). The results suggest that for wines with low tannin concentration, changes to tannin structure that occur during fermentation and ageing may change the cold stability of a wine. This opens a possible alternative role for commercial grape tannin additions during red winemaking, to improve cold stability.
Gerbaud, V., Gabas, N., Blouin, J., Pellerin, P., Moutounet, M. 1997. Influence of wine polysaccharides and polyphenols on the crystallization of potassium hydrogen tartrate. OENO One, 31(2): 65-83.
Mierczynska-Vasilev, A., Wahono, S., Smith, P., Bindon, K., Vasilev, K. 2019. Using zeolites to protein stabilize white wines. ACS Sustain. Chem. Eng. 7(14): 12240-12247.